Capillary Exchange and Edema, Animation

(USMLE topics, cardiology) Mechanisms of capillary exchange: diffusion, transcytosis, filtration and reabsorption; cause of edema.
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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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[@richardsantacruz7129]

where do capillary beds in which location (liver, kidney, pancreas, brain, bone marrow) have the greatest control over osmosis and diffusion?

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How can we increase either the capillaries, capillary density or capillary biogenesis for improved physical output?

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[@willow8749]

Do capillaries exchange substances between the blood and the interstitial fluid in the same way through- out the body?

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When a patient presents with edema/swelling in the extremities and face are their chances of having a "enlarged" internal organ also increased? besides possibly having hypertrophy of the heart. What other organs could be affected?

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We already have that video, look for it in our channel!

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Is there any change in osmotic pressure between arterial and venous end of capillaries? I saw somewhere that at venous end of capillary osmotic pressure increase due to hemoconcentration. Confused.. 🤪🙄

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My teacher said that osmotic pressure barely changes at either end. No sure what country your from though so maybe we need to know different thing idk

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Does blood diffuse through the capillaries to the tissue?

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I have a question what does the quantity of protein has tp do with water mvt?

[@Alilamedicalmedia]

Water (like anything else) moves from high to low concentration. When the inside of a compartment has more protein (or any other substance such as salt, sugar...) than outside, the concentration of water inside that compartment is lower than outside - water will flow from outside in.

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Is that osmotic pressure ?

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i just cant understand, the pressure is supposed to be 13, and then+ 7??? how?? and why? wont it explore?

[@sneha3488]

thats the net flow inwards and outwards ; net filteration = (BHP + IFOP) - (BCOP + IFHP) where BHP = blood hydrostatic pressure , IFOP = intersitiutal fluid osmotic pressure, this part of the eq in brackets represents the facotrs which allow filteration to occur and BCOP = blood colloid osmotic pressure and IFHP = intersital fluid hydorstatic pressure which allow reabsorption to occur. The two values are different because the values of BHP and BCOP values at the venous end and arterioal end are different to each other, which would result in different net filteration pressure hence the different numbers. IFOP = 0 and IFHP = 1, these values dont change. How? the differences are caused through the pressure and osmotic gradients itself as the video above mentions. Hydrostatic pressure moves lymph fluid out of the capillaries and then intersitial fluid osmotic pressure combats against it, however the IFOP is negligible since its a small number so fluid is pushed out of the capillary as a result when blood is moving down the the venous end of the cappilary, the lymph fluid is reabsorbed back in as the BCOP pulls fluid back into the cappilary since it has a higher plasma protein concentration inside the cappilary since IFHP force is also considered negligible to push fluid out as it is so small. If you understand this concept what i've said above should help clarifiy as certain numbers need to be higher or lower to accommodate.

[@cometali1398]

15% of the fluid is left in the tissues and not on the interstitial fluid? sorry thanks

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Wait what...

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The interstitial fluid is a part of the tissue! You need to study more.

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What is capillary permeability? 🙈

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How much the capillary can let solutes flow through its membrane.

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