21 - Capillary Circulation Flashcards
What are the major two functions of the capillaries?
- Nutrient exchange
- Waste exchange
What nutrients do the capillaries exchange?
- Glucose
- Oxygen
- Free fatty acids
- Other nutrients
What waste products do the capillaries exchange?
- CO2
- Lactate and ammonia
- Other metabolic byproducts
What characteristics of capillaries allow for these exchange processes to occur?
- Thin walls
- High total surface area
- Slow velocity of flow
How thin are the capillary walls?
One cell thick
This means that the distance for diffusion is very short
Why is the flow through the capillary slow?
Due to the large surface area
This allows for time for diffusion
What three factors are important in controlling the opening and closing of pre-capillary sphincters?
- Generation of metabolites locally
- Blood borne neurotransmitters
- Blood borne polypeptides
What metabolites can be generated to control precapillary sphincters?
- Decreased PO2
- Increased PCO2
- Decreased pH
- Adenosine
Are the metabolites that control precapillary sphincters uniform in each region/organ of the body?
No
What metabolites control capillary sphincters in skeletal muscle?
Decrease in PO2 is the main effector
What metabolites control capillary sphincters in the heart?
Increase in adenosine
What metabolites control capillary sphincters in the brain?
Increase in PCO2
What blood borne neurotransmitters can control the opening and closing of precapillary sphincters?
- Epinephrine
- Norepinephrine
- Dopamine
What blood borne polypeptides can control the opening and closing of precapillary sphincters?
- Angiotensin
- Kinins
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?
Open more capillaries
Why is opening more capillaries more beneficial?
By opening more capillaries, you decrease the velocity of blood flow, increasing the amount of time that is available for nutrients to exchange
Why is forcing more blood through open capillaries not as beneficial?
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
What is the Starling equation?
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
What are the four Starling forces?
1 - Capillary hydrostatic pressure
2 - Intestinal fluid oncotic pressure
3 - Capillary oncotic pressure
4 - Interstitial fluid hydrostatic pressure
What is capillary hydrostatic pressure?
The force in the capillaries is due to blood pressure that tends to drive fluid outward through the endothelial pores
What is interstitial fluid oncotic pressure?
The influence of proteins in the interstitial fluid “pulling” water out of the capillaries
What is capillary oncotic pressure?
The influence of proteins within the capillary lumen that keeps fluid in the capillaries
What is interstitial fluid hydrostatic pressure?
Pressure in the interstitium that would drive fluid into the capillaries
Each Starling force either favors __________ or ____________
Filtration or reabsorption
Which two starling forces favor reabsorption?
- Capillary oncotic pressure
- Interstitial fluid hydrostatic pressure
Which two Starling forces favor filtration?
- Capillary hydrostatic pressure
- Interstitial fluid oncotic pressure
What the conditions that may lead to edema?
- Decrease in the production of plasma proteins
- Vasodilation
- Obstruction in circulation (venous clot)
- Congestive heart failure
- Proteins leaking out of the post-capillary venule
Why does a decrease in the production of plasma proteins cause edema?
Because when you decrease plasma proteins, there is a lower capillary oncotic pressure
This results in more filtration than reabsorption
What happens to the excess filtrate in this case?
It is typically picked up by the lymphatics, and in this case, the lymphatic system will be overwhelmed
Why does vasodilation cause edema?
Because blood will enter the capillary at a higher osmotic pressure, which will result in more filtration than reabsorption
What does the amount of edema depend on?
Again, this depends on the ability of the lymphatics to pick up the extra filtrate
If they can, little or no edema will occur
How does an obstruction in the circulation (a venous clot) cause edema?
A clot will cause a backup in pressure, and therefore an increased blood pressure
This results in more filtration than reabsorption, causing edema
Why does congestive heart failure cause edema?
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
Why does proteins leaking out of post-capillary venules cause edema?
Because the proteins leaking causes an altered Starling force that favors more filtration than reabsorption and therefore edema
What are the two functions of the lymphatic capillaries?
1 - Remove excess fluid from the interstitial space
2 - Remove excess proteins from the interstitial space
