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Roger Evans; Week 11 MED1022; Physiology

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Capillaries have no smooth muscle or elastic tissue. They are highly permeable to water and small solutes 5-8um (single file). There are pores between adjacent endothelial cells. It is not permeable to large proteins. Velocity of blood flow through capillaries is slow due to large total cross sectional area due to branching, allows exchange of substances. Precapillary sphincters can close off individual capillaries, at rest multiple capillaries are closed. Lymphatics are required because some capillaries are leaky (fenestrated kidneys). Capillaries have hydrostatic pressure and the interstitial space also has hydrostatic pressure (-3mmHg). Both capillary blood and interstitial fluid have colloid osmotic pressure. Plasma has an oncotic pressure of 27mmHg, ISF 7mmHg. Movement through permeable membrane is due to gradients in concentration, electrical charge and pressure.


Starling's forces determine net filtration. (Pc - Pif) - (colloid p - colloid if) = net filtration pressure. Pressure difference (0.3mmHg) driving net filtration pressure results in 2-4 hours of fluid accumulation in the extracellular space in a 24hour period and loss of 25-50% of the plasma proteins from fenestrated capillary beds.

Lymph capillaries are not in CNS, cartilage, cornea, bone and bone marrow. They are found in close relation to blood capillaries- closed at one end, large gaps in basal lamina, overlapping edges of endothelial cells, drains into larger vessels. Walls of larger lymph vessels contain smooth muscle and internal one way valves. Lymphangion is the segment between the valves. Lymph movement occurs through rhythmic contraction of lymph vessels and from external pressure during skeletal muscle contraction and respiratory movements. Lymphatic cells are NK cells, T lymphocytes, B lymphocytes, macrophages, dendritic cells, reticular cells. Lymphatic tissues are aggregations of lymphocytes in connective tissues of mucus membranes and other organs. Lymphangion is the segment between lymphatic valves.

Balance between filtration and absorption can be changed by altering contriction or dilation of preceding arterioles or changes in systemic arterial pressure. If arteriole is dilated or artery pressure increases, this will lead to more filtration out of the capillary. If artery is constricted or pressure decreases, there will be less filtration out of the capillary. Changes in venous pressure also alter capillary pressure. In left heart failure, backup of pulmonary venous pressure leads to pulmonary oedema. In right heart failure, backup of venous pressure leads to peripheral oedema. Plasma protein concentration also affects- in liver cirrhosis there is reduced production of plasma proteins, kidney disease can have increased excretion of proteins. Lymphatic obstruction causes lymphoedema, elephantitis (Brugia malayi worm).

Continuous (non-fenestrated) capillaries are found in BBB, fenestrated in endocrine, intestines, pancreas, kidney. Sinusoidal is in bone marrow, lymph nodes and adrenal glands. 9/10 of fluid that leaves capillaries re-enters at venous end.

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