capillary exchange

• Capillary structure

  • Short diffusion distance (1um)
  • Slow blood flow
  • Large surface are (10+ billion capillaries)
  • Types of capillaries
  • Continuous (majority)
  • Fenestrated (endocrine organs, intestine, kidneys): water filled poresallowing rapid exchange of water and solutes via fenestrations
  • Sinusoid (endocrine organs, liver, bone marrow, spleen): large clefts between endothelial cells and an incomplete basement membraneallowing exchange of water and larger solutes (ie. plasma proteins)
• Transport mechanisms in capillary exchange

  • Substance exchange across capillary walls occurs as molecules move down aconcentration or pressure gradients

  1. DIFFUSION: directly through endothelial cell membranes, ion channels or clefts/ pores (down conc gradient)
  2. BULK FLOW: through clefts/ pores (down pressure gradient by filtration/ osmosis)
  3. TRANSCYTOSIS: by vesicular transport
• • Blood flow to tissues is regulated by SNS and local metabolic factors as a minimum pressure (capillary hydrostatic pressure) is required to exchange substances/ fluids across capillary networks
  • cells rely on capillary exchange to:

  1. obtain nutrients and oxygen
  2. remove metabolic wastes (e.g. CO2)
• NET FILTRATION PRESSURE

  • bulk flow between capillaries and interstitial fluid is determined by the net pressure difference across capillary walls

  1. CAPILLARY HYDROSTATIC PRESSURE: blood pressure exerted on capillary walls "pushing" fluid out
  2. INTERSTITIAL FLUID PRESSURE: pressure exerted on the outer capillary walls by the IF pushing fluid in
  3. BLOOD COLLOID OSMOTIC PRESSURE: plasma osmotic pressure pulling fluid in
  4. INTERSTITIAL FLUID COLLOID OSMOTIC PRESSURE: osmotic pressure of IF pulling fluid out
• NET FILTRATION PRESSURE = NET HYDROSTATIC PRESSURE - NET OSMOTIC PRESSURE
• NET HYDROSTATIC PRESSURE = CAPILLARY HYDROSTATIC PRESSURE (decreases along capillary) - INTERSTITIAL FLUID HYDROSTATIC PRESSURE (generally negligible)
• NET OSMOTIC PRESSURE = BLOOD COLLOID OSMOTIC PRESSURE (affected by blood volume) - INTERSTITIAL FLUID COLLOID OSMOTIC PRESSURE (negligible)
• DYNAMICS OF CAPILLARY EXCHANGE

  • capillary hydrostatic pressure > blood colloid osmotic pressure = positive NFP (filtration)
  • capillary hydrostatic pressure < blood colloid osmotic pressure = negative NFP (reabsorption)
  • CHP declines from arterial to venous end (35 to 18 mmHg)
  • BCOP is constant along capillary (*25 mmHg)
• MORE FILTRATION TAKES PLACE ALONG THE CAPILLARY THAN ABSORPTION...

  • filtration = 24 L/day
  • reabsorption = 20.4 L/day
  • lymphatic system = 3.6 L/day
  • transition point is located towards the venous end
• NET FILTRATION PRESSURES

  (CHP - IHP) - (BCOP - ICOP) = NFP

  • increased CHP = increased NFP (increased filtration, fluid collects in extremities- systemic oedema)
  • decreased CHP = decreased NFP (increased reabsorption, fluid recalled from tissue into bloodstream- increases BP and CO)
  • increased BCOP = decreased NFP (increased reabsorption, fluid recalled from tissue into bloodstream- delays onset of symptoms)
  • increased ICOP = increased NFP (increased filtration, plasma proteins leak into IF, increasing ICOP- oedema)
• SPECIAL CIRCUMSTANCES: PULMONARY CIRCULATION

  • in lungs, arterioles constrict in regions of low O2 to shunt blood flow to O2 rich areas
  • pulmonary vascular resistance is very low- arterioles are short, wider and have thinner walls
  • CHP is lower than in systemic circulation- 10 mmHg rather than 35 mmHg
  • arterioles are more distensible- can accommodate increased CO with little increase in pressure
  • from arteriole to venule, CHP < BCOP, so fluid is absorbed along entire capillary
  • if pulmonary CHP exceeds 25 mmHg, then fluid leaks into alveoli impacting respiration --> pulmonary oedema
• SPECIAL CIRCUMSTANCES: CORONARY CIRCULATION

  • ADRENALINE- promotes coronary artery vasodilation, coronary flow increases when vasoconstriction predominates elsewhere
  • coronary blood flow is affected by the cardiac cycle
  • coronary flow is restricted during systole due to compression of the left coronary artery
  • coronary flow is highest during diastole enabled by arterial elastic recoil
  • to compensate:

  1. cardiomyocytes have high O2 reserves
  2. the myocardium has high capillary density --> increases O2 extraction
  3. capillaries have few arterial collaterals
• SPECIAL CIRCUMSTANCES: CEREBRAL CIRCULATION

  • blood flow to the brain must be preserved at all times
  • in emergencies, there is vasodilation of cerebral vessels while there is vasoconstriction in there periphery
  • THE BRAIN:

  1. consumes 12% CO for 2% body mass
  2. flow rate = 750 ml/min
  3. 4 arteries used to supply the brain which anatomies inside cranium