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PinkMonkey Online Study Guide-Biology

The heart is enclosed in a tough, double walled membrane called the pericardium. The outer tough layer of pericardium is fibrous and forms a protective sheath. It is attached by ligaments to the breastbone and spine, anchoring the heart in position. Inside the fibrous pericardium is a thin but tough double membrane, the parietal pericardium. The pericardial fluid between the two membranes prevents friction, protects it from external shocks and over-distention of the heart with blood in extreme conditions.

The heart (Figures 18.3 & 18.4) consist of four chambers, two atria (auricles) and two ventricles.

The atria are thin walled and receive blood from four pulmonary veins (auricles) and two ventricles. The atria are thin-walled and receive blood from four pulmonary veins and two vena cavae.

The main tasks of the atria are to prime (fill) the ventricles between "pumps", and to help monitor blood pressure in the vascular system. The ventricles are thick walled and muscular. The walls of the left ventricle are at least three times thicker than those of the right, as it pumps blood to the whole body through the aorta.

Two thick-walled muscular vessels arise from the heart -- the aorta from the left side, which pumps blood to the body, and the pulmonary artery from the right side of the heart. The left and right coronary arteries arise from the aorta and supply oxygenated blood to the atria and ventricles. The coronary arteries divide several times to form a capillary network over heart muscles. Blood is collected from the capillaries by the coronary veins and returned to the right atrium. When defects in coronary arteries prevent blood flow to the heart muscles, that is when a heart attack occurs.

(B) Internal Structure of the Heart

Internally, the heart is divided into two sides (right and left) by two muscular partitions, and these two sides work in unison but independently as a double pump.

The auricles. The two atria or auricles are separated by a thin inter atrial septum. The left atrium receives oxygenated blood by four pulmonary veins, which opens into its upper part. Their openings are without valves. The left atrium opens into the left ventricle by atrio-ventricular orifice (opening ), guarded by the bicuspid or mitral valve.

The right atrium receives deoxygenated blood from the body through the superior vena cava (opens into the upper part), and inferior vena cava, which opens into the lower part of the atrium. Its opening is guarded by a rudimentary valve. A small opening of coronary sinus (vein), guarded by a valve, is present between the orifice of the inferior vena cava and atri-ventricular opening. The fossa ovalis is a small oval depression above and to the left of the opening of the inferior vena cava. The right atrium opens into the right ventricle, and this opening is guarded by tricuspid valves to prevent back-flow of the blood from the ventricle to the atrium.

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Table of Contents

18.0 - Introduction
18.1 - Closed Vascular System
18.2 - Heart
18.3 - Arterial Blood Pressure
18.4 - Blood

Chapter 19


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