Capillaries
Capillaries are the smallest blood vessels in the human body and play a crucial role in the circulatory system. They form an extensive network that connects arterioles, the smallest branches of arteries, to venules, the smallest branches of veins. This network facilitates the exchange of oxygen, nutrients, carbon dioxide, and waste products between blood and surrounding tissues.
Structurally, capillaries are composed of a single layer of endothelial cells, which are thin and flat, allowing for efficient exchange of substances. This single-cell thickness is essential for their primary function: to enable the diffusion of gases, nutrients, and waste products. The thin walls of capillaries allow oxygen and nutrients to pass from the blood into tissues, while carbon dioxide and metabolic waste products move from tissues into the blood for removal.
Capillaries are classified into three types based on their permeability and structure: continuous, fenestrated, and sinusoidal. Continuous capillaries have uninterrupted endothelial cells and are found in muscle, skin, and the central nervous system. They allow for the selective exchange of substances. Fenestrated capillaries have pores in their endothelial cells, which increase permeability and are found in areas requiring rapid exchange, such as the kidneys, intestines, and endocrine glands. Sinusoidal capillaries have larger openings and are found in the liver, bone marrow, and spleen, allowing for the passage of larger molecules and cells.
The distribution and density of capillaries vary depending on the metabolic activity of tissues. Highly active tissues, such as muscles and the brain, have a dense capillary network to meet their high oxygen and nutrient demands. In contrast, less active tissues have fewer capillaries.
Capillaries also play a role in thermoregulation. By adjusting blood flow through capillary networks, the body can regulate heat loss. For instance, in response to cold temperatures, capillaries constrict to reduce blood flow to the skin, conserving body heat. Conversely, in warm conditions, capillaries dilate to increase blood flow and facilitate heat dissipation.
In medical contexts, capillaries are significant in understanding various conditions. For example, capillary damage or dysfunction can lead to issues such as edema, where fluid leaks into tissues, or diabetic retinopathy, where capillary damage in the retina affects vision. Understanding capillary function is also crucial in pharmacology, as drug delivery often relies on capillary networks to reach target tissues.
In summary, capillaries are vital components of the circulatory system, facilitating essential exchanges between blood and tissues, contributing to thermoregulation, and playing a role in various physiological and pathological processes. Their structure and function are integral to maintaining homeostasis and supporting the body’s metabolic needs.
| Capillary | |
|---|---|
 Diagram of a capillary | |
 A simplified illustration of a capillary network | |
| Details | |
| Pronunciation | US: /ˈkæpəlɛri/, UK: /kəˈpɪləri/ |
| System | Circulatory system |
| Identifiers | |
| Latin | vas capillare |
| MeSH | D002196 |
| TA98 | A12.0.00.025 |
| TA2 | 3901 |
| TH | H3.09.02.0.02001 |
| FMA | 63194 |
| Anatomical terminology | |
A capillary is a small blood vessel, from 5 to 10 micrometres in diameter, and is part of the microcirculation system. Capillaries are microvessels and the smallest blood vessels in the body. They are composed of only the tunica intima (the innermost layer of an artery or vein), consisting of a thin wall of simple squamous endothelial cells. They are the site of the exchange of many substances from the surrounding interstitial fluid, and they convey blood from the smallest branches of the arteries (arterioles) to those of the veins (venules). Other substances which cross capillaries include water, oxygen, carbon dioxide, urea, glucose, uric acid, lactic acid and creatinine. Lymph capillaries connect with larger lymph vessels to drain lymphatic fluid collected in microcirculation.