Capillary Permeability Flashcards
what percent of total circulating blood is in the capillaries at any given time?
- 5%
- this amount is the most important part of the blood volume
what is the capillary wall made up of?
-a semipermeable membrane composed of a single layer of endothelial cells to promote efficient exchange
what components are exchanged between blood and cells during capillary exchange?
- all nutrients
- gases
- metabolites
- water
why are capillaries efficient sites for gas and nutrient exchange?
- blood velocity is low, allowing time for exchange
- huge surface area (over 10 mil capillaries in the human body)
what are the 3 main exchange mechanisms between capillaries and interstitial space?
- diffusion
- bulk flow
- vesicular transport
describe the diffusion exchange mechanism
- passive movements of nutrients, O2, CO2 and lipid soluble substances
- driven by osmotic gradients
- critical point - plasma proteins cannot cross capillary wall
describe the bulk flow exchange mechanism
- ultrafiltration
- movement of water soluble substances and extracellular fluid
- occurs through water-filled pores or intercellular clefts
describe the vesicular transport exchange mechanism
-larger, exchangeable macromolecules cross capillary endothelium via vesicular trancytosis
name 4 factors that affect the rate of diffusion
1) faster at higher temps (due to Brownian movement)
2) faster with higher concentration gradient
3) faster for smaller solutes
4) slower in more viscous solutions
T or F:
diffusion can only occur if membrane is permeable to the solute
true
T or F:
lipid solubles materials do not pass through the plasma membrane of endothelial cells
false
they pass directly through the plasma membrane
how can water molecules cross the plasma membrane?
-pass through spaces/pores between endothelial cells via bulk flow or vesicular transport
how can exchangeable proteins cross the plasma membrane?
vesicular transport
what are hydrostatic and oncotic pressures?
- different net pressures that operate at capillary beds to cause most of the plasma’s fluid to be filtered out at the arteriolar end
- the fluid is then mostly reabsorbed at the venous end
- the small amount of fluid that remains in the interstitial space becomes part of the interstitial fluid compartment
what do lymphatic vessels do?
- collect excess interstitial fluid and return it to the venous bloodstream
- fluid is called lymph once it enters the lymphatic vessels
T or F:
lymphatic vessels form a one-way system in which lymph only flows back toward the heart
true
what 3 pressures make up starling forces?
- crystalline osmotic pressure
- oncotic pressure
- hydrostatic pressure
what is crystalline osmotic pressure?
- oncotic pressure due to small molecules in plasma
- small molecules are water-soluble, so concentrations are equivalent on either side of capillary wall, so there is no effect on water flow
what is oncotic pressure?
- osmotic pressure exerted by impermeable plasma proteins in a blood vessel’s plasma that pull water into the circulatory system from interstitial fluid
- albumin is main plasma protein
- increases the length of the capillary
what is hydrostatic pressure?
-force that is directed out of the capillary by a fluid pushing against the capillary wall
What is Starling’s Law?
-fluid leaves (via filtration) or re-enters (via reabsorption) the capillary depending on how the opposing pressures in the capillary and in the interstitial spaces relate to one another (hydrostatic and oncotic mainly)
what is the equation of Starling’s Law?
NFP = (net forces pushing out) - (net forces pushing in)
NFP = (HPc + OPif) - (HPif + OPc)
NFP = net filtration pressure HPc = capillary hydrostatic pressure HPif = interstitial fluid hydrostatic pressure OPc = oncotic pressure due to protein concentration OPif = osmotic pressure due to interstitial fluid protein concentration
describe velocity of blood flow in the aorta and capillaries
- speed of blood flow changes as it passes through systemic circulation
- fastest in aorta
- slowest in capillaries
- remember, reduced velocity facilitates exchange
what is hydrostatic pressure?
- force per unit area exerted on the wall of a blood vessel by the blood
- this pressure gradient provides the driving force that keeps blood moving from higher to lower pressure areas
how is capillary microcirculation regulated?
- capillary beds
- vascular shunts
- precapillary sphincter
T or F:
fluid moves through the interstitial space at the arterial level, and fluid and lymphatics are reabsorbed at the venule level
true
T or F:
pinocytosis and trancytosis are used in the diffusion exchange mechanism
false
they are used in the vesicular transport exchange mechanism
what determines the bulk flow of protein-free plasma and interstitial fluid?
the relative hydrostatic and oncotic pressures on either side of the capillary wall (starling forces)
describe 5 characteristics of lymphatic capillaries
1) blind-ended sacs in interstitial space
2) relatively low pressure
3) absorb fluids from interstitial space only; unidirectional toward the heart
4) wider than blood capillaries
5) colorless
describe 5 characteristics of blood capillaries
1) join to arterioles on one end and venules on the other end
2) higher pressure than lymphatic capillaries
3) filter and then reabsorb most of the fluids by bulk flow into interstitial space
4) narrower than lymphatic capillaries
5) reddish and easy to observe
what 4 events can potentially cause edema?
- increased capillary blood pressure
- decreased plasma colloid osmotic (oncotic) pressure
- increased capillary permeability
- obstruction/disruption of lymphatics (lymphedema)
