Unit Six. Animal Life
26. Maintaining the Internal Environment
Animals must also carefully monitor the water content of their bodies. The first animals evolved in seawater, and the physiology of all animals reflects this origin. Approximately two-thirds of every vertebrate’s body is water. If the amount of water in the body of a vertebrate falls much lower than this, the animal dies. Animals use various mechanisms for osmoregulation, the regulation of the body’s osmotic composition, or how much water and salt it contains. The proper operation of many vertebrate organ systems of the body requires that the osmotic concentration of the blood—the concentration of solutes dissolved within it—be kept within a narrow range.
Animals have evolved a variety of mechanisms to cope with problems of water balance. In many animals and single- celled organisms, the removal of water or salts from the body is coupled with the removal of metabolic wastes through the excretory system. Protists, like the Paramecium in figure 26.3, employ contractile vacuoles for this purpose, as do the cells of sponges. Water and metabolic wastes are collected by the endoplasmic reticulum and pass through feeder canals to the contractile vacuole. The water and wastes are expelled when the vacuole contracts and releases its contents out a pore. Multicellular animals have a system of excretory tubules (little tubes) that expel fluid and wastes from the body.
Figure 26.3. Contractile vacuoles in Paramecium.
In flatworms, these tubules are called protonephridia (the green structures you see in figure 26.4). They branch throughout the body into bulblike flame cells, shown in the enlargement. While these simple excretory structures open to the outside of the body, they do not open to the inside of the body. Rather, the beating action of cilia within the flame cells draw in fluid from the body, which is passed into a collecting tube. Water and metabolites are then reabsorbed, and the substances to be excreted are expelled through excretory pores.
Figure 26.4.The protonephridia of flatworms.
A branching system of tubules, bulblike flame cells, and excretory pores make up the protonephridia of flatworms.
Other invertebrates have a system of tubules that open both to the inside and to the outside of the body. In the earthworm, these tubules are known as nephridia, the blue structures you see in figure 26.5. The nephridia obtain fluid from the body cavity through a process of filtration into funnel-shaped structures called nephrostomes. The term filtration is used because the fluid is formed under pressure and passes through small openings, so that molecules larger than a certain size are excluded. This filtered fluid is isotonic (having the same osmotic concentration) to the fluid in the coelom, but as it passes through the tubules of the nephridia, NaCl is removed by active transport processes. A general term for transport out of the tubule and into the surrounding body fluids is reabsorption. Because salt is reabsorbed from the filtrate, the urine excreted is more dilute than the body fluids (meaning it is hypotonic).
Figure 26.5. The nephridia of annelids.
Most invertebrates, such as the annelid shown here, have nephridia. These consist of tubules that receive a filtrate of coelomic fluid, which enters the funnel-like nephrostomes. Salt can be reabsorbed from these tubules, and the fluid that remains, urine, is released from pores into the external environment.
The excretory organs in insects are called Malpighian tubules, the green structures you see in figure 26.6. Malpighian tubules are extensions of the digestive tract that branch off anterior to the hindgut. Urine is not formed by filtration in these tubules because there is no pressure difference between the blood in the body cavity and the tubule. Instead, waste molecules and potassium ions (K+) are secreted into the tubules by active transport. In secretion, ions or molecules are transported from the body fluid into the tubule. The secretion of K+ creates an osmotic gradient that causes water to enter the tubules by osmosis from the body’s open circulatory system. Most of the water and K+ is then reabsorbed into the circulatory system through the epithelium of the hindgut, leaving only small molecules and waste products to be excreted from the rectum along with feces. Malpighian tubules thus provide a very efficient means of water conservation.
Figure 26.6. The Malpighian tubules of insects.
(a) The Malpighian tubules of insects are extensions of the digestive tract that collect water and wastes from the body's circulatory system.
(b) K+ is secreted into these tubules, drawing water with it osmotically. Much of this water and K+ is reabsorbed across the wall of the hindgut.
Kidneys are the excretory organs in vertebrates and are discussed in more detail in the rest of the chapter. Unlike the Malpighian tubules of insects, kidneys create a tubular fluid by filtration of the blood under pressure. In addition to containing waste products and water, the filtrate contains many small molecules that are of value to the animal, including glucose, amino acids, and vitamins. These molecules and most of the water are reabsorbed from the tubules into the blood, while wastes remain in the filtrate. Additional wastes may be secreted by the tubules and added to the filtrate, and the final waste product, urine, is eliminated from the body.
It may seem odd that the vertebrate kidney should filter out almost everything from blood plasma (except proteins, which are too large to be filtered) and then spend energy to take back or reabsorb what the body needs. But selective reabsorption provides great flexibility; various vertebrate groups have evolved the ability to reabsorb different molecules that are especially valuable in particular habitats. This flexibility is a key factor underlying the successful colonization of many diverse environments by the vertebrates.
Key Learning Outcome 26.2. Many invertebrates filter fluid into a system of tubules and then reabsorb ions and water, leaving waste products for excretion. Insects create an excretory fluid by secreting K+ into tubules, which draws water osmotically. The vertebrate kidney produces a filtrate that enters tubules from which water is reabsorbed.