CIRCULATION

Capillary Dynamics

Before the capillaries, nothing is happening; other blood vessels are mostly conduits

Opening of precapillary sphincter is determined by O2 needs and [CO2]

Vasal motion:

Flow Through CapillariesFlow is not continuous or pulsatile through capillaries but intermittent.Contraction of precapillary sphincters can close off capillaries and stop flow.Not all capillaries are open at rest.Capillaries oscillate between open and closed.Blood cells must deform to pass through narrow capillaries.

Capillary StructureCapillaries are approximately 5-9 mm in diameter. (compared with erythrocytes 8x2 mm)

Capillary Walls have intercellular clefts approximately 6-7 nm wide.

Clefts are absent in the brain with tight junctions between endothelial cells blood brain barrier.

Clefts are larger in the liver.

Transport Across CapillariesLipid soluble substances pass across the cell membranes, e.g. O2, CO2, N2, anesthetics, ethanol.

Small-medium sized water soluble substances diffuse between endothelial cells, e.g. water, Na+, Cl-, Ca2+, urea, glucose, lactate, ADH (vasopressin), insulin.

Large substances can be moved by pinocytosis e.g. lipoproteins.

Plasma proteins like albumin dont normally cross the capillary wall.

STARLINGS HYPOTHESISThe balance of hydrostatic and oncoticpressures across the capillary endothelium.

Blood hydrostatic pressure (BHP)

Blood colloid osmotic pressure (BOP)(oncotic pressure)

Tissue hydrostatic pressure (THP)

Tissue colloid osmotic pressure (TOP)

Filtration forces:BHPTOP

Reabsorption forces:BOPTHP

BHPBOPTHPTOPFILTRATIONREABSORPTION

BHP=35 mmHgBOP=25 mmHgTHP=5 mmHgTOP=1 mmHgFILTRATIONREABSORPTIONAt arterial end of capillary,Total filtration pressure = 35+1 or 36 mmHgTotal reabsorptive pressure = 25+5 or 30 mmHgNet filtration pressure = 36-30 or 6 mmHg

BHP=15 mmHgBOP=25 mmHgTHP=5 mmHgTOP=1 mmHgFILTRATIONREABSORPTIONAt venous end of capillary,Total filtration pressure = 15+1 or 16 mmHgTotal reabsorptive pressure = 25+5 or 30 mmHgNet reabsorptive pressure = 30-16 or 14 mmHg

Bulk FlowNormally 7,200 l/day flows through the capillaries.

20 l/day is filtered out of the capillaries

17 l/day is reabsorbed into the capillaries

3 l/day is collected by the lymphatic system

Edema: accumulation of fluid in the interstitial space.

Causes of edema:

1. A reduced concentration of plasma proteins, e.g. kwashiorkor

2. Increased permeability of the capillary walls, e.g. blisters, hives

3. Increased venous pressure, e.g. pregnancy

4. Blockage of lymph, e.g. elephantiasis

The Microcirculation

Edema from Reduced Lymph FlowElephantiasis (Filariasis) is produced by blockage of lymphatic vessels and reduced lymph flow.

The blockage is produced by parasitic worms colonizing the lymphatic system and lymph nodes.

BHP=50BOP=25THP=5TOP=1BHP=30BOP=25THP=5TOP=1ARTERIOLAR VASODILATION:ARTERIAL ENDNET FILTRATION PRESSURE = 51-30 or 21 mmHgVENOUS ENDNET FILTRATION PRESSURE = 31-30 or 1 mmHgFILTRATION ALONG THE LENGTH OF THE CAPILLARY.

BHP=15BOP=25THP=5TOP=1BHP=1BOP=25THP=5TOP=1ARTERIOLAR VASOCONSTRICTION:ARTERIAL ENDNET REABSORTIVE PRESSURE = 16-30 or -14 mmHgVENOUS ENDNET REABSORTIVE PRESSURE = 2-30 or -28 mmHgREABSORPTION ALONG THE LENGTH OF THE CAPILLARY.

BHP=8 mmHgBOP=25 mmHgTHP=5 mmHgTOP=1 mmHgFILTRATIONREABSORPTIONFor the pulmonary circulation,Total filtration pressure = 8+1 or 9 mmHgTotal reabsorptive pressure = 25+5 or 30 mmHgNet reabsorptive pressure = 9-30 or -21 mmHg

Changes in Filtration & AbsorptionIncreased filtrationEdemaVasodilationArterial HypertensionVenous HypertensionIncreased plasma leakagePlasma protein deficiency

Increase absorption

VasoconstrictionArterial HypotentionVenous HypotensionDehydrationHemorrhage