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Sir M.Farooque Soomro

RN, BSN (LUMHS), B.Com (UOS)

CARDIOVASCULAR

SYSTEM

UNIT-VIII

Anatomy & Physiology

Generic BSN 1^st year, 1^st semester

OBJECTIVES

At the completion of this unit, students will be able to:

1. Define blood and list its functions.
2. Describe the composition, sites of production and functions of cellular parts of blood and plasma.
3. Briefly explain the ABO blood groups & Rh factor.
4. Describe the location, structure and functions of the heart and its great blood vessels.
5. Discuss the blood flow through the heart.
6. Describe the structure and functional features of the conducting system of the heart.
7. Describe the principal events of a cardiac cycle.
8. Explain the structure and function of Arteries, Veins & Capillaries.
9. Describe the following types of blood circulation: Pulmonary circulation & Systemic circulation (coronary & hepatic portal circulation).

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Cardiovascular System

• The cardiovascular (cardio – heart, vascular – blood vessels) system, also known as the circulatory system is a network of organs and vessels that transport blood throughout the body. It divided into three main parts:
• The heart
• The blood
• The blood vessels

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1. Define blood and list its functions

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The blood

• Blood is a fluid connective tissue, it circulates continually around the body.
• Blood in the blood vessels is always in motion because of the pumping action of the heart. The continual flow maintains a fairly constant environment for body cells.
• Study of blood is called hematology.

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Blood’s functions

1. Oxygen transport: Delivers oxygen from lungs to body tissues.
2. Nutrient transport: Carries nutrients, vitamins, and minerals to cells.
3. Waste removal: Removes waste products, such as carbon dioxide and urea.
4. Regulation: Maintains body temperature (100.4⁰F ), pH (7.35 to 7.45) and water balance.
5. Immune response: Transports WBCs to fight infections.
6. Clotting: Platelets help form blood clots to stop bleeding.

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2. Describe the composition, sites of production and functions of cellular parts of blood and plasma

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Composition of Blood & Plasma

• Blood makes up about 7% of body weight (about 5.6 litres in a 70 kg man). This proportion is less in women
• Plasma (55%): liquid portion, moslty water, proteins, and nutrients.
• Cellular content of blood (45%)
• Red blood cells (41%)
• White blood cell (3%)
• Platelets (1%)

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A. The proportions of blood cells and plasma in whole blood separated by gravity. B. A blood clot in serum.

Plasma (55%)

• The constituents of plasma are water (90–92%) and dissolved and suspended substances, including:
1. Plasma proteins
2. Inorganic salts
3. Nutrients
4. Waste materials
5. Hormones
6. Gases

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• Plasma Protien: Make up 7% of plasma, Responsible for osmotic pressure to keep blood within circulation & Plasma viscosity is due to plasma protein mainly albumin and fibrinogen.
• Albumin: (54%) formed in liver, main function is to maintain osmotic pressure. Albumin also acts as carrier molecule for lipids and steroid hormones.
• Fibrinogen: (7%) is synthesis in liver and essential for blood coagulation.
• Electrolytes: These have a range of functions, including muscle contraction (e.g. Ca²⁺), transmission of nerve impulses (e.g. Ca²⁺ and Na⁺), and maintenance of acid–base balance (e.g. phosphate).
• Nutrients: he products of digestion, e.g. glucose, amino acids, fatty acids and glycerol, are absorbed from the alimentary tract. Together with mineral salts and vitamins they are used by body cells for energy, heat, repair and replacement, and for the synthesis of other blood components and body secretions.

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• Waste product: Urea, creatinine and uric acid are the waste products of protein metabolism. They are formed in the liver and carried in blood to the kidneys for excretion. Carbon dioxide from tissue metabolism is transported to the lungs for excretion.
• Hormones: These are chemical messengers synthesised by endocrine glands. Hormones pass directly from the endocrine cells into the blood, which transports them to their target tissues and organs elsewhere in the body, where they influence cellular activity.
• Gases: Oxygen, carbon dioxide and nitrogen are transported round the body dissolved in plasma. Oxygen and carbon dioxide are also transported in combination with haemoglobin in red blood cells. Most oxygen is carried in combination with haemoglobin and most carbon dioxide as bicarbonate ions dissolved in plasma). Atmospheric nitrogen enters the body in the same way as other gases and is present in plasma but it has no physio­logical function.

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Cellular content of blood 45%

• There are three types of blood cell
• Erythrocytes (red blood cells) 41%
• Leukocytes (white blood cells) 3%
• Platelets (Thrombocytes) 1%

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A blood smear, showing erythrocytes, a monocyte, a neutrophil, a lymphocyte and a platelet.

Erythrocytes (RBCs)

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The red blood cell. A. Under the light microscope. B. Drawn from the front. C. Drawn in section. D. Coloured scanning electron micrograph of a group of red blood cells travelling along an arteriole.

Life cycle of the erythrocyte

• Erythrocytes (RBCs): The most abundant type of blood cell; 99% of all blood cells are erythrocytes.
• They are biconcave discs with no nucleus, and their dia­meter is about 7 µm.
• Their main function is in gas transport, mainly of oxygen, but they also carry some carbon dioxide.
• Life span 120 days, then destroyed in spleen (graveyard of RBCs).
• 1 RBC contains 280 million hemoglobin molecules.
• There are approximately 30 trillion red blood cells in the average human body.
• The immature cells are released into the bloodstream as reticulocytes, and mature into erythrocytes over a day or two within the circulation. During this time, they lose their nucleus and therefore become incapable of division 

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Haemoglobin (Hb)

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The haemoglobin molecule.

• Haemoglobin is a large, complex molecule containing a globular protein (globin) and a pigmented iron-containing complex called haem. Each haemoglobin molecule contains four globin chains and four haem units, each with one atom of iron. As each atom of iron can combine with an oxygen molecule, this means that a single haemoglobin molecule can carry up to four molecules of oxygen. When all four oxygen-binding sites on a haemoglobin molecule are full, it is described as saturated. Haemoglobin binds reversibly to oxygen to form oxyhaemoglobin, according to the equation:

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• Blood rich in oxygen (usually arterial blood) is bright red because of the high levels of oxyhaemoglobin it contains, compared with blood with lower oxygen levels (usually venous blood), which is dark bluish in colour because it is not saturated.

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Eukocytes (WBCs): These cells have an important function in defence and immunity. Leukocytes are the largest blood cells but they account for only about 1% of the blood volume. They contain nuclei and some have granules in their cytoplasm. There are two main types:

1. Granulocytes (polymorphonuclear leukocytes)
• Neutrophils
• Eosinophils
• Basophils
2. Agranulocytes
• Monocytes
• Lymphocytes

Rising white cell numbers in the bloodstream usually indicate a physiological problem, e.g. infection, trauma or malignancy.

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Granulocytes (polymorphonuclear leukocytes):

• During their formation, granulopoiesis, they follow a common line of development through myeloblast to myelocyte before differentiating into the three types.
• All granulocytes have multilobed nuclei in their cytoplasm.
• Their names represent the dyes they take up when stained in the laboratory.
• Eosinophils take up the red acid dye.
• Basophils take up alkaline methylene blue.
• Neutrophils are purple because they take up both dyes.

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• Neutrophils: These small, fast and active scavengers protect the body against bacterial invasion, and remove dead cells and debris from damaged tissues. lobes, and their granules are lysosomes containing enzymes to digest engulfed material. Neutrophils live on average 6–9 hours in the bloodstream. Pus that may form in an infected area consists of dead tissue cells, dead and live microbes, and phagocytes killed by microbes.
• Eosinophils: Capable of phagocytosis, are less active in this than neutrophils; their specialised role appears to be in the elimination of parasites, such as worms, which are too big to be phagocytosed. They are equipped with certain toxic chemicals, stored in their granules, which they release when the eosinophil binds to an infecting organism. Eosinophils are often found at sites of allergic inflammation, such as the asthmatic airway and skin allergies. There, they promote tissue inflammation by releasing their array of toxic chemicals, but they may also dampen down the inflammatory process through the release of other chemicals, such as an enzyme that breaks down histamine.

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• Basophils: Closely associated with allergic reactions, contain cytoplasmic granules packed with heparin (an anticoagulant), histamine (an inflammatory agent) and other substances that promote inflammation.
• Usually the stimulus that causes basophils to release the contents of their granules is an allergen (an antigen that causes allergy) of some type.
• This binds to antibody-type receptors on the basophil membrane. A cell type very similar to basophils, except that it is found in the tissues, not in the circulation, is the mast cell.
• Mast cells release their granule contents within seconds of binding an allergen, which accounts for the rapid onset of allergic symptoms following exposure to, for example, pollen in hay fever.

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Agranulocytes

• The monocytes and lymphocytes make up 25 to 50% of the total leukocyte count.
• They have a large nucleus and no cytoplasmic granules.

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Monocytes

• These are the largest of the white blood cells. Some circulate in the blood and are actively motile and phagocytic while others migrate into the tissues where they develop into macrophages. Both types of cell produce interleukin 1, which:
• acts on the hypothalamus, causing the rise in body temperature associated with microbial infections
• stimulates the production of some globulins by the liver enhances the production of activated T-lymphocytes.
• Macrophages have important functions in inflammation and immunity.

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The monocyte macrophage system

• The monocyte–macrophage system, also known as the mononuclear phagocyte system (MPS), is a Important part of the immune system that consists of monocytes in the bloodstream and their differentiated form, macrophages, in tissues.
• This system plays an essential role in both innate and adaptive immunity.

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Lymphocytes

• Lymphocytes are smaller than monocytes and have large nuclei. They circulate in the blood and present in great numbers in lymphatic tissue such as lymph nodes and the spleen. Lymphocytes develop from pluripotent stem cells in red bone marrow and from precursors in lymphoid tissue, then travel in the blood to lymphoid tissue elsewhere in the body where they are activated, i.e. they become immuno competent which means they are able to respond to antigens.
• Examples of antigens include:
1. cells regarded by lymphocytes as abnormal.
2. pollen from flowers and plants fungi
3. bacteria
4. some large molecule drugs, e.g. penicillin, aspirin.

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Platelets (thrombocytes)

• These are very small non-nucleated discs, 2 to 4 μm in diameter, derived from the cytoplasm of megakaryocytes in red bone marrow.
• They contain a variety of substances that promote blood clotting, which causes haemostasis (cessation of bleeding).
• The normal blood platelet count is between 200 000 to 350 000/mm3.
• The control of platelet production is not yet entirely clear but one stimulus is a fall in platelet count.
• The kidneys release a substance called thrombopoietin, which stimulates platelet synthesis.
• The life span of platelets is between 8 and 11 days and those not used in haemostasis are destroyed by macrophages, mainly in the spleen.
• About a third of platelets are stored within the spleen rather than in the circulation; this is an emergency store that can be released as required to control excessive bleeding.

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3. Briefly explain the ABO blood groups & Rh factor.

The ABO blood group system and the Rh factor are essential components of human blood typing, which are important for safe blood transfusions and other medical procedures.

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ABO blood group system

• The ABO blood group system classifies human blood into four main types (A, B, AB, and O) based on the presence or absence of antigens (A and B) on the surface of red blood cells.
• Type A: Has A antigens on the surface of red blood cells and anti-B antibodies in the plasma.
• Type B: Has B antigens on the surface and anti-A antibodies in the plasma.
• Type AB: Has both A and B antigens on the surface and no anti-A or anti-B antibodies (universal recipient).
• Type O: Has no A or B antigens on the surface but has both anti-A and anti-B antibodies in the plasma (universal donor).

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ABO blood group system

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ABO blood group system

• The Rh factor refers to another antigen, the Rh antigen (D antigen), present on red blood cells.
• If a person has the Rh antigen, they are Rh-positive (Rh+); if not, they are Rh-negative (Rh−).
• The combination of ABO type and Rh factor (e.g., A+, O−) determines a person’s complete blood type.
• Very important for pregnancy because Rh incompatibility between an Rh-negative mother and an Rh-positive fetus can lead to complications such as hemolytic disease of the newborn.
• Also important in transfusions to ensure compatibility.

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4. Describe the location, structure and functions of the heart and its great blood vessels

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Location of the Heart

• The heart is located in the thoracic cavity, specifically in the mediastinum, which is the central compartment of the chest betw’p;een the lungs.

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• It sits slightly left of the midline, with about two-thirds of its mass on the left side.

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• The apex points downward and to the left, while the base is directed upward.

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Base of heart

Apex of heart

Structure of the Heart

The heart is a muscular organ responsible. It is composed of four main chambers and several key structures.

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Structure of the Heart

• Layers: these are the three layer of the heart
1. Pericardium: A protective sac surrounding the heart, consisting of an outer fibrous layer and an inner serous layer.
2. Myocardium: The thick, muscular middle layer responsible for the pumping action.
3. Endocardium: The smooth inner layer that lines the chambers and valves.

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Structure of the Heart

• Chambers: Four chambers two atria (upper chambers) and two ventricles (lower chambers).
1. Right Atrium: Receives deoxygenated blood from the body via the superior and inferior vena cava.
2. Right Ventricle: Pumps deoxygenated blood to the lungs through the pulmonary arteries.
3. Left Atrium: Receives oxygenated blood from the lungs via the pulmonary veins.
4. Left Ventricle: Pumps oxygenated blood to the body through the aorta.

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Structure of the Heart

• Valves: these are the four valve of the heart
1. Tricuspid Valve: Between the right atrium and right ventricle.
2. Pulmonary Valve: Between the right ventricle and the pulmonary artery.
3. Mitral (Bicuspid) Valve: Between the left atrium and left ventricle.
4. Aortic Valve: Between the left ventricle and the aorta.

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Structure of the Heart

• Blood Vessels Associated with the Heart

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• Aorta: The main artery carrying oxygen-rich blood from the left ventricle to the body.
• Pulmonary Arteries: Carry deoxygenated blood from the right ventricle to the lungs.
• Pulmonary Veins: Bring oxygenated blood from the lungs to the left atrium.
• Superior and Inferior Vena Cava: Large veins that carry deoxygenated blood from the body to the right atrium.

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Heart Structure

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Function of the Heart

• Pumping Blood: The heart functions as a dual pump: Right Side: Pumps deoxygenated blood to the lungs for oxygenation. Left Side: Pumps oxygenated blood to the entire body.

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• Regulating Blood Flow: Valves ensure unidirectional blood flow and prevent backflow.

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• Maintaining Blood Pressure: The strong contraction of the ventricles generates pressure that propels blood through the arteries.

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5. Discuss the blood flow through the heart

Blood flow through the heart is an important process that ensures that oxygenated and deoxygenated blood is efficiently circulated through the body and lungs.

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• Deoxygenated blood from the body enters the heart through the superior vena cava (from the upper body) and inferior vena cava (from the lower body).
• The blood flows into the right atrium, which is the first chamber that deoxygenated blood encounters.
• When the right atrium contracts, blood passes through the tricuspid valve into the right ventricle.
• The right ventricle fills with blood and contracts to push the blood into the pulmonary circuit.
• Oxygenated blood from the lungs flows back to the heart via the pulmonary veins.
• The blood enters the left atrium, which is the receiving chamber for oxygen-rich blood.
• The left atrium contracts, and blood flows through the mitral valve into the left ventricle.
• The left ventricle, with its thick muscular walls, contracts powerfully to pump blood into the systemic circulation.

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• As the oxygen-rich blood reaches tissues and organs, oxygen and nutrients are exchanged for carbon dioxide and metabolic waste.
• The deoxygenated blood returns to the heart through the venous system, completing the cycle.
• Summary of Blood Flow Path
• Body → Superior/Inferior Vena Cava → Right Atrium
• Right Atrium → Tricuspid Valve → Right Ventricle
• Right Ventricle → Pulmonary Valve → Pulmonary Arteries → Lungs
• Lungs → Pulmonary Veins → Left Atrium
• Left Atrium → Mitral Valve → Left Ventricle
• Left Ventricle → Aortic Valve → Aorta → Body

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Direction of blood flow through the heart

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6. Describe the structure and functional features of the conducting system of the heart

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The conducting system of the heart

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• The heart possesses the property of autorhythmicity, which means it generates its own electrical impulses and beats independently of nervous or hormonal control, i.e. it is not reliant on external mechanisms to initiate each heartbeat.

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• It is supplied with both sympathetic and parasympathetic autonomic nerve fibres, which increase and decrease respectively the intrinsic heart rate.

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• In addition, the heart responds to a number of circulating hormones, including adrenaline (epinephrine) and thyroxine.

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• Small groups of specialised neuromuscular cells in the myocardium initiate and conduct impulses, causing coordinated and synchronised contraction of the heart muscle.

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Components and their structure

Sinoatrial node (SA node)

• This small mass of specialised cells lies in the wall of the right atrium near the opening of the superior vena cava.
• The sinoatrial cells generate these regular impulses because they are electrically unstable. This instability leads them to discharge (depolarise) regularly, usually between 60 and 80 times a minute.
• This depolarisation is followed by recovery (repolarisation), but almost immediately their instability leads them to discharge again, setting the heart rate. Because the SA node discharges faster than any other part of the heart, it normally sets the heart rate and is called the pacemaker of the heart.
• Firing of the SA node triggers atrial contraction.

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Atrioventricular node (AV node)

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• This small mass of neuromuscular tissue is situated in the wall of the atrial septum near the atrioventricular valves.

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• Normally, the AV node merely transmits the electrical signals from the atria into the ventricles. There is a delay here; the electrical signal takes 0.1 of a second to pass through into the ventricles. This allows the atria to finish contracting before the ventricles start.

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• The AV node also has a secondary pacemaker function and takes over this role if there is a problem with the SA node itself, or with the transmission of impulses from the atria. Its intrinsic firing rate, however, is slower than that set by the SA node (40–60 bpm).

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Atrioventricular bundle (AV bundle or bundle of His)

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• This is a mass of specialised fibres that originate from the AV node.

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• The AV bundle crosses the fibrous ring that separates atria and ventricles then, at the upper end of the ventricular septum, it divides into right and left bundle branches.

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• Within the ventricular myocardium the branches break up into fine fibres, called the Purkinje fibres.

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• The AV bundle, bundle branches and Purkinje fibres transmit electrical impulses from the AV node to the apex of the myocardium where the wave of ventricular contraction begins, then sweeps upwards and outwards, pumping blood into the pulmonary artery and the aorta.

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Nerve supply to the heart

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• As mentioned earlier, the heart is influenced by autonomic (sympathetic and parasympathetic) nerves originating in the cardiovascular centre in the medulla oblongata.

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• The vagus nerves (parasympathetic) supply mainly the SA and AV nodes and atrial muscle. Parasympathetic stimulation reduces the rate at which impulses are produced, decreasing the rate and force of the heartbeat.

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• The sympathetic nerves supply the SA and AV nodes and the myocardium of atria and ventricles. Sympathetic stimulation increases the rate and force of the heartbeat.

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The main factors affecting heart rate

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• Gender
• Autonomic (sympathetic and parasympathetic) nerve activity
• Age
• Circulating hormones, e.g. adrenaline (epinephrine), thyroxine
• Activity and exercise
• Temperature
• The baroreceptor reflex
• Emotional states

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7. Describe the principal events of a cardiac cycle

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• At rest, the healthy adult heart is likely to beat at a rate of 60–80 bpm.
• During each heartbeat, or cardiac cycle, the heart contracts and then relaxes.
• The period of contraction is called systole and that of relaxation, diastole.
• Taking 74 bpm as an example, each cycle lasts about 0.8 of a second and consists of:
• Atrial systole – contraction of the atria
• Ventricular systole – contraction of the ventricles
• Complete cardiac diastole – relaxation of the atria and ventricles.

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The stages of the cardiac cycle

• The SA node triggers a wave of contraction that spreads over the myocardium of both atria, emptying the atria and completing ventricular filling (atrial systole 0.1 s).
• When the electrical impulse reaches the AV node it is slowed down, delaying atrioventricular transmission. This delay means that the mechanical result of atrial stimulation, atrial contraction, lags behind the electrical activity by a fraction of a second. This allows the atria to finish emptying into the ventricles before the ventricles begin to contract. After this brief delay, the AV node triggers its own electrical impulse, which quickly spreads to the ventricular muscle via the AV bundle, the bundle branches and Purkinje fibres.
• This results in a wave of contraction which sweeps upwards from the apex of the heart and across the walls of both ventricles pumping the blood into the pulmonary artery and the aorta (ventricular systole 0.3 s).
• The high pressure generated during ventricular contraction is greater than that in the aorta and forces the atrioventricular valves to close, preventing backflow of blood into the atria.

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• After contraction of the ventricles there is complete cardiac diastole, a period of 0.4 seconds, when atria and ventricles are relaxed. During this time the myocardium recovers in preparation for the next heartbeat, and the atria refill in preparation for the next cycle.

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• The valves of the heart and of the great vessels open and close according to the pressure within the chambers of the heart.

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• The AV valves are open while the ventricular muscle is relaxed during atrial filling and systole.

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• When the ventricles contract there is a rapid increase in the pressure in these chambers, and when it rises above atrial pressure the atrioventricular valves close.

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• When the ventricular pressure rises above that in the pulmonary artery and in the aorta, the pulmonary and aortic valves open and blood flows into these vessels.

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• When the ventricles relax and the pressure within them falls, the reverse process occurs. First the pulmonary and aortic valves close, then the atrioventricular valves open and the cycle begins again.

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• This sequence of opening and closing valves ensures that the blood flows in only one direction. This figure also shows how the walls of the aorta and other elastic arteries stretch and recoil in response to blood pumped into them.

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The elasticity of the wall of the aorta.

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Heart Sounds

• Sound 1 (S1): "Lubb" sound due to the closing of AV valves (beginning of ventricular systole).
• Sound 2 (S2): "Dubb" sound due to the closing of semilunar valves (beginning of diastole).

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Electrical changes in the heart (ECG)

• P Wave: Atrial depolarization leading to atrial systole.
• QRS Complex: Ventricular depolarization leading to ventricular systole.
• T Wave: Ventricular repolarization during diastole.

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Cardiac output

• The cardiac output is the amount of blood ejected from each ventricle every minute. The amount expelled by each contraction of each ventricle is the stroke volume. Cardiac output is expressed in litres per minute (l/min) and is calculated by multiplying the stroke volume by the heart rate (measured in beats per minute):

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Cardiac output = Stroke volume x Heart rate

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• In a healthy adult at rest, the stroke volume is approximately 70 ml and if the heart rate is 72 per minute, the cardiac output is 5 l/minute. This can be greatly increased to meet the demands of exercise to around 25 l/minute, and in athletes up to 35 l/minute. This increase during exercise is called the cardiac reserve.

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• When increased blood supply is needed to meet increased tissue requirements of oxygen and nutrients, heart rate and/or stroke volume can be increased 

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8. Explain the structure and function of Arteries, Veins & Capillaries

Arteries, veins, and capillaries are the three main types of blood vessels in the circulatory system, each with a unique structure and function.

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Blood Vessels

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Arteries

Structure

1. Arteries have thick, muscular walls composed of three layers:
2. Tunica intima: The innermost layer, lined with smooth endothelial cells, helps reduce friction as blood flows through.
3. Tunica media: The middle layer, made up of smooth muscle and elastic tissue, allows the artery to expand and contract, maintaining blood pressure.
4. Tunica adventitia: The outer layer of connective tissue provides structural support.

Functions

1. Arteries carry oxygenated blood away from the heart to various tissues and organs. (The pulmonary arteries are an exception; they carry deoxygenated blood from the heart to the lungs for oxygenation.)
2. Due to their thick walls, arteries can handle the high pressure generated by the heart during blood ejection (systole).
3. Arteries branch into smaller arterioles, which then connect to capillaries.

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Veins

Structure

1. Veins have thinner walls compared to arteries and are less muscular.
2. Tunica intima: The innermost layer is smooth, similar to arteries, but thinner.
3. Tunica media: The muscle layer is much thinner than in arteries, since veins do not need to withstand high pressure.
4. Tunica adventitia: The outer layer is also composed of connective tissue but is less robust than that of arteries.
5. Veins contain valves to prevent backflow of blood, especially in the limbs, because blood pressure in veins is much lower.

Functions

1. Veins carry deoxygenated blood from the tissues back to the heart. (The pulmonary veins are an exception; they carry oxygenated blood from the lungs to the heart.)
2. Blood flow in veins is assisted by muscle contractions (skeletal muscle pump) and the action of the valves to ensure one-way flow.

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Capillaries

Structure

1. Capillaries are the smallest and thinnest of all blood vessels. They have walls that are only one cell thick, consisting of a single layer of endothelial cells.
2. Their thin walls allow for the exchange of materials between blood and tissues.

Functions

1. Capillaries are the site of gas, nutrient, and waste exchange between blood and tissues. Oxygen and nutrients from the blood diffuse through the capillary walls into the surrounding tissues, while carbon dioxide and metabolic wastes move from the tissues into the blood.
2. Capillaries connect arterioles to venules, forming a bridge between the arterial and venous systems.

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Summary

• Arteries: Carry oxygenated blood away from the heart at high pressure.

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• Veins: Return deoxygenated blood to the heart at low pressure, often with the help of valves.

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• Capillaries: Facilitate the exchange of oxygen, nutrients, and wastes between blood and tissues.

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Together, these vessels form a network that ensures the efficient circulation of blood, delivering oxygen and nutrients to tissues and removing waste products.

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9. Describe the following types of blood circulation: Pulmonary circulation & Systemic circulation (coronary & hepatic portal circulation)

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Blood circulation is the process by which the heart pumps blood throughout the body, delivering oxygen and nutrients to cells and removing waste products like carbon dioxide.

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The heart pumps blood into two anatomically separate systems of blood vessels i.e: the pulmonary circulation & the systemic circulation.

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Pulmonary Circulation

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Pulmonary Circulation: this is the circulation of blood from the right ventricle of the heart to the lungs and back to the left atrium. In the lungs, carbon dioxide is excreted and oxygen is absorbed.

Arterial supply

• The pulmonary artery or trunk, carrying deoxygenated blood, leaves the upper part of the right ventricle of the heart. It passes upwards and divides into left and right pulmonary arteries at the level of the 5th thoracic vertebra.
• The left pulmonary artery runs to the root of the left lung where it divides into two branches, one passing into each lobe.
• The right pulmonary artery passes to the root of the right lung and divides into two branches. The larger branch carries blood to the middle and lower lobes, and the smaller branch to the upper lobe.
• Within the lung these arteries divide and subdivide into smaller arteries, arterioles and capillaries. The exchange of gases takes place between capillary blood and air in the alveoli of the lungs. In each lung the capillaries containing oxygenated blood join up and eventually form two pulmonary veins.

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Venous drainage

• Two pulmonary veins leave each lung, returning oxygenated blood to the left atrium of the heart.
• During atrial systole this blood is pumped into the left ventricle, and during ventricular systole it is forced into the aorta, the first artery of the general circulation.

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Systemic or general circulation

• The blood pumped out from the left ventricle is carried by the branches of the aorta around the body and returns to the right atrium of the heart by the superior and inferior venae cavae.

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The general positions of the aorta and the main arteries of the limbs.

The venae cavae and the veins of the limbs.

The aorta

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The aorta

• The aorta begins at the upper part of the left ventricle and, after passing upwards for a short way, it arches backwards and to the left.
• It then descends behind the heart through the thoracic cavity a little to the left of the thoracic vertebrae.
• At the level of the 12th thoracic vertebra it passes behind the diaphragm then downwards in the abdominal cavity to the level of the 4th lumbar vertebra, where it divides into the right and left common iliac arteries.
• Throughout its length the aorta gives off numerous branches. Some of the branches are paired, i.e. there is a right and left branch of the same name, for instance, the right and left renal arteries supplying the kidneys, and some are single or unpaired, e.g. the coeliac artery.
• The aorta will be described here according to its location: Thoracic aorta & Abdominal aorta.

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Thoracic aorta

• This part of the aorta lies above the diaphragm and is described in three parts:
• Ascending aorta.
• Arch of the aorta.
• Descending aorta in the thorax.

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1. Ascending aorta

• This is the short section of the aorta that rises from the heart. It is about 5 cm long and lies behind the sternum.
• The right and left coronary arteries are its only branches and they arise from the aorta just above the level of the aortic valve.
• These important arteries supply the myocardium.

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2. Arch of the aorta

• The arch of the aorta is a continuation of the ascending aorta. It begins behind the manubrium of the sternum and runs upwards, backwards and to the left in front of the trachea. It then passes downwards to the left of the trachea and is continuous with the descending aorta.
• Three branches are given off from its upper aspect:
• Brachiocephalic artery or trunk
• Left common carotid artery
• Left subclavian artery.

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Brachiocephalic artery

• The brachiocephalic artery is about 4 to 5 cm long and passes obliquely upwards, backwards and to the right.
• At the level of the sternoclavicular joint it divides into the right common carotid artery and the right subclavian artery.

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Circulation of blood to the head and neck

• The paired arteries supplying the head and neck are the common carotid arteries and the vertebral arteries.

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Main arteries of the left side of the head and neck.

The right vertebral artery.

Carotid arteries

• The right common carotid artery is a branch of the brachiocephalic artery. The left common carotid artery arises directly from the arch of the aorta. They pass upwards on either side of the neck and have the same distribution on each side.
• At the level of the upper border of the thyroid cartilage each divides into an internal carotid artery and an external carotid artery.

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• The carotid sinuses are slight dilations at the point of division (bifurcation) of the common carotid arteries into their internal and external branches. The walls of the sinuses are thin and contain numerous nerve endings of the glossopharyngeal nerves. These nerve endings, or baroreceptors, are stimulated by changes in blood pressure in the carotid sinuses. The resultant nerve impulses initiate reflex adjustments of blood pressure through the vasomotor centre in the medulla oblongata.
• The carotid bodies are two small groups of specialised cells, called chemoreceptors, one lying in close association with each common carotid artery at its bifurcation. They are supplied by the glossopharyngeal nerves and their cells are stimulated by changes in the carbon dioxide and oxygen content of blood. The resultant nerve impulses initiate reflex adjustments of respiration through the respiratory centre in the medulla oblongata.

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External carotid artery

• This artery supplies the superficial tissues of the head and neck, via a number of branches:
• The superior thyroid artery supplies the thyroid gland and adjacent muscles.
• The lingual artery supplies the tongue, the lining membrane of the mouth, the structures in the floor of the mouth, the tonsil and the epiglottis.
• The facial artery passes outwards over the mandible just in front of the angle of the jaw and supplies the muscles of facial expression and structures in the mouth. The pulse can be felt where the artery crosses the jaw bone.
• The occipital artery supplies the posterior part of the scalp.
• The temporal artery passes upwards over the zygomatic process in front of the ear and supplies the frontal, temporal and parietal parts of the scalp. The pulse can be felt in front of the upper part of the ear.
• The maxillary artery supplies the muscles of mastication and a branch of this artery, the middle meningeal artery, runs deeply to supply structures in the interior of the skull.

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Internal carotid artery

• This is a major contributor to the circulus arteriosus (circle of Willis), which supplies the greater part of the brain. It also has branches that supply the eyes, forehead and nose. It ascends to the base of the skull and passes through the carotid foramen in the temporal bone.
• The circulus arteriosus is formed by:
• 2 anterior cerebral arteries
• 2 internal carotid arteries
• 1 anterior communicating artery
• 2 posterior communicating arteries
• 2 posterior cerebral arteries
• 1 basilar artery.
• From this circle, the anterior cerebral arteries pass forward to supply the anterior part of the brain, the middle cerebral arteries pass laterally to supply the sides of the brain, and the posterior cerebral arteriessupply the posterior part of the brain.
• Branches of the basilar artery supply parts of the brain stem.

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Venous return from the head and neck

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Veins of the left side of the head and neck.

• The venous blood from the head and neck is returned by deep and superficial veins.
• Superficial veins with the same names as the branches of the external carotid artery return venous blood from the superficial structures of the face and scalp and unite to form the external jugular vein.
• The venous blood from the deep areas of the brain is collected into channels called the dural venous sinuses.
• The main venous sinuses are listed below:
• The superior sagittal sinus
• The inferior sagittal sinus
• The straight sinus
• The transverse sinuses
• The sigmoid sinuses

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• The brachiocephalic veins are situated one on each side in the root of the neck. Each is formed by the union of the internal jugular and the subclavian veins. The left brachiocephalic vein is longer than the right and passes obliquely behind the manubrium of the sternum, where it joins the right brachiocephalic vein to form the superior vena cava 

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• The superior vena cava, which drains all the venous blood from the head, neck and upper limbs, is about 7 cm long. It passes downwards along the right border of the sternum and ends in the right atrium of the heart.

Circulation of blood to the upper limb

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The main arteries of the right arm.

The subclavian arteries

• The right subclavian artery arises from the brachiocephalic artery; the left branches from the arch of the aorta. They are slightly arched and pass behind the clavicles and over the first ribs before entering the axillae, where they continue as the axillary arteries.
• Before entering the axilla, each subclavian artery gives off two branches: the vertebral artery, which passes upwards to supply the brain, and the internal thoracic artery, which supplies the breast and a number of structures in the thoracic cavity.
• The axillary artery is a continuation of the subclavian artery and lies in the axilla. The first part lies deeply; then it runs more superficially to become the brachial artery.

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• The brachial artery is a continuation of the axillary artery. It runs down the medial aspect of the upper arm, passes to the front of the elbow and extends to about 1 cm below the joint, where it divides into the radial and ulnar arteries.
• The radial artery passes down the radial or lateral side of the forearm to the wrist. Just above the wrist it lies superficially and can be felt in front of the radius, as the radial pulse. The artery then passes between the first and second metacarpal bones and enters the palm of the hand.
• The ulnar artery runs downwards on the ulnar or medial aspect of the forearm to cross the wrist and pass into the hand.
• There are anastomoses between the radial and ulnar arteries, called the deep and superficial palmar arches, from which palmar metacarpal and palmar digital arteries arise to supply the structures in the hand and fingers.

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Venous return from the upper limb

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The main veins of the right arm. Dark blue indicates deep veins.

The veins of the upper limb are divided into two groups: deep and superficial veins.

1. The deep veins follow the course of the arteries and have the same names:
1. Palmar metacarpal veins
2. Deep palmar venous arch
3. Ulnar and radial veins
4. Brachial vein
5. Axillary vein
6. Subclavian vein.
2. The superficial veins begin in the hand and consist of the following:
1. Cephalic vein
2. Basilic vein
3. Median vein
4. Median cubital vein.

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3. Descending aorta in the thorax

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The aorta and its main branches in the thorax

• Descending aorta: This part of the aorta is continuous with the arch of the aorta and begins at the level of the 4th thoracic vertebra. It extends downwards on the anterior surface of the bodies of the thoracic vertebrae to the level of the 12th thoracic vertebra, where it passes behind the diaphragm to become the abdominal aorta.
• The descending aorta in the thorax gives off many paired branches which supply the walls of the thoracic cavity and the organs within the cavity, including the:
• Bronchial arteries that supply the bronchi and their branches, connective tissue in the lungs and the lymph nodes at the root of the lungs
• Oesophageal arteries, supplying the oesophagus
• Intercostal arteries, that run along the inferior border of the ribs and supply the intercostal muscles, some muscles of the thorax, the ribs, the skin and its underlying connective tissues.

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Venous return from the thoracic cavity

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The superior vena cava and the main veins of the thorax

• Most of the venous blood from the organs in the thoracic cavity is drained into the azygos vein and the hemiazygos vein. Some of the main veins that join them are the bronchial, oesophageal and intercostal veins. The azygos vein joins the superior vena cava and the hemiazygos vein joins the left brachiocephalic vein. At the distal end of the oesophagus, some oesophageal veins join the azygos vein, and others the left gastric vein.

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Abdominal aorta

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The abdominal aorta and its branches

• The abdominal aorta is a continuation of the thoracic aorta. The name changes when the aorta enters the abdominal cavity by passing behind the diaphragm at the level of the 12th thoracic vertebra. It descends in front of the bodies of the vertebrae to the level of the 4th lumbar vertebra, where it divides into the right and left common iliac arteries.

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When a branch of the abdominal aorta supplies an organ it is only named here and is described in more detail in association with the organ. However, illustrations showing the distribution of blood from the coeliac, superior and inferior mesenteric arteries are presented here

Many branches arise from the abdominal aorta, some of which are paired and some unpaired.

Paired branches

• Inferior phrenic arteries supply the diaphragm.
• Renal arteries supply the kidneys and give off branches, the suprarenal arteries, to supply the adrenal glands.
• Testicular arteries supply the testes in the male.
• Ovarian arteries supply the ovaries in the female.
• The testicular and ovarian arteries are much longer than the other paired branches, because these organs begin their development in the region of the kidneys. As they grow, they descend into the scrotum and the pelvis respectively, and are accompanied by their blood vessels.

Unpaired branches

• The coeliac artery is a short thick artery about 1.25 cm long. It arises immediately below the diaphragm and divides into three branches:
• The left gastric artery supplies the stomach
• The splenic artery supplies the pancreas and the spleen
• The hepatic artery supplies the liver, gall bladder and parts of the stomach, duodenum and pancreas.
• The superior mesenteric artery branches from the aorta between the coeliac artery and the renal arteries. It supplies the whole of the small intestine and the proximal half of the large intestine.
• The inferior mesenteric artery arises from the aorta about 4 cm above its division into the common iliac arteries. It supplies the distal half of the large intestine and part of the rectum.

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Venous return from the abdominal organs

• The inferior vena cava is formed when the right and left common iliac veins join at the level of the body of the 5th lumbar vertebra. This is the largest vein in the body, and it carries blood from all parts of the body below the diaphragm to the right atrium of the heart. It passes through the central tendon of the diaphragm at the level of the 8th thoracic vertebra.
• Paired testicular, ovarian, renal and adrenal veins join the inferior vena cava.
• Blood from the remaining organs in the abdominal cavity passes through the liver via the portal circulation before entering the inferior vena cava

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Circulation to the pelvis and lower limb

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The femoral artery and its main branches.

Common iliac arteries: The right and left common iliac arteries are formed when the abdominal aorta divides at the level of the 4th lumbar vertebra. In front of the sacroiliac joint each divides into the internal and the external iliac arteries.

• The internal iliac artery runs medially to supply the organs within the pelvic cavity. In the female, one of the largest branches is the uterine artery, which provides the main arterial blood supply to the reproductive organs.
• The external iliac artery runs obliquely downwards and passes behind the inguinal ligament into the thigh where it becomes the femoral artery.
• The femoral artery begins at the midpoint of the inguinal ligament and extends downwards in front of the thigh; it then turns medially and eventually passes round the medial aspect of the femur to enter the popliteal space where it becomes the popliteal artery. It supplies blood to the structures of the thigh and some superficial pelvic and inguinal structures.

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• The popliteal artery passes through the popliteal fossa behind the knee, where the pulse can be felt. It supplies the structures in this area, including the knee joint. At the lower border of the popliteal fossa it divides into the anterior and posterior tibial arteries.
• The anterior tibial artery passes forwards between the tibia and fibula and supplies the structures in the front of the leg. It lies on the tibia, runs in front of the ankle joint and continues over the dorsum (top) of the foot as the dorsalis pedis artery.
• The dorsalis pedis artery is a continuation of the anterior tibial artery and passes over the dorsum of the foot, where the pulse can be felt, supplying arterial blood to the structures in this area. It ends by passing between the first and second metatarsal bones into the sole of the foot where it contributes to the formation of the plantar arch.

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• The posterior tibial artery runs downwards and medially on the back of the leg. Near its origin it gives off a large branch called the peroneal artery, which supplies the lateral aspect of the leg. In the lower part it becomes superficial and passes medial to the ankle joint to reach the sole of the foot, where it continues as the plantar artery.
• The plantar artery supplies the structures in the sole of the foot. This artery, its branches and the dorsalis pedis artery form the plantar arch from which the digital branches arise to supply the toes.

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 The right popliteal artery and its main branches.

Venous return

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Superficial veins of the leg.

• There are both deep and superficial veins in the lower limb. Blood entering the superficial veins passes to the deep veins through communicating veins.
• Movement of blood towards the heart is partly dependent on contraction of skeletal muscles. Backward flow is prevented by a large number of valves. Superficial veins receive less support from surrounding tissues than deep veins.

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Summary of the main blood vessels

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The aorta and main arteries of the body

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 The venae cavae and main veins of the body

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Coronary Circulation

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Coronary Circulation

Arterial supply

• Coronary circulation refers to the flow of blood to and from the tissues of the heart itself.
• The heart is supplied with arterial blood by the right and left coronary arteries, which branch from the aorta immediately distal to the aortic valve.
• The coronary arteries receive about 5% of the blood pumped from the heart, although the heart comprises a small proportion of body weight. This large blood supply, especially to the left ventricle, highlights the importance of the heart to body function.
• The coronary arteries traverse the heart, eventually forming a vast network of capillaries.
• The main coronary arteries are the right coronary artery (RCA) and the left coronary artery (LCA), which further divides into the left anterior descending artery (LAD) and the circumflex artery.

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Venous drainage

• Most of the venous blood is collected into a number of cardiac veins that join to form the coronary sinus, which opens into the right atrium.
• The remainder passes directly into the heart chambers through little venous channels.

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Portal circulation 

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Portal circulation

Venous drainage from the abdominal organs, and the formation of the portal vein.

• Portal circulation: In all parts of circulation described so far, venous blood passes from the tissues to the heart by the most direct route through only one capillary bed.
• In the portal circulation, venous blood passes from the capillary beds of the abdominal part of the digestive system, the spleen and pancreas to the liver.
• It then passes through a second capillary bed, the hepatic sinusoids, in the liver before entering the general circulation via the inferior vena cava.
• In this way, blood with a high concentration of nutrients, absorbed from the stomach and intestines, goes to the liver first.
• In the liver certain modifications take place, including the regulation of blood nutrient levels.

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• Portal vein: This is formed by the union of several veins, each of which drains blood from the area supplied by the corresponding artery:
• The splenic vein drains blood from the spleen, the pancreas and part of the stomach.
• The inferior mesenteric vein returns the venous blood from the rectum, pelvic and descending colon of the large intestine. It joins the splenic vein.
• The superior mesenteric vein returns venous blood from the small intestine and the proximal parts of the large intestine, i.e. the caecum, ascending and transverse colon. It unites with the splenic vein to form the portal vein.
• The gastric veins drain blood from the stomach and the distal end of the oesophagus, then join the portal vein.
• The cystic vein, which drains venous blood from the gall bladder, joins the portal vein.

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The portal vein origin and termination

• Hepatic veins: These are very short veins that leave the posterior surface of the liver and, almost immediately, enter the inferior vena cava.

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If you have any…!

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Questions…?

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Confusion…?

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Good luck

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