Circulation and Gas Exchange

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7

Blood pressure is generally measured for an artery in the arm at the same height as the heart.

Blood pressure for a healthy 20 year old at rest is 120 mm Hg at systole / 70 mm Hg at diastole.

Slide 42

Question: How do endothelial cells control vasoconstriction?

Ser

RESULTS

Ser

Ser

Cys

Cys

—NH3+

Leu

Met

Asp

Lys

Glu

Cys

Val

Tyr

Phe

Cys

His

Leu

Asp

Ile

Ile

Trp

—COO–

Endothelin

Parent polypeptide

Trp

Cys

Endothelin

53

73

1

203

Slide 43

Measurement of blood pressure: sphygmomanometer

Pressure in cuff

greater than

120 mm Hg

Rubber

cuff

inflated

with air

Artery

closed

120

120

Pressure in cuff

drops below

120 mm Hg

Sounds

audible in

stethoscope

Pressure in

cuff below

70 mm Hg

70

Blood pressure reading: 120/70

Sounds

stop

Slide 44

Fainting is caused by inadequate blood flow to the head.

Animals with longer necks require a higher systolic pressure to pump blood a greater distance against gravity.

Blood is moved through veins by smooth muscle contraction, skeletal muscle contraction, and expansion of the vena cava with inhalation.

One-way valves in veins / heart prevent backflow of blood.

Slide 45

Blood flow in veins

Direction of blood flow

in vein (toward heart)

Valve (open)

Skeletal muscle

Valve (closed)

Slide 46

Capillary Function

Capillaries in major organs are usually filled to capacity. Blood supply varies in many other sites.

Two mechanisms regulate distribution of blood in capillary beds:

Contraction of the smooth muscle layer in the wall of an arteriole constricts the vessel.

Precapillary sphincters control flow of blood between arterioles and venules.

Slide 47

Blood flow in capillary beds

Precapillary sphincters

Thoroughfare

channel

Arteriole

Capillaries

Venule

(a) Sphincters relaxed

(b) Sphincters contracted

Arteriole

Venule

Slide 48

The critical exchange of substances between the blood and interstitial fluid takes place across the thin endothelial walls of the capillaries.

The difference between blood pressure and osmotic pressure drives fluids out of capillaries at the arteriole end and into capillaries at the venule end.