Capillaries are the smallest blood vessels in the body, serving a function distinct from the major transportation roles of arteries and veins. These microscopic vessels form an expansive network that acts as the bridge between the arterial system and the venous system. While arteries and veins are built for high-volume transport, the primary purpose of capillaries is not movement but exchange. It is within these vast capillary beds that the continuous movement of gases, nutrients, and waste products occurs between the bloodstream and the surrounding body tissues.
Structural Traits Crucial for Exchange
Each capillary is an extremely narrow tube, typically measuring only 5 to 10 micrometers in diameter, which is just wide enough for red blood cells to pass through in single file. This narrowness slows blood flow and ensures that every red blood cell comes into close contact with the vessel wall, thereby maximizing the time and surface area available for transfer.
The vessel wall is composed of a single layer of flattened endothelial cells (tunica intima) resting upon a thin basement membrane. This composition makes the capillary wall exceptionally thin, minimizing the distance substances must travel to move between the blood and the interstitial fluid of the tissues. The sheer number of capillaries form dense, interweaving networks called capillary beds that provide an enormous total surface area for exchange throughout the body. These beds connect the smallest arterioles to the smallest venules, completing the circulatory pathway at the tissue level.
Facilitating Gas and Nutrient Transfer
The capillary network’s fundamental function is the exchange of gases required for cellular metabolism. Oxygen, carried primarily by red blood cells, must be delivered from the blood into the tissue fluid for energy production. Simultaneously, the metabolic waste product carbon dioxide is collected from the tissues and moved into the capillary blood to be transported back to the lungs for exhalation.
Capillaries also serve as the delivery system for energy substrates and regulatory molecules. Nutrients absorbed from the digestive system, such as glucose and fatty acids, are released to fuel the surrounding cells. Hormones secreted by endocrine glands also enter the bloodstream and are delivered to their target cells via these extensive networks.
In addition to delivering resources, capillaries are responsible for collecting the byproducts of cellular activity. These waste products include urea from protein metabolism and lactic acid produced during anaerobic respiration. The collection of these substances is necessary for their eventual processing and elimination by organs like the kidneys and liver, maintaining the overall chemical balance, or homeostasis, of the body.
The Physics of Capillary Exchange
The movement of substances across the capillary wall is governed by two primary physical mechanisms: diffusion and bulk flow, which involves filtration and reabsorption. Diffusion is the passive movement of substances like oxygen, carbon dioxide, and small ions from an area of higher concentration to an area of lower concentration. Since the blood entering the capillaries has a higher concentration of oxygen and nutrients than the surrounding tissue, these substances readily diffuse out of the blood.
Bulk flow, which dictates the movement of fluid, is determined by the balance of two opposing forces known as Starling forces: hydrostatic pressure and colloid osmotic pressure. Hydrostatic pressure, the force exerted by the blood pushing against the capillary wall, tends to push fluid out of the vessel. At the arteriolar end of the capillary, this pressure is relatively high, leading to net filtration of fluid, along with dissolved small solutes, into the interstitial space.
Colloid osmotic pressure (oncotic pressure) is a force that tends to pull fluid back into the capillary due to the presence of large plasma proteins, particularly albumin, that cannot easily cross the capillary wall. As fluid filters out at the arteriolar end, the concentration of these proteins increases, and the capillary hydrostatic pressure decreases significantly as blood moves toward the venous end. By the time the blood reaches the venular end, the hydrostatic pressure has dropped below the colloid osmotic pressure, causing a shift to net reabsorption of fluid back into the capillary. This dynamic pressure gradient ensures that most of the filtered fluid is recovered, with any remaining excess being collected by the lymphatic system.
Specialized Roles in Organ Systems
Capillaries are not structurally identical throughout the body; their permeability varies depending on the specific needs of the organ they serve. These variations are categorized into three main types: continuous, fenestrated, and sinusoidal. Continuous capillaries are the most common type and are characterized by tight junctions between endothelial cells that limit the passage of molecules. A highly specialized version of this type forms the blood-brain barrier, where exceptionally tight junctions restrict almost all exchange, protecting the sensitive neural tissue from substances in the blood.
Fenestrated capillaries possess small pores, or fenestrations, that allow for the rapid movement of water and small solutes. This structure is found in organs where rapid filtration or absorption is necessary, such as the kidneys for filtering blood and the small intestine for absorbing digested nutrients.
The third type, sinusoidal capillaries, are the most permeable, featuring large gaps between cells and an incomplete basement membrane. These large openings allow even blood cells and large plasma proteins to pass through, which is necessary for the production and modification of blood components in organs like the liver, spleen, and bone marrow.