Capillary exchange 2 Flashcards
what** 2 things** does molecular permeability depend on?
- lipid solubility (the more lipid soluble)
- size of molecule
what** 3 classes** do solutes fall into?
- lipid soluble
- small lipid insoluble
- large lipid insoluble
How do lipid soluble molecules eg O2 and CO2 diffuse through the membrane?
- as these molecules are lipid soluble, the entire capillary surface is available for diffusion - ie the permeability is extremely high
- they diffuse through the endothelial cell membrane
How do small lipid insoluble or hydrophillic (water loving) molecules go through capillary walls?
- small lipid insoluble molecules such as glucose, lactate & amino acids cannot difffuse through the lipid endothelial membrane
- they require aqueous channels (ie water filled pathway)
- also through convective transport
- and paracellular exchange (between cells)
What is the ‘small pore’ theory?
- scientists measured the solute diffusion across the endothelium & concluded that the endothelium was penetrated by a small set of aqueous channels that occupy a small amount of the capillary SA
- as the size of the hydrophillic molecules increased, the capillary permeability fell faster than the free diffusion co efficient
What is the** fibre matrix theory** of capillary permeability?
- this theory proposes that long biopolymer chains of the glycocalyx form a fine network to act as size selective pores
- the size limiting pores are actually the spaces within the glycocalyx
According to the fibre matrix theory, what effect does the glycocalyx have on the width of the junctions and fenestrae? Why?
- the glycocalyx reduces the effective width of the junctions and fenestrae
- the glycocalyx causes electrostatic repulsion ( ie as it is made up of negative macromolecules, it causes negative proteins etc to repel this charge) & steric hinderance ( molecules find it difficult to pass through pore due to their shape etc)
Why is there a close relationship between** hydrophillic solute permeability** & hydraulic conductance?
- capillary permeability to hydrophillic solutes changes in diff areas in the body - eg fenestrated capillaries are much more permeable than continous capillaries due to the collection of small pores / fenestrae
- hydrophillic solute permeability & hydraulic conductance correlate linerarly - ie if one is doubled, so is the other
- this proves that differences in permeability are not due to differences in pore radius,** they must be due to differences in pore numbers **
what is convective transport?
- the transport of solutes (eg glucose) are swept along in the water that is continously flowing out through a capillary wall
- transport of solute through a ‘solvent drag’
How do** large hydrophillic molecules** transport across a capillary wall?
- through large pores
- transcellular route
- vesicular transport (via vesciles through the cell)
Describe the permeability of cerebral capillaries
- capillaries in the brain are highly permeable to lipophillic (hydrophobic) solutes eg oxygen & CO2
- they are impermeable to hydrophillic solutes eg plasma proteins
- cerebral capillaries form a blood brain barrier
How are hydrophillic solutes transported across a brain capillary?
give example
via specific carrier proteins in the endothelial cell membrane - transcellular
* eg ) carriers for glucose, lactate etc
what are the** 2 basic states** in relaton to the effect of blood flow on solute transfer?
- flow limited exchange** - the solute flow across the capillary wall is limited by the rate at which blood is deliverying solute to the capillary
- Diffusion limited exchange - the solute flow across the capillary wall is limited due to the rate at which the solute can pass the wall
Describe** flow limited exchange**
- this describes a mechanism in which the capillary wall is very permeable to a solute - eg lipophillic O2 and CO2
- there is a rapid equilibration between the plasma and the surrounding tissue so that the venous concentration of the solute is equal to the interstitial fluid conc
- an increase in blood flow will increase the amount of solute exchange with the surrounding tissues
* EG respiratory gases
Describe how gas exchange is a good example of flow limited exchange
- the Co2 and O2 in the pulmonary capillary blood equilibrate with the alveolar gas long before the end of the capillary is reached
- and an increase in pulmonary blood flow ie cardiac output causes a direct increase in oxygen uptake and CO2 removal
