CVS: Capillary Structure and Function, and Solute and Fluid Movement Flashcards
What does metabolism create a need for?
Transport of solutes + fluids
To do this, solutes + fluids must move across cell membranes which often acts as barriers
What controls the rate of solute transport?
- Properties of passive and active transport across membranes
- Fick’s law
- Properties of capillaries
Together, these form the concept of permeability - Allowing solutes/fluids to cross capillary membranes
Describe passive transport
- Movement molecules down conc/pressure/osmotic gradient
- DOESN’t need energy
- Simple (O2/CO2) or facilitated (ions,glucose)
Describe active transport
- Movement molecules against conc gradient
- NEEDS energy (ATP)
- e.g. ATP-dependent pumps, endocytosis, exocytosis
Describe the 4 passive transport processes
- Diffusion
- Concentration gradient -e.g. O2 uptake from lungs into blood
- Regulated by distance, time taken ∝ distance squared
- Fast over micro m and slow >1 mm
- Convection
- Pressure gradient -e.g. Circulation
- Requires pressure gradient, functioning heart - appropriate CO etc.
- Osmosis
- Osmotic pressure (water) gradient - e.g. water uptake by cells
- Requires balance of filtration, reabsorption and functioning lymphatics
- Electrochemical flux
- Electrical and concentration gradient - e.g. Ion flow during AP
- Requires - Active/other transport mechanisms to create electrochemical gradients and ion channels to provide ion movement across membranes
What does Fick’s law describe?
Properties of solutes and membranes affecting transport
Solute movement - Mass per unit time, m/t (Js)
Determined by 4 factors:
Js = - D A (deltaC/x)
D = Diffusion coefficient of solute – how easy it moves through solvent
A = Area
**DeltaC / x = Concentration gradient (C1-C2) across distance
x, negative value : flowing ‘down’ a concentration gradient**
What controls diffusion rate?
Linked to Fick’s law:
- Increased blood flow
- Increases concentration solutes transported to capillaries
- Fall in intracellular concentration (more solute used, metabolism)
- Increase concentration difference (greater concentration gradient)
- Recruitment of capillaries
- Dilation of arterioles - ⬆️number capillaries perfused
- Increases total SA for diffusion (Fick’s law)
- Shortens diffusion distance (faster diffusion)
At what vessel does most solute and fluid movement occur? Describe it
Capilaries:
- Smallest diameter BV
- Extension of inner lining of arterioles
- Endothelium only 1 cell thick
- Semi-permeable
- Vessels that connect arterioles to venules
- Found near every cell in body but higher density in highly active tissue
- Solute movement (due to passive/active transport and filtration), e.g. O2, glucose, amino acids, hormones, drugs
- Fluid movement (due to pressure gradients, osmotic pressure), e.g. regulation of plasma, interstitial, intracellular fluid
Describe the properties and function of continuous capillaries
- Moderate permeability
- Tight gaps b/w neighbouring cells
- Constant basement membrane e.g. blood-brain barrier
Describe the properties and function of 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 the properties and function of discontinuous capillaries
- Very large fenestration structures
- Disrupted basement membrane
- E.g. when movement of cells is requires such as RBCs in liver, spleen, bone marrow
How does permeability change as you go from continuous → fenestrated → discontinuous capillaries?
Increasing permeability to solutes + fluids
What are the general properties of all capillaries that can influence solute transfer?
- Intracellular cleft - 10-20nm wide
- Glycocalyx - Covers endothelium charged material, acts as sieve for solute permeation
- Caveolae + vesicles - Movement of large molecules, e.g. plasma proteins, lipoproteins
What are the different routes of solute transport?
- Big gaps in inflammation
- Trans-cellular channels
- Vesicles
- Trans-cellular
- Inter-cellular
- Fenestral route
- Water channels
What is the dominant route of solute transport?
- Diffusion, for example, filtration only accounts for 2% glucose transport
