112 Arteries

Artery Function

Arteries are high-pressure blood vessels that carry oxygenated blood away from the heart to all other tissues and organs.

Arteries are blood vessels that carry blood away from the heart under pressure. This blood is usually oxygenated, with the exception of that in the pulmonary artery, which carries deoxygenated blood to the lungs.

As with veins, arteries are comprised of three layers: the tunicae intima, media, and externa. In arteries, the tunica media, which contains smooth muscle cells and elastic tissue, is thicker than that of veins so it can modulate vessel caliber and thus control and maintain blood pressure.

Arterial pressure varies between the peak pressure during heart contraction, called the systolic pressure, and the minimum or diastolic pressure between contractions, when the heart expands and refills. This pressure variation within the artery produces the observable pulse that reflects heart activity. The pressure in the arterial system decreases steadily, highest in the aorta and lowest in the venous system, as blood approaches the heart after delivery of oxygen to tissues in the systemic circulation.

Arteries of the systemic circulation can be subdivided into muscular or elastic types according to the the relative compositions of elastic and muscle tissue in their tunica media. Larger arteries are typically elastic and smaller arteries are more likely to be muscular. These arteries deliver blood to the arterioles, which in turn deliver blood to the capillary networks associated with the body’s tissues.

Elastic Arteries

An elastic or conducting artery has a large number of collagen and elastin filaments in the tunica media.

Elastic arteries contain larger numbers of collagen and elastin filaments in their tunica media than muscular arteries do, giving them the ability to stretch in response to each pulse.

Elastic arteries include the largest arteries in the body, those closest to the heart, and give rise to the smaller muscular arteries. The pulmonary arteries, the aorta, and its branches together comprise the body’s system of elastic arteries. In these large arteries, the amount of elastic tissue is considerable and the smooth muscle fiber cells are arranged in 5 to 7 layers in both circular and longitudinal directions.

Arterial elasticity gives rise to the Windkessel effect, which through passive contraction after expansion helps to maintain a relatively constant pressure in the arteries despite the pulsating nature of the blood flow from the heart.

The Aorta

Due to position as the first part of the systemic circulatory system closest to the heart and the resultant high pressures it will experience, the aorta is perhaps the most elastic artery, featuring an incredibly thick tunica media rich in elastic filaments. The aorta is so thick that it requires its own capillary network to supply it with sufficient oxygen and nutrients to function, the vasa vasorum.

When the left ventricle contracts to force blood into the aorta, the aorta expands. This stretching generates the potential energy that will help maintain blood pressure during diastole, when the aorta contracts passively. Additionally, the elastic recoil helps conserve the energy from the pumping heart and smooth the flow of blood around the body through the Windkessel effect.

Muscular Arteries

Distributing arteries are medium-sized arteries that draw blood from an elastic artery and branch into resistance vessels.

Muscular or distributing arteries are medium-sized arteries that draw blood from an elastic artery and branch into resistance vessels, including small arteries and arterioles. In contrast to the mechanism elastic arteries use to store and dissipate energy generated by the heart’s contraction, muscular arteries contain layers of smooth muscle providing allowing for involuntary control of vessel caliber and thus control of blood flow. Muscular arteries can be identified by the well-defined elastic lamina that lies between the tunicae intima and media.

The splenic artery (lienal artery), the blood vessel that supplies oxygenated blood to the spleen, is an example of a muscular artery. It branches from the celiac artery and follows a course superior to the pancreas. The splenic artery branches off to the stomach and pancreas before reaching the spleen and gives rise to arterioles that directly supply capillaries of these organs.

Anastomoses

A circulatory anastomosis is a connection or looped interaction between two blood vessels.

An anastomosis refers to any join between two vessels. Circulatory anastomoses are named based on the vessels they join: two arteries (arterio-arterial anastomosis), two veins (veno-venous anastomosis), or between an artery and a vein (arterio-venous anastomosis).

Anastomoses between arteries and anastomoses between veins result in a multitude of arteries and veins serving the same volume of tissue. Such anastomoses occur normally in the body in the circulatory system, serving as backup routes for blood to flow if one link is blocked or otherwise compromised, but may also occur pathologically.

Examples of Anastomoses

Arterio-arterial anastomoses include actual joins (e.g. palmar arch, plantar arch) and potential ones, which may only function if the  normal vessel is damaged or blocked (e.g. coronary arteries and cortical branch of cerebral arteries). Important examples include:

• The circle of Willis in the brain.
  The arrangement of the brain’s arteries into the circle of Willis creates redundancies for the cerebral circulation. If one part of the circle becomes blocked or narrowed or one of the arteries supplying the circle is blocked or narrowed, blood flow from the other blood vessels can often preserve the cerebral perfusion well enough to maintain function.
• Joint anastomoses. Almost all joints receive anastomotic blood supply from more than one source. Examples include the knee and geniculate arteries, shoulder and circumflex humeral, and hip and circumflex iliac.
• Coronary artery anastomoses. The coronary arteries are functionally end arteries, so these meetings are referred to as anatomical anastamoses, which lack function. As blockage of one coronary artery generally results in death of the heart tissue due to lack of sufficient blood supply from the other branch, when two arteries or their branches join, the area of the myocardium receives dual blood supply. If one coronary artery is obstructed by an atheroma, a degradation of the arterial walls, the second artery is still able to supply oxygenated blood to the myocardium. However, this can only occur if the atheroma progresses slowly, giving the anastomosis time to form.

Pathological anastomoses result from trauma or disease and are usually referred to as fistulae. They can be very severe if they result in the bypassing of key tissues by the circulatory system.

Arterioles

An arteriole is a small diameter blood vessel in the microcirculation system that branches out from an artery and leads to capillaries.

An arteriole is a small-diameter blood vessel which forms part of the microcirculation that extends from an artery and leads to capillaries.

The microcirculation involves the flow of blood in the smallest blood vessels, including arterioles, capillaries, and venules.

Arterioles have muscular walls that usually consist of one or two layers of smooth muscle. They are the primary site of vascular resistance. This reduces the pressure and velocity of blood flow to enable gas and nutrient exchange to occur within the capillaries. Arterioles are innervated and also respond to various circulating hormones and other factors such as pH in order to regulate their caliber, thus modulating the amount of blood flow into the capillary network and tissues.

Capillaries

Capillaries, the smallest blood vessels in the body, are part of the microcirculation.

Capillaries, which form part of the micro-circulation, are the smallest of the body’s blood vessels at between 5-10
μm in diameter with the endothelial vessel wall of only one cell thick. They are surrounded by a thin basal lamina of connective tissue.

Capillary Function

Capillaries form a network through body tissues that connects arterioles and venules and facilitates the exchange of water, oxygen, carbon dioxide, and many other nutrients and waste substances between blood and surrounding tissues.
The thin wall of the capillary and close association with its resident tissue allow for gas and lipophilic molecules to pass through without the need for special transport mechanisms. This allows bidirectional diffusion depending on osmotic gradients.

Formation of New Capillaries

During embryological development, new capillaries are formed by vasculogenesis, the process of blood vessel formation occurring by de novo production of endothelial cells and their formation into vascular tubes. The term angiogenesis denotes the formation of new capillaries from pre-existing blood vessels.

The Capillary Bed

Capillaries do not function independently. The capillary bed is an interwoven network of capillaries that supplies an organ. The more metabolically active the cells, the more capillaries required to supply nutrients and carry away waste products.

A capillary bed can consist of two types of vessels: true capillaries, which branch mainly from arterioles and provide exchange between cells and the circulation, and vascular shunts, short vessels that directly connect arterioles and venules at opposite ends of the bed, allowing for bypass.

Types of Capillaries

There are three main types of capillaries:

• Continuous: Endothelial cells provide an uninterrupted lining, only allowing small molecules like water and ions to diffuse through tight junctions. This leave gaps of unjoined membrane called intercellular clefts.
• Fenestrated: Fenestrated capillaries have pores in the endothelial cells (60-80 nanometers in diameter) that are spanned by a diaphragm of radially-oriented fibrils. They allow small molecules and limited amounts of protein to diffuse.
• Sinusoidal: Sinusoidal capillaries are a special type of fenestrated capillaries that have larger openings (30–40 μm in diameter) in the endothelium. These types of blood vessels allow red and white blood cells (7.5μm–25μm diameter) and various serum proteins to pass using a process aided by a discontinuous basal lamina. Sinusoid blood vessels are primarily located in the bone marrow, lymph nodes, and adrenal gland. Some sinusoids are special in that they do not have tight junctions between cells. These are called discontinuous sinusoidal capillaries, present in the liver and spleen where greater movement of cells and materials is necessary.

Control of Flow

Capillary beds may control blood flow via autoregulation. This allows an organ to maintain constant flow despite a change in central blood pressure. This is achieved by myogenic response and by tubuloglomerular feedback in the kidney. When blood pressure increases, the arterioles that lead to the capillary bed are stretched and subsequently constrict to counteract the increased tendency for high pressure to increase blood flow. In the lungs, special mechanisms have been adapted to meet the needs of increased necessity of blood flow during exercise. When heart rate increases and more blood must flow through the lungs, capillaries are recruited and are distended to make room for increased blood flow while resistance decreases.