The body must rid itself of the waste products of cellular activity. The process of removing metabolic wastes, called excretion, is just as vital as digestion in maintaining the body’s internal environment. Thus, the urinary system not only excretes wastes but also helps maintain homeostasis by regulating the content of water and other substances in the blood.
The main waste products that the body must eliminate are carbon dioxide, from cellular respiration, and nitrogenous compounds, from the breakdown of proteins. The lungs excrete most of the carbon dioxide, and nitrogenous wastes are eliminated by the kidneys. The excretion of water is necessary to dissolve wastes and is closely regulated by the kidneys, the main organs of the urinary system.
Humans have two bean-shaped kidneys, each about the size of a clenched fist. The kidneys are located one behind the stomach and the other behind the liver. Together, they regulate the chemical composition of the blood.
STRUCTURE of KIDNEY
Figure 48-15 shows the three main parts of the kidney. The renal cortex, the outermost portion of the kidney, makes up about a third of the kidney’s tissue mass. The renal medulla is the inner two-thirds of the kidney. The renal pelvis is a funnel-shaped structure in the center of the kidney. Also, notice in Figure 48-15 that blood enters the kidney through a renal artery and leaves through a renal vein. The renal artery transports nutrients and wastes to the kidneys. The nutrients are used by kidney cells to carry out their life processes. One such process is the removal of wastes brought by the renal artery.
The most common mammalian metabolic waste is urea (yoo-REE-uh), a nitrogenous product made by the liver. Nitrogenous wastes are initially brought to the liver as ammonia, a chemical compound of nitrogen so toxic that it could not remain long in the body without harming cells. The liver removes ammonia from the blood and converts it into the less harmful substance urea. The urea enters the bloodstream and is then removed by the kidneys.
The substances removed from the blood by the kidneys—toxins, urea, water, and mineral salts—form an amber-colored liquid called urine. Urine is made in structures called nephrons (NEF-RAHNZ), the functional units of the kidney. Nephrons are tiny tubes in the kidneys. One end of a nephron is a cup-shaped capsule surrounding a tight ball of capillaries that retains cells and large molecules in the blood and passes wastes dissolved in water through the nephron. The cup-shaped capsule is called Bowman’s capsule.
Within each Bowman’s capsule, an arteriole enters and splits into a fine network of capillaries called a glomerulus (gloh-MER-yoo-luhs). Take a close look at the structure of the nephron, shown in Figure 48-15. Notice the close association between a nephron of the kidney and capillaries of the circulatory system. Initially, fluid passes from the glomerulus into a Bowman’s capsule of the nephron. As the fluid travels through the nephron, nutrients that passed into the Bowman’s capsule are reabsorbed into the bloodstream.
What normally remains in the nephron are waste products and some water, which form urine that passes out of the kidney. Each kidney consists of more than a million nephrons. If they were stretched out, the nephrons from both kidneys would extend for 80 km (50 mi). As you read about the structure of a nephron, locate each part in Figure 48-15.
Each nephron has a cup-shaped structure, called a Bowman’s capsule, that encloses a bed of capillaries. This capillary bed, called a glomerulus, receives blood from the renal artery. Fluids are forced from the blood through the capillary walls and into the Bowman’s capsule. The material filtered from the blood then flows through the renal tubule, which consists of three parts: the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule. Blood remaining in the glomerulus then flows through a network of capillaries. The long and winding course of both the renal tubule and the surrounding capillaries provides a large surface area for the exchange of materials.
As the filtrate flows through a nephron, its composition is modified by the exchange of materials among the renal tubule, the capillaries, and the extracellular fluid. Various types of exchanges take place in the different parts of the renal tubule. To understand how the structure of each part of the nephron is related to its function, we will examine the three major processes that take place in the nephron: filtration, reabsorption, and secretion. Figure 48-16 shows the site of each of these processes in the nephron.
Materials from the blood are forced out of the glomerulus and into the Bowman’s capsule during a process called filtration. Blood in the glomerulus is under relatively high pressure. This pressure forces water, urea, glucose, vitamins, and salts through the thin capillary walls of the glomerulus and into the Bowman’s capsule.
About one-fifth of the fluid portion of the blood filters into the Bowman’s capsule. The rest remains in the capillaries, along with proteins and cells that are too large to pass through the capillary walls. In a healthy kidney, the filtrate—the fluid that enters the nephron—does not contain large protein molecules.
REABSORPTION AND SECRETION
The body needs to retain many of the substances that were removed from the blood by filtration. Thus, as the filtrate flows through the renal tubule, these materials return to the blood by being selectively transported through the walls of the renal tubule and into the surrounding capillaries. This process is called reabsorption. Most reabsorption occurs in the proximal convoluted tubule. In this region, about 75 percent of the water in the filtrate
returns to the capillaries by osmosis. Glucose and minerals, such as sodium, potassium, and calcium, are returned to the blood by active transport. Some additional reabsorption occurs in the distal convoluted tubule.
When the filtrate reaches the distal convoluted tubule, some substances pass from the blood into the filtrate through a process called secretion. These substances include wastes and toxic materials. The pH of the blood is adjusted by hydrogen ions that are secreted from the blood into the filtrate.
FORMATION of URINE
The fluid and wastes that remain in the distal convoluted tubule form urine. The urine from several renal tubules flows into a collecting duct. Notice in Figure 48-17 that the urine is further concentrated in the collecting duct by the osmosis of water through the wall of the duct. This process allows the body to conserve water. In fact, osmosis in the collecting duct, together with reabsorption in other parts of the tubule, returns to the blood about 99
of every 100 mL (about 3.4 oz) of water in the filtrate.
The Loop of Henle
The function of the loop of Henle (HEN-lee) is closely related to that of the collecting duct. Water moves out of the collecting duct because the concentration of sodium chloride is higher in the fluid surrounding the collecting duct than it is in the fluid inside the collecting duct. This high concentration of sodium chloride is created and maintained by the loop of Henle. Cells in the wall of the loop transport chloride ions from the filtrate to the fluid between the loops and the collecting duct. Positively charged sodium ions follow the chloride ions into the fluid. This process ensures that the sodium chloride concentration of the fluid between the loops and the collecting duct remains high and thus promotes the reabsorption of water from the collecting duct.
ELIMINATION OF URINE
Urine from the collecting ducts flows through the renal pelvis and into a narrow tube called a ureter (yoo-REET-uhr). A ureter leads from each kidney to the urinary bladder, a muscular sac that stores urine. Muscular contractions of the bladder force urine out of the body through a tube called the urethra (yoo-REE-thruh). Locate the ureters, urinary bladder, and urethra in Figure 48-18.
At least 500 mL (17 oz) of urine must be eliminated every day because this amount of fluid is needed to remove potentially toxic materials from the body and to maintain homeostasis. A normal adult eliminates from 1.5 L (1.6 qt) to 2.3 L (2.4 qt) of urine a day, depending on the amount of water taken in and the amount of water lost through respiration and perspiration.
THE EXCRETORY ORGANS
Although the kidneys, lungs, and skin belong to different organ systems, they all have a common function: waste excretion. The kidneys are the primary excretory organs of the body. They play a vital role in maintaining the homeostasis of body fluids.
The lungs are the primary site of carbondioxide excretion. The lungs carry out detoxification, altering harmful substances so that they are not poisonous. The lungs are also responsible for the excretion of the volatile substances in onions, garlic, and other spices.
The skin helps the kidneys control the salt composition of the blood. Some salt, water, nitrogen waste and other substances are excreted through perspiration. A person working in extreme heat may lose water through perspiration at the rate of 1 L per hour. This amount of perspiration represents a loss of about 10 to 30 grams of salt per day. Both the water and salt must be replenished to maintain normal body functions. Figure 48-19 summarizes some waste substances and the organ(s) that excrete them.
Notice that undigested food is not in the figure. Undigested food is not excreted in the scientific sense; it is eliminated, meaning it is expelled as feces from the body without ever passing through a membrane or being subjected to metabolic processes. The term excretion correctly refers to the process during which substances must pass through a membrane to leave the body.
Source: Modern Biology: Hopson & Postlethwait