Circulation and Gas Exchange

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Capillaries have thin walls, the endothelium plus its basement membrane, to facilitate the exchange of materials.

Arteries and veins have an endothelium, smooth muscle, and connective tissue.

Arteries have thicker walls than veins to accommodate the high pressure of blood pumped from the heart.

In the thinner-walled veins, blood flows back to the heart mainly as a result of muscle action.

Slide 36

Blood Flow Velocity

Physical laws governing movement of fluids through pipes affect blood flow and blood pressure.

Velocity of blood flow is slowest in the capillary beds, as a result of the high resistance and large total cross-sectional area.

Blood flow in capillaries is necessarily slow for exchange of materials.

Slide 37

The interrelationship of cross-sectional area of blood vessels, blood flow velocity, and blood pressure.

5,000

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1,000

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120

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Area (cm2)

Velocity

(cm/sec)

Pressure

(mm Hg)

Aorta

Arteries

Arterioles

Capillaries

Venules

Veins

Venae cavae

Diastolic

pressure

Systolic

pressure

Slide 38

Blood Pressure

Blood pressure is the hydrostatic pressure that blood exerts against the wall of a vessel.

In rigid vessels blood pressure is maintained; less rigid vessels deform and blood pressure is lost.

Slide 39

Changes in Blood Pressure During the Cardiac Cycle

Systolic pressure is the pressure in the arteries during ventricle contraction /systole; it is the highest pressure in the arteries.

Diastolic pressure is the pressure in the arteries during relaxation /diastole; it is lower than systolic pressure.

A pulse is the rhythic bulging of artery walls with each heartbeat.

Slide 40

Regulation of Blood Pressure

Blood pressure is determined by cardiac output and peripheral resistance due to constriction of arterioles.

Vasoconstriction is the contraction of smooth muscle in arteriole walls; it increases blood pressure.

Vasodilation is the relaxation of smooth muscles in the arterioles; it causes blood pressure to fall.

Slide 41

Vasoconstriction and vasodilation help maintain adequate blood flow as the body’s demands change.

The peptide endothelin is an important inducer of vasoconstriction.