Introduction

The heart is a muscular pump that serves two functions: (1) to collect blood from the tissues of the body and pump it to the lungs and (2) to collect blood from the lungs and pump it to all tissues of the body. The human heart lies in the protective thorax, posterior to the sternum and costal cartilages, and rests on the superior surface of the diaphragm. The heart assumes an oblique position in the thorax, with two-thirds to the left of midline. it occupies a space between the pleural cavities called the middle mediastinum, defined as the space inside the pericardium, the covering around the heart. This serous membrane has inner and outer layers, with a lubricating fluid in between. The fluid allows the inner visceral pericardium to "glide" against the outer parietal pericardium.

The internal anatomy of the heart reveals four chambers composed of cardiac muscle or myocardium. The two upper chambers (or atria) function mainly as collecting chambers; the two lower chambers (ventricles) are much stronger and function to pump blood. The role of the right atrium and ventricle is to collect blood from the body and pump it to the lungs. The role

From: Handbook of Cardiac Anatomy, Physiology, and Devices Edited by: P. A. Iaizzo © Humana Press Inc., Totowa, NJ

of the left atrium and ventricle is to collect blood from the lungs and pump it throughout the body. There is a one-way flow of blood through the heart; this flow is maintained by a set of four valves. The atrioventricular valves (tricuspid and bicuspid) allow blood to flow only from atria to ventricles. The semilunar valves (pulmonary and semilunar) allow blood to flow only from the ventricles out of the heart and through the great arteries.

A number of structures that can be observed in the adult heart are remnants of fetal circulation. In the fetus, the lungs do not function as a site for the exchange of oxygen and carbon dioxide, and the fetus receives all of its oxygen from the mother. In the fetal heart, blood arriving to the right side of the heart is passed through specialized structures to the left side. shortly after birth, these specialized fetal structures normally collapse, and the heart takes on the "adult" pattern of circulation. However, in rare cases, some fetal remnants and defects can occur.

Although the heart is filled with blood, it provides very little nourishment and oxygen to the tissues of the heart. Instead, the tissues of the heart are supplied by a separate vascular supply committed only to the heart. The arterial supply to the heart arises from the base of the aorta as the right and left coronary arteries (running in the coronary sulcus). The venous drainage is via cardiac veins that return deoxygenated blood to the right atrium.

Fig. 1. Position of the heart in the thorax. The heart lies in the protective thorax, posterior to the sternum and costal cartilages, and rests on the superior surface of the diaphragm. The heart assumes an oblique position in the thorax, with two-thirds to the left of midline. It is located between the two lungs, which occupy the lateral spaces called the pleural cavities. The space between these two cavities is referred to as the mediastinum. The heart lies obliquely in a division of this space, the middle mediastinum, surrounded by the pericardium. (Figs. 18.2 a, b, c, p. 523 from Human Anatomy, 3rd Ed. by Elaine N. Marieb and Jon Mallatt. © 2001 by Benjamin Cummings. Reprinted by permission of Pearson Education, Inc.)

Fig. 1. Position of the heart in the thorax. The heart lies in the protective thorax, posterior to the sternum and costal cartilages, and rests on the superior surface of the diaphragm. The heart assumes an oblique position in the thorax, with two-thirds to the left of midline. It is located between the two lungs, which occupy the lateral spaces called the pleural cavities. The space between these two cavities is referred to as the mediastinum. The heart lies obliquely in a division of this space, the middle mediastinum, surrounded by the pericardium. (Figs. 18.2 a, b, c, p. 523 from Human Anatomy, 3rd Ed. by Elaine N. Marieb and Jon Mallatt. © 2001 by Benjamin Cummings. Reprinted by permission of Pearson Education, Inc.)

The heart is a muscular pump that serves two functions: (1) to collect oxygen-poor blood from the tissues of the body and pump this blood to the lungs to pick up oxygen and release carbon dioxide and (2) to collect oxygen-rich blood from the lungs and pump this blood to all of the tissues of the body.

It is important to note that, besides pumping oxygen-rich blood to the tissues of the body for exchange of oxygen for carbon dioxide, the blood also circulates many other important substances. Nutrients from digestion are collected from the small intestine and pumped through the circulatory system to be delivered to all cells of the body. Hormones are produced from one type of tissues and distributed to all cells of the body. The circulatory system carries waste materials (salts, nitrogenous wastes, and excess water) from cells to the kidneys, where they are extracted and passed to the bladder. The pumping of interstitial fluid from the blood into the extracellular space is an important function of the heart. Excess interstitial fluid is then returned to the circulatory system via the lymphatic system.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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