Download CHAPTER 16 URINARY SYSTEM AND EXCRETION

CHAPTER 16 URINARY SYSTEM AND EXCRETION
Structure and function of the urinary system organs, including the microscopic structure of the kidney, is
followed by a details of urine formation and role of the kidneys in water-salt and acid-base balance.
LEARNING OBJECTIVES
A student should learn the following concepts:
1. The urinary system has organs specialized to produce, store, and rid the body of urine.
2. Kidneys excrete nitrogenous wastes and maintain the water-salt and the acid-base balance of the
blood within normal limits.
3. Kidneys have a macroscopic anatomy and a microscopic anatomy.
4. Urine is produced by many microscopic tubules called nephrons.
5. Urine formation is a multistep process.
6. Kidneys are under hormonal control as they regulate the water-salt balance of blood.
7. Kidneys excrete hydrogen ions and reabsorb bicarbonate ions to regulate the pH of blood.
CHAPTER OUTLINE
16.1
Urinary System
The Urinary Organs
The kidneys are at the back of the abdominal wall. They produce urine which is conducted by the
ureters to the bladder where it is stored before being released through the urethra.
Urination
As the bladder fills with urine, sensory impulses go to the spinal cord where motor nerve
impulses return and cause the bladder to contract and sphincters to open. With maturation, the
brain controls this reflex and delays urination.
Functions of the Urinary System
Excretion and Homeostasis
Defecation eliminates feces through the digestive tract. Excretion eliminates metabolic wastes
that were cell metabolites. Kidneys also regulate water-salt and acid-base balance of blood.
Excretion of Metabolic Wastes
Kidneys excrete nitrogenous wastes, including urea, uric acid, and creatinine. Collection of uric
acid in joints causes gout.
Maintenance of Water-Salt Balance
Kidneys maintain the water-salt balance of the body which also regulates blood pressure. Kidneys
also maintain correct levels of potassium, bicarbonate, and calcium ions in blood.
Maintenance of Acid-Base Balance
Kidneys help keep the blood pH within normal limits by excreting hydrogen ions and reabsorbing
bicarbonate ions as needed.
Secretion of Hormones
Kidneys secrete erythropoietin to stimulate red blood cell production, help modify vitamin D to
calcitriol needed for calcium reabsorption in digestion, and release renin that leads to the
secretion of aldosterone.
16.2
Kidney Structure
Anatomy of a Kidney
On a large scale, a kidney is divided into an outer renal cortex, an inner renal medulla, and the
central space called the renal pelvis. Microscopically, each contains over one million nephrons.
Anatomy of a Nephron
Each nephron has its own blood supply. An afferent arteriole approaches the glomerular capsule
and divides to become the glomerulus, a knot of capillaries. The permeability of the glomerular
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capsule allows small molecules to enter the capsule from the glomerulus. The efferent arteriole
leaves the capsule and branches into the peritubular capillary network.
Parts of a Nephron
The closed end of the nephron is a cuplike glomerular capsule. The spaces between podocytes of
the glomerular capsule allow small molecules to enter the capsule from the glomerulus, a
capillary knot. The cuboidal epithelial cells of the proximal convoluted tubule have many
mitochondria and microvilli or a brush border to carry out active transport (following passive
transport) from the tubule to blood. The descending loop of the nephron allows water to leave and
the ascending portion extrudes salt. The cuboidal epithelial cells of the distal convoluted tubule
have numerous mitochondria but lack microvilli. They carry out active transport from the blood
to the tubule or tubular secretion. Collecting ducts gather in the renal medulla and form the renal
pyramids.
16.3
Urine Formation
Glomerular Filtration
During glomerular filtration, small molecules including water, wastes, and nutrients move from
the glomerulus to the inside of the glomerular capsule. Blood cells and large proteins do not move
across.
Tubular Reabsorption
During tubular reabsorption, nutrients and water and some urea moves from the proximal
convoluted tubule into the blood of the peritubular capillary network.
Tubular Secretion
During tubular secretion, specific substances such as hydrogen ions, creatinine, and drugs such as
penicillin move from the blood into the distal convoluted tubule.
16.4
Maintaining Water-Salt Balance
Reabsorption of Water
Salt passively diffuses out of the lower portion of the ascending limb of the loop; the upper thick
portion actively extrudes salt into the tissue of the outer renal medulla. Water is reabsorbed from
all parts of the tubule. The ascending limb of loop of the nephron establishes an osmotic gradient
that draws water from the descending limb of the nephron and also the collecting duct. The
permeability of the collecting duct is under the control of antidiuretic hormone (ADH). Diuresis
is an increase in urine flow and antidiuresis is a decrease.
Reabsorption of Salt
Kidneys regulate salt balance in blood by controlling excretion and reabsorption of ions. Two
hormones, aldosterone and atrial natriuretic hormone (ANH), control the kidneys’ reabsorption of
sodium (Na). The juxtaglomerular apparatus detects low blood volume, it secretes renin that
eventually results in the adrenal cortex releasing aldosterone that restores blood volume and
pressure through reabsorption of sodium ions. Reabsorption of salt increases blood volume and
pressure because more water is also reabsorbed. ANH is secreted by the atria of the heart when
cardiac cells are stretched by blood. ANH inhibits secretion of renin; the resulting excretion of
sodium also excretes water and drops blood volume.
Diuretics
Alcohol inhibits secretion of ADH. Caffeine increases the glomerular filtration rate and decreases
tubular reabsorption of sodium.
16.5
Maintaining Acid-Base Balance
Kidneys rid the body of acidic and basic substances. If the blood is acidic, hydrogen ions are
excreted and bicarbonate ions are reabsorbed. If the blood is basic, hydrogen ions are not excreted
and bicarbonate ions are not reabsorbed. Hydrogen ions combine with ammonia (NH3). The
breathing system also ties up hydrogen ions when carbon dioxide is exhaled.
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TENTH EDITION CHANGES
This chapter remains essentially as it was previously.
New/Revised Text:
Section 16.1 Urinary System. The definition of excretion has been expanded. The role of the kidneys
in homeostasis is better explained.
New/Revised Figures: 16.4 Nephron anatomy.
TECHNOLOGY CONNECTIONS
American Journal of Kidney Diseases is at http://www.ajkd.org/
Cystocele (Fallen Bladder) is authoritatively discussed at
http://www.niddk.nih.gov/health/urolog/summary/cystocel/index.htm
Kidney Stones are described by UrologyChannel at http://urologychannel.com/kidneystones/index.shtml
The Kidney Transplant/Dialysis Association is at http://www.ultranet.com/~ktda/
National Institute of Diabetes and Digestive and Kidney Diseases has a kidney information sector at its
NIH-based website at http://www.niddk.nih.gov/health/kidney/kidney.htm
National Kidney Foundation is at http://www.kidney.org
RenalNet is a Kidney Information Clearinghouse at http://www.renalnet.org/
World Kidney Fund is at http://www.worldkidneyfund.org/
TECHNOLOGY RESOURCES [BOLD = RECOMMENDED]
ABGAME (JB), Win
ABASE: A Program for Teaching Acid Base Regulation (C-BE), Mac
A.D.A.M. Interactive Physiology: Fluids & Electrolytes/Acids & Bases (CYBER) (NIMCO), Mac, Win
CD, 1997
A.D.A.M. Interactive Physiology: Urinary System (CYBER) (NIMCO), Mac, Win CD, 1997
Anatomy/Physiology: The Urinary System (NIMCO), 28 min. video [slide series on video]
Anatomy/Physiology: The Metabolism: Fluid & Electrolyte Balance (NIMCO), 28 min. video [slide
series on video]
Body Language: Urinary System (PLP), Win, Mac
Comprehensive Review in Biology: Digestion and Excretion (Q), Mac, Win
Excretory System (IM) 19 min. video
Graphic Human Anatomy and Physiology Tutor: Urinary System (PLP), Win
The Human Body Series: Excretory System (PHO), 14 min. video
Incredible Voyage (CRM), 26 min. video
The Kidney (PLP), Mac
Kidney Functions (AIMS), Mac, Win CD, 5 min. video, laserdisc
The Kidney and Homeostasis (BSCS Classic Inquiry) (MDA), videodisc
The Living Body: Water! [urinary system] (FH), 20 min. video
The Mammalian Kidney (ei), slides
The Mammalian Kidney (Biology: Form and Function) (CPB) (IM), 24 min. video
Osmoregulation (FH), 10 min. video
The Physiology of the Kidney (PH), 7 min. video
Problems in Fluid Compartment Re-Distribution (C-BE), Win
The Renal System [Simulations in Physiology] (NRCLSE), Win, Mac
Respiration and Waste (WARDS), 8 min. video
The Urinary Tract: Water (FH), 26 min. video
The Work of the Kidneys (EBE), 29 min. video
Work of the Kidneys (IM) 23 min. video
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LECTURE ENRICHMENT INCLUDING TOPICS AND PROJECTS UTILIZING
SCIENTIFIC REASONING
16.1
Urinary System
1. Students may recoil at old concepts of using warm urine to melt earwax or shipwreck survivors
surviving on urine. This is a place to emphasize that, in contrast to the digestive system, the urinary
system is a filtered and sterile system unless there is a urinary tract infection. The first human trials
on the antibiotic penicillin, produced in small quantities, involved extracting the penicillin in the
patient’s urine and recycling it. “Smelly-urine” restrooms are the result of bacterial action on urine
waste molecules after the urine has exited the body.
2. Birds must remain lightweight and cannot store large quantities of heavy water. Lecture question:
What is the likely system in birds? Note the variation in toxicity and solubility of ammonia, urea, and
uric acid pastelike wastes.
3. Bears in hibernation still undergo metabolism although at a slower rate. Students may be able to
postulate the problems that must be overcome. The NOVA: All American Bear provides excellent
graphics on how the bear overcomes the problem of metabolic waste buildup in hibernation.
4. The variations in length of the urethra is related to the ease of entry of bacterial infections; students
should be able to speculate on whether males or females are more prone to urinary infections.
5. Students may confuse “ureter” and “urethra” due to their similarity; a phonics distinction is that
“ureters” sounds correct in the plural but “urethras” does not.
6. Textbook and overhead outlines of sagittal sections of the pelvic organs usually illustrate the
substantially smaller space allotted for the female urinary bladder, thus accounting for a very real
difference in mileage between bathroom stops during long-distance trips. Lecture question: What is
likely to happen in “bladder space” after a hysterectomy?
7. Term paper topics: comparative structure and function of flame cells, protonephridia, and Malpighian
tubules; boxing regulations concerning the “kidney punch,” history and rationale.
16.2
Kidney Structure
1. Use models to demonstrate the gross anatomy of the kidney, bladder, etc.
2. On occasion, a person develops four kidneys in the same space as two; however, this means there are
twice as many renal arteries and veins. Ask what the consequence would be in processing time for a
soft drink, for instance.
3. Lecture question: Why was the kidney one of the first organs to be successfully transplanted? It is
encapsulated and has a limited number of connections (artery, vein, ureter).
4. Term paper topics: comparative structure and function of flame cells, protonephridia, and kidney;
glomerulonephritis, prevalence of gout in history, diabetes mellitus.
16.3
Urine Formation
1. The complexity of the nephron processes again makes good visuals a necessity.
2. Have the students keep an approximated record of their fluid intake and urinary output for a 24-hour
period and explain any discrepancies between the two values.
3. Albumin is a large protein molecule in blood plasma; students will relate to it and its “thickness” as a
molecule in simple raw egg white.
4. Lecture question: Why must a person continue to take tablets or capsules for a medication to remain
in the blood at levels to be effective? The rate at which a medication is removed is one of the critical
items in drug research, and is the basis for prescription twice-a-day, once-a-day, etc.
16.4
Maintaining Water-Salt Balance
1. The concept of “countercurrent” is not readily apparent because there are no general visible
experiences that operate as countercurrents in daily life. Therefore, it is necessary to walk through the
two reversed gradients and show the effect with and without the countercurrent process.
2. Lecture question: Tea is a common diuretic; what is its effect on urination?
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3. Term paper topics: juxtaglomerular apparatus, kidney dialysis techniques, dietary limitations for
patients on hemodialysis, countercurrent mechanisms in other organisms and other organ systems.
16.5
Maintaining Acid-Base Balance
1. Many blood pressure medicines act through changing the blood volume and amount of urine
produced. At the end of this lecture, students should be able to deduce why. A diagram showing the
kidneys tapping into a major portion of the systemic circulation may also help—the kidneys intercept
approximately 20% of blood flow.
2. Hemodialysis involving artificial mechanisms is not effective in removing all metabolic waste
molecules. Lecture question: Why do patients on dialysis machines have to adhere to limited diets?
3. Chapter concepts here should now make evident what a physician is looking for with urine tests for
sugar, protein, etc.
ESSAY QUESTIONS WITH ANSWERS
1. How are kidneys related to blood pressure?
Answer: Nephrons regulate the water-salt balance of the blood. The more salts in the blood, the
greater the diffusion of water into the blood and greater blood volume requires the heart to work
harder and causes greater blood pressure.
2. Three steps (or major processes) are required in order to form urine. Briefly describe each of these
processes and give the location where each of these processes takes place in the nephron.
Answer: Pressure filtration takes place between the glomerulus and the Bowman’s capsule of the
nephron. Under the influence of glomerular blood pressure, small molecules move from the
glomerulus to the Bowman’s capsule. The filtered substances include water, salts, nutrients, and
waste molecules, such as urea. Selective reabsorption is another process that takes place in the
proximal convoluted tubule of the nephron and by both passive and active transport processes, water
and other molecules move from the tubule into the surrounding peritubular capillary. Finally, tubular
excretion takes place in the distal convoluted tubule of the nephron. Hydrogen and potassium ions,
creatinine, and others are molecules that are transported from the blood of the peritubular
capillaries into the distal convoluted tubule.
3. List the organs of the urinary system according to the path that urine takes, and give the major
functions of each.
Answer: The kidneys produce the urine that contains nitrogenous end products. The ureters then
convey the urine from the kidneys toward the urinary bladder. The urinary bladder acts as a
reservoir to store the urine. Finally, the urethra transports the urine from the bladder to the outside
of the body.
4. Why does a small change in the percentage of water reabsorbed produce a large change in the volume
of urine produced?
Answer: If 99% of the water is being reabsorbed, the urine constitutes the 1% not reabsorbed. If
reabsorption drops 1% to 98% the urine will constitute 2%, or double the urine production.
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