1. Systemic arteries
Arteries carry blood away from the heart and distribute it to all parts of the body. Oxygenated blood is pumped out of the heart through the body’s main artery- the aorta. Arteries that branch off the aorta transport blood throughout the body, supplying tissues with oxygen and nutrients.
Arteries have thicker and stronger walls and under higher pressure than those of veins.
Arterioles
Arteriols are the smallest branches of the arteries. They are the sites of highest resistance in the cardiovascular system.
2. Veins
Veins carry blood back toward the heart. Tiny vessels called capillaries in organs and tissues of the body deliver deoxygenated blood into small veins called venules, which join to form veins. Blood flows through the veins to the body’s 2 main veins (called the superior and inferior vena cavae), which deliver the blood back into the heart.
Veins have the lowest pressure. Some of them contain valves that prevent backflow.
Venules
Venules are the smallest branches of the veins.
3. Capillaries
Capillaries are the exchange sites where oxygen and nutritive materials from the oxygenated blood diffuse across the wall of the arterial end of the capillaries into tissue spaces, and the waste products and carbon dioxide diffuse from the tissue spaces into the blood through the wall of the venous end. Capillaries are thin-walled (one layer). They are absent in the cornea, epidermis, and hyaline cartilage and some areas where arterioles and venules have direct connections (this arteriovenous anastomoses called AV shunts and are present in the skin of the nose, lips, fingers, ears, where they conserve body heat.
4. Sinusoids
Sinusoids are wider than capillaries. Serve as a substitute for capillaries in the spleen, red bone marrow, liver, parathyroid glands, adenohypophysis, suprarenal cortex.
5. Circulatory loops
Pulmonary circulation
Pumps blood from the right Ventricle (RV) to
the Lungs through the Pulmonary arteries and returns it to the left Atrium (LA) of the Heart through the Pulmonary veins.
Systemic circulation
Pumps blood from the Left Ventricle (LV) through the Aorta to all parts of the body and returns it to the right Atrium (RA) through the Superior and Inferior Vena Cavae and the Cardiac Veins.
6. The Heart
Heart is hollow muscular organ that pumps blood through the body. The heart’s weight is about a pound and is shaped like an upside-down cone with the size of a closed fist and is located inside the chest cavity (Thorax) in the mediastenum, anetrior (in front) to the spine and posterior (behind) to the sternum (breast bone). The top of the heart, called the base, and the bottom of the heart, called the apex. Apex is attached to diafragm. This attachment plays important role in heart physiology.
Attachment allows heart ventricles to contract in width in two directions and in length just in one direction, with shifting AV septum (septum dividing atria and ventricles) down during ventricle contraction when in expanded atria difference in pressure “sucks” in blood from the superior and inferior vena cavae; and up, during ventricle relaxation when in expanded ventricles difference in pressure “sucks” in blood from the atria through valves into the ventricles. This contributes 90% of blood at rest and 50% during physical activity to compare with contraction of the atria contributing 10% and 50% respectively of blood entering ventricles through AV valves.
The heart’s the right and left sides pumps connected by the pulmonary and systemic circulation.
Heart chambers
Heart has four chambers. The superior (upper) two chambers, the right and left atria, are receiving chambers for blood. They collect blood from the veins. The heart’s inferior (lower) two chambers, the right and left ventricles, are the powerful pumping chambers. The ventricles pump blood into arteries. The atria have relatively thin walls compared to the ventricles.
Heart wall
Heart wall consists of three layers: outer – Epicardium, middle- Myocardium, and inner- Endocardium. Lining of endocardium helps blood flow smoothly and prevents blood clots from forming inside the circulatory system.
Pericardium envelops the heart. It comprise of the fibrous and the serous pericardium. The fibrous layer of the pericardium is attached to the breastbone (Sternum) and
other structures in the chest cavity and helps hold the heart in place. Between the two layers of the serous pericardium (visceral and parietal) is a thin space filled with a watery pericardial fluid that helps prevent these layers from rubbing against each other and cushions the heart when it beats.
The heart is separated by a wall of tissue (Septum) into right and left sides. Each side pumps blood through a different circuit of blood vessels.
Coronary blood supply
The heart is nourished not by the blood passing through its chambers but by a specialized network of blood vessels (coronary arteries), these blood vessels encircle the heart like a crown. Three main coronary arteries-the right, the left circumflex, and the left anterior descending-nourish different regions of the heart muscle. Veins running through the heart muscle converge to form a large channel called the coronary sinus, which returns blood to the right atrium.
Heart Valves
The heart has four valves which help prevent blood from flowing backward in the heart. The valves open easily in the direction of blood flow.
Two of the valves are located between the atria and ventricles, and are known as
atrioventricular valves. The right atrioventricular valve is formed from three flaps of tissue and is called the tricuspid valve, while the left atrioventricular valve has two flaps and is called the bicuspid or mitral valve. Tricuspid is most audible over the right lower part of the body of the breastbone (sternum). Mitral valve is most audible over the left fifth intercostals space at the midclavicular line.
The other two valves are located between the ventricles and arteries. They are called semilunar valves because they each consist of three half-moon-shaped flaps of tissue. The right semilunar valve, between the right ventricle and pulmonary trunk, is also called the pulmonary (or pumonic) valve. The left semilunar valve, between the left ventricle and aorta, is also called the aortic (or aortal) valve. Pulmonary valve is most audible over the left second intercostals space. Aortic valve is most audible over the right second intercostals space.
Although the right and left halves of the heart are separate, they both contract in unison, producing a single heartbeat.
The Cardiac Cycle
The sequence of events from the beginning of one heartbeat to the beginning of the next is called the cardiac cycle. The cardiac cycle has two phases: diastole, when the heart’s chambers are relaxed, and systole, when the chambers contract to move blood. During the systolic phase, the atria contract first, followed by contraction of the ventricles. During diastole, both atria and ventricles are relaxed, and the atrioventricular valves are open. Blood pours from the veins into the atria, and from there into the ventricles. Systole then begins as the atria contract to complete the filling of the ventricles. Next, the ventricles contract, forcing blood out through the semilunar valves and into the arteries, and the atrioventricular valves close to prevent blood from flowing back into the atria. As pressure rises in the arteries, the semilunar valves snap shut to prevent blood from flowing back into the ventricles. Diastole then begins again as the heart muscle relaxes-the atria first, followed by the ventricles-and blood begins to pour into the heart once more.
Heart sounds
An instrument known as a stethoscope is used to detect internal body sounds, including the sounds produced by the heart as it is beating. The characteristic heartbeat sounds are made by the valves in the heart-not by the contraction of the heart muscle itself. The sound comes from the leaflets of the valves slapping together. The closing of the atrioventricular valves, just before the ventricles contract, makes the first heart sound. The second heart sound is made when the semilunar valves snap closed. The first heart sound is generally longer and lower than the second, producing a heartbeat that sounds like “lub-dup, lub-dup, lub-dup.
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Ramaz Mitaishvili, MD
Copyright 2009 by the author of this book (Ramaz Mitaishvili, MD). The book author retains sole copyright to his contributions to this book.
