Chapter 20
Urinary System
20.1 Introduction
1. Explain why the urinary system is necessary for survival. (p. 775)
The urinary system maintains the composition, pH, and volume of body fluids within normal ranges. It also removes metabolic wastes and excretes foreign substances.
2. Identify the organs of the urinary system and list their general functions. (p. 775)
a. Kidneys—remove substances from the blood, form urine, and help regulate various metabolic processes.
b. Ureters—tubes that transport urine away from kidneys to the urinary bladder.
c. Urinary bladder—saclike organ that serves as a urine reservoir.
d. Urethra—tube that transports urine to the outside the body.
20.2 Kidneys
3. Describe the external and internal structure of a kidney. (p. 776)
Externally, a kidney is a reddish brown, bean-shaped organ about 12 centimeters long, 6 centimeters wide, and is enclosed in a smooth, tough, fibrous capsule (tunic fibrosa). The lateral surface of the kidney is convex, while its medial side is concave. The hollow depression (renal sinus) of the concave side allows blood and lymphatic vessels, and the ureter to pass through at the hilum. The ureter opens up inside the sinus to become the renal pelvis. The pelvis then divides into two or three tubes (major calyces [sing. calyx]) that further divide into eight to fourteen minor calyces.
The kidney has two distinct regions, the inner renal medulla and the outer renal cortex. The renal medulla contains conical masses of tissue (renal pyramids) between the renal columns, arranged so that the bases face the convex side of the kidney and the apexes (renal papillae) project into the renal sinus. The papillae have many openings that lead into the minor calyces. The cortex is granular, and surrounds the medulla. Its tissue dips into the medulla between the renal pyramids, forming renal columns. The random arrangement of tiny tubules associated with the nephrons gives the cortex its granular appearance.
4. List the functions of the kidneys. (p. 777)
The functions of the kidneys are:
a. regulate the volume, composition, and the pH of body fluids,
b. remove metabolic wastes from the blood and excrete them,
c. aid control of the rate of red blood cell production by secreting erythropoietin,
d. regulate blood pressure by secreting renin, and
e. regulate calcium ion absorption by activating vitamin D.
5. List in correct order the vessels through which blood passes as it travels from the renal artery to the renal vein. (p. 779)
The route the blood follows is: renal artery to several interlobar arteries, to arciform (arcuate) arteries, to afferent arterioles, to efferent arterioles, to peritubular capillaries, to interlobular veins, to arciform (arcuate) veins, to interlobar veins, to a renal vein.
6. Distinguish between a renal corpuscle and a renal tubule. (p. 781)
A renal corpuscle is a tangled cluster of blood capillaries called a glomerulus. The glomerulus surrounds a glomerular capsule that marks the enlarged, closed end of a renal tubule. The renal tubule contains the fluids secreted by the blood in the renal corpuscle.
7. Name in correct order the structures through which fluid passes from the glomerulus to the collecting duct. (p. 781)
Once the fluid is secreted into the renal tubule, it travels into a highly coiled proximal convoluted tubule.
This tubule turns toward the renal pelvis and straightens to become the descending nephron loop. The tubule then curves back toward the renal corpuscle as the ascending nephron loop. This branch again becomes highly coiled, and is called the distal convoluted tubule. Several of these tubules converge in the renal cortex to form a collecting duct that joins with others to eventually empty into a minor calyx.
8. Describe the location and structure of the juxtaglomerular apparatus. (p. 782)
A juxtaglomerular apparatus is located at the point were a distal convoluted tubule passes between, and contacts, the afferent and efferent arterioles. At this point, the tubule’s epithelial cells narrow and pack tightly together to form the macula densa. Nearby, the wall of the afferent arteriole contains large, vascular smooth muscle cells called juxtaglomerular cells. The macula densa and the juxtaglomerular cells, together, make a juxtaglomerular apparatus.
9. Distinguish between cortical and juxtamedullary nephrons. (p. 784)
Nephrons located in the renal cortex are called cortical nephrons. The nephrons located close to the renal medulla have nephron loops extending into the medulla, and are thus called juxtamedullary nephrons.
20.3 Urine Formation
10. Distinguish among filtration, tubular reabsorption, and tubular secretion as they relate to urine formation. (p. 785)
Filtration moves chemicals from the glomerulus into the renal tubules by pressure. Reabsorption moves some of these chemicals back into the blood. Secretion moves other chemicals from the peritubular capillaries into renal tubules by various mechanisms.
11. Which of the following is abundant in blood plasma, but present only in small amounts in glomerular filtrate? (p. 788)
d. protein.
12. Define filtration pressure. (p. 788)
Filtration pressure is the net pressure that causes substances to move from the capillaries into the renal tubules. The calculation takes into account glomerular hydrostatic pressure (blood pressure inside the glomerulus), capsular hydrostatic pressure (fluid pressure inside the glomerular capsule), and the glomerular osmotic pressure (the amount of force the substances have in the blood to keep water inside).
13. Explain how the diameters of the afferent and efferent arterioles affect the rate of glomerular filtration. (p. 788)
The rate of glomerular filtration is directly proportional to the filtration pressure. So, if the afferent arteriole constricts, blood flow decreases, and the filtration pressure drops. On the other hand, if the efferent arteriole constricts, the resulting back pressure will cause an increase in filtration pressure.
14. Explain how changes in the osmotic pressure of the blood plasma affect the glomerular filtration rate. (p. 789)
Osmotic pressure is the amount of attraction of substances to water. When hydrostatic pressure forces water out, the remaining blood proteins exert more force to pull water back in. When the osmotic pressure is high enough, filtration stops. If blood protein concentration is low, filtration increases. If the blood flow is rapid, osmotic pressure stays low and filtration rates remain high. If the blood flow is slow, the osmotic pressure changes because the blood has more time to be filtered.
15. Explain how the hydrostatic pressure of a glomerular capsule affects the rate of glomerular filtration. (p. 789)
The hydrostatic pressure in the glomerular capsule opposes the hydrostatic pressure in the glomerulus. If the glomerular capsule pressure is too great, as with an obstruction, filtration will decrease.
16. Define autoregulation. (p. 790)
Filtration usually, and continuously, occurs simply by the physical properties of osmosis and hydrostatic pressure. This process is called autoregulation.
17. Describe the two mechanisms by which the body regulates glomerular filtration rate. (p. 790)
Glomerular filtration rate (GFR) can be controlled by the renin-angiotensin system, which controls sodium excretion. Renal baroreceptors sense changes in blood pressure and act with the sympathetic nervous system to cause the juxtaglomerular cells to secrete renin when the pressure drops. The macula densa sense concentrations of sodium, potassium, and chloride ions and cause renin secretion if any of these drop too low. The atrial natriuretic hormone (ANH) can also control GFR. When the atria stretch because of excessive pressure, this process is called autoregulation.
18. Discuss how tubular reabsorption is selective. (p. 791)
Tubular reabsorption causes the composition of the filtrate to change before it is excreted as urine. For instance, glucose is present in the filtrate, but is absent in the urine. Urea and uric acid are considerably more concentrated in the urine than they are in the glomerular filtrate. This is accomplished through the epithelium of the renal tubule.
19. Explain how the peritubular capillary is adapted for tubular reabsorption. (p. 791)
Because the efferent arteriole is narrower than the peritubular capillary, the pressure of the blood in the capillary is relatively low. Also, the walls of the capillary are more permeable than those of other capillaries. These factors enhance the rate of fluid reabsorption.
20. Explain how epithelial cells of the proximal convoluted tubule are adapted for tubular reabsorption. (p. 791)
The epithelial cells of the proximal convoluted tubule contain microvilli that greatly increase their surface area. For this reason, most tubular reabsorption occurs here.
21. Explain why active transport mechanisms have limited transport capacities. (p. 791)
Active transport is a highly selective mechanism. The carrier molecules can only transport certain amounts of very specific passenger molecules. For this reason, the active transport mechanism has a limited transport capacity.
22. Define renal plasma threshold and explain its significance in tubular reabsorption. (p. 792)
Under normal conditions there are sufficient carrier molecules to transport glucose out of the filtrate. If the glucose concentration reaches a point where active transport can no longer handle it, it is said to have reached its renal plasma threshold. Because no more glucose can be carried out, the excess will remain in the urine. This is significant because glucose draws water into the renal tubule by osmosis and results in increased urine volume.
23. Explain how amino acids and proteins are reabsorbed. (p. 792)
Amino acids are thought to be reabsorbed by using three different active transport systems in the proximal convoluted tubules. Each active transport system reabsorbs a different group of amino acids with a similar molecular structure. The filtrate is usually free of proteins except albumin. Albumin has a relatively small structure and is reabsorbed by pinocytosis in the microvilli of the proximal convoluted tubules. From there, it is converted by enzymatic actions into amino acids and moved back into the blood of the peritubular capillaries.
24. Describe the effect of sodium reabsorption on the reabsorption of negatively charged ions. (p. 792)
Because sodium is a positively charged ion, its reabsorption causes negatively charged ions to follow it out of the filtrate. The sodium is reabsorbed by active transport in the proximal area of the renal tubule. The negatively charged ions of chloride, phosphate, and bicarbonate follow through in passive transport systems.
25. Explain how sodium ion reabsorption affects water reabsorption. (p. 792)
As sodium and its negative ions are reabsorbed, the solute concentration of the peritubular capillaries increases. This causes water to move into the peritubular capillary by osmosis in order to equalize the solute concentrations on both sides, and this reduces the fluid volume within the renal tubule.
26. Explain how the renal tubule is adapted to secrete hydrogen ions. (p. 793)
Secretory mechanisms are active transport mechanisms that work in the opposite direction. The epithelium of the proximal convoluted segment actively secretes hydrogen ions along with other organic compounds.
27. Explain how potassium ions may be secreted passively. (p. 794)
The active reabsorption of sodium ions out of the tubular fluid produces a negative charge within the tube.
Because of this, in the distal portions of the proximal convoluted tubule and the collecting duct, potassium ions (which are positively charged) move from the tubular epithelium and into the tubular fluid by electrical attraction.
28. Explain how hypotonic fluid is produced in the ascending limb of the nephron loop. (p. 795)
The ascending limb of the nephron loop is impermeable to water but diffuses electrolytes by transport mechanisms. This causes its fluid to be hypotonic with respect to the interstitial fluid surrounding it.
29. Explain why fluid in the descending limb of the nephron loop is hypertonic. (p. 795)
Because the descending limb of the nephron loop is very permeable to water yet almost impermeable to solutes, its fluid is hypertonic with respect to the interstitial fluid surrounding it.
30. The major action of ADH on the kidneys is to (p. 796)
c. increase water reabsorption by the collecting duct.
31. Explain how urine may become concentrated as it moves through the collecting duct. (p. 796)
In response to certain metabolic conditions such as decreasing blood or fluid volume, the posterior lobe of the pituitary gland releases ADH (antidiuretic hormone). In the kidney, ADH causes increased permeability of the epithelial linings of the distal convoluted tubule and collecting duct. This moves water rapidly out of these segments. Thus, the urine becomes more concentrated because water is conserved by the body.
32. Compare the processes by which urea and uric acid are absorbed. (p. 796)
About fifty percent of urea is reabsorbed by passive diffusion in the renal tubule. The rest is secreted in the urine. Uric acid is reabsorbed by active transport. Although all of the uric acid is reabsorbed, about ten percent is secreted in the urine. This seems to be a result of the acid being secreted back into the renal tubule.
33. List the common constituents of urine and their sources. (p. 798)
a. Water—excess intake and cellular metabolism.
b. Urea—a by-product of amino acid catabolism.
c. Uric acid—a by-product of organic base and nucleic acid metabolism.
d. Amino acids—protein synthesis or breakdown.
e. Electrolytes—various body processes.
34. List some of the factors that affect the daily urine volume. (p. 798)
These factors include: amount of fluid intake, environmental temperature, relative humidity of surrounding air, the person’s emotional condition, respiratory rate, and body temperature.
20.4 Elimination of Urine
35. Describe the structure and function of a ureter. (p. 798)
A ureter is a tubular organ, approximately 25 centimeters long. It begins as the funnel-shaped renal pelvis, extends downward behind the parietal peritoneum and parallel to the vertebral column. In the pelvic cavity, it moves medially and anteriorly, to join the urinary bladder from underneath. The wall of the ureter has three layers:
a. Mucous coat—the inner layer, it is continuous with the linings of the renal tubules and the urinary bladder.
b. Muscular coat—the middle layer consisting largely of smooth muscle.
c. Fibrous coat—the outer layer, it consists of connective tissue.
The ureter functions to pass urine from the kidneys into the urinary bladder.
36. Explain how the muscular wall of the ureter helps move urine. (p. 798)
The smooth muscle layer produces peristaltic waves originating from the renal pelvis and forcing urine along the ureter into the urinary bladder.
37. Describe what happens if a ureter becomes obstructed. (p. 799)
If the ureter becomes obstructed, peristaltic waves strengthen at the proximal end of the tube helping to push the obstruction into the bladder. Concurrently, the ureterorenal reflex constricts the renal arterioles and decreases urine production in that kidney.
38. Describe the structure and location of the urinary bladder. (p. 799)
The urinary bladder is a hollow, distensible, muscular organ located within the pelvic cavity, just posterior to the symphysis pubis, and inferior to the parietal peritoneum. It is somewhat spherical in shape and when empty, contains many folds. Internally, the floor of the bladder contains a triangular area called the trigone that has openings at each of the three points. At the base of the trigone, are the openings for the ureters, and anteriorly, the bladder has a short, funnel shaped extension called the neck of the bladder that contains the opening to the urethra. The bladder wall consists of four layers:
a. Mucous coat—the innermost layer, it contains several layers of transitional epithelial cells.
b. Submucous coat—the second layer, it consists of connective tissue and elastic fibers.
c. Muscular coat—this layer contains coarse bundles of smooth muscle fiber. Together these muscles comprise the detrusor muscle.
d. Serous coat—the outermost layer, it is synonymous with the parietal peritoneum. This layer occurs only on the upper surface of the bladder. Elsewhere, the outer coat is comprised of fibrous connective tissue.
39. Define detrusor muscle. (p. 800)
The detrusor muscle is the muscle layer of the bladder. Part of this muscle surrounds the neck of the bladder and forms the internal urethral sphincter. The sustained contraction of this muscle prevents urine from passing into the urethra until the pressure within the bladder reaches a certain level.
40. Distinguish between the internal and external urethral sphincters. (p. 800)
The internal urethral sphincter is composed of smooth muscle that surrounds the neck of the bladder. It is controlled involuntarily. The external urethral sphincter is part of the urogenital diaphragm and is located about three centimeters below the bladder and surrounding the urethra. It is composed of skeletal muscle tissue and is therefore controlled voluntarily.
41. Compare the urethra of a female with that of a male. (p. 800)
Structurally, urethras in both males and females are internally lined with a mucous membrane containing numerous mucous glands (urethral glands) and are surrounded by a thick layer of longitudinal smooth muscle fibers. The female urethra is about 4 cm long and travels forward from the bladder, below the symphysis pubis, and empties from the external urethra orifice (urinary meatus)—anterior to the vaginal opening and 2.5 cm posterior to the clitoris. In the male, the urethra functions as a urinary canal and a passageway for secretions and cells of the reproductive organs. It contains three sections:
a. Prostatic urethra—about 2.5 cm long, it passes from the bladder through the prostate gland. Ducts from the reproductive structures empty here.
b. Membranous urethra—about 2 cm long, it begins just distal to the prostate gland passing through the urogenital diaphragm. It is surrounded by fibers of the external urethral sphincter muscle.
c. Penile urethra—about 15 cm long, it passes through the corpus spongiosum of the penis and ends at the tip of the penis with the external urethral orifice.
42. Describe the micturition reflex. (p. 802)
The micturition reflex (urination) involves the contraction of the detrusor muscle and relaxation of the external urethral sphincter. When the bladder is sufficiently distended with urine, stretch receptors in the wall of the bladder signal the micturition reflex center in the spinal cord to send a motor impulse along the parasympathetic nerves to the detrusor muscle.
43. Which movement involves skeletal muscle? (p. 802)
b. contraction of the external urethral sphincter.
20.5 Life-Span Changes
44. Describe changes in the urinary system with age. (p. 803)
The kidneys are slower to remove nitrogenous wastes and toxins and to compensate for changes to maintain homeostasis.
From the outside, the kidneys change with age, appearing scarred and grainy as arterioles serving the cortex constrict, and fibrous connective tissue accumulates around the capsules. On the inside, kidney cells begin to die as early as age 20 years, but the gradual shrinkage is not generally noticeable until after age 40. By 80 years, the kidneys have lost about a third of their mass.
Further along the nephron, the renal tubules thicken, accumulating coats of fat.
The bladder, ureters, and urethra change with the years too. These muscular organs lose elasticity and recoil with age, so that in the later years, the bladder holds less than half of what it did in young adulthood, and may retain more urine after urination. In the elderly, the urge to urinate may become delayed, so that when it does happen, it is sudden. Older individuals have to urinate at night more than younger people.
Shier, Butler, and Lewis: Hole’s Human Anatomy and Physiology, 12th ed.
A. The organs of the urinary system are kidneys, ureters, urinary bladder, and urethra.
B. The functions of the kidneys are to remove substances from blood, form urine, and to regulate certain metabolic processes.
C. The function of the ureter is to carry urine from the kidneys to the bladder.
D. The function of the bladder is to store urine.
E. The function of the urethra is to convey urine from the bladder to the outside.
A. Introduction
1. A kidney is reddish brown in color and bean shaped.
2. A kidney is enclosed by a tough, fibrous capsule.
B. Location of Kidneys
1. The kidneys are located on either side of the vertebral column in a depression high on the posterior wall of the abdominal cavity. They are positioned about the level of the 1st three lumbar vertebrae.
2. Retroperitoneally means behind the parietal peritoneum and against the deep muscles of the back.
C. Kidney Structure
1. The renal sinus is a hollow chamber on the medial side of each kidney.
2. The renal pelvis is the expansion of the ureters in the kidney.
3. The renal pelvis is divided into major calyces.
4. Major calyces are divided into minor calyces.
5. Renal papillae are small projections that extend into each minor calyx.
6. The renal medulla is an inner region composed of conical masses called renal pyramids.
7. Renal pyramids are divisions of the renal medulla.
8. The renal cortex is the outer region of a kidney.
9. Renal columns are cortical tissues between renal pyramids.
10. The renal capsule is a fibrous membrane surrounding a kidney.
D. Functions of the Kidneys
1. The main functions of the kidneys are to regulate the volume, composition, and pH of body fluids and to remove metabolic wastes from the blood and excrete them to the outside.
2. Erythropoietin functions to regulate the production of red blood cells.
3. Renin regulates blood pressure.
4. Hemodialysis is an artificial means of removing substances from blood that would normally be excreted in urine.
E. Renal Blood Vessels
1. Renal arteries arise from the abdominal aorta.
2. At rest, the renal arteries contain 15% to 30% of the total cardiac output.
3. Renal arteries branch into interlobar arteries, which pass between renal pyramids.
4. Interlobar arteries branch into arcuate arteries.
5. Arcuate arteries branch into interlobular arteries.
6. Interlobular arteries branch into afferent arterioles.
7. Afferent arterioles lead to the nephrons.
8. Venous blood of the kidneys is returned through a series of vessels that generally correspond to the arterial pathways.
9. Renal veins join the inferior vena cava.
F. Nephrons
1. Structure of a Nephron
a. Functional units of the kidneys are called nephrons.
b. Each nephron consists of a renal corpuscle and renal tubules.
c. A renal corpuscle consists of a glomerulus and glomerular capsule.
d. A glomerulus is cluster of capillaries.
e. A glomerular capsule is a thin walled, saclike structure that surrounds a glomerulus.
f. Afferent arterioles give rise to glomeruli, which lead to efferent arterioles.
g. The two layers of the glomerular capsule are a visceral layer and a parietal layer.
h. Podocytes are located in the visceral layer.
i. Slit pores are cleft between podocytes.
j. The renal tubule leads away from the glomerular capsule.
k. The parts of the renal tubule are proximal convoluted tubule, nephron loop, and distal convoluted tubules.
l. Distal convoluted tubules merge together to form a collecting duct, which empties into a minor calyx.
2. Juxtaglomerular Apparatus
a. The macula densa is comprised of epithelial cells of the distal convoluted tubule that contact and are between the afferent and efferent arterioles.
b. The juxtaglomerular cells are vascular smooth muscle cells in the walls of an afferent arteriole near its attachment to a glomerulus.
c. The juxtaglomerular apparatus is composed of juxtaglomerular cells and macula densa cells.
d. The juxtaglomerular apparatus is important in regulating the secretion of renin.
3. Cortical and Juxtamedullary Nephrons
a. Cortical nephrons have relatively short nephron loops that do not reach the renal medulla.
b. Juxtamedullary nephrons have corpuscles that extend deep into the medulla.
c. The juxtamedullary nephrons are important in regulating water balance.
4. Blood Supply of a Nephron
1. Blood enters a glomerulus through an afferent arteriole.
2. Blood leaves a glomerulus through an efferent arteriole.
3. An efferent arteriole delivers blood to the peritubular capillary system.
4. A peritubular capillary system is located around the renal tubule.
5. Vasa recta are capillary loops that are closely associated with juxtamedullary nephrons.
6. Blood leaves the peritubular capillary system through the venous system of the kidney.
A. Introduction
1. The main function of the nephrons is to control the composition of body fluids and remove wastes from the blood.
2. Urine is the product produced by kidneys and contains wastes, excess water, and electrolytes.
3. The three processes involved in urine formation are glomerular filtration, tubular reabsorption, and tubular secretion.
4. In glomerular filtration, blood plasma is filtered.
5. The function of tubular reabsorption is to return most of the products filtered from plasma back to the blood.
6. The function of tubular secretion is to put waste products into the filtrate to be excreted from the kidney.
B. Glomerular Filtration
1. Glomerular filtration is the process in which water and other small dissolved molecules and ions are filtered out of the glomerular capillary plasma and into the glomerular capsule.
2. Glomerular filtrate is the fluid in the glomerular capsule.
3. The normal composition of glomerular filtrate is mostly water and the same solutes as in blood plasma, except for the larger protein molecules.
C. Filtration Pressure
1. The main force that moves substances through the glomerular capillary wall is the hydrostatic pressure of the blood inside.
2. Glomerular filtration is also influenced by the osmotic pressure of the blood plasma in the glomerulus and the hydrostatic pressure inside the glomerular capsule.
3. Net filtration pressure is the net effect of all forces that influence glomerular filtration and normally favors filtration at the glomerulus.
4. Net filtration can be calculated by subtracting forces opposing filtration from forces favoring filtration.
D. Filtration Rate
1. The glomerular filtration rate is directly proportional to the net filtration pressure.
2. The factors that affect glomerular filtration are glomerular hydrostatic pressure, glomerular plasma osmotic pressure, or hydrostatic pressure in the glomerular capsule.
3. Normally the most important factor affecting net filtration pressure and GFR is glomerular hydrostatic pressure.
4. If the afferent arteriole constricts, net filtration pressure decreases and the filtration rate drops.
5. If the efferent arteriole constricts, net filtration pressure increases and the filtration rate rises.
6. Factors that can change the hydrostatic pressure in the glomerular capsule are obstructions in the glomerular capsule.
7. If hydrostatic pressure in the glomerular capsule becomes too high, net filtration pressure will decrease.
E. Control of Filtration Rate
1. GFR may increase when body fluids are in excess and decrease when the body must conserve fluid.
2. If blood pressure and volume drop, vasoconstriction of the afferent arterioles results, which leads to a decrease in filtration pressure and GFR.
3. If excess body fluids are detected, vasodilation of the afferent arterioles results, which leads to an increase in filtration pressure and GFR.
4. Renin is secreted by the juxtaglomerular cells in response to stimulation from sympathetic nerves and pressure-sensitive cells.
5. Renal baroreceptors detect pressure.
6. In the bloodstream, renin reacts with angiotensinogen to form angiotensin I.
7. Angiotensin I is used to make angiotensin II.
8. The effects of angiotensin II are vasoconstriction, increased aldosterone secretion, increased ADH secretion, and increased thirst.
9. The functions of ANP are to stimulate sodium excretion through a number of mechanisms, including increasing GFR.
F. Tubular Reabsorption
1. Introduction
a. Tubular reabsorption is the process by which substances are transported out of the tubular fluid, through the epithelium of the renal tubule, and into the interstitial fluid and then into the peritubular capillaries.
b. Tubular reabsorption returns substances to the internal environment.
c. In tubular reabsorption, substances must first cross the cell membrane facing the inside of the tubule and then the cell membrane facing the interstitial fluid.
d. Active tubular reabsorption requires ATP.
e. The factors that enhance the rate of fluid reabsorption from the renal tubule are the low pressure in the peritubular capillary, the increased permeability of the peritubular capillary wall, and the colloid osmotic pressure of the peritubular capillary plasma.
f. Tubular reabsorption occurs throughout the renal tubules but most of it is in the proximal convoluted portion.
g. Microvilli in the proximal convoluted tubule function to greatly increase the surface area exposed to the glomerular filtrate and enhance reabsorption.
h. Segments of the renal tubule are adapted to reabsorb specific substances, using particular modes of transport.
i. Usually all of the glucose in glomerular filtrate is reabsorbed because there are enough carrier molecules to transport it.
j. The renal plasma threshold is when the plasma concentration of a substance increases to a critical level in which more substances are in the filtrate than the active transport mechanisms can handle.
k. Glucose is excreted in urine when its concentration exceeds the renal plasma threshold.
l. Diuresis is any increase in urine volume.
m. Osmotic diuresis is when non-reabsorbed glucose in the tubular fluid increases the osmotic concentration of the tubular fluid reducing the amount of water reabsorbed by osmosis from the proximal tubule, thus increasing urine volume.
n. Examples of substances that are reabsorbed through renal tubules are glucose, amino acids, small proteins, creatine, lactic acid, citric acid, uric acid, ascorbic acid, and many ions.
2. Sodium and Water Reabsorption
a. Water reabsorption occurs by osmosis and is closely associated with the active reabsorption of sodium ions.
b. If sodium reabsorption increases, water reabsorption increases.
c. Much of the sodium reabsorption occurs in the proximal segment of the renal tubule by active transport.
d. When sodium ions move through the tubular wall, negatively charged ions move with them.
e. About 70% of water and sodium may be reabsorbed before urine is excreted.
f. Two hormones that affect sodium and water reabsorption are ADH and aldosterone.
G. Tubular Secretion
1. In tubular secretion, substances move from the plasma of the peritubular capillary into the fluid of the renal tubule.
2. Examples of substances that are secreted into renal tubules are drugs, histamine, ammonia, and various ions.
3. To summarize, urine forms as a result of glomerular filtration of materials from blood plasma, reabsorption of substances, and secretion of substances.
H. Regulation of Urine Concentration and Volume
1. Aldosterone and ANP affect the solute concentration of urine.
2. The cells lining the later portion of the distal convoluted tubule and the collecting ducts are impermeable to water unless ADH is present.
3. A countercurrent mechanism ensures that the medullary interstitial fluid becomes hypertonic.
4. Chloride ions are reabsorbed in the in the ascending limb and sodium ions follow the chloride ions.
5. Tubular fluid in the ascending limb becomes hypotonic as it loses solutes.
6. Water leaves the descending limb by osmosis and NaCl enters the descending limb by diffusion.
7. Tubular fluid in the descending limb becomes hypertonic as it loses water and gains NaCl.
8. As NaCl repeats the circuit, its concentration in the medulla increases.
9. The vasa recta countercurrent mechanism helps maintain the NaCl concentration in the medulla.
I. Urea and Uric Acid Excretion
1. Urea is a by-product of amino acid catabolism in the liver.
2. Urea enters the renal tubule through filtration.
3. Up to 80% of urea is recycled.
4. Uric acid is a product of the metabolism of certain nucleic acid bases.
5. Uric acid is completely reabsorbed.
6. About 10% of the reabsorbed uric acid ends up in urine because it is secreted into the renal tubule.
J. Urine Composition
1. Urine is normally composed of water, urea, uric acid, creatinine, trace amounts of amino acids, and various electrolytes.
2. Factors that change urine composition are fluid intake, environmental temperature, relative humidity of surrounding air, a person’s emotional condition, respiratory rate, and body temperature.
K. Renal Clearance
1. Renal clearance is the rate at which a particular substance is removed from the plasma.
2. The inulin clearance test is used to calculate the rate of glomerular filtration.
3. The creatinine clearance test is used to calculate GRF and to determine amount of renal failure.
4. The para-aminohippuric acid test is used to calculate the rate of plasma flow through the kidneys.
A. Introduction
1. After forming in the nephrons, urine passes from the collecting ducts through openings in renal papillae and enters the calyces of the kidney.
2. From the renal calyces, urine passes through the renal pelvis into a ureter, and into the urinary bladder.
B. Ureters
1. Ureters are located posterior to the parietal peritoneum and parallel to the vertebral column. In the pelvic cavity, they course forward and medially to join the bladder.
2. The three layers of the wall of a ureter are an inner mucous coat, a middle muscular coat, and an outer fibrous coat.
3. Urine is moved through ureters by peristaltic waves.
4. A renal calculus is a kidney stone.
5. The effects of ureter obstruction are to stimulate constriction of the renal arterioles and to reduce urine output.
C. Urinary Bladder
1. The urinary bladder is located within the pelvic cavity, posterior to the symphysis pubis, and inferior to the parietal peritoneum.
2. The trigone of the bladder consists of the opening of the urethra and the two openings of the ureters.
3. The neck of the bladder is a funnel shaped extension of the bladder that contains the opening into the urethra.
4. The four layers of the wall of the bladder are an inner mucous coat, a submucous coat, a muscular coat, and an outer serous coat.
5. The mucous coat is composed of transitional epithelial cells.
6. The submucosa consists of connective tissue and elastic fibers.
7. The muscular coat is composed of smooth muscle fibers.
8. The detrusor muscle is the collection of smooth muscle fibers in the wall of the urinary bladder.
9. The internal urethral sphincter is located in the neck of the bladder and functions to prevent the bladder from emptying until the pressure within the bladder increases to a certain level.
10. The serous coat is composed of the parietal peritoneum.
D. Urethra
1. The urethra conveys urine from the bladder to the outside.
2. Urethral glands are located in the urethral wall and function to secrete mucus into the urethral canal.
3. The external urethral sphincter is located as part of the urogenital diaphragm and functions to voluntarily control urination.
4. The three parts of the male urethra are prostatic, membranous, and penile.
E. Micturition
1. Micturition is urination reflex.
2. The muscles that contract during micturition are the destrusor muscle, abdominal wall muscles, pelvic floor muscles, and the diaphragm.
3. The micturition reflex center is located in the sacral portion of the spinal cord.
4. The urgency to urinate occurs when the bladder wall distends as it fills with urine.
5. Micturition is usually under voluntary control because the external urethral sphincter is under voluntary control.
A. With age, changes of the kidneys include shrinkage, scarring, loss of glomeruli, and a decreased ability to remove nitrogenous wastes and toxins.
B. Changes of the nephron include thickening of renal tubules, shortening of renal tubules, and a decreased ability to clear drugs and other substances from the blood.
C. Changes of the bladder, ureters, and urethra include loss of elasticity.
D. Common reasons for incontinence are loss of muscle tone in the bladder, urethra, and ureters; and the atrophy of bladder sphincters. In males, an enlarged prostate may lead to incontinence.
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