THE LIVING WORLD
Unit Six. Animal Life
Blood has been called the river of life. Among all the vertebrate body’s tissues, blood is the only liquid tissue, a fluid highway transporting gases, nutrients, hormones, antibodies, and wastes throughout the body. The material that makes blood a liquid is a protein-rich fluid called plasma that makes up approximately 55% of blood. The other 45% is made up mostly of red and white blood cells. Red blood cells like those seen above are the oxygen transporters of the vertebrate circulatory system. There are approximately 5 million of them in each microliter (1 ml) of blood! Each red blood cell is shaped like a rounded cushion, squashed in the center, and is crammed full of a protein called hemoglobin, an iron-containing molecule that gives blood its red color. Oxygen binds easily to the iron in hemoglobin, making red blood cells efficient oxygen carriers. A single red blood cell contains about 250 million hemoglobin molecules, and each red blood cell can carry about 1 billion molecules of oxygen at one time. The average life span of a red blood cell is only 120 days—about 2 million new ones are produced in the bone marrow every second to replace those that die or are worn out. In this chapter we will examine the ways in which vertebrates use cells like these, and the fluid surrounding them, to transport oxygen, food, and information throughout the body.
Every cell in the vertebrate body must acquire the energy it needs for living from organic molecules outside the body. Like residents of a city whose food is imported from farms in the countryside, the cells of the body need trucks to carry the food, highways for the trucks to travel on, and a means to cook the food when it arrives. In the vertebrate body, the trucks are blood, the highways are blood vessels, and oxygen molecules are used to cook the food. Remember from chapter 7 that cells obtain energy by “burning” sugars like glucose, using up oxygen and generating carbon dioxide. In animals, the organ system that provides the trucks and highways is called the circulatory system, while the organ system that acquires the oxygen fuel and disposes of the carbon dioxide waste is called the respiratory system. We discuss the functions of the circulatory system in this chapter and of the respiratory system in chapter 24.
Not all eukaryotes have circulatory systems. Among the unicellular protists, oxygen and nutrients are obtained directly by simple diffusion from the aqueous external environment.
Animals amplify the power of diffusion with a body cavity. Cnidarians, such as Hydra, and flatworms, such as Planaria (pictured above), have cells that are directly exposed to either the external environment or to a body cavity called the gastrovascular cavity that functions in both digestion and circulation, delivering nutrients and oxygen directly to the tissue cells by diffusion from the digestive cavity. The gastrovascular cavity of Hydra extends even into the tentacles (see page 532), and that of Planaria, colored green in the figure above, branches extensively to supply every cell with oxygen and the nourishment obtained by digestion.
Larger animals, however, have tissues that are several cell layers thick, so many cells are too far away from the body surface or digestive cavity to exchange materials directly with the environment. Instead, oxygen and nutrients are transported from the environment and digestive cavity to the body cells by an internal fluid within a circulatory system.
There are two main types of circulatory systems: open and closed. In an open circulatory system, such as that found in arthropods and many mollusks, there is no distinction between the circulating fluid (blood) and the extracellular fluid of the body tissues. This fluid is thus called hemolymph. Insects, like the fly pictured here, have a muscular tube that serves as a heart to pump the hemolymph through a network of open-ended channels that empty into the cavities in the body (downward-pointing arrows). There, the hemolymph delivers nutrients to the cells of the body. It then reenters the circulatory system through pores in the heart (upward-pointing arrows). The pores close when the heart pumps to keep the hemolymph from flowing back out into the body cavity.
In a closed circulatory system, the circulating fluid, or blood, is always enclosed within blood vessels that transport blood away from and back to a pump, the heart. Blood pumped to and from the hearts always remains within this system of vessels. Annelids and all vertebrates have a closed circulatory system. In annelids, such as the earthworm seen here, a dorsal blood vessel contracts rhythmically to function as a pump. Blood is pumped through five small connecting vessels called lateral hearts, which also function as pumps, to a ventral blood vessel, which transports the blood posteriorly (lower arrows) until it eventually reenters the dorsal blood vessel (upper arrows). Smaller vessels branch between the ventral and dorsal blood vessels to supply the tissues of the earthworm with oxygen and nutrients and to carry away waste products.
In vertebrates, blood vessels form a tubular network that permits blood to flow from the heart to all the cells of the body and then back to the heart. Arteries carry blood away from the heart, whereas veins return blood to the heart. Blood passes from the arterial to the venous system in capillaries, which are the thinnest and most numerous of the blood vessels.
As blood plasma passes through capillaries, the pressure of the blood forces some of this fluid out of the capillary walls. Fluid derived this way is called interstitial fluid. Some of this fluid returns directly to capillaries, and some enters into lymph vessels, located in the connective tissues around the blood vessels. This fluid, now called lymph, is returned to the venous blood at specific sites. The lymphatic system is considered a part of the circulatory system and is discussed later in this chapter.
The Functions of Vertebrate Circulatory Systems
The functions of the circulatory system can be divided into three areas: transportation, regulation, and protection.
1. Transportation. Substances essential for cellular functions are transported by the circulatory system. These substances can be categorized as follows:
Respiratory. Red blood cells, or erythrocytes, transport oxygen to the tissue cells. In the capillaries of lungs or gills, oxygen attaches to hemoglobin molecules within the erythrocytes and is transported to the cells for aerobic respiration. Carbon dioxide produced by cell respiration is carried by the blood to the lungs or gills for elimination.
Nutritive. The digestive system is responsible for the breakdown of food so that nutrients can be absorbed through the intestinal wall and into the blood vessels of the circulatory system. The blood then carries these absorbed products of digestion through the liver and to the cells of the body.
Excretory. Metabolic wastes, excessive water and ions, and other molecules in plasma (the fluid portion of blood) are filtered through the capillaries of the kidneys and excreted in urine.
Endocrine. The blood carries hormones from the endocrine glands, where they are secreted, to the distant target organs they regulate.
2. Regulation. The cardiovascular system participates in temperature regulation in two ways:
Temperature regulation. In warm-blooded vertebrates, or homeotherms, a constant body temperature is maintained, regardless of the surrounding temperature. This is accomplished in part by blood vessels located just under the epidermis. When the ambient temperature is cold, the superficial vessels constrict to divert the warm blood to deeper vessels. When the ambient temperature is warm, the superficial vessels dilate so that the warmth of the blood can be lost by radiation.
Heat exchange. Some vertebrates also retain heat in a cold environment by using a countercurrent heat exchange. Figure 23.1 shows how a countercurrent heat exchange system works in the flipper of a killer whale. In this process, a vessel carrying warm blood from deep within the body (colored red) passes next to a vessel carrying cold blood from the surface of the body (colored blue). The warm blood going out heats the cold blood returning from the body surface (heat indicated by the red arrows in the cross section), so that this blood is no longer cold when it reaches the interior of the body, helping to maintain a stable core body temperature.
Figure 23.1 Countercurrent heat exchange.
Many marine mammals, such as this killer whale, limit heat loss in cold water by countercurrent flow that allows heat exchange between arteries and veins. The warm blood pumped from within the body in arteries warms the cold blood returning from the skin in veins, so that the core body temperature can remain constant in cold water. The cutaway portion in the figure shows how the veins surround the artery, maximizing the heat exchange between the artery and the veins.
3. Protection. The circulatory system protects against injury and foreign microbes or toxins introduced into the body:
Blood clotting. The clotting mechanism protects against blood loss when vessels are damaged. This clotting mechanism involves both proteins from the blood plasma and blood cell structures called platelets (discussed in section 23.4).
Immune defense. The blood contains proteins and white blood cells, or leukocytes, that provide immunity against many disease-causing agents. Some white blood cells are phagocytic, some produce antibodies, and some act by other mechanisms to protect the body.
Key Learning Outcome 23.1. Circulatory systems may be open or closed. All vertebrates have a closed circulatory system, in which blood circulates away from the heart in arteries and back to the heart in veins. The circulatory system serves a variety of functions, including transportation, regulation, and protection.