L7: Capillary circulation and dynamics of capillary filtrations

Question 1

What is the definition of capillaries?

Answer 1

• Capillaries are the smallest blood vessels in the body, connecting the smallest arteries to the smallest veins. These vessels are often referred to as the “microcirculation.”

Question 2

What is the function of capillaries?

Answer 2

• They are responsible for facilitating the transport and exchange of gases, fluids, and nutrients in the body.

Question 3

What do meta-arterioles branch into and what is found at the junction of arterioles and capillaries?

Answer 3

• Metarterioles branch into the capillary beds. At the junction of the arterioles and capillaries is a smooth muscle band called the precapillary sphincter.

Question 4

do capillaries have smooth muscles?

Answer 4

• True capillaries “those of nutritional benefit” do not have smooth muscle; they consist of a single layer of endothelial cells surrounded by a basement membrane.

Question 5

What do Clifts between the endothelial cells allow?

Answer 5

• Clefts (pores) between the endothelial cells allow passage of water- soluble substances. The clefts represent a very small fraction of the surface area.

Question 6

How do substances pass through the capillary walls?

Answer 6

1. Lipid-soluble substances: include O2 and CO2; cross the membranes of the capillary endothelial cells by simple diffusion.
2. Small water-soluble substances: include water, glucose, and amino acids; cross via the water-filled clefts between the endothelial cells.
   - Generally, protein molecules are too large to pass freely through the clefts. - In the brain, the clefts between endothelial cells are exceptionally tight (blood–brain barrier).
   - In the liver and intestine, the clefts are exceptionally wide and allow passage of protein. These capillaries are called sinusoids.
3. Large water-soluble substances: can cross by pinocytosis.

Question 7

How does blood flow in the capillaries?

Answer 7

• Blood usually does not flow at a continuous rate through capillaries, but it flows intermittently. The cause of this intermittency is the phenomenon called vasomotion.

Question 8

What is vasomotion?

Answer 8

• is intermittent contraction of the metarterioles and precapillary sphincters➔alternating cycle of constriction and relaxation for 5-10 times/minute so that only 10-20% of cap are opened at a time.

Question 9

What regulates vasomotion?

Answer 9

it is controlled mainly by O2 concentration in the tissues.
-Decrease O2 concentration leads to:
✓ Increase intermittent periods of BF.
✓ Increase duration of each period. “To supply more o2”

Which Allows blood to carry increased quantities of O2 to tissues.

Question 10

What is the capillary blood pressure?

Answer 10

• At the arterial end, the pressure is about 30-35 mmHg.
• At the venous end it is about 10-15 mmHg.
• At the middle of the capillary is 25 mmHg.

Question 11

What are the factors affecting capillary blood pressure and capillary blood flow?

Answer 11

The capillary BP may be influenced passively by extracapillary factors, and actively by the contraction of the capillaries themselves.

Question 12

What are the extra Capillary factors that affect the capillary blood pressure?

Answer 12

a. The diameter of arterioles: Arteriolar VD→ Inc. capillary BF and—>capillary BP and vice versa.
b. The venous pressure:the venous pressure→ Inc. capillary BP and dec. capillary BF. “Due to the closure of the Venous outflow and continuous flow from arterioles”
c. Gravity: in areas above the heart→ Dec. capillary BP, however in areas below the heart→ Inc. capillary BP by antagonizing venous return.

Question 13

What are the active factors that affect capillary blood pressure?

Answer 13

The actual cause of capillary contraction is not yet known, but it may be due to:

a. Contraction of the precapillary sphincter.
b. Contraction of smooth muscle fibers in the wall of the metarterioles.

Question 14

What are the two forces that govern fluid flux across the capilllaries?

Answer 14

-Fluid flux across the capillary is governed by the 2 fundamental forces that cause water flow:

➢ Hydrostatic force, which is simply the pressure of the fluid.
➢ Osmotic (oncotic) force, which represents the osmotic force created by solutes that do not cross the membrane.

• Each force exists on both sides of the membrane. Filtration is the movement of fluid from the plasma into the interstitium, while absorption is movement of fluid from the interstitium into the plasma.

Question 15

What is the amount of fluid filtered every day?

Answer 15

• About 20 liters of fluid are filtered every day at the arterial ends of capillaries, 18 liters of them are reabsorbed back at the venous ends, and the remaining 2 liters are drained by the lymphatic system.

Question 16

What are the forces which are called Starling forces and what does starling principle state?

Answer 16

1. The capillary hydrostatic pressure (Pc). “The main filtering force”
2. The interstitial fluid hydrostatic pressure (Pi).
3. The plasma colloid osmotic pressure (πc).
4. The interstitial fluid colloid osmotic pressure (πi).

Starling ‘s principle states that: “ the rate & direction of fluid movement is proprotional to the algabric sum of hydrostatic and osmotic forces”

Question 17

What does increase in hydrostatic pressure in the capillary favor?

Answer 17

An increase in Pc favors filtration out of the capillary.

Question 18

What is the hydrostatic pressure in the Capillary in determind by?

Answer 18

■ Pc is determined by arterial and venous pressures and resistances.

■ An increase in either arterial or venous pressure produces an increase in Pc; increases in venous pressure have a greater effect on Pc.

Question 19

What is hydrostatic pressure in the capillary?

Answer 19

■ It is 30 mmHg in the arterial end, 10 mmHg in the venous end. The functional mean capillary pressure is about 17.3 mmHg (i.e. it is the average effective pressure).

Question 20

What does increase in osmotic force in the interstitium favor?

Answer 20

An increase in πi favors filtration out of the capillary.

Question 21

What determines the osmotic force in the interstitium?

Answer 21

• is determined by the concentration of protein in the interstitial fluid. Normally the small amount of protein that leaks to the interstitium is minor and is removed by the lymphatics. Under most conditions, this is not an important factor influencing the exchange of fluid.

Question 22

What is the average protein concentration in the interstitium and what pressure does it result in?

Answer 22

• The average protein concentration of the interstitial fluid is about 3 gm/100 ml.
• resulting in average colloid osmotic pressure of about 8 mmHg. “Small effect”

Question 23

What does increase in osmotic pressure of plasma favor?

Answer 23

An increase in πc opposes filtration out of the capillary.

Question 24

What increases or decreases the osmotic pressure of plasma?

Answer 24

■ πc is increased by increases in the protein concentration in the blood (e.g., dehydration).

■ πc is decreased by decreases in the protein concentration in the blood (e.g., nephrotic syndrome, protein malnutrition, liver failure).

■ Small solutes do not contribute to πc.

Question 25

What is the most abundant plasma protein?

Answer 25

■ Albumin is the most abundant plasma protein and thus the biggest contributor to this force.

Question 26

What is the value of the osmotic pressure of plasma?

Answer 26

■ The colloidal osmotic pressure or the oncotic pressure of normal human plasma average about 28 mmHg.

Question 27

What does increase in the hydrostatic pressure of the interstitium favor?

Answer 27

■ An increase in Pi opposes filtration out of the capillary.

Question 28

What is the value of the hydrostatic pressure in the interstitium?

Answer 28

■ It is about -3 mmHg “causes suction as it is connected to lymphatics that drain fluids” and is called negative interstitial fluid pressure.

Question 29

What are the forces affecting the fluid the arterial end of capillaries?

Answer 29

A. Forces moving the fluid outward:
- Capillary pressure 30
- Negative interstitial fluid pressure 3
- Interstitial fluid colloid osmotic pressure 8
Total outward force 41

B. Forces moving the fluid inward
- Plasma colloid osmotic pressure 28

Summation of the forces:
Outward force 41
Inward force 28
Net outward force: 13

Thus 13 mmHg filtration pressure occurs at the arterial ends of the capillaries.

Question 30

What are the forces affecting the fluid at the Venous end of the capillaries?

Answer 30

At the venous end of the capillary:
A. Forces tending to move fluid outward:
- Capillary pressure 10
- Negative interstitial fluid pressure 3
- Interstitial fluid colloid osmotic pressure 8
Total outward force 21

B. Forces tending to move fluid inward:
- Plasma colloid osmotic pressure 28

Summation of forces:
Outward force 21
Inward force 28
Net inward force: 7

-Thus 7 mmHg is the reabsorbing pressure at the venous ends of the capillaries.

Question 31

How does blood filtration equal blood absorption even though the reabsorbing pressure is lower than the filtration pressure?

Answer 31

the venous capillaries are more numerous and more permeable so it reabsorbs about 9/10 of the fluid, the remainder flows into the lymph vessels.