Capillary exchange 1 Flashcards
Describe the structure of capillaires
- capillaries are a single layer of endothelial cells located** upon a basement membrane **
- there is no media layer ie no muscle, so therefore capillaries cannot regulate their own diameters to control blood flow
Describe the 3 types of capillaries
- fenestrated - similar to continous capillaries, but the endothelial cells are perforated (ie pores), the pores running through them are called fenestrae, eg intestine
- discontinous- discontinous basement membrane, contain** wide gaps** between adjacent endothelial cells & large fenestrations - v leaky, found in liver etc
- **continous ** - most common, found in skeletal muscle, very thin, contain gap junctions between the endothelial cells
what does the permeability of a capillary wall to a particular molecule depend on?
- the size of the particle - smaller = higher permeability
- how lipid soluable it is (can dissolve in fat ) - the more lipid soluble it is, the more permeable (as cell membrane is a lipid environment)
By what process, do solutes mainly ** transport** across the capillary wall?
- by diffusion
- a** passive process **- ie no energy required as the solute is moving down it’s concentration gradient
How do carbon dioxide and oxygen pass through the endothelial cell?
- as these 2 solutes are very lipid permebale, they pass through freely through transcellular exchange
how do water soluble substances (hydrophillic) diffuse across the capillary wall?
- water soluble substances such as glucose diffuse through small spaces between the cells (paracellular exchange) and through fenestrations
what forces does fluid moving in or out of a capillary depend on?
- the balance of the** 4 starling forces **
- 2 hydrostatic forces (pressure exerted by liquid) & 2 osmotic forces
what are the** 2 hydrostatic forces** in the starling forces?
- Capillary pressure Pc
- PIF - interstitial fluid pressure
what are the 2 osmotic pressures in the starling forces?
-
oncotic pressure of the plasma (osmotic pressure exerted by the plasma proteins) Pi sign p
* oncotic pressure of the interstitial fluid (osmotic pressure exerted by the proteins in the interstitial fluid) Pi sign IF
How do the starling forces act?
- Capillary pressure acts to pull fluid in an **outward direction **
- the intersitial pressure acts to** force fluid inwards **
- plasma oncotic pressure acts to** pull fluid in**
- interstitial oncotic pressure acts to **pull fluid out **
What is NFP?
- the net filtration pressure
- it refers to the balance of the 4 starling forces
- it is usually in favour of filtration (fluid flowing out of capillaries) and not absorption (fluid flowing in)
How is the equation for NFP written out?
NFP = (Pc + intersitial fluid oncotic pressure) - (PIF + plasma oncotic pressure)
Note both Pc and IF oncotic pressure act to push fluid out and PIF and plasma oncotic pressure act to pull fluid in
What does it mean if the NFP value is + vs -?
- If NFP is** positive, the net movement of fluid is out of the capillaries - ie filtration**
- if NFP is negative, the net movement of fluid is into the capillaries, ie absorption
How do the tone of arteries influence capillary pressures?
- arterial constriction reduces the capillary pressure Pc and therefore favours reabsorption
- arterial dilatation increases the capillary pressure Pc and therefore favours filtration
what is vasomotion?
the muscle tone of a vessel changes constantly
* ie does not stay relaxed or contracted for long
* eg resting skeletal muscle vasomotion has a regular rhythm vs other tissues have a more irregular vasomotion
What is the hydrualic conductance of a capillary?
- hydrualic conductance (Lp) (water permebility of the capillary wall)
what is hydraulic conductance determined by?
- the number, width and length of the channels - ie if there are more channels or wider/shorter channels, it will be easier for fluid to flow through these channels
What is the reflection co efficient?
note - same sign as standard deviation
- a measure of the membranes molecular sensitivity
- capillaries are not perfect semi-permeable membranes ( ie dissolved substances can flow through)
what does the reflection co efficient modify?
it modifies the balance of the starling forces
what is the glycocalyx in capillaries?
- the glycocalyx is a layer of macromolecules eg glycoproteins
- this layer impacts the movements of water and solutes across the capillary wall
what is intracellular cleft osmotic pressure ?
note : this is a new pressure
- note - the intracellular cleft is the channel between 2 cells through which molecules may travel
- oncotic intracellular cleft pressure is the pressure present in the intracellular cleft
what is ficks law?
- a law which dictates the** amount of substance moving from one region to another **
- the amount of substance moved depends on the area available for diffusion, the **concentration gradient **and the **diffusion co efficient **
what is the equation for fick’s law?
- Js - quantity moved
- D- the diffusion co efficient
- s - surface area
- Delta C - conc difference
- Delta X - diffusion distance
why is there a negative sign in the ficks law equation?
the negative sign indicates that diffusion occurs from a region of high conc to region of low conc
what is the diffusion co efficient dependent on?
depends on the **temperature, solvent viscocity & solute size **
what effect does temperature, solute size & solvent viscocity have on the rate of diffusion?
- the warmer the temperature, the higher the rate of diffusion
- the smaller the solute size, the higher the rate of diffusion
- the less the solvent viscocity, the higher the rate of diffusion
what effect does the membrane have on the rate of diffusion?
- it slows diffusion
- ie they act as physical barriers
- why? - the reduce the SA by replacing area with pores and the membrane causes an increase in the path length
