Blood Vessels

• Delivery system of dynamic structures that begins and ends at heart
• Arteries: carry blood away from heart; oxygenated except for pulmonary circulation and umbilical vessels of fetus
• Capillaries: contact tissue cells; directly serve cellular needs
• Veins: carry blood toward heart

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Structure of Blood Vessel Walls

• Lumen
• Central blood-containing space
• Three wall layers in arteries and veins
• Tunica intima, tunica media, and tunica externa
• Capillaries
• Endothelium with sparse basal lamina

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Physiology of Circulation: Definition of Terms

• Blood flow
• Volume of blood flowing through vessel, organ, or entire circulation in given period
• Measured as ml/min
• Equivalent to cardiac output (CO) for entire vascular system
• Relatively constant when at rest
• Varies widely through individual organs, based on needs

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Physiology of Circulation: Definition of Terms

• Blood pressure (BP)
• Force per unit area exerted on wall of blood vessel by blood
• Expressed in mm Hg
• Measured as systemic arterial BP in large arteries near heart
• Pressure gradient provides driving force that keeps blood moving from higher to lower pressure areas

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Physiology of Circulation: Definition of Terms

• Resistance (peripheral resistance)
• Opposition to flow
• Measure of amount of friction blood encounters with vessel walls, generally in peripheral (systemic) circulation
• Three important sources of resistance
• Blood viscosity
• Total blood vessel length
• Blood vessel diameter

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Resistance

• Factors that remain relatively constant:
• Blood viscosity
• The "stickiness" of blood due to formed elements and plasma proteins
• Increased viscosity = increased resistance
• Blood vessel length
• Longer vessel = greater resistance encountered

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Resistance

• Blood vessel diameter
• Greatest influence on resistance

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Arterial Blood Pressure

• Reflects two factors of arteries close to heart
• Elasticity (compliance or distensibility)
• Volume of blood forced into them at any time
• Blood pressure near heart is pulsatile

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Arterial Blood Pressure

• Systolic pressure: pressure exerted in aorta during ventricular contraction
• Averages 120 mm Hg in normal adult
• Diastolic pressure: lowest level of aortic pressure
• Pulse pressure = difference between systolic and diastolic pressure
• Throbbing of arteries (pulse)

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Capillary Blood Pressure

• Ranges from 17 to 35 mm Hg
• Low capillary pressure is desirable
• High BP would rupture fragile, thin-walled capillaries
• Most very permeable, so low pressure forces filtrate into interstitial spaces

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Venous Blood Pressure

• Changes little during cardiac cycle
• Small pressure gradient; about 15 mm Hg
• Low pressure due to cumulative effects of peripheral resistance
• Energy of blood pressure lost as heat during each circuit

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Factors Aiding Venous Return

1. Muscular pump: contraction of skeletal muscles "milks" blood toward heart; valves prevent backflow
2. Respiratory pump: pressure changes during breathing move blood toward heart by squeezing abdominal veins as thoracic veins expand
3. Venoconstriction under sympathetic control pushes blood toward heart

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Maintaining Blood Pressure

• Requires
• Cooperation of heart, blood vessels, and kidneys
• Supervision by brain
• Main factors influencing blood pressure
• Cardiac output (CO)
• Peripheral resistance (PR)
• Blood volume

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Maintaining Blood Pressure

• Blood pressure = CO × PR (and CO depends on blood volume)
• Blood pressure varies directly with CO, PR, and blood volume
• Changes in one variable quickly compensated for by changes in other variables

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Cardiac Output (CO)

• CO = SV × HR; normal = 5.0-5.5 L/min
• Determined by venous return, and neural and hormonal controls
• Resting heart rate maintained by cardioinhibitory center via parasympathetic vagus nerves
• Stroke volume controlled by venous return (EDV)

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Cardiac Output (CO)

• During stress, cardioacceleratory center increases heart rate and stroke volume via sympathetic stimulation
• ESV decreases and MAP increases

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Figure 19.8 Major factors enhancing cardiac output.

Exercise

BP activates cardiac centers in medulla

Activity of respiratory pump

(ventral body cavity pressure)

​

Activity of muscular pump

(skeletal muscles)

​

Sympathetic venoconstriction

Sympathetic activity

Parasympathetic activity

Venous return

Contractility of cardiac muscle

Epinephrine in blood

EDV

ESV

Stroke volume (SV)

Heart rate (HR)

Cardiac output (CO = SV x HR)

Initial stimulus

Physiological response

Result

Control of Blood Pressure

• Short-term neural and hormonal controls
• Counteract fluctuations in blood pressure by altering peripheral resistance and CO
• Long-term renal regulation
• Counteracts fluctuations in blood pressure by altering blood volume

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Short-term Mechanisms: Neural Controls

• Neural controls of peripheral resistance
• Maintain MAP by altering blood vessel diameter
• If low blood volume all vessels constricted except those to heart and brain
• Alter blood distribution to organs in response to specific demands

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Short-term Mechanisms: Neural Controls

• Neural controls operate via reflex arcs that involve
• Baroreceptors
• Cardiovascular center of medulla
• Vasomotor fibers to heart and vascular smooth muscle
• Sometimes input from chemoreceptors and higher brain centers

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The Cardiovascular Center

• Clusters of sympathetic neurons in medulla oversee changes in CO and blood vessel diameter
• Consists of cardiac centers and vasomotor center
• Vasomotor center sends steady impulses via sympathetic efferents to blood vessels 🡪 moderate constriction called vasomotor tone
• Receives inputs from baroreceptors, chemoreceptors, and higher brain centers

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Short-term Mechanisms: Baroreceptor Reflexes

• Baroreceptors located in
• Carotid sinuses
• Aortic arch
• Walls of large arteries of neck and thorax

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Short-term Mechanisms: Baroreceptor Reflexes

• Increased blood pressure stimulates baroreceptors to increase input to vasomotor center
• Inhibits vasomotor and cardioacceleratory centers, causing arteriole dilation and venodilation
• Stimulates cardioinhibitory center
• 🡪 decreased blood pressure

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Short-term Mechanisms: Baroreceptor Reflexes

• Decrease in blood pressure due to
• Arteriolar vasodilation
• Venodilation
• Decreased cardiac output

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Short-term Mechanisms: Baroreceptor Reflexes

• If MAP low
• 🡪 Reflex vasoconstriction 🡪 increased CO 🡪 increased blood pressure
• Ex. Upon standing baroreceptors of carotid sinus reflex protect blood to brain; in systemic circuit as whole aortic reflex maintains blood pressure
• Baroreceptors ineffective if altered blood pressure sustained

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Short-term Mechanisms: Chemoreceptor Reflexes

• Chemoreceptors in aortic arch and large arteries of neck detect increase in CO₂, or drop in pH or O₂
• Cause increased blood pressure by
• Signaling cardioacceleratory center 🡪 increase CO
• Signaling vasomotor center 🡪 increase vasoconstriction

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Short-term Mechanisms: Influence of Higher Brain Centers

• Reflexes in medulla
• Hypothalamus and cerebral cortex can modify arterial pressure via relays to medulla
• Hypothalamus increases blood pressure during stress
• Hypothalamus mediates redistribution of blood flow during exercise and changes in body temperature

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Short-term Mechanisms: Hormonal Controls

• Short term regulation via changes in peripheral resistance
• Long term regulation via changes in blood volume

​

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Short-term Mechanisms: Hormonal Controls

• Cause increased blood pressure
• Epinephrine and norepinephrine from adrenal gland 🡪 increased CO and vasoconstriction
• Angiotensin II stimulates vasoconstriction
• High ADH levels cause vasoconstriction
• Cause lowered blood pressure
• Atrial natriuretic peptide causes decreased blood volume by antagonizing aldosterone

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Long-term Mechanisms: Renal Regulation

• Baroreceptors quickly adapt to chronic high or low BP so are ineffective
• Long-term mechanisms control BP by altering blood volume via kidneys
• Kidneys regulate arterial blood pressure
1. Direct renal mechanism
2. Indirect renal (renin-angiotensin-aldosterone) mechanism

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Direct Renal Mechanism

• Alters blood volume independently of hormones
• Increased BP or blood volume causes elimination of more urine, thus reducing BP
• Decreased BP or blood volume causes kidneys to conserve water, and BP rises

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Indirect Mechanism

• The renin-angiotensin-aldosterone mechanism
• ↓ Arterial blood pressure → release of renin
• Renin catalyzes conversion of angiotensinogen from liver to angiotensin I
• Angiotensin converting enzyme, especially from lungs, converts angiotensin I to angiotensin II

​

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Functions of Angiotensin II

• Increases blood volume
• Stimulates aldosterone secretion
• Causes ADH release
• Triggers hypothalamic thirst center
• Causes vasoconstriction directly increasing blood pressure

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Direct renal mechanism

Indirect renal mechanism (renin-angiotensin-aldosterone)

Arterial pressure

Arterial pressure

Inhibits baroreceptors

Sympathetic nervous

system activity

Renin release

from kidneys

Angiotensinogen

Angiotensin I

Angiotensin II

Angiotensin converting

enzyme (ACE)

Urine formation

Filtration by kidneys

Blood volume

Adrenal cortex

ADH release by

posterior pituitary

Secretes

Aldosterone

Sodium reabsorption

by kidneys

Water reabsorption

by kidneys

Water intake

Blood volume

Mean arterial pressure

Vasoconstriction;

peripheral resistance

Thirst via

hypothalamus

Mean arterial pressure

Initial stimulus

Physiological response

Result

Figure 19.10 Direct and indirect (hormonal) mechanisms for renal control of blood pressure.

Monitoring Circulatory Efficiency

• Vital signs: pulse and blood pressure, along with respiratory rate and body temperature
• Pulse: pressure wave caused by expansion and recoil of arteries
• Radial pulse (taken at the wrist) routinely used
• Pressure points where arteries close to body surface
• Can be compressed to stop blood flow

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Measuring Blood Pressure

• Systemic arterial BP
• Measured indirectly by auscultatory method using a sphygmomanometer
• Pressure increased in cuff until it exceeds systolic pressure in brachial artery
• Pressure released slowly and examiner listens for sounds of Korotkoff with a stethoscope

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Measuring Blood Pressure

• Systolic pressure, normally less than 120 mm Hg, is pressure when sounds first occur as blood starts to spurt through artery
• Diastolic pressure, normally less than 80 mm Hg, is pressure when sounds disappear because artery no longer constricted; blood flowing freely

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Variations in Blood Pressure

• Transient elevations occur during changes in posture, physical exertion, emotional upset, fever.
• Age, sex, weight, race, mood, and posture may cause BP to vary

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Alterations in Blood Pressure

• Hypertension: high blood pressure
• Sustained elevated arterial pressure of 140/90 or higher
• Prehypertension if values elevated but not yet in hypertension range
• May be transient adaptations during fever, physical exertion, and emotional upset
• Often persistent in obese people

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Homeostatic Imbalance: Hypertension

• Prolonged hypertension major cause of heart failure, vascular disease, renal failure, and stroke
• Heart must work harder 🡪 myocardium enlarges, weakens, becomes flabby
• Also accelerates atherosclerosis

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Primary or Essential Hypertension�

• 90% of hypertensive conditions
• No underlying cause identified
• Risk factors include heredity, diet, obesity, age, diabetes mellitus, stress, and smoking
• No cure but can be controlled
• Restrict salt, fat, cholesterol intake
• Increase exercise, lose weight, stop smoking
• Antihypertensive drugs

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Homeostatic Imbalance: Hypertension

• Secondary hypertension less common
• Due to identifiable disorders including obstructed renal arteries, kidney disease, and endocrine disorders such as hyperthyroidism and Cushing's syndrome
• Treatment focuses on correcting underlying cause

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Alterations in Blood Pressure

• Hypotension: low blood pressure
• Blood pressure below 90/60 mm Hg
• Usually not a concern
• Only if leads to inadequate blood flow to tissues
• Often associated with long life and lack of cardiovascular illness

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Blood Flow Through Body Tissues

• Tissue perfusion involved in
• Delivery of O₂ and nutrients to, and removal of wastes from, tissue cells
• Gas exchange (lungs)
• Absorption of nutrients (digestive tract)
• Urine formation (kidneys)
• Rate of flow is precisely right amount to provide proper function

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Autoregulation

• Two types of autoregulation
• Metabolic controls
• Myogenic controls
• Both determine final autoregulatory response

​

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Blood Flow Through Capillaries

• Vasomotion
• Slow, intermittent flow
• Reflects on/off opening and closing of precapillary sphincters

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Capillary Exchange of Respiratory Gases and Nutrients

• Diffusion down concentration gradients
• O₂ and nutrients from blood to tissues
• CO₂ and metabolic wastes from tissues to blood
• Lipid-soluble molecules diffuse directly through endothelial membranes
• Water-soluble solutes pass through clefts and fenestrations
• Larger molecules, such as proteins, are actively transported in pinocytotic vesicles or caveolae

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Fluid Movements: Bulk Flow

• Fluid leaves capillaries at arterial end; most returns to blood at venous end
• Extremely important in determining relative fluid volumes in blood and interstitial space
• Direction and amount of fluid flow depend on two opposing forces: hydrostatic and colloid osmotic pressures

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Hydrostatic Pressures

• Capillary hydrostatic pressure (HP_c) (capillary blood pressure)
• Tends to force fluids through capillary walls
• Greater at arterial end (35 mm Hg) of bed than at venule end (17 mm Hg)
• Interstitial fluid hydrostatic pressure (HP_if)
• Pressure that would push fluid into vessel
• Usually assumed to be zero because of lymphatic vessels

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Colloid Osmotic Pressures

• Capillary colloid osmotic pressure (oncotic pressure) (OP_c)
• Created by nondiffusible plasma proteins, which draw water toward themselves
• ~26 mm Hg
• Interstitial fluid osmotic pressure (OP_if)
• Low (~1 mm Hg) due to low protein content

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Hydrostatic-osmotic Pressure Interactions: Net Filtration Pressure (NFP)

• NFP—comprises all forces acting on capillary bed
• NFP = (HP_c—HP_if)—(OP_c—OP_if)
• Net fluid flow out at arterial end
• Net fluid flow in at venous end
• More leaves than is returned
• Excess fluid returned to blood via lymphatic system

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The big picture

Fluid filters from capillaries at their arteriolar

end and flows through the interstitial space.

Most is reabsorbed at the venous end.

For all capillary beds,

20 L of fluid is filtered

out per day—almost 7

times the total plasma

volume!

• Due to fluid pressing against a

boundary

• HP “pushes” fluid across the

boundary

• In blood vessels, is due to blood

pressure

• Due to nondiffusible solutes that

cannot cross the boundary

• OP “pulls” fluid across the

boundary

• In blood vessels, is due to

plasma proteins

Piston

Boundary

Solute

molecules

(proteins)

Boundary

“Pushes”

“Pulls”

Hydrostatic pressure (HP)

Osmotic pressure (OP)

17 L of fluid per

day is reabsorbed

into the capillaries

at the venous end.

Lymphatic

capillary

Venule

About 3 L per day

of fluid (and any

leaked proteins) are

removed by the

lymphatic system

(see Chapter 20).

Arteriole

Fluid moves through

the interstitial space.

Net filtration pressure (NFP) determines the

direction of fluid movement. Two kinds of

pressure drive fluid flow:

Figure 19.17 Bulk fluid flow across capillary walls causes continuous mixing of fluid between the plasma and the interstitial fluid compartments, and maintains the interstitial environment. (1 of 5)