Blood Vessels are tubular structures that conduct blood throughout the body and form an extensive branching network connecting the heart with tissues and returning blood to the heart. They include arteries, arterioles, capillaries, venules and veins, each adapted to particular pressure, flow and exchange requirements. Arteries generally carry blood away from the heart, while veins generally carry blood towards it, so the distinction is based on direction rather than oxygen content.
Pulmonary arteries carry oxygen depleted blood to the lungs, whereas pulmonary veins carry oxygenated blood back to the heart. Vessel walls contain varying proportions of endothelial cells, connective tissue and smooth muscle. Large arteries have substantial elastic and muscular components that help withstand and regulate high pressure. Capillaries have extremely thin walls suited to exchange. Veins generally have thinner walls and larger internal diameters than corresponding arteries and can contain valves that help prevent reverse flow.
Arteries experience the highest pressures within the systemic circulation and possess walls adapted to repeated mechanical stress. Elastic arteries close to the heart contain abundant elastic tissue that allows them to expand during ventricular ejection and recoil between heartbeats, helping smooth pulsatile flow. Muscular arteries contain more smooth muscle and distribute blood to particular organs and regions. Arterioles are smaller vessels that provide major control over vascular resistance and tissue perfusion.
Contraction of arteriolar smooth muscle narrows the lumen and can reduce local blood flow, while relaxation increases diameter and facilitates greater flow. Local metabolic conditions can influence arteriolar tone so that active tissues receive appropriate blood supply. Hormonal and nervous signals also modify vascular diameter. This layered organisation means that blood distribution can be controlled both centrally and locally.
Capillaries are the principal sites of exchange between blood and surrounding tissues. Their walls are generally composed of a single layer of endothelial cells supported by a thin basement membrane, producing a short diffusion distance. Oxygen, carbon dioxide, water, nutrients, hormones and waste products can move between blood and interstitial fluid through different mechanisms. Some substances cross endothelial cells or junctions between them, while others require specialised transport processes. Capillary structure varies among organs.
Some tissues require highly permeable capillaries to permit extensive exchange, whereas other organs maintain tighter barriers. Blood flow through capillary beds is regulated by upstream vessels and local physiological conditions. Because capillaries have a very large total cross sectional area, blood velocity is relatively low compared with large arteries. This lower velocity increases the time available for exchange. The enormous total length and number of capillaries ensure that most cells are located sufficiently close to a blood supply for diffusion to support their metabolic requirements.
Venules collect blood leaving capillary beds and converge into progressively larger veins. Veins operate at substantially lower pressure than arteries and therefore rely on several mechanisms to assist venous return. Valves in many veins prevent blood from flowing backwards, particularly in the limbs. Contraction of surrounding skeletal muscles can compress veins and push blood towards the heart, while changes in pressure during breathing can also influence venous return.
Veins contain relatively compliant walls and can accommodate substantial changes in blood volume. This makes the venous system an important blood reservoir. Changes in venous tone can redistribute blood between peripheral tissues and the central circulation. If venous return falls significantly, cardiac filling may decrease and cardiac output can be affected. The structure of veins therefore reflects both their low pressure environment and their important role in returning blood to the heart.
The vascular endothelium is a biologically active layer that regulates interactions between blood and vessel walls. Endothelial cells influence vessel permeability, smooth muscle behaviour, inflammation, platelet interactions and coagulation. They can release substances that promote vessel dilation or constriction and can respond to changes in pressure, chemical signals and blood flow. Mechanical forces generated by flowing blood influence endothelial behaviour and can contribute to long term vascular adaptation.
Damage or dysfunction of the endothelium can increase inflammatory activity, alter permeability and promote abnormal interactions with platelets and circulating proteins. Atherosclerotic processes involve complex interactions among lipids, inflammatory cells, endothelial function and vascular wall changes. Blood vessels therefore cannot accurately be understood as inert tubes. Their cellular lining actively participates in maintaining normal circulation and responding to physiological and pathological conditions.
Vessel diameter has a major influence on resistance to blood flow. Small changes in the radius of a vessel can produce large changes in resistance, making arterioles especially effective regulators of regional blood supply. Blood pressure progressively decreases as blood travels through the systemic circulation, with the greatest pressure drop occurring across high resistance vessels. Flow also depends on pressure differences and blood viscosity.
Changes in blood viscosity can occur with altered cell concentrations or plasma composition and can influence circulatory resistance. The vascular network is extensively branched, allowing the high volume of blood leaving the heart to be distributed among enormous numbers of small vessels. The branching pattern reduces the distance between capillaries and most cells, supporting efficient exchange. These principles are important in physiology, pathology and medical engineering because alterations in vessel diameter, wall properties or branching can significantly affect tissue perfusion.
Blood vessels are affected by disorders including atherosclerosis, aneurysms, thrombosis, inflammation and abnormal vascular constriction or dilation. Atherosclerosis involves accumulation of lipid rich and inflammatory material within arterial walls and can reduce the effective lumen or destabilise plaques. An aneurysm involves abnormal local dilation of a vessel and may become dangerous if the wall weakens sufficiently to rupture. Thrombosis can obstruct blood flow within a vessel, while an embolus can travel through the circulation and lodge elsewhere.
Chronic hypertension can increase mechanical stress on vascular walls and contribute to structural changes. Peripheral vascular disease can reduce blood supply to limbs and other tissues. Treatment depends on the underlying problem and may involve risk factor modification, medicines, catheter based procedures or surgery. Vascular health is therefore central to the function of virtually every organ.
Blood vessels are studied through physical examination, blood pressure measurement, ultrasound, angiography, computed tomography, magnetic resonance imaging and other methods. Doppler techniques can estimate blood movement and identify abnormalities in flow. Vascular biology also examines endothelial signalling, smooth muscle behaviour, extracellular matrix composition and inflammatory processes. Research into blood vessel formation has important implications for development, cancer biology, wound healing and tissue engineering because new vessels are required to supply growing tissues. Conversely, abnormal vessel formation can contribute to disease. Understanding blood vessels therefore extends beyond cardiovascular anatomy into developmental biology, molecular medicine and regenerative research. Their organisation demonstrates how microscopic cellular structures produce large scale transport networks capable of supplying every region of the body.
