Cardiovascular Physiology 28 Flashcards
What are the ways that material are exchanged across the walls of Cappilaries?
- Diffusion
- Vesicle transport
- Bulk flow
Why does blood flow very slowly in the capillaries?
To maximize the time for the exchange of substances between the plasma and the interstitial fluid
What is Diffusion?
The movement of a substance from a region of high concentration to a region of low concentration down its concentration gradient
Why is Diffusion the most important mechanism?
Because it is able to exchange nutrients and metabolic end products
How do Lipid soluble substances go through capillary cell walls?
Through diffusion
How do Lipid insoluble substances go through capillary cell walls?
Water filled channels
•Intercellular cleft
•Fenestrae
•Fused vesicle channels
Which substances are lipid soluble in Capillaries?
Oxygen and Carbon Dioxide
Which substances are lipid insoluble in Capillaries?
Ions and Glucose
What is Transcytosis?
The use of vesicles to cross endothelial cells
What is a fused vesicle channel?
When endocytic and exocytic vesicles form a water-filled channel across the cell
How do substances move through a fused vesicle channel?
By diffusion
What is Bulk Flow?
The movement of protein free plasma across the capillary wall
What are the Capillary walls highly permeable to?
Plasma and all plasma solutes, except proteins
What cannot cross a capillary wall?
Plasma proteins
What crosses the capillary in Bulk during bulk flow?
Various constituents of a fluid such as sodium and potassium are crossing the capillary wall in bulk or as a unit
What is the function of Bulk Flow?
The distribution of the extracellular fluid volume
What does the Extracellular fluid include?
The plasma and the interstitial fluid
What is Filtration?
The movement of protein-free plasma from the capillary to IF
What is Reabsorption?
The movement of protein-free plasma from IF to capillary
What is Bulk flow driven by?
Different pressures
Why does bulk flow occur?
Because of differences in the hydrostatic and colloid osmotic pressures between the capillary plasma and the interstitial fluid
What is Hydrostatic pressure?
The force of a fluid against a membrane
What drives bulk flow?
Hydrostatic pressure
What is Capillary Hydrostatic Pressure (Pc)?
The pressure exerted by the blood on the inside of the capillary walls
What does Capillary Hydrostatic Pressure (Pc) tend to do?
It tends to force protein free plasma out of the capillaries into the interstitial fluid
What is Interstitial Fluid Hydrostatic Pressure (PIF)?
The pressure exerted on the outside of the capillary wall by the interstitial fluid
What does Interstitial Fluid Hydrostatic Pressure (PIF) tend to do?
Tends to force fluid into the capillaries, but is very negligible
Which Hydrostatic Pressure is very negligible and does not contribute significantly to bulk flow?
Interstitial Fluid Hydrostatic Pressure (PIF)
What is the Colloid Osmotic Pressure?
The osmotic pressure due to the presence of impermeable proteins
Why isn’t the water concentration between the Plasma and the Interstitial Fluid equal?
Proteins that are impermeable and thus draw water towards them inside the capillary
What is the Blood Colloid Osmotic Pressure (πc)?
The Osmotic Pressure due to the presence of a large number of non-permeating plasma proteins such as albumin in the blood
Why don’t sodium and potassium influence water concentration in the plasma and the interstitial fluid?
Because the are found in equal amounts in the plasma and interstitial fluid
What do the Plasma Proteins do in terms of water?
The pull water into the capillaries
What is the Interstitial Fluid Colloid Osmotic Pressure (πIF) due to?
The small amount of plasma proteins that escape the capillaries into the interstitial fluid
What does the Interstitial Fluid Colloid Osmotic Pressure (πIF) favor?
Movement of fluid out of the capillary, but it is negligible
Which Pressures are negligible in bulk flow?
- Interstitial Fluid Colloid Osmotic Pressure (πIF)
* Interstitial fluid Hydrostatic Pressure (PIF)
What equation calculates the net exchange pressure at a given point across a capillary wall?
Net exchange pressure = Pc + πIF - PIF - πc
What are the outward pressures in Bulk flow?
- Capillary Hydrostatic Pressure
* Interstitial fluid osmotic pressure
What are the Inward Pressures in Bulk flow?
- The osmotic force due to the plasma protein concentration
* The interstitial fluid hydrostatic pressure
Which forces have a positive value?
Forces directed out of the capillary
Which forces have a negative value?
Forces directed into the capillary
How is the Hydrostatic pressure affected as blood moves from the arterial end to the venous end?
Hydrostatic pressure decreases going from the arterial end to the venous end
How is the colloid osmotic pressure (πc) affected as blood moves down a capillary?
Colloid Osmotic Pressure (πc) remains the same along the length of the capillary
What does it mean if the Net Exchange Pressure is a positive number?
It favours filtration from the capillary into the interstitial fluid at the arterial end of the capillary
What does it mean if the Net Exchange Pressure is a negative number?
It favours absorption from the interstitial fluid into the capillary
What are the Starling Forces?
- Blood Hydrostatic Pressure (Pc)
- Blood Colloid Osmotic Pressure (πc)
- Interstitial fluid hydrostatic pressure (PIF)
- Interstitial fluid colloid osmotic pressure (πIF)
Where do the Starling forces favor filtration?
At the arterial end a of a capillary
Where do the Starling forces favor absorption?
At the venous end of a capillary
Which Starling force changes along the length of a capillary and why?
The capillary hydrostatic pressure. It decreases along the length of a capillary due to friction
When and where does the transition between filtration and reabsorption occur?
It occurs when the sum of the outward pressures is equal to the sum of the inward pressures. Closer to the venous end of the capillary
Why does more filtration than absorption occur in capullaries?
Because the filtration transition to absorption occurs closer to the venous end
