21 - Capillary Circulation

Question 1

What are the major two functions of the capillaries?

Answer 1

• Nutrient exchange

- Waste exchange

Question 2

What nutrients do the capillaries exchange?

Answer 2

• Glucose
• Oxygen
• Free fatty acids
• Other nutrients

Question 3

What waste products do the capillaries exchange?

Answer 3

• CO2
• Lactate and ammonia
• Other metabolic byproducts

Question 4

What characteristics of capillaries allow for these exchange processes to occur?

Answer 4

• Thin walls
• High total surface area
• Slow velocity of flow

Question 5

How thin are the capillary walls?

Answer 5

One cell thick

This means that the distance for diffusion is very short

Question 6

Why is the flow through the capillary slow?

Answer 6

Due to the large surface area

This allows for time for diffusion

Question 7

What three factors are important in controlling the opening and closing of pre-capillary sphincters?

Answer 7

• Generation of metabolites locally
• Blood borne neurotransmitters
• Blood borne polypeptides

Question 8

What metabolites can be generated to control precapillary sphincters?

Answer 8

• Decreased PO2
• Increased PCO2
• Decreased pH
• Adenosine

Question 9

Are the metabolites that control precapillary sphincters uniform in each region/organ of the body?

Answer 9

No

Question 10

What metabolites control capillary sphincters in skeletal muscle?

Answer 10

Decrease in PO2 is the main effector

Question 11

What metabolites control capillary sphincters in the heart?

Answer 11

Increase in adenosine

Question 12

What metabolites control capillary sphincters in the brain?

Answer 12

Increase in PCO2

Question 13

What blood borne neurotransmitters can control the opening and closing of precapillary sphincters?

Answer 13

• Epinephrine
• Norepinephrine
• Dopamine

Question 14

What blood borne polypeptides can control the opening and closing of precapillary sphincters?

Answer 14

• Angiotensin

- Kinins

Question 15

What is more beneficial to the body, opening more capillaries for increased blood flow or pushing more blood through the capillaries that are already open?

Answer 15

Open more capillaries

Question 16

Why is opening more capillaries more beneficial?

Answer 16

By opening more capillaries, you decrease the velocity of blood flow, increasing the amount of time that is available for nutrients to exchange

Question 17

Why is forcing more blood through open capillaries not as beneficial?

Answer 17

You would be forcing more blood flow through the same space, so the only thing that would happen is that there is a decreased amount of time for the exchange of nutrients to occur

Question 18

What is the Starling equation?

Answer 18

The Starling equation is an equation that illustrates the role of hydrostatic and oncotic forces (the so-called Starling forces) in the movement of fluid across capillary membranes

Question 19

What are the four Starling forces?

Answer 19

1 - Capillary hydrostatic pressure
2 - Intestinal fluid oncotic pressure
3 - Capillary oncotic pressure
4 - Interstitial fluid hydrostatic pressure

Question 20

What is capillary hydrostatic pressure?

Answer 20

The force in the capillaries is due to blood pressure that tends to drive fluid outward through the endothelial pores

Question 21

What is interstitial fluid oncotic pressure?

Answer 21

The influence of proteins in the interstitial fluid “pulling” water out of the capillaries

Question 22

What is capillary oncotic pressure?

Answer 22

The influence of proteins within the capillary lumen that keeps fluid in the capillaries

Question 23

What is interstitial fluid hydrostatic pressure?

Answer 23

Pressure in the interstitium that would drive fluid into the capillaries

Question 24

Each Starling force either favors __________ or ____________

Answer 24

Filtration or reabsorption

Question 25

Which two starling forces favor reabsorption?

Answer 25

• Capillary oncotic pressure

- Interstitial fluid hydrostatic pressure

Question 26

Which two Starling forces favor filtration?

Answer 26

• Capillary hydrostatic pressure

- Interstitial fluid oncotic pressure

Question 27

What the conditions that may lead to edema?

Answer 27

• Decrease in the production of plasma proteins
• Vasodilation
• Obstruction in circulation (venous clot)
• Congestive heart failure
• Proteins leaking out of the post-capillary venule

Question 28

Why does a decrease in the production of plasma proteins cause edema?

Answer 28

Because when you decrease plasma proteins, there is a lower capillary oncotic pressure

This results in more filtration than reabsorption

Question 29

What happens to the excess filtrate in this case?

Answer 29

It is typically picked up by the lymphatics, and in this case, the lymphatic system will be overwhelmed

Question 30

Why does vasodilation cause edema?

Answer 30

Because blood will enter the capillary at a higher osmotic pressure, which will result in more filtration than reabsorption

Question 31

What does the amount of edema depend on?

Answer 31

Again, this depends on the ability of the lymphatics to pick up the extra filtrate

If they can, little or no edema will occur

Question 32

How does an obstruction in the circulation (a venous clot) cause edema?

Answer 32

A clot will cause a backup in pressure, and therefore an increased blood pressure

This results in more filtration than reabsorption, causing edema

Question 33

Why does congestive heart failure cause edema?

Answer 33

The left ventricle is not effectively pumping blood out of the heart, so there is an increase in pulmonary pressure

Fluid will leak out into the lungs

Question 34

Why does proteins leaking out of post-capillary venules cause edema?

Answer 34

Because the proteins leaking causes an altered Starling force that favors more filtration than reabsorption and therefore edema

Question 35

What are the two functions of the lymphatic capillaries?

Answer 35

1 - Remove excess fluid from the interstitial space

2 - Remove excess proteins from the interstitial space