this lecture is a brief overview of capillaries and their filtration and absorptive functions. the first section describes the physical characteristics of capillaries and the three major types of capillaries: continuous, fenestrated, and discontinuous. continuous is found in the skin, muscle, lungs, CNS, and connective tissue, and contain small clefts that pass water and small solutes, but not proteins. fenestrated capillaries have a more extensive pore network, up to 10 times larger than continuous, and are found in the renal glomeruli, exocrine ducts, and the choroid plexus. discontinuous capillaries have huge holes which let proteins and rbc pass and are located in the spleen, liver, and bone marrow.
the next section delves into the details about capillary absorption/filtration. starling forces are the balance of hydrostatic and oncotic forces. hydrostatic forces are basically created from the transmural pressure from the fluid flow, and oncotic pressure is created by the osmotic pressure across the endothelium from the proteins inside the capillaries. the starling force equation describes the net flow of fluid either in or out of the capillary according to the interaction of these two forces: V=K[(Pc-Pi)-π(pc-pi)]. as the fluid flows down the length of the vessel, hydrostatic force gradually decreases (force lost to maintaining pressure of vessel?) and eventually becomes less than the oncotic pressure, causing the net flow to go from filtration to absorption (NFP describes the filtration pressure and NAP describes the absorption pressure)
a few more random details about capillaries are thrown in: the interstitial space around capillaries is composed of collagen, proteoglycans (hyaluronic acid gel), and rivulets of free fluid. capillary growth happens via pseudopods that are extended into surrounding CT, in response to nearby tissue O2 demand or injury.
lymph is then introduced: the remaining 10% of the extravasated interstitial fluid that is not reabsorbed by the venous sections of capillaries. lymphatics reabsorb fluids and proteins, absorbed into terminal lymphatics via the hydrostatic pressure in the interstitial fluid. lymph is transported through lymph vessels through valves, mediated by smooth muscle that is innervated by the sympathetic nervous system. the deep lymph layer runs alongside the aorta and the superficial lymph layer runs along the veins and coalesces into the axillary, cervical, and inguinal nodes. lymph drains back into the venous system, via either the thoracic drain, which collects from the lower 3/4 of the body, and the right lymphatic drain, which drains the upper right 1/4. both drain into the subclavian and internal jugular veins.
the last section describes some pathologies related to interstitial fluid buildup, otherwise known as edema. these pathologies involve either an excess hydrostatic pressure or a deficient oncotic pressure, which both cause excess filtration or deficient reabsorption (leading to fluid buildup) the first type mentioned is venous edema, which occurs because of excess hydrostatic pressure in the lower limbs due to occlusion or CHF. the second type is hypoalbuminemic enema, which is when the lack of proteins in the capillaries reduces the encotic pressure inside, leading to reduced reabsorption. the third type is inflammatory edema, when protein leaks out into the interstitial space, raising its encotic pressure and thereby reducing absorption. the last is lymphatic edema, where uptake into the lymph vessels is blocked by infection or damage.
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