The Circulatory System

Introduction

• The main function of a circulatory system is TRANSPORT.
• Diffusion is not sufficient to meet the metabolic demands of cells.
• The circulatory system ensures the cells of large organs are supplied with nutrients and oxygen and cleansed of wastes and carbon dioxide.

Introduction

• This unit will consist of 4 parts:

1. blood vessels

2. the heart

3. human circulation

4. blood

Blood Vessels

• The circulatory system consists of five types of blood vessels:
• Arteries
• Arterioles
• Capillaries
• Venules
• Veins

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1. Arteries

• most carry OXYGENATED BLOOD, but all carry blood AWAY from the heart
• Structure:
• thickest and strongest walls, consisting of three layers
• outer layer of CONNECTIVE TISSUE
• middle layer of ELASTIC TISSUE and SMOOTH MUSCLE
• inner layer of ENDOTHELIUM

Arteries (continued)

• thick elastic and muscle fibres enable artery to stretch (heart pumps) and recoil (heart relaxes) → this pressure is called BLOOD PRESSURE (BP)
• arteries branch into ARTERIOLES

2. Arterioles

• smaller arteries
• just visible to the naked eye
• endothelium wrapped in smooth muscle

Arterioles (continued)

• able to constrict (contract) and dilate (relax) → this affects the BP
• dilation (relaxed) → increase inside of arteriole, decrease BP
• constriction (contracted) → decrease inside of arteriole, increase BP
• arterioles branch into CAPILLARIES

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3. Capillaries

• smallest and narrowest vessel in the body
• blood cells pass through them one at a time
• every body cell is within 0.13 mm of a capillary
• composed of ONE layer of endothelial cells

Capillaries (continued)

• CAPILLARY BEDS: these networks of capillaries are present in all regions of the body
• it is in these beds that exchange of nutrients and oxygen for waste and carbon dioxide occurs
• one cell thick wall allows materials to diffuse through

Capillaries (continued)

• not all capillary beds are open at the same time
• “SHUNTING” of blood occurs due to opening and closing of sphincter muscles
• when contracted, the sphincter muscles prevent blood from entering the capillary bed
• when relaxed, the sphincter muscles allow blood to enter the capillary bed
• example → exercise vs digestion

4. Venules

• drain blood from capillaries
• join together to form veins

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5. Veins

• most carry DEOXYGENATED BLOOD, but all carry blood TOWARDS the heart
• same layers as arteries, but much thinner
• middle (muscle) layer poorly developed, so walls are less elastic, but will stretch more (less resistance)
• an empty vein will collapse (not so for an artery)
• larger lumen (opening) than arteries

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Veins (continued)

• when blood reaches the veins, the BP is close to zero due to resistance in the capillary beds
• this means blood needs help to get back to the heart (especially from the lower extremities where it is going against gravity)
• 2 helpers:
• one way valves prevent backflow
• veins are usually sandwiched between muscles which help move the blood when they contract

Summary

The Heart

• cone shaped, muscular organ
• fist sized
• contracts at regular intervals to pump blood
• composed of cardiac muscle

Heart Structure

• composed of 4 chambers (2 atria and 2 ventricles)
• separated into a right and left side by a SEPTUM
• lies within the PERICARDIUM (specialized sac) which contains a liquid for lubrication

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Atria

• singular → atrium
• 2 thin-walled chambers
• located at the top of the heart
• receive blood from veins

Ventricles

• 2 thick-walled chambers
• located at the bottom of the heart
• pump blood into the arteries

Heart Structures and Functions

• Anterior (superior) Vena Cava → collects DEOXYGENATED blood from upper body (head, chest, arms) and carries it to the right atrium
• Posterior (inferior) Vena Cava → collects DEOXYGENATED blood from lower body and carries it to the right atrium

Heart Structures and Functions

• Right Atrium → receives DEOXYGENATED blood from the body and heart (via coronary veins) and sends it to the right ventricle
• Right Atrioventricular (tricuspid) Valve → prevents backflow of blood from right ventricle to right atrium

→ tricuspid because it has three cusps or flaps

Chordae Tendinae

• strong, fibrous strings
• attached to muscular projections in the right and left ventricular walls
• support the tricuspid valves and prevent inversion (flipping backwards)

Heart Structures and Functions

• Right Ventricle → receives deoxygenated blood from the right atrium and pumps it into the pulmonary circuit via the pulmonary trunk
• Pulmonary Semilunar Valve → prevents backflow from pulmonary trunk into right ventricle
• → 3 cusps, but looks like a halfmoon

Heart Structures and Functions

• Pulmonary Trunk → carries DEOXYGENATED blood to the pulmonary arteries
• Pulmonary Arteries → 2 arteries which carry DEOXYGENATED blood to the lungs (only arteries to carry deoxygenated blood)

Heart Structures and Functions

• Pulmonary Veins → carry OXYGENATED blood from the lungs to the left atrium (only veins to carry oxygenated blood)
• Left Atrium → receives OXYGENATED blood and sends it to the right ventricle

Heart Structures and Functions

• Left Atrioventricular (bicuspid/mitral) Valve → prevents backflow of blood from the left ventricle to the left atrium
• Left Ventricle → most muscular chamber of the heart, receives OXYGENATED blood from the right atrium and pumps it to systemic circuit via the aorta

Heart Structures and Functions

• Aortic Semilunar Valve → prevents backflow of blood from aorta to left ventricle
• Aorta → largest artery in the body, receives OXYGENATED blood from the left ventricle and delivers it to the body

Heart Structures and Functions

• Septum
• the wall separating the right and left sides of the heart
• prevents mixing of oxygen-rich blood (left side) and oxygen-poor blood (right side)
• allows each side of the heart to maintain its own pressure (left side more muscular)

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Heart Structures and Functions

• Coronary (cardiac) Arteries → first branch off of aorta, supply heart with oxygenated blood, lie on surface of heart

→ a heart attack occurs when the coronary arteries are blocked due to atherosclerosis (fatty deposits inside the arteries) or blood clots

• Coronary (cardiac) Veins → drains deoxygenated blood from heart into the right atrium

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Blood Flow Through the Heart

• anterior/posterior vena cava → R atrium → R AV valve → R ventricle → pulmonary semilunar valve → pulmonary trunk → pulmonary arteries → lungs → pulmonary veins → L atrium → L AV valve → L ventricle → aortic semilunar valve → aorta → systemic circuit → anterior/posterior vena cava
• http://www.guidant.com/condition/heart/heart_bloodflow.shtml

Cardiac Cycle

• resting heart rate → about 70 bpm (5 liters of blood/minute)
• strenuous exercise → about 170 bpm (20 liters/minute)
• a 75 year old heart has beat about 2.8 billion times and pumped about 390 million liters of blood

Cardiac Cycle

• 1 heartbeat = 1 cardiac cycle
• at rest, 1 cardiac cycle takes about 0.85 seconds
• 2 contractions (atria first, then ventricles)
• Diastole = relaxation of heart muscle
• Systole = contraction of heart muscle

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Diastole

• semilunar valves close (dupp/dub sound)
• AV valves are both open
• heart is relaxed
• all 4 chambers fill with blood
• lasts about 0.4 seconds

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Systole

• consists of 2 parts
• atria briefly contract

→ this contraction completely fills the ventricles

→ lasts about 0.1 seconds

Systole

2. ventricles contract

→ AV valves close (lub sound)

→ semilunar valves open

→ blood is pumped into the aorta (from L ventricle) and the pulmonary trunk (from R ventricle)

* some blood enters atria

Heart Sounds

• lub-dupp → sound of heartbeat is actually the sound of valves closing → can be heard using a stethoscope
• http://www.guidant.com/condition/heart/heart_bloodflow.shtml
• heart murmur → “slush” sound as blood flows back through valves (faulty valves)
• pulse → alternating expansion and recoil of arterial wall due to contraction of the L ventricle (best taken at carotid artery – neck or radial artery – wrist)

Control of the Heartbeat

1. Intrinsic Control → the heartbeat will beat independently of stimulation, there is no nervous control

→ a number of structures are involved in this process

Control of the Heartbeat

Sinoatrial (SA) Node

• small region of heart muscle
• area of spontaneous electrical activity
• upper dorsal wall of the right atrium
• initiates each heartbeat
• cause atrial contraction (start of systole)
• impulse travels to AV node

Control of the Heartbeat

Atrioventricular Node

• second area of electrical activity
• located at the base of R atrium near the septum
• when AV impulse arrives, ventricular contraction is stimulated via the PURKINJE FIBERS

Control of the Heartbeat

2. Extrinsic Control → the heartbeat can be controlled extrinsically by the nervous system or by chemicals
1. Autonomic Nervous System – cardiac center in the medulla oblongata (brain) can alter the heartbeat

Control of the Heartbeat

• Parasympathetic Nervous System: promotes normal body responses, slows heart down
• Sympathetic Nervous System: promotes stress responses (fight or flight), speeds up heart rate

Control of the Heartbeat

2. Chemical control
1. epinephrin
2. hormone secreted by the adrenal gland
3. secreted when under stress
4. increases heart rate

Control of the Heartbeat

3. Other factors

→ the relative need for oxygen, exercise, other chemicals (caffeine, nicotine, amphetamines), temperature, emotions and health can also effect heart rate

Electrocardiogram (ECG)

• an ECG (or EKG) indicates the conduction of electrical impulses through the heart during the cardiac cycle
• on a graph of millivolts vs milliseconds, Control of the Heartbeat each cardiac cycle produces three distinct waves

Electrocardiogram (ECG)

1. P wave → atrial contraction (systole)
2. QRS wave → ventricular contraction (systole)
3. T wave → ventricular relaxation (diastole) (atrial relaxation occurs at same time but is not observed)

Blood Pressure

• the pressure of blood against the wall of blood vessel
• decreases with distance from L ventricle due to increase in total cross sectional area
• dissipated once blood is in veins
• velocity of blood also varies
• high in arteries due to BP
• lowest in capillaries due to high cross sectional area (important for diffusion)
• slight increase in veins due to skeletal muscle contraction (not BP)

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