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Capillary Exchange and Edema, Animation

October 8, 2019


The major purpose of the circulatory system
is to bring oxygen and nutrients to body tissues and remove wastes. This exchange happens in the smallest blood
vessels called the capillaries. The walls of capillaries consist of a single
layer of endothelial cells. Substances move between the blood and surrounding
tissue in several ways: – Diffusion through the plasma membranes of
endothelial cells: the hydrophobic nature of the cell membrane makes it intrinsically
permeable to small lipid-soluble molecules and small gases. Oxygen moves down its concentration gradient,
from the blood to the surrounding tissue, while carbon dioxide diffuses in the reverse
direction. Glucose and other small water-soluble molecules
move, in part, by facilitated diffusion: they use special channels, called transporters,
to cross the cell membrane. Water moves by osmosis. – Transcellular vesicle transport, or transcytosis:
some proteins and hormones are packaged into lipid vesicles and transported through endothelial
cells by endocytosis and exocytosis. – In most tissues, however, the bulk exchange
of fluids and solutes is through the gaps between endothelial cells, called intercellular
clefts; and, in some tissues, through the pores of so-called fenestrated capillaries. Blood plasma containing nutrients moves out
of capillaries at the arterial end of capillary beds, in a process called filtration, while
tissue fluid containing wastes reabsorbs back in at the venous end. This movement, called bulk flow, is driven
by the balance between two forces: – Hydrostatic force, generated by the difference
in hydrostatic pressures inside and outside the capillaries. Hydrostatic pressure is defined as the pressure
of fluids in a closed space. Inside capillaries, this is the same as capillary
blood pressure. As tissues generally contain much less fluid
than blood, hydrostatic pressure from inside capillaries is considerably higher than that
from outside. Thus, hydrostatic force drives fluids, and
blood solutes, out of capillaries. – Hydrostatic force is opposed by osmotic
force. Osmotic force, also called oncotic pressure,
is generated mainly by the difference in protein concentrations between the blood and interstitial
tissue. The blood has a much higher protein content,
due to albumin, and this draws water into blood vessels. Because the arterial end of a capillary bed
is relatively closer to the heart than the venous end, capillary blood pressure and,
by extension, hydrostatic pressure, is higher at the arterial end. With osmotic pressure remaining the same throughout,
the balance shifts from net outward flow at the arterial end to net inward flow at the
venous end. Note that the net outward filtration pressure
is greater than the net inward reabsorption pressure. This means more fluid is filtered out than
reabsorbed back in. In fact, about 15% of the fluid is left in
the tissues after capillary exchange. This fluid is picked up by the lymphatic system
and returned to the circulation at a later point. Edema refers to abnormal accumulation of excess
fluid in a tissue. It manifests as external swelling or enlarged
internal organs. There are 3 principal groups of causes:
– Increased filtration, either from increased blood pressure or increased capillary permeability,
– Decreased reabsorption due to reduced blood albumin concentrations,
– and obstruction of lymphatic drainage. Excess fluid hinders the exchange of nutrient/waste
and gases and may lead to tissue necrosis. Severe edema may also be accompanied by critically
reduced blood volume which may result in circulatory shock.

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4 Comments

  • Reply Alila Medical Media September 30, 2019 at 5:36 pm

    If this video is helpful to you, please consider supporting our next projects. As a token of our appreciation, we also offer early access to our videos and free image downloads in return, please check us out here: https://www.patreon.com/AlilaMedicalMedia/posts

  • Reply ڕۆژی پڕشنگدار September 30, 2019 at 5:40 pm

    can you present action potential , i need it , thanks for this attention

  • Reply AyAyRon September 30, 2019 at 7:52 pm

    Interesting.. Didn't know the albumin was so useful

  • Reply Chan Valentine October 1, 2019 at 1:49 am

    Thank you very much. I was recently diagnosed with lipodema with lymphedema. This is helpful in visually unwinding the skein of medical mysteries I'm dealing with.

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