Understanding Capillary Fluid Exchange

Capillary with red blood cells — Capillaries are so small that blood cells can only move through them in single file. Ed Reschke / Getty Images

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Updated on August 19, 2019

A capillary is an extremely small blood vessel located within the tissues of the body that transports blood from arteries to veins. Capillaries are most abundant in tissues and organs that are metabolically active. For example, muscle tissues and the kidneys have a greater amount of capillary networks than do connective tissues.

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Capillary Size and Microcirculation

Capillary bed — OpenStax College / Wikimedia Commons / CC BY 3.0

Capillaries are so small that red blood cells can only travel through them in single file. Capillaries measure in size from about 5 to 10 microns in diameter. Capillary walls are thin and are composed of endothelium (a type of simple squamous epithelial tissue). Oxygen, carbon dioxide, nutrients, and wastes are exchanged through the thin walls of the capillaries.

Capillary Microcirculation

Capillaries play an important role in microcirculation. Microcirculation deals with the circulation of blood from the heart to arteries, to smaller arterioles, to capillaries, to venules, to veins and back to the heart.
The flow of blood in the capillaries is controlled by structures called precapillary sphincters. These structures are located between arterioles and capillaries and contain muscle fibers that allow them to contract. When the sphincters are open, blood flows freely to the capillary beds of body tissue. When the sphincters are closed, blood is not allowed to flow through the capillary beds. Fluid exchange between the capillaries and the body tissues takes place at the capillary bed.

Capillary to Tissue Fluid Exchange

Capillary Microcirculation — Kes47 / Wikimedia Commons / Public domain

Capillaries are where fluids, gasses, nutrients, and wastes are exchanged between the blood and body tissues by diffusion. Capillary walls contain small pores that allow certain substances to pass into and out of the blood vessel. Fluid exchange is controlled by blood pressure within the capillary vessel (hydrostatic pressure) and osmotic pressure of the blood within the vessel. The osmotic pressure is produced by high concentrations of salts and plasma proteins in the blood. The capillary walls allow water and small solutes to pass between its pores but does not allow proteins to pass through.

As blood enters the capillary bed on the arteriole end, the blood pressure in the capillary vessel is greater than the osmotic pressure of the blood in the vessel. The net result is that fluid moves from the vessel to the body tissue.
At the middle of the capillary bed, blood pressure in the vessel equals the osmotic pressure of the blood in the vessel. The net result is that fluid passes equally between the capillary vessel and the body tissue. Gasses, nutrients, and wastes are also exchanged at this point.
On the venule end of the capillary bed, blood pressure in the vessel is less than the osmotic pressure of the blood in the vessel. The net result is that fluid, carbon dioxide and wastes are drawn from the body tissue into the capillary vessel.

Blood Vessels

Arteries—transport blood away from the heart.
Veins—transport blood to the heart.
Capillary—transport blood from arteries to the veins.
Sinusoids—vessels found in certain organs including the liver, spleen, and bone marrow.