Capillaries are the smallest and most numerous blood vessels in the body, forming an extensive network vital for sustaining life at the cellular level.

These microscopic vessels, often only about 5 micrometers in diameter, serve as critical sites of exchange between the bloodstream and tissues.

Capillary Structure: Design for Efficient Exchange

Capillaries are composed of a single layer of endothelial cells surrounded by a thin basement membrane. This minimal structural barrier allows for efficient diffusion of gases, nutrients, and waste products between blood and body cells. The permeability of the capillary walls varies depending on their location and function.

Three main types of capillaries—continuous, fenestrated, and sinusoidal—differ in their level of leakiness, adapting to specific tissue demands.

Continuous capillaries, found in muscles, skin, and the central nervous system, have tightly joined endothelial cells that restrict passage to small molecules, maintaining selective filtration. Fenestrated capillaries possess pores allowing increased permeability and are prevalent with high exchange rates such as kidneys and intestines.

Sinusoidal capillaries feature larger gaps permitting passage of cells and large molecules, characteristic of the liver, spleen, and marrow.

Essential Exchange of Substances

Capillaries serve as the primary interface for exchanging oxygen, carbon dioxide, nutrients, hormones, and metabolic wastes. Oxygen-rich blood flows from arterioles into capillaries, where oxygen diffuses through endothelial walls into surrounding tissues. Concurrently, carbon dioxide and metabolic by-products travel from tissues into capillaries to be carried away for excretion.

Nutrients such as glucose, amino acids, and lipids also traverse capillary walls through diffusion, facilitated transport, or vesicular transport (pinocytosis), nourishing cells throughout the body. Hormones and signaling molecules exit or enter the bloodstream here as well, enabling rapid physiological communication and homeostatic regulation.

Fluid balance is maintained by the interplay of hydrostatic pressure pushing plasma out of capillaries and osmotic pressure pulling fluid back in, ensuring tissues receive adequate hydration without undue swelling.

Regulation of Blood Flow and Autoregulation

Capillaries regulate local blood flow through precapillary sphincters—smooth muscle rings at capillary entrances. These sphincters respond to tissue oxygen demand, metabolic waste levels, and neural or chemical signals, adjusting blood distribution dynamically.

For example, during exercise, active muscles release vasodilatory substances like nitric oxide, causing sphincters to relax and increase blood flow, supplying oxygen and nutrients to meet heightened metabolic needs.

Role in Immune Surveillance and Inflammation

Capillaries also participate actively in immune defense. Endothelial cells express adhesion molecules allowing white blood cells to exit circulation at sites of infection or injury through a process called extravasation. These immune cells then combat pathogens, remove debris, and facilitate tissue repair.

During inflammation, capillary permeability increases due to signaling molecules such as histamine and prostaglandins, enabling plasma proteins and immune cells to access affected sites. While critical for defense, excessive permeability can cause edema and tissue damage if unregulated.

Clinical Significance and Pathologies

Damage or dysfunction of capillaries can have profound health effects. Capillary rupture produces petechiae, small skin red spots, while compromised capillary networks can lead to ischemia or failure. Diseases such as diabetes induce capillary basement membrane thickening, impairing exchange and contributing to complications like retinopathy and nephropathy.

Dr. Coral Murrant states "Proper blood flow to every cell is critical for maintaining healthy tissues and a healthy body. For the longest time we've been operating under the assumption that larger blood vessels, like arteries, are the main players dictating blood flow."

Tiny capillaries perform indispensable roles by serving as the junctions between blood vessels and tissues, facilitating critical exchange of gases, nutrients, wastes, and signaling molecules. Their specialized structure accommodates varied permeability demands, while their capacity for autoregulation ensures adequate tissue perfusion across diverse physiological conditions.

Additionally, capillaries contribute to immune defense and inflammation control, underscoring their multifaceted importance. Dysfunction of capillaries can precipitate serious health problems, reflecting their fundamental role in maintaining the body's internal environment.