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Hydrostatic and osmotic pressure are opposing factors that drive capillary dynamics. Learning Objectives
Key Points
Key Terms
Capillary exchange refers to the exchange of material between the blood and tissues in the capillaries. There are three mechanisms that facilitate capillary exchange: diffusion, transcytosis, and bulk flow. Capillary Exchange MechanismsDiffusion, the most widely-used mechanism, allows the flow of small molecules across capillaries such as glucose and oxygen from the blood into the tissues and carbon dioxide from the tissue into the blood. The process depends on the difference of gradients between the interstitium and blood, with molecules moving to low-concentrated spaces from high-concentrated ones. Transcytosis is the mechanism whereby large, lipid-insoluble substances cross the capillary membranes. The substance to be transported is endocytosed by the endothelial cell into a lipid vesicle which moves through the cell and is then exocytosed to the other side. Bulk flow is used by small, lipid-insoluble solutes in water to cross the the capillary wall. The movement of materials across the wall is dependent on pressure and is bi-directional depending on the net filtration pressure derived from the four Starling forces that modulate capillary dynamics. Capillary DynamicsThe four Starling forces modulate capillary dynamics.
The net filtration pressure derived from the sum of the four forces described above determines the fluid flow into or out of the capillary. Movement from the bloodstream into the interstitium is favored by blood hydrostatic pressure and interstitial fluid oncotic pressure. Alternatively, movement from the interstitium into the bloodstream is favored by blood oncotic pressure and interstitial fluid hydrostatic pressure. Capillary Dynamics: Oncotic pressure exerted by proteins in blood plasma tends to pull water into the circulatory system. Due to the pressure of the blood in the capillaries, blood hydrostatic pressure is greater than interstitial fluid hydrostatic pressure, promoting a net flow of fluid from the blood vessels into the interstitium. However, because large plasma proteins, especially albumin, cannot easily cross through the capillary walls, their effect on the osmotic pressure of the capillary interiors will to some extent balance the tendency for fluid to leak from the capillaries.In conditions where plasma proteins are reduced (e.g. from being lost in the urine or from malnutrition), or blood pressure is significantly increased, a change in net filtration pressure and an increase in fluid movement across the capillary result in excess fluid build-up in the tissues (edema). 18.8A: Capillary Dynamics is shared under a CC BY-SA license and was authored, remixed, and/or curated by LibreTexts. What causes hydrostatic pressure in capillaries?Hydrostatic pressure is the pressure that is exerted by a fluid at a point, because of the weight of above the fluid. This pressure in vessels of blood is due to the weight above the vessels. As blood is a fluid hence it has hydrostatic pressure.
What is blood capillary hydrostatic pressure?The pressure that blood exerts in the capillaries is known as blood pressure. The force of hydrostatic pressure means that as blood moves along the capillary, fluid moves out through its pores and into the interstitial space.
What is meant by hydrostatic pressure?What is Hydrostatic Pressure? Hydrostatic pressure is defined as. “The pressure exerted by a fluid at equilibrium at any point of time due to the force of gravity”. Hydrostatic pressure is proportional to the depth measured from the surface as the weight of the fluid increases when a downward force is applied.
What is the difference between hydrostatic pressure and osmotic pressure in capillaries?Whereas hydrostatic pressure forces fluid out of the capillary, osmotic pressure draws fluid back in. Osmotic pressure is determined by osmotic concentration gradients, that is, the difference in the solute-to-water concentrations in the blood and tissue fluid.
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