Medic-CE

Anatomy: The Vessels

Introduction:

All of the cells in our body require a constant supply of oxygen and nutrients in order to function. This supply of oxygen and nutrients is provided by the constant passage of blood through the vessels, which is known as perfusion.

Structure:

Blood vessels are made up of a vessel wall, which surrounds a lumen (or opening).

Most vessel walls are made up of three layers of cells, with the exception of the capillaries, which consist of a single layer of cells. The innermost lining of a blood vessel is known as the intima, which is composed of a very thin layer of cells. The next layer is called the media, which consists of elastic fibers and muscle fibers. The media gives vessels their contractile properties and strength. Due to increased strain, the media of an artery is much thicker than the media of a vein. The outermost layer of a vessel is called the adventitia, which serves as a dense fibrous covering. It gives the vessel the strength to withstand the pressures generated by the heart’s contractile force.

The size of the central opening (or lumen) varies considerably among the different types of vessels and is directly related to the amount of blood that can be transported – the larger the diameter of the vessel, the greater the flow of blood.

There are two main types of blood vessels. They are differentiated based on the oxygen status of the blood they carry. In general, vessels that carry oxygenated blood are known as arteries, whereas vessels that carry deoxygenated blood are known as veins.

****NOTE: The pulmonary vasculature is an exception to this rule. The pulmonary artery transports deoxygenated blood to the lungs, whereas the pulmonary vein transports oxygenated blood to the heart.

The Arterial System:

The arterial system, which functions at very high pressures, carries oxygenated blood away from the heart.

The largest vessels in this system are called arteries. The arteries branch into smaller vessels called arterioles. The arterioles divide into smaller, single-walled vessels called capillaries, which act as connecting points between the arterial and venous systems. The exchange of gases, fluids, nutrients, and waste products occurs through the very thin capillary walls.

The Venous System:

The venous system transports deoxygenated blood from the peripheral circulation back to the heart. It functions under much lower pressure than the arterial system and, consequently, these vessels have much thinner walls.

Blood enters the venous system through capillaries, which drain into larger venules. The venules then drain into thicker-walled veins. These veins then drain into the vena cavae. As previously stated, the vena cavae empty into the right atrium.

The blood traveling from the capillaries to the venules and back to the heart is under very low pressure. To facilitate the return of blood to the heart, the vessels of the venous system contain one-way valves to prevent the back flow of slow moving blood. Furthermore, the contractions of muscles, which are near veins, function to “squeeze” blood back to the heart. This system is highly coordinated such that you do not feel pulsations through the veins; rather, the blood flow through veins is slow and steady.

The Great Arteries:

-The Vena Cavae:

The superior and inferior vena cavae deliver deoxygenated blood directly to the right atrium of the heart. The superior vena cava delivers deoxygenated blood from the head, neck, and upper extremities. The inferior vena cava delivers deoxygenated blood from tissues located below the level of the heart. From the right atrium, the blood then flows into the right ventricle.

-The Pulmonary Artery and Pulmonary Veins:

From the right ventricle, the deoxygenated blood is pumped through the pulmonic valve and into the pulmonary artery. The pulmonary artery delivers blood to the lungs. After becoming oxygenated in the lungs, the blood passes through the pulmonary veins and enters the left atrium. This oxygen-rich blood then flows from the left atrium through the mitral valve and into the left ventricle.

-The Aorta:

The left ventricle then pumps the blood through the aortic valve, into the aorta, and out to the peripheral circulation.

The aorta is given several different names as it courses through the chest and abdomen. The ascending aorta is the first segment of the aorta. It arises directly from the heart and ends at the aortic arch. The aortic arch is the “U” shaped portion of the aorta and signifies the point where the aorta transitions from ascending to descending. The descending aorta is given two names – the thoracic aorta and the abdominal aorta. The thoracic aorta extends from the aortic arch to the diaphragm. Once the aorta travels below the diaphragm and into the abdomen, it is classified as the abdominal aorta.

The Coronary Arteries:

The coronary arteries supply the heart with oxygen and nutrients. They branch off the ascending aorta, just above the aortic valve. The major coronary vessels are located on the surface of the heart and feed small arteries that penetrate the myocardium.

The coronary vessels are constricted when the heart is contracting; therefore, blood mainly flows through the coronary vessels when the heart is in diastole (also known as the period of time when the heart is relaxed and is filling with blood).

The right coronary artery wraps from front to back around the heart and branches off into the posterior descending artery and the marginal artery. The right coronary artery primarily supplies the right atrium, the right ventricle, and part of the heart’s conduction system.

The left coronary artery is located anteriorly on the heart’s surface and branches into the anterior descending artery and the circumflex artery. The left coronary artery supplies the left ventricle, part of the right ventricle, the interventricular septum, and a portion of the heart’s conduction system.

Just as in the rest of the circulatory system, veins carry deoxygenated blood away from the heart. Deoxygenated blood from the left coronary arteries enters the anterior great cardiac and lateral marginal veins and empties into the coronary sinus. The right coronary system empties into the right coronary vein, which then drains directly into the right atrium.

Although the heart is supplied primarily by the right and left coronary vessels, there are also very important anastamoses (or communication points between two or more vessels) among the various branches of the coronary arteries that allows for collateral circulation. Collateral circulation is an important protective mechanism that provides alternative pathways for blood to flow in the event that there is a blockage in any part of the system.