The kidney is an exceptionally sophisticated and efficient purification system that cleanses the blood of unwanted byproducts produced by the body. (These byproducts are called metabolites.) Although we have two kidneys, we need only one kidney to live. In fact, people can lose most of their kidney function without becoming ill.
Most organs in the body control only one function. The heart pumps blood, and the stomach digests food. But kidneys not only filter blood, they also regulate a number of other body functions:
• Balancing the amount of water and salts (called electrolytes) retained by the body
• Controlling blood pressure
• Maintaining the proper balance of acidity in the blood
• Regulating the production of red blood cells (called erythrocytes) that carry oxygen to the various organs of the body
• Controlling the level of phosphate in the blood
• Activating vitamin D
The kidneys function to keep conditions in the body within a normal range, known as homeostasis. All of these functions can be affected when kidneys fail.
Humans are not the only animals with kidneys. All vertebrates (animals that have a spine) have kidneys. The earliest vertebrates lived in water. Because fish take a lot of water into their bodies, they need a mechanism to eliminate excess amounts. Saltwater fish also require a means to eliminate excess salt that they absorb. If saltwater fish could not expel excess water and salt from their bodies, they would blow up like a balloon and eventually explode. Kidneys may have evolved in animals to regulate water and salt balance.
Kidneys in vertebrates like us also eliminate waste products. Waste products are produced when we digest proteins (like those found in meat, fish, or dairy products). Carbohydrates (sugars and starches) are eventually broken down (metabolized) into water and carbon dioxide. However, when glucose (or sugar) exceeds a certain level in the blood, the kidney begins eliminating the excess. When a person’s body expels sugar in the urine, it can be a sign that the person has diabetes (see chapter 3).
Kidneys are the body’s simple filtration system. A simple system for filtering liquids removes particles that are greater than a certain size, whether they be coffee grounds or microbes. Filters usually have four parts: (1) a reservoir into which the liquid passes, like a funnel; (2) the filter itself, like porous paper, membranes, or cheesecloth; (3) the funnel stem, like a hose or straw; and (4) a collection receptacle, like a bottle or jar. We use filters every day when we percolate coffee or purify tap water. Most filtration systems use paper or activated charcoal as filters. Kidneys are a bit more complicated, but their filtration system works in a similar way.
The kidney is bean-shaped, approximately the size of an adult’s fist, and weighs about half a pound. Like a simple filtration system, the kidney has four basic parts. Looking at figure 2.1, which compares the kidneys with a funnel, we see that blood containing wastes first enters the kidney from a branch of the renal artery, which is like the reservoir of the funnel. The blood passes through the filtration apparatus, called the nephron. Each kidney contains about one million nephrons. Nephrons are composed of the glomerulus, the tubular system, and the collecting duct.
The first part of the nephron is the glomerulus (the filter), which has a large surface area to provide efficient filtration. The larger or thicker the filter, the more efficiently it traps bigger particles. The glomerulus allows small molecules to pass through it while retaining large substances that the body needs, like various blood cells and protein molecules, and eliminates waste products the body does not need. In the kidney, filtration occurs as the blood is forced through the walls of the glomerulus and numerous small vessels, through which blood cells cannot pass, into the tubular system (the stem of the funnel). Filtration produces a plasma-like fluid called filtrate. Substances that the body still needs pass through the glomerulus.
The filtrate leaves the glomerulus and enters the tubules and collecting ducts, where the useful substances are reabsorbed. Unlike the hollow stem of a simple funnel, tubules and collecting ducts are highly complex, with specialized structures that remove waste products while reabsorbing nutrients and salts that the body needs. What is left is urine. The collecting duct connected to the bladder (the collection receptacle, akin to the coffee pot) provides the final filtration step for urine before the body eliminates it.
Figure 2.1. The Funneling Properties of the Kidney
Let’s take a closer look at the tubular system. As we see in figure 2.1, the tubules attach to the glomerulus and loop through the kidney. Tubules eliminate urea, the main byproduct of protein breakdown from the body. However, the body needs some of the salts, water, and other nutrients that also pass from the blood through the kidney’s filter (glomerulus) into the filtrate. The tubules at various points on the loops process the filtrate further to reabsorb into the capillaries the salts, water, and nutrients the body needs back into the blood. Any excess salts and water not needed by the body remain in the filtrate to be eliminated as urine. So, the entire process of filtration that takes place in the kidneys involves filtering and reabsorbing—until everything useful has been reabsorbed and everything else is sent to the bladder to be eliminated from the body as urine.
In addition to filtering, kidneys help keep blood pressure from dropping too low. They do so by making and releasing an enzyme called renin. Maintaining blood pressure with renin involves several organs in the body, including the liver, lungs, and adrenal glands (see figure 2.2).
Renin prevents the body from developing dangerously low sodium (salt) concentrations, leading to low blood pressure. Such a condition can occur in hot weather, when the body sweats profusely, or with substantial blood loss. To prevent low blood pressure during salt depletion, the kidney releases renin into the bloodstream. When renin reaches the liver, it reacts with a protein called angiotensinogen to produce a biologically inactive protein called angiotensin I.
Figure 2.2. How Renin Regulates Blood Pressure
When angiotensin I leaves the liver, it travels to the lungs, where it is converted in the veins to angiotensin II. While traveling through the body, angiotensin II constricts blood vessels and raises blood pressure. In addition, angiotensin II acts on the adrenal glands, two small endocrine glands, one located on top of each kidney. In the adrenal gland, angiotensin II stimulates the release of the hormone called aldosterone, which can direct the kidneys to retain sodium and water. It’s easy to see how the overproduction of renin can contribute to high blood pressure (see chapter 3).
The body maintains the blood’s narrow range of acidity with buffers. The buffering process is regulated predominantly by a balance between carbonic acid and bicarbonate (baking soda) in the blood and by the acidity of the urine. In the kidney, acid along the tubular membrane swaps places with sodium (one of the salts) and bicarbonate. Various conditions can change the acidity of the blood. If you breathe too hard and too fast, the blood can become less acidic because of a reduction of carbon dioxide. If the kidney does not function properly, acid can accumulate, a condition known as acidosis.
Bone marrow makes red blood cells (erythrocytes) that carry oxygen throughout the body. Red blood cells live about four months and then must be replaced. The kidneys produce a hormone called erythropoietin, which controls the rate at which red blood cells form. When the kidney senses too little oxygen in the blood, it releases erythropoietin to stimulate the bone marrow to make more red blood cells. When kidney function is degraded or lost and the kidneys make insufficient erythropoietin, patients may have too few red blood cells, a condition called anemia.
Phosphate is essential for the body to produce energy. Dairy products are a major source of phosphate. Numerous chemical reactions in the body use phosphate, but our diets generally provide more phosphate than we need. The kidney is the only means of eliminating excess phosphate carried in the blood. If the kidney malfunctions, several problems can result from an excess of phosphate.
Phosphate readily combines with calcium. When excess phosphate binds to enough calcium, the body thinks it does not have enough calcium in the blood, prompting bones to release calcium into the bloodstream. When calcium is released into the bloodstream, people may develop osteoporosis and form calcium phosphate plaques in their organs, possibly leading to organ failure (see chapter 4).
Excess phosphate in the blood is not the only cause of bone demineralization. Vitamin D, a fat-soluble vitamin, plays an important role in the body’s absorption of calcium to maintain strong bones and teeth. Making vitamin D is a complicated process involving active and less active forms of vitamin D (see figure 2.3).
Figure 2.3. How Vitamin D Is Produced
The body makes active forms of vitamin D from cholesterol. One way is from sunlight: ultraviolet light stimulates the formation of cholecalciferol, a derivative of cholesterol, in the skin. Activated cholecalciferol then passes through the liver and becomes an even more active form of vitamin D called calcidiol. The final activation of vitamin D occurs in the kidney. When calcidiol enters the kidney, it is converted to calcitriol, the only form of vitamin D that the body actually uses. Some dietary supplements contain cholecalciferol, bypassing the need for the sun’s activation of less active forms of vitamin D. Failing kidneys may affect the body’s ability to absorb vitamin D, which can lead to bone loss.
Many of the body’s essential activities depend on normal kidney function. When kidneys fail, there are major consequences for the body. In the following chapter we discuss why kidneys may fail.
