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Physiology (114)
PSL301H1 (21)


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Shaun Burns

REGULATION OF CARDIOVASCULAR FUNCTION • Baroreceptor reflex o Increased cardiac output and peripheral resistance increases mean arterial pressure  Baroreceptor quickly returns pressure to normal • Exchange at the Capillaries o Capillary density is directly related to the metabolic activity of tissue’s cells  Higher metabolic rate  need more nutrients and oxygen • Higher capillary network density o Composed of single layer of endothelial cells, supported by basal lamina  Narrow diameter, forcing RBC into single file  Cell junctions between endothelial cells vary, determine leakiness of capillary o Most common capillaries are continuous capillaries  Endothelial cells joined to each other with leaky junctions  Found in muscle, neural and connective tissue  Form blood-brain barrier o Fenestrated capillaries  Allow high volumes of fluid to pass quickly between plasma and interstitial fluid • Primarily in intestine and kidneys  absorptive transporting epithelia o Sinusoids  Found in marrow, liver and spleen  do not have capillaries • Found in areas where blood cells and plasma proteins need to cross endothelium to enter blood  Up to five times wider than a capillary  Sinusoid endothelium has fenestrations, haps between cell walls as well • Velocity of Blood Flow is Lowest in the Capillaries o Rate of blood flow plays significant role in efficiency exchange between interstitial fluid and blood o At constant flow rate, velocity is higher in a smaller diameter tube than larger  However, the primary determinant for velocity is not diameter, but rather total cross-sectional area of all the capillaries • ↑total cross-sectional area = ↓velocity of flow • Capillaries have much larger total cross-sectional area than arteries and veins combines o Low velocity of flow • Most Capillary Exchange Takes Place by Diffusion and Transcytosis o Exchange between plasma and interstitial fluid takes place either by  movement between endothelial cells (paracellular transport)  movement through cells (endothelial transport)  smaller dissolved solutes and gases move by diffusion between or through endothelial cells  larger dissolved solutes move via vesicular transport o diffusion rate determined by plasma-interstitial fluid concentration gradient  oxygen and carbon dioxide diffuse freely across thin endothelium  diffuse until plasma and interstitial fluid reaches equilibrium o in continuous capillaries, blood cells and plasma proteins are unable to pass through junctions between endothelial cells  larger molecules are transported through via transcytosis • Capillary Filtration and Absorption Take Place by Bulk Flow o Bulk flow is the mass movement of fluid due to hydrostatic pressure gradients  Into capillary  absorption  Out of capillary  filtration • Caused by hydrostatic pressure that forces fluid out like a leaky hose • Most capillaries show a transition from net filtration at arterial end to net absorption at venous end  Two forces regulate bulk flow • Hydrostatic pressure (P ) H o Pushes fluid against walls, out through capillary pores o Capillary hydrostatic pressure decreases along the length of the capillary as energy is lost to friction  P Hf interstitial fluid (IF) is very low, assume to be 0  Water movement due to P is Hhus directed outward, with pressure gradient decreasing from the arterial end to venous end • Osmotic pressure o Determined by solute concentration of a compartment  Colloid osmotic pressure (oncotic pressure) (π) • Osmostic pressure gradient created
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