Capillaries I- Solute Exchange Flashcards
What do the cell membranes consist of?
Cell membranes are a barrier to solute transport
Consists of two layers of amphipathic phospholipids
Phosphate head is polar (hydrophilic)
Fatty acid tail is non-polar (hydrophobic)
Form bilayers in solution
What are the roles of the cell membrane?
Provide support and protection
Cell-to-cell recognition e.g. immune system
Controls what enters or leaves the cell e.g. ion movement in nerves
Regulates cell function e.g. insulin-mediated glucose uptake
What is the difference between passive and active transport?
Passive transport:
Movement of molecules down a gradient
Concentration/pressure/osmotic/electrical
Does not require energy
Simple (O2/CO2) or facilitated (ions, glucose)
Active transport:
Movement of molecules against a gradient
Requires energy (uses ATP)
E.g. ATP-dependent pumps, endocytosis, exocytosis
What are the different passive transport processes?
Diffusion: Concentration gradient E.g. O2 uptake from lungs into blood Convection: Pressure gradient E.g. blood flow from heart to blood vessels Osmosis: Osmotic pressure gradient E.g. water uptake by cells Electrochemical flux: Electrical and concentration gradient E.g. ion flow across cell membranes
How does solute and fluid exchange occur at capillaries?
Connect terminal arterioles to venules- extension of inner lining of arterioles
Smallest diameter blood vessels endothelium, 1 cell thick, semi-permeable
Found near every cell in the body but higher density in highly active tissue (muscles, liver, heart, kidney, brain etc.)
Solute exchange (passive diffusion)- electrolytes, O2, glucose, amino acids, hormones, drugs
Fluid exchange along pressure gradients
Regulation of plasma and interstitial fluid volumes
What affects the rate of solute transport?
Properties of passive diffusion- e.g. concentration rate, distance
Properties of solutes and membranes e.g. Fick’s law
Properties of capillaries- affect movement
Ultimately combining all these- concept of permeability, or how easy it is to cross the membrane
What are the properties of passive diffusion?
Does not require energy
Molecules move randomly
Move from area of high to low concentration
Great for transport of lipid-soluble solutes over very short distances e.g. O2, CO2
Why only short distances?
Time taken (t) for one randomly moving molecule to move a net distance (x) in one specific direction increases with the distance squared
D= diffusion coefficient for molecule within the medium e.g. D for O2 in water and O2 in are different
What are the properties of solutes and membranes that affect transport?
Properties of the solute: Concentration gradient Size of solute Lipid solubility of solute (lipophilic, lipophobic nature Properties of the membrane: Membrane thickness/composition Aqueous pores in the membrane Carrier-mediated transport Active transport mechanisms
What is Fick’s law?
Properties of solutes and membranes affecting transport
How much of a substance is transported per time?
Solute movement- mass per unit time m/t (Js) determined by four factors:
D= diffusion coefficient of solute- ease of movement through solvent
A= area
ΔC= concentration gradient (C1-C2)
x= distance (between C1 and C2)
It has a negative value= flowing ‘down’ a concentration gradient
What properties of different capillaries increase permeability to solutes and fluid?
Continuous capillaries:
Moderate permeability; tight gaps between neighbouring cells; constant basement membrane
Blood-brain barrier
Muscle, skin, fat, connective tissue
Fenestrated capillaries:
High water permeability, fenestration structures, modest disruption of membrane
‘high water turnover’ tissues e.g. salivary glands, kidney, synovial joints, anterior eye, choroid plexus (cerebrospinal fluid), gut mucosa
Discontinuous capillaries
Very large fenestration structures disrupted membrane
When movement of cells is required; RBCs in liver, spleen, bone marrow
What 3 structural features influence solute transfer?
Intercellular cleft:
10-20nm wide
Caveolae and vesicles:
Large pore system
Glycocalyx:
Cover endothelium, negatively charged material, blocks solute permeation and access to transport mechanisms, highly regulated
The glycocalyx is very dynamic; can be broken down and remade as required
The cells can regulate this and these processes are currently an active research area
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What is permeability?
Permeability is the rate of solute transfer by diffusion across unit are of membrane per unit concentration difference i.e. how freely a solute crosses a membrane
A porous membrane interferes with the diffusion of lipid insoluble solute in multiple ways:
Reduction in area for diffusion (A)
Increased path length through membrane (x)
Restricted diffusion in pore produces hydrostatic issues (D)
All factors affecting diffusion go into one term- permeability (P)
Js= - D A (DeltaC/x)
Which is the dominant route of transport; diffusion or filtration? (includes glucose as
Diffusion is going to be passive down its concentration gradient whereas filtration is going to be going through pores, gaps and fenestrations etc. so as fluid moves through dissolved glucose go through it
Example- glucose transport from blood plasma to tissues
Glucose concentration in plasma is 1g/litre
Total volume of plasma filtrate flowing into tissues per day= 8 litres
Maximum filtration of glucose= 8g/ day
BUT glucose consumption of human adult is 400g/ day
Therefore filtration transport only accounts for 2% glucose transport
98% of glucose transport into interstitial space via passive diffusion- via GLUT transporter system
How is diffusion rate controlled?
Blood flow:
More blood brings more solutes.
Increased blood volume means less time for equilibration to occur across capillaries.
E.g. Flow limited diffusion where there is slow flow in a long capillary.
O2/CO2 equilibrate over proximal section and then there is little diffusion over the rest of the vessel.
Sometimes occurs in sepsis where blood pressure and flow are low, can lead to ischaemia.
Fall in interstitial concentration:
During metabolism more solute is used up thereby increasing the concentration difference.
Also, metabolism increases blood flow - increased O2 delivery, controlled by the arterioles.
Recruitment of capillaries:
Dilation of arterioles leads to increased number of capillaries perfused which increases total surface area A for diffusion (Fick’s law).
Also this shortens diffusion distance between capillary and cell.
