Interaction b/w cardiac output and capillary blood flow depends on Pressure, P, and Resistance, R.
Inc. Pressure = Inc. Flow, and Inc. Resistance = Dec. Flow
Pressure gradient is the difference in Pressure from one end of the vessel to the other
Mean blood pressure decreases as you move away from the heart
Resistance in the Venous system is very small and is thus able to return blood to the heart with ease.
Capillary Exchange – transfer of liquid and solutes between blood and interstitial fluid; such as Oxygen, CO2, waste.
Pressure: BP= blood pressure, CHP = capillary hydrostatic pressure – P in capillary beds (35-18 mmHg), Venous pressure – P in venous system; very low, venule to right atrium is 18 mmHg
**∆P across entire systemic circuit = ‘Circulatory Pressure’ ~ 100mmHg, must overcome total ‘Peripheral Resistance’
Arterial network has Highest pressure gradient @ ~65 mmHg, due to high resistance of arterioles
Resistance: Total Peripheral Resistance is due to: Vascular Resistance, Viscosity, & Turbulence
Vascular Resistance = resistance of blood vessels; due to friction b/w blood and vessel walls; depends on diameter of vessel: 1Vessel Length, 2Vessel Diameter, 3Viscosity, 4Turbulence (eddies and swirls due to shape irregularities)
As arterial branching occurs, the TOTAL cross-sectional area increases and blood pressure falls rapidly; mostly @ small arteries/arterioles
Pulmonary vessels are much shorter; thus Pulmonary Circuit has LOWER pressure than Systemic Circuit
Arterial Blood Pressure – systolic = peak, diastolic = minimium e.g; 110/75
‘Pulse’ – rhythmic pressure oscillation w/ each heartbeat
Pulse pressure = difference in systolic & diastolic pressure
MAP (mean arterial pressure) = Diastolic P + (1/3)(Pulse Pressure)
Abnormally High = hyper tension ; above 140/90; (increased load on Left Ventricle) & Abnormally Low = Hypotension
Elastic Rebound – push generated as arteries recoil, forcing blood toward capillaries while LV is in Diastole.
Venous Pressure & Return - blood pressure is low as it enters the inf.vena-cava; overcomes gravity by:
Muscular Compression – cycles of comp/relaxation assist venous return; but when you stop walking and stand, its lost = reduced cardiac output
Respiratory Pump – action of inhaling/exhaling expands/compresses the thoracic cavity aiding in pushing venous blood into Right Atrium.
Capillary Exchange – materials are moved across capillary walls are: Diffusion, Filtration, Reabsorption
Diffusion = ions move from Hi to Lo concentration (gradient); occurs when Distance is small, Gradient is large, Ions/molecules are small
Filtration = removal of solutes as fluid moves across porous membrane; solutes too large to move through; due to hydrostatic pressure: water pushed from Hi to Lo pressure areas
Reabsorption = occurs by Osmosis – water molecules diffuse across selectively permeable membrane to higher solute concentration
**Remember – Hydrostatic Filtration pushes water OUT of solution, and Osmotic Pressure draws water INTO solution.
Filtration/Reabsorption – net hydrostatic pressure tends to push water/solutes out of capillaries into interstitial fluid.
NHP is the difference b/w capillary-HP (CHP) and interstitial-HP (IHP)
If CHP > BCOP, then (+)NFP = Filtration, out of capillary near the Arterial end.
If BCOP > CHP, then (-)NFP = Reabsorption, into capillary near the Venule end.
** IHP and ICOP are usually 0 mmHg;
Thus, NFP = NHP – NCOP = (CHP – IHP) – (BCOP – ICOP)
85% of daily 24L of blood is reabsorbed thus water is moving out of capillaries and then back