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TRANSPORT Ch. 14DATE______

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CIRCULATION AND IMMUNITY

One-celled organisms can obtain nutrients directly from surrounding water and discharge metabolic wastes back into the water. In multicellular organisms, however, this is impossible. Most of the cells are not in contact with the environment, but instead are surrounded by other cells. Thus, all multicellular organisms must have some method of bringing food, oxygen, and other needed substances to the cells and of removing metabolic wastes. This is done by the transport, or circulatory system.

In this chapter we are going to discuss the transport system of humans. However, the transport systems of most other vertebrates are very similar.

ORGANS OF TRANSPORT. Since the human body has millions and millions of cells, the transport system must be very extensive to reach all of them. The system includes the heart, blood, and blood vessels, as well as the lymphatic system, which will be discussed later in the chapter.

Humans, like all other vertebrates, have a closed transport system. This means that the transporting fluid---the blood--- is enclosed within vessels throughout the system. Some invertebrates, on the other hand, have open transport systems. In an open system the blood leaves the vessels at some point and directly bathes the body tissues. Eventually, it returns to the heart or into the vessels. Open transport systems are much slower and less efficient than closed ones.

It is possible for you to see blood circulating inside vessels. This can be done using a goldfish wrapped in wet toweling or cotton. Place the tail of the goldfish on the stage of your microscope, and focus on it. Because the covering tissues are thin and the vessels close to the surface, you will be able to see the blood flowing.

THE HEART. The heart is the most important organ in the transport system. It is a fist-sized, highly muscular organ. The heart is found in the middle of the chest, but because it tilts to the left, it is often thought to be on the left side. Its strong, rhythmic contractions pump the blood through the many miles of vessels in the body. Over the course of a lifetime, the human heart pumps enough blood to fill a good-sized lake.

The human heart has four chambers (see Figure 14-1). The top chambers, the right and left atria (singular atrium) are relatively thin-walled structures. The bottom chambers, the right and left ventricles, have strong, muscular walls. The chambers on the right side of the heart are separated from those on the left by a wall, or septum. There are complex valves separating the ventricles from the atria. These valves allow the blood to flow in only one direction. Blood vessels leaving the heart also have valves that prevent the backflow of blood into the heart.


Although the heart beats continually, the rate is highly variable, and it is constantly adjusted to meet the needs of the body. Depending on the individual, the normal heart rate is generally from sixty to ninety beats per minute. However, with strenuous exercise, the rate may increase to over one hundred and fifty beats per minute. The rate of the heartbeat is controlled by nerves from the brain.


The heart muscle relaxes only between beats. Because of this constant hard work, the heart muscle must have a plentiful and uninterrupted supply of blood, bringing it oxygen and nutrients. This flow is supplied by the coronary arteries, which are vessels that serve only the heart. If these vessels or any of their many branches that cover the heart are blocked in any way, the muscle that they serve does not receive its blood supply. This blood stoppage damages or kills that part of the heart muscle. This is what happens in a heart attack. If the affectedarea is large enough, the heart stops functioning.

Heart disease is a leading cause of death in the United States. It is caused by genetic factors, as well as overweight, smoking, air pollution, emotional stress, and poor muscle tone. To keep your heart in good condition, eat moderately, exercise regularly, and avoid smoking.


BLOOD VESSELS. Blood passes from the heart to all parts of the body and back again through a system of tubelike vessels. These vessels are of three basic types---arteries, veins, and capillaries.

  1. ARTERIES. The arteries are the vessels that carry oxygen-containing blood from the heart to all parts of the body. These vessels have thick, elastic, muscular walls that enable them to expand and contract.

With each contraction of the heart, a certain amount of blood is forced into the arteries. This spurt of blood causes the arterial walls to expand. As the blood passes and while the heart is filling for the next contraction, the arterial walls contract. This alternating expansion and contraction of the artery walls helps maintain a constant blood pressure.

The pulsating of the arteries in time with the heartbeat can be felt at certain points in the body. Generally, arteries run deep in the body.

However, at these points, the pressure points, the arteries run close to the surface of the body and in front of a bone. When you press down on the vessels at these points, you can feel them pulsating. This your pulse. The most common pressure point for checking the pulse rate (and thus the heart rate) is on the inside of the wrist. If any artery is cut, the blood comes out in rhythmic spurts corresponding to the contractions of the heart.


  1. VEINS. Veins carry waste-containing blood from the cells of the body back to the heart. Veins are relatively thin-walled vessels. Since much of the venous blood moves uphill against the force of gravity, the veins have valves that prevent the backflow of blood.

Unlike the arteries, veins do not show a pulse, and the blood in the veins is not moved by the force of the heartbeat. Instead, the movement of blood in the veins is brought about by body movements, such as breathing. In many places the veins run through muscles. When the muscles contract, the veins are compressed, and this forces the blood to move.

Venous blood is dark compared to the bright red arterial blood. The veins themselves, which in many places you can see through your skin, appear blue in color. They are actually white, but the combination of dark red blood and light reflecting off the skin cause the apparent bluish color.


  1. CAPILLARIES. The microscopic capillaries are the most numerous and in some ways the most important of all blood vessels. There are more miles of capillaries in the circulatory system than arteries and veins put together. The walls of capillaries are only one cell thick. It is through these walls that materials diffuse between the blood and the body cells.

PATHWAYS OF THE BLOOD. We will begin our description at the point where the blood leaves the heart (see Figure 14-5). With each heartbeat, oxygen-carrying blood is forced from the left ventricle into the aorta, the largest artery in the body. Many smaller arteries branch off the aorta, and these, in turn, branch into still smaller vessels. The smallest arterial branches give rise to the capillaries. Several capillaries come off each tiny artery. They form an interconnected network of capillaries called a capillary bed. All body cells must lie fairly close to part of a capillary bed.


In the internal spaces of the body and surrounding the cells is the intercellular fluid. Oxygen, nutrients, and other substances from the blood diffuse into the intercellular fluid, and from there into the cells. Wastes diffuse out of the cells into the intercellular fluid, and then into the capillary blood. The clear, light-colored fluid inside a blister is intercellular fluid.

At the end of the bed, the capillaries merge into a single vessel, which is the smallest vein. The tiny veins, in turn, merge into larger and larger vessels. Eventually, all the blood is emptied into two large veins, which enter the right atrium of the heart. These two veins are the superior vena cava, which collects blood from the upper part of the body, and the inferior vena cava, which collects blood from the lower part of the body.

Blood entering the right atrium passes down into the right ventricle. With the contraction of the heart, blood is pumped from the right ventricles into the pulmonary arteries, which carry it to the lungs. Here, carbon dioxide diffuses out of the blood into the lungs. Oxygen from the lungs diffuses into the blood; the oxygenated blood flows back to the heart through the pulmonary veins. These veins enter the left atrium. The blood flows from the left atrium into the left ventricle, and is again pumped to the rest of the body.

There is a special circulatory route involving blood flow from the intestines to the liver. After digestion of a heavy meal, large quantities of sugar diffuse into the blood in the intestinal capillaries. Some cells, particularly nerve cells, are sensitive to high concentrations of sugar. Therefore, this blood cannot enter the general circulation. Instead, it passes into the portal vein, which carries it to the liver. In the liver the sugar is removed from the blood and stored as glycogen. When the body needs sugar, the liver glycogen is broken down, and the sugar is released into the blood.

BLOOD. Blood is the medium of transport in the body. Although it is fluid, blood is classified as a tissue because it contains specialized types of cells performing certain, specialized function. Blood is approximately fifty-five percent fluid and forty-five percent solid materials.

  1. PLASMA. Plasma is the clear, yellow fluid that remains when all the solid components are removed from blood. Plasma is about ninety percent water. The remaining materials include various salts, nutrients, cellular waste products, and proteins. There are more than fifty different types of proteins present, and they are known collectively as the plasma proteins. Among these are proteins involved in immunity and in blood clotting. If the clotting factors are removed from the plasma, the remaining fluid is called serum.
  1. RED BLOOD CELLS (ERYTHROCYTES). Red blood cells have two functions---they carry oxygen from the lungs to the cells of the body, and they carry carbon dioxide from the cells back to the lungs. The special ability of the red cells to carry oxygen is due to the presence of an iron-containing pigment called hemoglobin.

Hemoglobin combines readily with oxygen. But just as important, it combines loosely and reversibly with oxygen. Thus, in the lungs, where the concentration of oxygen is high, the hemoglobin and oxygen combine. In the body tissues. Where the oxygen concentration is low, the hemoglobin gives up its oxygen, which can then diffuse into the cells where it is needed. If hemoglobin formed a strong bond with oxygen, it would be useless as a respiratory pigment. It would pick up oxygen in the lungs, but the oxygen would not be easily released in the tissues where it is needed.

In the blood going from the cells to the heart and lungs (venous blood), the waste gas carbon dioxide is carried dissolved in the red cells and in the plasma.

In adults, the red blood cells (as well as platelets and most white blood cells) are formed in the marrow of certain bones. Mature red blood cells differ from other human cells in that they contain no nucleus. When viewed from above they are round, but from the side they are dumbbell-shaped. This is because they are thicker around the edges than they are in the middle.

Because mature red cells have no nucleus, they cannot reproduce. They function for about one hundred and twenty days, and then they are broken down in the liver and spleen. In the average adult there are more than twenty-five trillion red blood cells. Millions of these cells are destroyed every second, while millions of new ones are released into the blood stream. If not enough red cells are produced, or if too many are being destroyed, or if the cells do not contain enough hemoglobin, a condition called anemia results.

  1. WHITE BLOOD CELLS (LEUCOCYTES). The white blood cells prevent infection in the body by engulfing bacteriaand other disease-causing organisms and rendering them harmless. Unlike red cells, white cells do have a nucleus, and they can move around on their own, rather like amebas. They can change their shape and squeeze out of the blood capillaries to fight invading organisms at the point where they enter the body. The pus often found in the site of an infection consists of dead and some living white cells, dead body cells dead and living microorganisms, and various other substances.

Although white cells are outnumbered by red cells six hundred to one, they can be produced quickly in the event of infection. In fact, doctors use the increase in the number of white cells as an indicator of infection somewhere in the body.

  1. PLATELETS (THROMBOCYTES). Platelets are oval or rounded structures involved in blood clotting. They are formed in the bone marrow by the breaking apart, or fragmentation, of a special type of blood cell.


BLOOD CLOTTING. If your blood did not clot, you could bleed to death from a small cut. This is what happens in an inherited disease called hemophilia. With normal blood, a cut initiates a complex series of reactions that ends with the formation of a clot that blocks off the cut vessels and prevents further bleeding. In hemophilia, one or more of the factors necessary for these reactions is missing, and no clot if formed.


The first step in clotting involves the release of a special substance by the injured cells and by blood platelets. After a series of reactions, fibrinogen, one of the plasma proteins, reacts with another substance to yield fibrin. Fibrin, in the form of a tangled mass of fibers, forms a tough clot over the wound. As the fibrin dries out, it forms a scab over the site of the cut.

THE LYMPHATIC SYSTEM. As we mentioned previously, the internal spaces of the body as well as the spaces between cells are filed with intercellular fluid. This fluid is mostly blood plasma that has diffused out of the capillaries. However, the plasma proteins cannot, for the most part, pass through the semipermeable capillary walls. Therefore, unlike plasma, the intercellular fluid contains little protein. Some intercellular fluid diffuses back into the capillaries. But much of it diffuses into another system of vessels, the lymphatic system. The fluid in these vessels is called lymph.

The lymphatic system begins as blind-ended lymphatic capillaries in the tissues of many parts of the body. The intercellular fluid enters these vessels by diffusion. The lymphatic capillaries merge into larger and larger vessels. In these vessels the lymph flows toward the upper chest. The lymph is eventually emptied into veins below the neck. Thus the fluid is returned to the bloodstream.

Since blood in the capillaries is under pressure, there is always a tendency for it to leak out into the surrounding tissues. Thus drainage by the lymphatic system prevents the buildup of fluids in the tissues of the body.

As we mentioned in Chapter 13, there are tiny lymphatic vessels in the villi of the small intestines. Fats absorbed through the intestinal wall tend to enter the lymphatic vessels rather than the blood capillaries of the villi. Along with the lymph, they are eventually emptied into the bloodstream.

Another important function of the lymphatic system is in preventing infection in the body. At numerous points in the system the lymphatic vessels enlarge to form lymph nodes. One type of white blood cell, the lymphocyte, is produced in the lymph nodes and other lymphatic tissues. Within the nodes, the lymphocytes engulf and destroy bacteria and other foreign particles present in the lymph. When there is an infection in the body, the lymph nodes that drain the infected area may become swollen and painful. The nodes in the neck and armpits are frequently affected. The lymph nodes are
also involved in the formation of antibodies.

BLOOD AND DISEASE---IMMUNITY. The human body has a wide range of general defense mechanisms, which successfully protect it against invasion by most disease-causing organisms. Unbroken skin plays a major role in keeping all foreign matter out of the body, and we have already discussed the activities of lymph nodes and white blood cells. Sometimes, however, these mechanisms fail, and the invading microorganisms multiply rapidly in the body. This activates the body's most powerful defense mechanism---the immune reaction. Unlike other defense mechanisms, the immune reaction is very specific. It can be activated by many types of foreign materials other than living organisms and each type is attacked separately.