Capillary Fluid Exchange

Talissa Nahass, Christopher Muscato
  • Author
    Talissa Nahass

    Talissa Nahass has taught various levels of secondary biological sciences for over 10 years. She has a B.S. in Biology/Secondary Education (with minors in Environmental Science & Psychology) from Marist College and M.A. in Educational Leadership from Montclair State University. She has the following NJ teaching certificates: Secondary Education (Biological Science), Supervisor, and Principal.

  • Instructor
    Christopher Muscato

    Chris has a master's degree in history and teaches at the University of Northern Colorado.

Explore the capillary exchange process. Learn the roles capillary osmotic and hydrostatic pressure, permeability, filtration, and membranes play in fluid exchange. Updated: 03/23/2022

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What are Capillaries?

Capillaries are tiny blood vessels that connect arteries to veins. Blood enters capillaries through the arteriole side and is released from the venule side. What happens to the fluid filtered from the blood capillaries? Capillaries allow the exchange of gasses, nutrients, hormones, and other molecules in the blood. These materials are exchanged with the cells of the body. Carbon dioxide and waste are transferred from the body to the capillaries. Capillaries exchange material with the extracellular fluid. Capillaries are present in a branching network between arteries and veins. There is an abundance of capillaries in areas of the body that have high metabolic activity (i.e., liver). In areas of the body with low metabolic activity (i.e., the eye), there are few to zero capillaries. Capillaries are arranged in capillary beds. Capillaries are very narrow, which allows for easy capillary diffusion to occur. The capillary membrane is an endothelial layer that is one cell thick. Since the capillary membrane is thin, gasses and nutrients can pass through easily.

There are three different types of capillaries. The three different types of capillaries are continuous capillaries, fenestrated capillaries, and sinusoid capillaries. Continuous capillaries are the most abundant capillary type in the body and have no intercellular or intracellular gaps. The tight junctions prevent any leakage from occurring. Fenestrated capillaries have fenestrations (pores) in the cells. These capillaries are more permeable than continuous capillaries. Fenestrated capillaries would be abundant in places that require high amounts of absorption or filtration (i.e., kidneys). Sinusoid capillaries are the biggest and most permeable capillaries. Sinusoid capillaries have large fenestrations and gaps. As a result of these gaps, large molecules can pass through, and these would be found in organs like the liver.


Image of Kidney Nephron - Capillaries #9- In this image of the kidney nephron, the peritubular capillaries are labeled at number 9. Capillaries are tiny blood vessels that connect arteries to veins. Capillaries are important to the circulation of materials throughout the body. Blood enters capillaries through the arteriole side and is released from the venule side.

Image of Kidney Nephron - Capillaries #9- In this image of the kidney nephron, the peritubular capillaries are labeled at number 9. Capillaries are tiny blood vessels that connect arteries to veins. Capillaries are important to the circulation of materials throughout the body. Blood enters capillaries through the arteriole side and is released from the venule side.


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  • 0:00 Capillaries
  • 1:06 Fluid Exchange
  • 2:11 Arteriole End of the…
  • 2:44 Middle of the Capillary Bed
  • 3:09 Venule End of the…
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Capillary Exchange

Capillary exchange involves exchanging materials through capillary membranes. The exchanged materials include gasses, nutrients, hormones, wastes, carbon dioxide, lipid-soluble substances, and other molecules in the blood. Capillaries exchange material with the extracellular fluid, cells of the body, or surrounding tissue. This capillary exchange occurs to bring certain materials into the blood (to transfer to the body) or to eliminate wastes out of the blood. Blood will flow from arteries, arterioles, capillaries, venules, and then veins. When blood flows through capillaries, it goes slowly so that capillary exchange can occur. There is a capillary exchange at the arterial and venule end of the capillaries but no exchange in the capillary center. There is a lack of capillary exchange in the middle because the capillary hydrostatic pressure and the blood osmotic pressure are equal. No diffusion will occur (no net movement) if no concentration gradient is present. Why do fluids leave capillaries at the arterial end? There is a capillary exchange at the arterial end, and the net movement is out of the capillary. The hydrostatic pressure inside the capillary is larger than the blood osmotic pressure. At the venule end of the capillary, the reverse is true. The net movement is of fluid into the capillary, and the blood osmotic pressure is higher than the capillary hydrostatic pressure. Three mechanisms by which substances are transferred between capillaries and interstitial fluid are diffusion, bulk flow, and transcytosis.

Capillary Diffusion

Capillary diffusion occurs when materials like oxygen and other lipid-soluble substances are exchanged. Diffusion is a type of passive transport that does not require energy. Oxygen and carbon dioxide will move down their concentration gradients (from high to low concentration). These substances are exchanged between the capillary and interstitial fluid by the driving force of a pressure gradient. The two pressure gradients important in determining capillary exchange are the capillary hydrostatic pressure and the blood osmotic pressure.

Bulk Flow

Bulk flow is a type of transport that occurs across capillary membranes that are driven by pressure. The hydrostatic pressure of the blood will push fluids and small molecules through the gaps or pores of the capillary. At the arteriole end, there is a net movement of materials toward the outside of the capillary because the hydrostatic pressure inside the capillary is more than the osmotic pressure outside. At the venule side of the capillary, the reverse is true, and there is a net movement toward the inside of the capillary. Bulk flow into the blood is called reabsorption, and bulk flow out of the capillary is called filtration. In continuous capillaries, bulk flow is restricted because there are no intercellular gaps or pores. In fenestrated capillaries, there is a more significant amount of bulk flow in fenestrated capillaries that have a lot of pores (fenestrations) and are the most permeable capillary type.

Net Filtration Pressure (NFP)

Net filtration pressure refers to the amount of pressure that determines capillary filtration. The Starling forces are hydrostatic pressure and osmotic pressure. There is a higher net filtration toward the outside of the capillary at the arterial end. At the venule end, the net movement of materials into the capillary is lower. Net filtration pressure is determined by the difference between the hydrostatic pressure in the capillary and the osmotic pressure outside of the capillary. In the middle of the capillary bed, the pressure evens out to equal the pressure of the surrounding tissues. Net filtration pressure is calculated with the following formula: NFP = Hydrostatic Pressure (inside capillary)- osmotic pressure.

Transcytosis

Transcytosis is when larger molecules and lipid insoluble molecules pass through a membrane. These larger molecules are moved through endocytosis or exocytosis (active transport) and are packaged into lipid vesicles to move across the membrane. Transcytosis is a crucial method to get larger molecules transported through the blood. Transcytosis is used to move larger molecules like glucose and amino acids. Transcytosis occurs in the epithelial cells that line the capillaries.

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Frequently Asked Questions

Why are capillaries called exchange vessels?

Capillaries are called exchange vessels because they allow the exchange of gasses, nutrients, hormones, and other molecules in the blood. Capillaries exchange material with the extracellular fluid.

How does capillary exchange occur?

Capillary fluid exchange is essential because this is how gasses and nutrients are supplied to the body's cells. At the venule end of the capillary, the carbon dioxide and wastes are reabsorbed into the blood. At the arteriole end, the blood osmotic pressure is lower than the hydrostatic pressure, and the net movement of substances is to the outside of the blood cells.

What determines capillary hydrostatic pressure?

Capillary hydrostatic pressure is determined by the pressure exerted on the capillary membrane by the blood within the narrow capillary. The hydrostatic capillary pressure is essentially capillary blood pressure.

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