Microcirculation and Venous Return

Question 1

Why don’t capillaries burst?

Answer 1

Due to the law of Laplace, which equates wall stress and (pressure*radius)/thickness. Because radius is so small, wall stress is reasonable even with high pressure.

Question 2

Wall stress in aorta vs capillaries

Answer 2

Aorta is 6x higher.

Question 3

Law of Laplace

Answer 3

Wall stress = (pressure*radius)/thickness

Question 4

How do gas/nutrients move across capillaries?

Answer 4

Via diffusion and filtration, thanks to slow velocity.

Question 5

Where does most oxygen exchange occur?

Answer 5

At the beginning of the capillary.

Question 6

Pc vs πc at beginning and end of capillary.

Answer 6

At beginning Pc (35mmHg) is higher than πc. At end, πc (25mmHg) is higher than Pc.

Question 7

Starling equation

Answer 7

Q=k[(Pc-Pi) - (πc-πi)]. Filtration occurs on arterial end, absorption occurs on venous end.

Question 8

Effect of Albumin in interstitial space?

Answer 8

Increased πi causing edema.

Question 9

Effect of increased capillary hydrostatic pressure

Answer 9

Increased Pc leading to edema.

Question 10

Will an increase in arteriolar pressure (MAP) lead to an increased Pc?

Answer 10

No, but increased pressure in veins will propagate back to capillaries increasing Pc.

Question 11

What is autoregulation? Where does it occur?

Answer 11

In kidneys, heart, brain, muscle. If an increase in BP is sensed (which causes increased local flow), vasoconstriction occurs.

Question 12

Active hyperemia

Answer 12

Metabolic activity, especially in skeletal muscle, causes the release of vasodilating agents

Question 13

What is the effect of increased sheer stress (turbulent flow) on the vasculature?

Answer 13

Causes endothelial cells to produce activate eNOS, which converts arginine into NO, which diffuses to smooth muscle cells leading to vasodilation (cGMP mediated inhibition of MLCK).

Question 14

eNOS

Answer 14

Turns arginine into NO. Activated in endothelial cells by increased sheer stress and CaM.

Question 15

Three ways laminar flow becomes turbulent.

Answer 15

High velocity (v=Q/A), low viscosity, narrow BVs

Question 16

How is fluid filtered out of capillaries returned?

Answer 16

Returned to venous system by lymphatics.

Question 17

Explain:

Answer 17

Starling curve. Cardiac output increases with increased preload. Indicative of the heart in isolation

Question 18

Draw the venous return curve.

Answer 18

Couples function of heart to entire circulation.

When atrial pressure is lower, it is easier fror blood to flow into the atrium.

Question 19

Relationship of venous return and cardiac output in steady state

Answer 19

The two must be equal!

Question 20

Draw cardiac output vs venous return graph

Answer 20

Question 21

What happens to cardiac output if contractility is increased/decreased. Draw cardiac vs venous function graph.

Answer 21

Increased output leads to an increased pressure drop in arterioles, which leads to decreased venous pressures (because everything slows down), which leads to decreased venous return.

Question 22

What happens to graph if there is a change in blood volume? Draw it.

Answer 22

Increased blood volume leads to increased cardiac output and increased right atrial pressure.

Question 23

What happens to cardiac function x venous return graph with arteriolar constriction? Draw it.

Answer 23

Total peripheral resistance increases, causes a rotation of venous return curve and changes in the starling curve. No change in X intercept!

Question 24

How will an increase in afterload change stroke volume and affect starling curve?

Answer 24

Decrease stroke volume and shift starling curve to the right.

Question 25

How will venoconstriction/change in venous compliance change the cardiac output x venous return graph.?

Answer 25

Change venous return curve. **No change in y-intercept**

Question 26

Three ways venous return can be altered

Answer 26

Change blood volume, change TPR, change dilation of veins

Question 27

What changes will increased blood volume cause on venous return graph?

Answer 27

Shift curve to the right. Will increase no flow pressure and increase maximal flow.

Question 28

What changes will decreased TPR cause on venous return graph?

Answer 28

Will shift curve around same X intercept. No change in no-flow atrial pressure because most of the blood is in veins. Maximal flow increases because resistence decreases.

Question 29

What changes will venoconstriction cause on venous return graph?

Answer 29

Same Y intercept, increased X intercept. No-flow resistance will be increased because most of the blood is in veins. No change in maximal flow because its determined by arteriolar constriction.

Question 30

Four changes that occur in heart failure

Answer 30

1) **Decrease in contractility due to damaged myocardium.** This causes a decrease in cardiac output and an increase in venous pressure 2) **Fluids are retained to increase preload further.** This will increase CO, but will also further increase venous pressure. 3) **Chronic sympathetic stimulation to improve myocardial performance.** This increases contractility and HR, but can damage myocardium further.

Question 31

What does increased LA pressure cause?

Answer 31

Increased pressures in pulmonary system, causing congestion.