Located on top of each kidney is an adrenal gland, also shown in Figure below. The two adrenal glands secrete several hormones. Hormones are chemical messengers in the body that regulate many body functions. The adrenal hormone aldosterone helps regulate kidney functions.
Figure 23.11
The human kidney.
In Figure above, you can see that the kidney has three layers. The outer layer is the renal cortex, and the middle layer is the renal medulla. The inner layer, the renal pelvis, is where the renal artery enters the kidney and the renal vein exits the kidney. The renal artery carries blood to the kidney to be filtered, and the renal vein carries the filtered blood away from the kidney. Structures in the kidney called nephrons are also seen in Figure above. Each nephron extends from the cortex down into the medulla.
Nephrons
Nephrons are the structural and functional units of the kidneys. A single kidney may have more than a million nephrons. The diagram in Figure below represents an individual nephron and shows its main structures and functions. The structures include the glomerulus, Bowman’s capsule, and renal tubule.
Figure 23.12
Nephron structures and functions.
The glomerulus is a cluster of arteries that filters substances out of the blood.
Bowman’s capsule is a cup-shaped structure around the glomerulus that collects the filtered substances.
The renal tubule is a long, narrow tube surrounded by capillaries that reabsorbs many of the filtered substances and secretes other substances.
Filtration, Reabsorption, and Secretion
The renal arteries, which carry blood into the kidneys, branch into the capillaries of the glomerulus of each nephron. The pressure of blood moving through these capillaries forces some of the water and dissolved substances in the blood through the capillary walls and into Bowman’s capsule. Bowman’s capsule is composed of layers. The space between the layers, called Bowman’s space, fills with the filtered substances.
The process of filtering substances from blood in the glomerulus is called filtration. The fluid that collects in Bowman’s space is called filtrate. It is composed of water, salts, glucose, amino acids, and urea. Larger structures in the blood—including protein molecules, blood cells, and platelets—do not pass into Bowman’s space. Instead, they return to the main circulation.
From Bowman’s space, the filtrate passes into the renal tubule. The main function of the renal tubule is reabsorption. Reabsorption is the return of needed substances in the filtrate back to the bloodstream. It is necessary because some of the substances removed from the blood by filtration—including water, salts, glucose, and amino acids—are needed by the body. About 75 percent of these substances are reabsorbed in the renal tubule.
As shown in Figure below, the renal tubule is divided into three parts: the proximal tubule, the Loop of Henle, and the distal tubule.
Figure 23.13
Parts of the renal tubule and other nephron structures.
Filtrate first enters the proximal tubule. This is where that most reabsorption takes place. Tiny projections called microvilli line the proximal tubule and increase the surface area for reabsorption. From the proximal tubule, the filtrate passes through the loop of Henle.
The loop of Henle carries the filtrate from the cortex down into the medulla and then back up to the cortex again. Its primary purpose is to reabsorb water and salt from the fluid. The remaining fluid enters the distal tubule.
The distal tubule carries the fluid, now called tubular fluid, from the loop of Henle to a collecting duct. As it transports the fluid, the distal tubule also reabsorbs or secretes substances such as calcium and sodium. The process of secreting substances into the tubular fluid is called secretion.
Urine Formation
The collecting ducts are the site of urine formation. This process is crucial for water conservation in the body. The collecting ducts reabsorb water from tubular fluid and return it to the blood. The remaining fluid, called urine, has a smaller volume and a greater concentration than tubular fluid. From the collecting ducts, urine enters a ureter and is eventually excreted from the body.
The reabsorption of water by the collecting ducts is controlled by a negative feedback mechanism. The mechanism involves a hormone secreted by the pituitary gland, called antidiuretic hormone (ADH). ADH makes the collecting ducts more permeable to water, allowing more water to be reabsorbed from tubular fluid. When there is not enough water in the blood, more ADH is secreted, more water is reabsorbed from tubular fluid, and less water is excreted in urine. The opposite happens when there is too much water in the blood.
Kidneys and Homeostasis
The kidneys play many vital roles in homeostasis. As you have already read, the kidneys filter blood and excrete liquid waste. In fact, the kidneys filter all the blood in the body about 16 times a day, producing approximately 180 liters of filtrate and about 1.5 liters of urine. The kidneys also control the amount of water in the blood by excreting more or less water in urine.
Balancing the Blood
The kidneys are responsible for maintaining balance in the blood in other ways, as well. For example, they control the acid-base balance in the blood, mainly by secreting hydrogen ions into tubular fluid and reabsorbing bicarbonate ions from tubular fluid as needed. The kidneys also regulate blood concentrations of many other ions—including sodium, potassium, calcium, and magnesium—by the controlling the amounts that are excreted in urine.
Secreting Hormones
The kidneys also secrete various hormones to help maintain homeostasis. Hormones secreted by the kidneys include erythropoietin and rennin.
Erythropoietin is secreted when the blood does not have enough red blood cells to carry adequate oxygen. The hormone stimulates the production of red blood cells by the bone marrow.
Rennin is secreted when blood pressure falls. The hormone stimulates the secretion of aldosterone by the adrenal gland. Aldosterone, in turn, stimulates the kidneys to reabsorb more sodium ions and water. This increases the volume of the blood, which causes an increase in blood pressure.
Kidney Disease and Dialysis
A person can live a normal, healthy life with just one kidney. However, at least one kidney must function properly to maintain life. Diseases that threaten the health and functioning of the kidneys include kidney stones, infections, and diabetes.
Kidney Stones
Kidney stones are crystals of dissolved minerals that form in urine inside the kidneys. They may start out as small as a grain of salt and grow to be as large as a grapefruit. There may be one large stone or many small ones. Small kidney stones often pass undetected through the urinary tract and out of the body in urine. However, kidney stones may grow large enough before passing to block a ureter. This can cause a buildup of urine above the blockage and severe pain. Large kidney stones can sometimes be broken into smaller pieces that wash out of the urinary tract in urine. The stones are shattered by high-intensity sound waves focused on them from outside the body. Another alternative is to remove kidney stones surgically.
Infections
Bacterial infections of the urinary tract are very common. In fact, urinary tract infections (UTI) are the second most common type of bacterial infections seen by health care providers. Typical organisms that cause UTIs include Escherichia coli and Staphylococcus saprophyticus. The organisms may infect any part of the urinary tract.
The most common type of UTIs are bladder infections. They can be treated with antibiotics prescribed by a doctor. However, if a bladder infection is not treated, it may spread to the kidney and cause a kidney infection, or pyelonephritis. This is the most serious type of UTI. It can damage the kidney and interfere with normal kidney function. Kidney infections can also be treated with antibiotics but may require other treatments as well.
Diabetes
Two different types of diabetes can involve the kidneys: diabetes insipidus and diabetes mellitus. Diabetes insipidus is a disease characterized b
y the inability to concentrate urine. A person with this disease typically produces many liters of very dilute urine each day. Diabetes insipidus can be caused by a deficiency of ADH (antidiuretic hormone) or by the kidneys failing to respond to ADH. If the cause of diabetes insipidus can be treated, it may cure the disease.
In diabetes mellitus, the kidneys try to reduce the high glucose level in the blood by excreting more glucose in urine. This causes frequent urination and increased thirst. If blood glucose levels are not controlled by medication or diet, they may damage capillaries of the glomerulus and interfere with the kidney’s ability to filter blood. Eventually, high glucose levels may lead to kidney failure, in which kidney function is greatly reduced. Kidney failure leads to high levels of urea and other wastes in the blood and may require treatment with dialysis.
Dialysis and Transplantation
Dialysis is a medical procedure in which blood is filtered with the help of a machine. One type of dialysis treatment is shown in Figure below. Blood from the patient’s vein enters the dialysis machine through a tube. Inside the machine, excess water, wastes, and other unneeded substances are filtered from the blood. The filtered blood is then returned to the patient’s vein through another tube. A dialysis treatment usually lasts three to four hours and must be repeated three times a week. Dialysis is generally performed on patients who have kidney failure. Dialysis helps them stay alive, but does not cure their failing kidneys.
Figure 23.14
Patient receiving dialysis treatment.
The only cure for most people with kidney failure is a kidney transplant. To be suitable for transplantation, the donated kidney must come from a donor who has the same blood and tissue types as the recipient. Even then, the recipient must take medication to suppress the immune system so it does not reject the new kidney.
Lesson Summary
Homeostasis is the body’s attempt to maintain a constant internal environment. Excretion helps achieve homeostasis by removing wastes, excess water, and other unneeded substances from the body. Both processes are essential for life.
The urinary system includes the kidneys and other structures that excrete liquid waste. The kidneys are the main organs of excretion of wastes in the blood, and nephrons are structural and functional units of the kidneys. The kidneys filter blood, reabsorb and secrete substances, and form urine.
The kidneys are the main organs of homeostasis. In addition to excretion, they regulate acid-base balance and ion concentrations in the blood. They also secrete hormones that control other body processes.
Diseases of the urinary system include kidney stones, infections, and diabetes, which may lead to kidney failure. Kidney failure can be treated with dialysis, in which a machine filters the blood.
Review Questions
What are homeostasis and excretion?
Identify three organs of excretion and one substance that each organ excretes.
Why do the kidneys reabsorb some of the substances they filter from the blood?
Describe how urine forms in the collecting ducts of the kidneys.
How does ADH control the amount of water in urine? How is this an example of negative feedback?
Does an otherwise healthy person with just one kidney need dialysis? Why or why not?
Summarize the processes and structures involved in excretion by the kidneys.
Contrast the effects on the kidneys of diabetes insipidus and diabetes mellitus.
Further Reading / Supplemental Links
Gold, Susan Dudley, The Digestive and Excretory Systems. Enslow Publishers, 2004.
O’Callaghan, Christopher A. and Caplan, Louis, The Renal System at a Glance. (2nd edition) Blackwell Publishing, 2006.
Tuso, Philip J., Who Stole My Kidneys? AuthorHouse, 2004.
Watson, Stephanie, The Urinary System. Greenwood Press, 2004.
http://www.estrellamountain.edu/faculty/farabee/biobk/BioBookEXCRET.html
http://www.ishib.org/journal/16-2s2/ethn-16-2s2-40ab.pdf
http://en.wikibooks.org/wiki/Human_Physiology/Homeostasis
http://en.wikibooks.org/wiki/Human_Physiology/The_Urinary_System
http://health.howstuffworks.com/kidney.htm
http://kidney.niddk.nih.gov/kudiseases/pubs/yourkidneys/
http://kidney.niddk.nih.gov/kudiseases/pubs/yoururinary/
http://library.thinkquest.org/22016/excretion/index.html
http://www.kidshealth.org/parent/general/body_basics/kidneys_urinary.html
http://en.wikipedia.org
Vocabulary
bladder
A hollow organ that stores urine.
Bowman’s capsule
A cup-shaped structure around the glomerulus that collects the filtered substances; part of the nephron.
control center
Involved in maintaining homeostasis; processes the information, determines the appropriate action, and sends a command to the effector.
dialysis
A medical procedure in which blood is filtered with the help of a machine.
effector
Involved in maintaining homeostasis; responds to the command and changes conditions in the internal environment.
erythropoietin
Hormone secreted by the kidney when the blood does not have enough red blood cells to carry adequate oxygen; stimulates the production of red blood cells by the bone marrow.
excretion
The process of removing wastes and excess water from the body.
filtration
The process of filtering substances from blood in the glomerulus.
glomerulus
Part of the nephron; a cluster of arteries that filters substances out of the blood.
homeostasis
The body’s attempt to maintain a constant internal environment.
kidney
Organ that filters the blood and forms urine.
kidney stones
Crystals of dissolved minerals that form in urine inside the kidneys.
loop of Henle
Carries the filtrate from the cortex down into the medulla and then back up to the cortex again; primary purpose is to reabsorb water and salt from the fluid.
negative feedback
A type of homeostatic mechanism in which change in one direction results in a counteractive change in the opposite direction; reverses the direction of change to bring conditions back to normal.
nephrons
The structural and functional units of the kidneys; includes the glomerulus, Bowman’s capsule, and renal tubule.
positive feedback
Accelerates or amplifies a change and pushes levels farther away from normal; not common in the human body.
reabsorption
The return of needed substances in the filtrate back to the bloodstream.
receptor
Involved in maintaining homeostasis; senses changes in the internal environment and sends the information to the control center.
renal tubule
A long, narrow tube surrounded by capillaries that reabsorbs many of the filtered substances and secretes other substances; part of the nephron.
rennin
Hormone secreted by kidney when blood pressure falls; stimulates the secretion of aldosterone by the adrenal gland. Aldosterone, in turn, stimulates the kidneys to reabsorb more sodium ions and water.
urethra
A muscular tube that carries urine out of the body.
urinary system
System in which the main function is to filter waste products and excess water from the blood and remove them from the body.
urine
The liquid waste product of the body that is excreted by the urinary system.
Points to Consider
A transplanted kidney may be rejected unless medication is taken to suppress the immune system. Why does the immune system reject transplanted organs?
How does the immune system recognize transplanted organs as foreign to the body?
&nbs
p; What happens when the immune system “attacks” a transplanted organ?
Chapter 24: Immune System and Disease
Lesson 24.1: Nonspecific Defenses
Lesson Objectives
Describe mechanical, chemical, and biological barriers that keep most pathogens out of the human body.
Explain how the inflammatory response and white blood cells help fight pathogens that enter the body.
Introduction
The immune system protects the body from "germs" and other harmful substances. The immune system is like a medieval castle. The outside of a medieval castle was protected by a moat and high stone walls. Inside the castle, soldiers were ready to defend the castle against any invaders that got through the outer defenses. Like a medieval castle, the immune system has a series of defenses. Only pathogens that are able to get through all the defenses can cause harm to the body.
CK-12 Biology I - Honors Page 117