Sheila M. Martin-Corbin
The Circulatory System
This is one system that students frequently know something about, but they often don’t completely understand the complexity of the system. It is important to discuss the anatomy of the heart, such as its chambers, valves and the vessels through which blood flows in and out of its various regions. The heart is a muscular organ which pumps blood through vast complex blood vessels which bring oxygen and nourishment to the cells of the body. . It has four chambers, an atrium and ventricle on each side. The atria receive blood returning to the heart from the veins and act as reservoirs between contractions of the heart. The ventricles pump blood into the arteries leaving the heart. The right side of the heart receives oxygen-poor blood, returning from the tissues and pumps it into the pulmonary circulation. Pulmonary arteries carry blood to the lungs, where gases are exchanged. Pulmonary veins then return oxygen-rich blood to the left side of the heart.
The left side of the heart pumps oxygen-rich blood into the systemic circulation, the network of blood vessels that serves all the body systems. There are three primary types of blood vessels. The arteries carry blood away from the heart and the vein carry blood toward the heart. Capillaries which interface the arteries and veins permit the exchange of gases, nutrients, waste products and other materials with body tissues. Arteries are under pressure and, therefore their walls are thicker and possess large amounts of elastic fibers. By contrast, most veins are not under significant pressure and, therefore, they are much thinner and less elastic. Capillaries are comprised of simple endothelial tissue that permits the easy movement of materials through these cells. Blood pressure is the force exerted by the blood against the inner wall of the vessel. Although the left side of the heart generates more pressure than the right side, approximately the same volume of blood is ejected from each side per beat. When blood is pumped from either atrium into the corresponding ventricle pressure in the ventricle becomes greater than in the atrium.
When the atrium relaxes, blood must be prevented from flowing backward into it. To prevent such backflow of blood, atrioventricular (AV) valves guards the passageway between each atrium and ventricle. The AV valve between the right atrium and the right ventricle is known as the tricuspid valve and the left AV valve is known as the tricuspid valve commonly called the mitral valve.
The cardiac cycle includes contraction and relaxation phases. During contraction, blood is forced out of the heart and then the heart is filled with blood during the period of relaxation. The period of contraction is known as
systole
and the period of relaxation is
diastole
. Each cardiac cycle begins with a muscle impulse that spreads from the SA node throughout the atria, resulting in the contraction of the atria. As the atria contract, the AV valves are open, and blood is forced from the atria into the ventricles. As this happens, the semilunar valves are closed. As the atria relax, they are filled with blood from the veins. During this time, the AV valves are closed and the ventricles are contracting forcing blood through the semilunar valves into the arteries. Then as the ventricles begin to relax, the semilunar valves close and the AV valves open. Blood flows into the ventricles and the cycle begins again.
Blood flow is a dynamic process. Blood flows through a continuous network of blood vessels that forms a double circuit: the pulmonary circulation which connects heart and lungs and the systemic circulation which connects the heart and all the organs and tissues. The left ventricle pumps blood into the systemic circulation, which brings oxygen-rich blood to all the different organs and tissues. Blood returns to the right atrium of the heart, poor in oxygen but loaded with carbon dioxide wastes. Then the blood is pumped by the right ventricle into the pulmonary circulation, where gases are exchanged. From the pulmonary circulation, blood is returned to the left atrium. It is pumped into the left ventricle, which pumps it back out into the systemic circulation. Therefore, blood flowing through the pulmonary circuit gains oxygen and loses carbon dioxide. Blood flowing through the systemic circuit loses oxygen and picks up carbon dioxide.
Investigating Breathing and Heart Rates in Humans
Cellular respiration is a metabolic process similar to burning fuel. While burning converts almost all of the energy in a fuel to heat, cellular respiration releases much of the energy in food to make ATP. This ATP provides cells with the energy they need to carry out the activities of life. Oxygen in the air you breathe makes the production of ATP more efficient, although some ATP is made without oxygen. Metabolic processes that require oxygen are called aerobic and metabolic processes that do not require oxygen are called anaerobic. Since oxygen is necessary for respiration, cells must receive a continuous supply via the circulatory system from the lungs. Carbon dioxide, a waste product which is released during respiration is transported to the lungs and released from the body. During periods of high ATP usage such as strenuous exercise, breathing and heart rates are accelerated to deliver more oxygen to body cells. On the contrary when ATP is in less demand, example during periods of rest, breathing and heart rates are lower and the oxygen concentration is not usually as high.
Regulation of breathing and heart rates involves the monitoring of blood pH levels. As carbon dioxide levels increases in the blood, more carbonic acid is produced and this tends to lower the pH of blood. The nervous system readily responds to this condition by stimulating cardiac and skeletal muscles to contract more rapidly increasing the rate of breathing and consequently an increase of heart rate. As a result more ATP is produced and more oxygen is delivered to body cells. When there is less demand for oxygen as energy demands decreased, the pH level of blood increases, resulting in a decrease of breathing and heart rate. Other factors such as age, drugs and physical fitness can affect heart and breathing rates and can lead to death.
