Circulatory System

The circulatory system transports blood to deliver important substances, such as oxygen, to cells and to remove wastes, such as carbon dioxide.

Functions of the Circulatory System

Cells must have oxygen and nutrients and must get rid of waste products. This exchange is done by the circulatory system—the body’s transport system. The circulatory system consists of blood, the heart, blood vessels, and the lymphatic system. Blood carries important substances to all parts of the body. The heart pumps blood through a vast network of tubes inside your body called blood vessels. The lymphatic system is considered part of the circulatory and immune systems. You will learn about the lymphatic system in Chapter 37. All of these components work together to maintain homeostasis in the body. The circulatory system transports many important substances, such as oxygen and nutrients. The blood also carries disease-fighting materials produced by the immune system. The blood contains cell fragments and proteins for blood clotting. Finally, the circulatory system distributes heat throughout the body to help regulate body temperature.

Blood Vessels

Highways have lanes that separate traffic. They also have access ramps that take vehicles to and from roads. Similarly, the body also has a network of channels—the blood vessels. Blood vessels circulate blood throughout the body and help keep the blood flowing to and from the heart. The fact that there are different kinds of blood vessels was first observed by Greek physician Praxagoras, as noted in

Arteries

Oxygen-rich blood, or oxygenated blood, is carried away from the heart in large blood vessels called arteries. These strong, thick-walled vessels are elastic and durable. They are capable of withstanding high pressures exerted by blood as it is pumped by the heart. As shown in Figure 34.2, arteries are composed of three layers: an outer layer of connective tissue, a middle layer of smooth muscle, and an inner layer of endothelial tissue. The endothelial layer of the artery is thicker than that of the other blood vessels. The endothelial layer of arteries needs to be thicker because blood is under higher pressure when it is pumped from the heart into the arteries.

Capillaries

Arteries branch through the body like the branches of a tree, becoming smaller in diameter as they grow farther away from the main vessel. The smallest branches are capillaries. Capillaries are microscopic blood vessels where the exchange of important substances and wastes occurs. Capillary walls are only one cell thick, as illustrated in Figure 34.2. This permits easy exchange of materials between the blood and body cells, through the process of diffusion. These tubes are so small that red blood cells move single-file through these vessels. The diameter of blood vessels changes in response to the needs of the body. For example, when you are exercising, muscle capillaries will expand, or dilate. This increases blood flow to working muscles, which brings more oxygen to cells and removes extra wastes from cells.

Veins

After blood moves through the tiny capillaries, it enters the largest blood vessels, called veins. Veins carry oxygen-poor blood, or deoxygenated blood, back to the heart. The endothelial walls of veins are much thinner than the walls of arteries. Pressure of the blood decreases when the blood flows through capillaries before it enters the veins. By the time blood flows into the veins, the heart’s original pushing force has less effect on making the blood move. So how does the blood keep moving? Because many veins are located near skeletal muscles, the contraction of these muscles helps keep the blood moving. Larger veins in your body also have f laps of tissue called valves, such as the one, that prevent blood from f lowing backward. Lastly, breathing movements exert a squeezing pressure against veins in the chest, forcing blood back to the heart.

The Heart

The heart is a muscular organ that is about as large as your fist and is located at the center of your chest. This hollow organ pumps blood throughout the body. The heart performs two pumping functions at the same time. The heart pumps oxygenated blood to the body, and it pumps deoxygenated blood to the lungs.

Structure of the heart

that the heart is made of cardiac muscle. It is capable of conducting electrical impulses for muscular contractions. The heart is divided into four compartments called chambers. The two chambers in the top half of the heart—the right atrium and the left atrium (plural, atria)—receive blood returning to the heart. Below the atria are the right and left ventricles, which pump blood away from the heart. A strong muscular wall separates the left side of the heart from the right side of the heart. The right and left atria have thinner muscular walls and do less work than the ventricles. Notice the valves in that separate the atria from the ventricles and keep blood flowing in one direction. Valves also are found in between each ventricle and the large blood vessels that carry blood away from the heart, such as the aortic valve shown in a closed position

How the heart beats

The heart acts in two main phases. In the first phase, the atria fill with blood. The atria contract, filling the ventricles with blood. In the second phase, the ventricles contract to pump blood out of the heart, into the lungs, and forward into the body. The heart works in a regular rhythm. A group of cells found in the right atrium, called the pacemaker or sinoatrial (SA) node, send out signals that tell the heart muscle to contract. The SA node receives internal stimuli about the body’s oxygen needs, and then responds by adjusting the heart rate. The signal initiated by the SA node causes both atria to contract. This signal then travels to another area in the heart called the atrioventricular (AV) node, This signal travels through fibers, causing both ventricles to contract. This two-step contraction makes up one complete heartbeat. Pulse The heart pulses about 70 times each minute. If you touch the inside of your wrist just below your thumb, you can feel a pulse in the artery in your wrist rise and fall. This pulse is the alternating expansion and relaxation of the artery wall caused by contraction of the left ventricle. The number of times the artery pulses is the number of times your heart beats.

Blood pressure

Blood pressure is a measure of how much pressure is exerted against the vessel walls by the blood. Blood-pressure readings can provide information about the condition of arteries. The contraction of the heart, or systole (SIS tuh lee), causes the blood pressure to rise to its highest point, and the relaxation of the heart, or dias tole (di AS tuh lee) brings the pressure down to its lowest point. A normal blood-pressure reading for a healthy adult is a reading below 120 (systolic pressure)/80 (diastolic pressure).

Blood flow in the body

First, the blood travels from the heart to the lungs and back to the heart. Then, the blood is pumped in another loop from the heart through the body and back. The right side of the heart pumps deoxygenated blood to the lungs, and the left side of the heart pumps oxygenated blood to the rest of the body. To the lungs and back When blood from the body flows into the right atrium, it has a low concentration of oxygen but a high concentration of carbon dioxide. This deoxygenated blood is dark red. The blood flows from the right atrium into the right ventricle and is pumped into the pulmonary arteries that lead to the lungs, as shown in Figure 34.6. Eventually, blood flows into capillaries in the lungs that are in close contact with the air that comes into the lungs. The air in the lungs has a greater concentration of oxygen than the blood in the capillaries does, so oxygen diffuses from the lungs into the blood. At the same time, carbon dioxide diffuses in the opposite direction—from the blood into the air space in the lungs. Oxygenated blood, which is now bright red, flows to the left atrium of the heart to be pumped out to the body. To the body and back The left atrium fills with oxygenated blood

from the lungs, beginning the second loop of the figure eight.  the blood then moves from the left atrium into the left ventricle. The left ventricle pumps the blood into the largest artery in the body called the aorta. Eventually, blood flows into the capillaries that branch throughout the body. Importantly, the capillaries are in close contact with body cells. Oxygen is released from the blood into the body cells by diffusion, and carbon dioxide moves from the cells to the blood by diffusion. The deoxygenated blood then flows back to the right atrium through veins.