S2: Capillaries: Solute Exchange Flashcards
Why is transport important?
- Metabolism creates a need to transport solutes and fluids:
- H20 solution containing O2, glucose, amino acids, hormones, immune responses, immune response etc.
- For energy, growth and repair
- H20 solution containing metabolic end products e.g co2 and urea
Role of cell membranes
- Barrier to solute transport
- Provide support and protection
- Cell to cell recognition e.g. immune system
- Controls what enters or leaves the cell barrier e.g. ion movement in nerves
- Regulate cell function e,g, insulin mediated glucose uptake
What is the structure of the cell membrane?
· Consists of two layers of amphipathic phospholipids
· Phosphate head is polar (hydrophilic)
· Fatty acid tails is non-polar (hydrophobic)
.Form bilayers in solution
What is passive transport?
· Movement of molecules DOWN a gradient
· Concentration / Pressure / Osmosis
· Does Not require energy
. Simple (O2/CO2) or facilitated (ions, glucose)
What is active transport?
· Movement of molecules AGAINST a gradient
· Requires energy (uses ATP)
e.g. ATP-dependent pumps, endocytosis, exocytosis
4 passive transport processes
1) DIFFUSION
Concentration gradient
e. g. O2 uptake from lungs into blood
2) CONVECTION
Pressure gradient
e. g. Circulation
3) OSMOSIS Osmotic pressure (water) gradient e. g. water uptake by cells
4) ELECTROCHEMICAL FLUX
Electrical and Concentration gradient
e. g. ion flow during an action potential in a nerve
Describe capillaries
- Smallest diameter blood vessels
- Extension of inner lining of arterioles
- Endothelium only – 1 cell thick
- Semi-permeable
Capillaries are vessels that connect terminal arterioles to venules. They are found near every cell in the body but at higher density in highly active tissue (muscles, liver, heart, kidney and brain).
involved in solute and fluid exchange
What is solute and fluid exchange?
Solute exchange (passive diffusion) : O2, glucose, amino acids, hormones, drugs
Fluid exchange (flow down pressure gradients) : regulation of plasma and interstitial fluid volumes
What controls the rate of solute transport?
- Properties of passive diffusion
- Properties of solutes and membranes
- Properties of capillaries
- Concept of permeability
Equation for diffusion
Time taken (t) for one randomly moving molecule to move a net distance (x) in one specific direction increases with the distance squared · t = x2 / 2D
D = diffusion coefficient for molecule within the medium e.g. D for O2 in water vs. O2 in air are different
Name properties of solute and membrane than can affect transport
Properties of the solute:
- Concentration gradient
- Size of solute
- Lipid solubility of solute
Properties of the membrane:
- Membrane thickness/composition
- Aqueous pores in the membrane
- Carrier-mediated transport
- Active transport mechanisms
What is Fick’s Law?
How much of a substance is transported per unit of time
· Solute movement - mass per unit time m/t (Js)
It is determined by 4 factors
Js = - DA <i></i>
What are the three types of capillaries?
- Continuous capillaries
- Fenestrated capillaries
- Discontinuous capillaries
Describe continuous capillaries
- Moderate permeability
- Tight gaps between neighbouring cells
- Constant basement membrane
e. g. blood-brain barrier which keeps things in and out of brain (NT not moved out of brain)
Describe fenestrated capillaries
- High water permeability
- Fenestration structures
- Modest disruption of basement membrane
e.g. ‘high water turnover’ tissues such as
salivary glands, kidney, synovial joints, choroid plexus (cerebrospinal fluid)
Describe discontinuous capillaries
- Very large fenestration structures
- Disrupted basement membrane
- Large substances like plasma proteins and cells can move out and in
e. g. When movement of cells is required such as RBCs in liver, spleen, bone marrow, Plasma proteins made in liver enter the blood circulation through discontinuous capillaries
3 other structural features of capillary walls that can influence solute transfer
Intercellular cleft: 10-20 nm wide
Glycocalyx: covers endothelium on lumen side, -ve charged material, blocking solute permeation and access to transport mechanisms and it is highly regulated acting as a barrier
· increasingly considered important to control movement of molecules -
Caveolae and vesicles: large pores system
What is permeability?
Permeability is the rate of solute transfer by diffusion across unit area of membrane per unit concentration difference
’how freely a solute crosses a membrane’
What is the modified Fick’s law for a porous membrane?
Because a porous membrane interferes with the diffusion of lipid insoluble solute in multiple ways (Fick’s law doesn’t really work for ion channels)
Js = - PAm <i></i>
Which is the dominant route of solute transport: diffusion or filtration?
Use glucose as an example
Diffusion
xample : glucose transport from capillaries into tissues can potentially occur by either passive diffusion or filtration through fenestrations/intercellular gaps etc.
e.g. filtration only accounts for 2% of glucose transport - Therefore, 98% of glucose transport into interstitial space via passive diffusion – via GLUT transporter system
What controls diffusion rate?
- Increased blood flow because increased concentration of solutes in capillaries means more exchange of O2/ CO2 occurs along capillaries in the lungs. There is then less time for equilibration of O2/CO2 to occur between interstitial spaces and capillaries – which would reduce exchange.
- Fall in interstitial concentration increases conc. difference due to more solute used (e.g. metabolism). Also, metabolism increases blood flow - metabolic hyperaemia - increases O2 delivery
- Recruitment of capillaries.
Dilation of arterioles - increases number of capillaries perfused
-Increases total surface area A for diffusion (Fick’s law)
Shortens diffusion distance x (faster diffusion)
