The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away from the tissues. Blood vessels are needed to sustain life because all of the body’s tissues rely on their functionality.[rx] There are five types of blood vessels: the arteries, which carry the blood away from the heart; the arterioles; the capillaries, where the exchange of water and chemicals between the blood and the tissues occurs; the venules; and the veins, which carry blood from the capillaries back towards the heart.
Blood vessels are fundamental components of the cardiovascular system, responsible for the dynamic transportation of matter and blood products to every cell in the body. The vascular network begins at the outlets of the heart, courses the entire body, and returns at the major venous inlets of the heart. This complex vascular highway functions to deliver blood cells, nutrients, oxygen, and pharmacological agents to tissue. Just as blood vessels direct material towards the tissue, they also facilitate the removal of cellular byproducts, carbon dioxide, and toxic chemicals from the tissue. Histologically the vasculature system is separated into macro vasculature and microvasculature. The macro vasculature being any vessel observable with the naked eye, and the microvasculature being vessels that are less than 100 microns.
The peripheral vascular system (PVS) includes all the blood vessels that exist outside the heart. The peripheral vascular system is classified as follows: The aorta and its branches:
The venules and veins returning blood to the heart
The function and structure of each segment of the peripheral vascular system vary depending on the organ it supplies. Aside from capillaries, blood vessels are all made of three layers:
The adventitia or outer layer which provides structural support and shape to the vessel
The tunica media or a middle layer composed of elastic and muscular tissue which regulates the internal diameter of the vessel
The tunic intima or an inner layer consisting of an endothelial lining which provides a frictionless pathway for the movement of blood
Within each layer, the amount of muscle and collagen fibrils varies, depending on the size and location of the vessel.
Arteries play a major role in nourishing organs with blood and nutrients. Arteries are always under high pressure. To accommodate this stress, they have an abundance of elastic tissue and less smooth muscle. The presence of elastin in the large blood vessels enables these vessels to increase in size and alter their diameter. When an artery reaches a particular organ, it undergoes a further division into smaller vessels that have more smooth muscle and less elastic tissue. As the diameter of the blood vessels decreases, the velocity of blood flow also diminishes. Estimates are that about 10% to 15% of the total blood volume is contained in the arterial system. This feature of high systemic pressure and low volume is typical of the arterial system.
There are two main types of arteries found in the body: (1) the elastic arteries, and (2) the muscular arteries. Muscular arteries include the anatomically named arteries like the brachial artery, the radial artery, and the femoral artery, for example. Muscular arteries contain more smooth muscle cells in the tunica media layer than the elastic arteries. Elastic arteries are those nearest the heart (aorta and pulmonary arteries) that contain much more elastic tissue in the tunica media than muscular arteries. This feature of the elastic arteries allows them to maintain a relatively constant pressure gradient despite the constant pumping action of the heart.
Arterioles provide blood to the organs and are chiefly composed of smooth muscle. The autonomic nervous system influences the diameter and shape of arterioles. They respond to the tissue’s need for more nutrients/oxygen. Arterioles play a significant role in systemic vascular resistance because of the lack of significant elastic tissue in the walls.
The arterioles vary from 8 to 60 micrometers. The arterioles further subdivide into meta-arterioles.
Capillaries are thin-walled vessels composed of a single endothelial layer. Because of the thin walls of the capillary, the exchange of nutrients and metabolites occurs primarily via diffusion. The arteriolar lumen regulates the flow of blood through the capillaries.
Venules are the smallest veins and receive blood from capillaries. They also play a role in the exchange of oxygen and nutrients for water products. There are post-capillary sphincters located between the capillaries and venules. The venue is very thin-walled and easily prone to rupture with excessive volume.
Blood flows from venules into larger veins. Just like the arterial system, three layers make up the vein walls. But unlike the arteries, the venous pressure is low. Veins are thin-walled and are less elastic. This feature permits the veins to hold a very high percentage of the blood in circulation. The venous system can accommodate a large volume of blood at relatively low pressures, a feature termed high capacitance. At any point in time, nearly three-fourths of the circulating blood volume is contained in the venous system. One can also find one-way valves inside veins that allow for blood flow, toward the heart, in a forward direction. Muscle contractions aid the blood flow in the leg veins. The forward blood flow from the lower extremities to the heart is also influenced by respiratory changes that affect pressure gradients in the abdomen and chest cavity. This pressure differential is highest during deep inspiration, but a small pressure differential is observable during the entire respiratory cycle.
Blood Vessel Structure
Blood vessels are flexible tubes that carry blood, associated oxygen, nutrients, water, and hormones throughout the body.
Blood vessels consist of arteries, arterioles, capillaries, venules, and veins. Vessel networks deliver blood to all tissues in a directed and regulated manner.
Arteries and veins are composed of three tissue layers.
The thick outermost layer of a vessel (tunica adventitia or tunica externa ) is made of connective tissue.
The middle layer ( tunica media ) is thicker and contains more contractile tissue in arteries than in veins. It consists of circularly arranged elastic fibers, connective tissue, and smooth muscle cells.
The inner layer ( tunica intima ) is the thinnest layer, comprised of a single layer of endothelium supported by a subendothelial layer.
Capillaries consist of a single layer of endothelium and associated connective tissue.
- tunica intima: The innermost layer of a blood vessel.
- tunica externa: The outermost layer of a blood vessel.
- capillary: Any of the small blood vessels that connect arteries to veins.
- tunica media: The middle layer of a blood vessel.
- anastomosis: The junction between blood vessels.
Blood vessels are key components of the systemic and pulmonary circulatory systems that distribute blood throughout the body. There are three major types of blood vessels: arteries that carry blood away from the heart, branching into smaller arterioles throughout the body, and eventually forming the capillary network. The latter facilitates efficient chemical exchange between tissue and blood. Capillaries in turn merge into venules, then into larger veins responsible for returning the blood to the heart. The junctions between vessels are called anastomoses.
Arteries and veins are comprised of three distinct layers while the much smaller capillaries are composed of a single layer.
The inner layer (tunica intima) is the thinnest layer, formed from a single continuous layer of endothelial cells and supported by a subendothelial layer of connective tissue and supportive cells. In smaller arterioles or venules, this subendothelial layer consists of a single layer of cells, but can be much thicker in larger vessels such as the aorta. The tunica intima is surrounded by a thin membrane comprised of elastic fibers running parallel to the vessel. Capillaries consist only of the thin endothelial layer of cells with an associated thin layer of connective tissue.
Surrounding the tunica intima is the tunica media, comprised of smooth muscle cells and elastic and connective tissues arranged circularly around the vessel. This layer is much thicker in arteries than in veins. Fiber composition also differs; veins contain fewer elastic fibers and function to control the caliber of the arteries, a key step in maintaining blood pressure.
The outermost layer is the tunica externa or tunica adventitia, composed entirely of connective fibers and surrounded by an external elastic lamina which functions to anchor vessels with surrounding tissues. The tunica externa is often thicker in veins to prevent the collapse of the blood vessel and provide protection from damage since veins may be superficially located.
A major structural difference between arteries and veins is the presence of valves. In arteries, the blood is pumped under pressure from the heart, so backflow cannot occur. However, passing through the capillary network results in a decrease in blood pressure, meaning that backflow of blood is possible in veins. To counteract this, veins contain numerous one-direction valves that prevent backflow.
Blood Vessel Function
Blood vessels carry nutrients and oxygen throughout the body and aid in gas exchange.
Systemic and pulmonary circulatory systems efficiently deliver oxygen to the tissues of the body and remove waste products such as carbon dioxide. Arterial blood (except in the pulmonary artery ) is highly saturated with oxygen and supplies oxygen to the body’s tissues.
Venous blood (except in the pulmonary vein ) is deoxygenated and returns to the heart to be pumped into the lungs for reoxygenation.
Nutrients carried in the blood are released to tissues via the permeable endothelium of blood vessels.
Immune cells move throughout the circulatory system and are able to rapidly permeate the walls of blood vessels to attend sites of injury or infection.
Blood vessels can increase or decrease blood flow near the surface of the body, either increasing or reducing the amount of heat lost as a means of regulating body temperature.
- thermoregulation: The maintenance of a constant internal temperature of an organism independent of the temperature of the environment
Blood plays many critical roles within the body: delivering nutrients and chemicals to tissues, removing waste products, and maintaining homeostasis and health. The circulatory system transports blood through the body to perform these actions, facilitated by the extensive network of blood vessels.
The circulatory system can be split into two sections, systemic and pulmonary. In the systemic circulatory system, highly oxygenated blood (95-100%) is pumped from the left ventricle of the heart and into the arteries of the body. Upon reaching the capillary networks, gas exchange between tissue and blood can occur, facilitated by the narrow walls of the capillaries. Oxygen is released from the blood into the tissues and carbon dioxide, a waste product of respiration, is absorbed. The capillaries merge into venules and then veins, carrying the deoxygenated blood (~75%) back to the right atrium of the heart at the end of the systemic circulatory system.
The much smaller pulmonary system reoxygenates the blood and facilitates the removal of carbon dioxide. After leaving the heart through the right ventricle, the blood passes through the pulmonary artery, the only artery in the body that contains deoxygenated blood, and into the capillary network within the lungs. The close association of the thin-walled alveoli with the equally thin-walled capillaries allows for rapid release of carbon dioxide and uptake of oxygen. After leaving the lungs through the pulmonary vein, the only vein which carries oxygenated blood, the blood enters the left atrium. This completes the pulmonary circulatory system.
Blood vessels also facilitate the rapid distribution and efficient transport of factors such as glucose, amino acids, or lipids into the tissues and the removal of waste products for processing elsewhere, such as lactic acid to the liver or urea to the kidneys. Additionally, blood vessels provide the ideal network for immune system surveillance and distribution. Numerous white blood cells circulate around the body, sensing for infection or injury. Once an injury is detected, they rapidly leave the circulatory system by passing through gaps in vessel walls to reach the affected area while signalling for a larger targeted immune response.
Mechanically the blood vessels, especially those near the skin, play a key role in thermoregulation. Blood vessels can swell to allow greater blood flow, allowing for greater radiant heat loss. Conversely, blood flow through these vessels can be lessened to reduce heat loss in colder climates.
The pathophysiology related to vascular physiology is rooted in the hemodynamic parameters described. Among several associated conditions, the following are common sequelae to consider:
Aging/Hypertension: Most notably with the aorta, there is a decrease in compliance with age due to a loss in elasticity.[rx] The stiffer vessel is also due to an increase in fibrotic changes with an increase in collagen fibers and their cross-linking. Thus with older age, higher blood pressures become prevalent. Likewise, the pathophysiology of hypertension is similar to risk factors such as diabetes, hypercholesterolemia, and cigarette smoking contributing to the same arteriosclerotic changes.
Vessel Aneurysm: With a loss or defective elastic components of a vessel, as noted with connective tissue diseases, the outward transmural pressure (distension) can be unbalanced with an inadequate tension force (as applied by Laplace), which allows for an aneurysm to form which is susceptible to rupture.[rx] The aneurysmal rupture in the brain results in life-threatening subarachnoid hemorrhage.
Atherosclerosis: In cases where the stress to move fluid forward (same stress that causes deformation of the vessel) is too large, due to changes in blood components, for example, smooth laminar flow is broken. Such flow can disrupt the endothelial layer of the vessel causing a pro-inflammatory response with the release of several pro-atherogenic factors (i.e., interleukin-1, tumor necrosis factor, and plasminogen activator inhibitor-1) that lead to the formation and progression of atherosclerotic plaque.[rx]
Dissection: When there is severe stress on the vessel wall that can lead to dissection of the blood vessel. Cervicocerebral dissection occurs commonly in young people. It can be due to various reasons, including trauma, fibromuscular dysplasia, Marfan syndrome, and several other causes. Up to one-half of the patients have a headache. Usually, extracranial dissection is associated with a good prognosis.
Reversible Cerebral Vasoconstriction Syndrome: This is associated with vasoconstrictive medications, pregnancy, illicit drugs, etc. Most patients have severe thunderclap headaches. It presents as subarachnoid hemorrhages, ischemic stroke, and lobar hemorrhages. Calcium channel blockers are the initial therapy. The recurrence is rare.
The following items are pertinent to the clinical assessment of the status of the vascular system:
Blood pressure (BP): normal BP – less than 120 (systolic [S]) and under 80 (diastolic [D]); elevated BP – 120 to 129 S and less than 80 D; hypertension stage 1: 130 to 139 S or 80 to 89 D; hypertension stage 2: over 140 S or greater than 90 D; hypertensive crisis: over 180 S and/or greater than 120 D.[ex]
Pulses – Palpation in the arterial regions of the carotid, femoral, popliteal, dorsalis pedis, posterior tibial, etc. for the distribution of blood flow.
Bruit – Auscultation for vibration sound localized to an arterial wall that is caused by turbulence and suggests a lesion.
Skin findings – Cyanosis, pallor, edema, and ulceration can indicate a vascular etiology. With ulceration, distal ulceration of the foot is more associated with an arterial etiology compared to ulceration of the malleolar region, which better suggests a venous cause.
Capillary refill – Refers to the amount of time required for refill after compression of a nailbed, and indicates perfusion status.
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