BIOLOGY Volume 2 - A Guide to General Biology - 2004

14. TRANSPORT IN ANIMALS

14.7. The Heart

14.7.1. Structure

The Heart is located in the thoracic cavity between the Lungs and behind the Sternum. It is enclosed in a tough, thin sac called the Pericardium, the outer layer of which consists of inelastic white fibrous tissue. The inner, or serous, layer is composed of two membranes. The inner (visceral) membrane is fused to the heart, forming its outer covering, the epicardium, while the outer (parietal) membrane is attached to the fibrous tissue. Between these membranes lies a fluid-filled cavity containing pericardial fluid, which reduces friction between the heart wall and surrounding Tissues during contractions. The inelastic Nature of the pericardium generally prevents the heart from overstretching or becoming overfilled with Blood.

The human heart consists of four chambers: two upper chambers with relatively thin walls, the atria, and two lower chambers with thick walls, the ventricles (Fig. 14.13). The atria receive blood from the Veins and pump it into the ventricles, which in turn pump it out into the Arteries. The walls of the atria are thin because their contractions only need to move blood over a very short distance.

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Fig. 14.13. A. Section through the heart. B. Simplified diagram of heart Structure. Note the varying thickness of the walls in different PARTS OF THE heart.

The right side of the heart is completely separated from the left, which is why specialists often refer to the right and left hearts. The right atrium receives deoxygenated (venous) blood from the systemic Circulation, while the left atrium receives oxygenated (arterial) blood from the lungs. The muscular wall of the left ventricle is at least three times thicker than the walls of the right ventricle. This difference is due to the fact that the right ventricle only pumps blood to the lungs, which are located right next to the heart in the thoracic cavity, whereas the left ventricle pumps blood throughout the entire body. Consequently, the blood entering the aorta from the left ventricle is under significantly higher pressure (approximately 14.0 kPa) than the blood entering the pulmonary artery (2.1 kPa). The movement of blood to and from the lungs constitutes the Pulmonary Circulation, while the movement of blood through the rest of the body forms the systemic circulation.

14.3. What are the additional advantages of lower blood pressure in the pulmonary circulation compared to the pressure in the systemic circulation?

When the atria contract, blood is forced into the ventricles. At the same time, circular Muscles located at the entrances of the venae cavae and Pulmonary veins into the atria contract, preventing blood from flowing backward into the veins. The left atrium is separated from the left ventricle by the bicuspid (mitral) valve, and the right atrium from the right ventricle by the tricuspid valve (Fig. 14.14, A). These are also known as the atrioventricular Valves. Attached to the cusps of these valves on the ventricular side are fibrous cords (chordae tendineae), which connect at their other end to cone-shaped papillary muscles—projections of the inner ventricular wall. The atrioventricular valves open when the atria contract, and when the ventricles contract, their cusps snap tightly shut, preventing the backflow of blood into the atria. Simultaneously, the papillary muscles contract, pulling taut the chordae tendineae and preventing the valve cusps from everting into the atria. At the Base of the aorta and the pulmonary artery are pocket-like semilunar valves (Fig. 14.14, B) that prevent blood from flowing backward from these vessels into the heart.

Immediately past the aortic valve, two coronary arteries branch off the aorta. These are the sole Blood Vessels supplying the heart walls with oxygenated blood.

Fig. 14.14. A. Tricuspid valve. B. Bicuspid valve with chordae tendineae connecting its cusps to papillary muscles. C. Open semilunar valve at the base of the pulmonary artery. D. Closed semilunar valve at the base of the pulmonary artery.

Structure of Cardiac Muscle

The heart wall is composed of cardiac muscle fibers, Connective Tissue, and small blood vessels. Each muscle fiber (cardiomyocyte) contains one or two nuclei, numerous large Cell/35.html">Mitochondria, and many parallel myofibrils. Myofibrils are made up of Actin and Myosin filaments (myofilaments) that enable the cardiomyocyte to contract in a manner similar to Skeletal Muscle (section 18.4). In principle, the Internal Structure of cardiomyocytes is identical to that of skeletal muscle fibers, which is why they also appear striated under a Microscope (Figs. 14.15 and 14.16). Dark bands called intercalated discs represent the specialized cell surface membranes that separate one muscle cell from another. These membranes are modified to allow ions to diffuse rapidly through them, which in turn ensures the rapid Propagation of Excitation (action potentials) throughout The cardiac muscle. Because these Cells are interconnected to form a complex network, an impulse generated in one cell instantly spreads across the entire myocardium, causing it to act as a single functional unit. This characteristic explains the absence of regulatory Neurons within the heart wall. The myocardium contracts more slowly than skeletal muscle and is much less prone to fatigue.

Fig. 14.15. Structure of cardiac muscle.

Fig. 14.16. Micrograph of a cardiac muscle section.