Chapter 14 Questions (Kidneys)
How does the composition of the glomerular filtrate compare with that of plasma?
Glomerular filtrate normally contains all plasma substances in virtually the same concentration as in plasma except for plasma proteins and red blood cells. The vast majority of plasma proteins are normally excluded from the filtrate.
All of the following substances are present in proximal tubular fluid in the kidney, but which one is NOT normally present in urine?
Glucose
where do Na/K+ ATPase pumps play an active role in reabsorbing Na+?
in the basolateral membrane of cells of the cortical collecting duct
After prolonged exertion in a hot climate, baroreceptors would ____firing, leading to _____ secretion of ______& thus _______renal re absorption of ______.
increase, increased; renin, increased; Na+
which stimulates vasopressin secretion?
increased plasma osmolarity
which of the following would be likely to increase GFR?
increasing efferent arteriolar vasoconstriction
Which correctly describes the composition of the glomerular filtrate?
it is highly similar to plasma, except it does not contain plasma proteins
Describe the distribution of water and Na+ between intracellular and extracellular fluids.
Na+ is the major extracellular solute. The concentration of sodium in intracellular fluid is very low. The concentration of water inside and outside the cells is the same, but the volume of intracellular fluid is twice that of the extracellular fluid.
Contrast the control of calcium and phosphate ion excretion by PTH.
PTH acts in kidneys to increase the reabsorption of calcium ions, and thus decrease excretion of calcium ions. In contrast, PTH decreases the reabsorption of phosphate ions and thus increases phosphate ion excretion.
about 2/3 of the reabsorption of sodium and chloride takes place in the structure of the nephron known as the
Proximal convoluted tubule or ascending limg
What is the effect of vasopressin on the renal tubules, and what are the sites affected?
Vasopressin stimulates the insertion, by exocytosis, of a particular group of aquaporin water channels (AQP2) into the luminal membrane. Vasopressin has this action on the cells of the cortical and medullary collecting ducts, and its effect is to increase the permeability of the collecting ducts to water, allowing for large amounts of water reabsorption from them.
Diagram the sequence of events leading to micturition. What are the sources of water gain and loss in the body? What are the sources of Na+ gain and loss?
Water is gained from ingestion of liquids and "solid" food, and is also produced by metabolism. Water is lost in urine, feces, sweat, and insensible loss through the skin and lungs, and, in menstruating women, in menstrual blood. Na+ is gained from ingestion of food. It is lost in urine, feces, and sweat.
a decrease in the total body content of sodium causes
a concurrent decrease in plasma volume, which induces a decrease in GFR and sodium reabsorption
Constriction of the ____decreases hydrostatic pressure in ____.
afferent arterioles, glomerular capillaries
The conoutercurrent multiplier system of the kidney
allows the kidneys to form hypertonic urine requires that the colecting ducts be near the loops of henle requires active transport of sodium and chloride out of the ascending limb of loop of Henle Would not function if the ascending limb of the loop of Henle were freely permeable to water
following ingestion of a large amount of salt
both urine osmolarity would increase and secretion of atrial natriuretic factor would occur
What three hormones/factors do the kidneys secrete into the blood?
erythropoietin, a hormone that controls erythrocyte production; renin, an enzyme that controls formation of angiotensin II, which in turn influences blood pressure and sodium balance; and 1,25-dihydroxyvitamin D, which influences calcium balance.
THe amount of a substance that is excerted in the urine is equal to the amount that is ___ + the amount that is ____- the amount that is ____.
filtered+secreted-reabsorbed
with the condition known as diabetes mellitus
plasma glucose concentration is always less than the transport maximum by the nephrons
which of the following is least likely to be filtered into Bowman's capsule in a normal healthy person?
plasma protein
which of the following most accurately describes the renal transport of sodium ion?
primary active transport of sodium ion allows for secondary active transport of glucose and amino acids in the proximal tubule
which is NOT a function of the kidneys in maintaining homeostasis a) regulation of extracellular fluid osmolarity b) regulation of blood hydrogen ion concentration c) regulation of blood glucose concentratoin
regulation of blood glucose concentration
List several diseases that diuretics can be used to treat.
Diuretics are used to treat diseases characterized by abnormal renal retention of salt and water such as congestive heart failure and hypertension.
What are the three inputs controlling renin secretion?
(1) The renal sympathetic nerves directly innervate the juxtaglomerular cells, and an increase in the activity of these nerves stimulates renin secretion by these cells. (2) The juxtaglomerular cells are located in the walls of the afferent arterioles; they are themselves sensitive to the pressure within these arterioles, and so they act as intrarenal baroreceptors. When blood pressure in the kidney decreases, as occurs when plasma volume is reduced, these baroreceptor cells are stretched less and, therefore, secrete more renin. (3) The macula densa is a layer of specialized epithelial cells located near the ends of the ascending loops of Henle, closely juxtaposed to the afferent arteriole and the juxtaglomerular cells. The macula densa is a chemoreceptor—it senses the Na+ and/or Cl- concentration in the tubule flowing past it. A decreased concentration of Na+/Cl- causes the macula densa to stimulate the juxtaglomerular cells to secrete renin.
What are the three basic renal processes that lead to the formation of urine?
(1) glomerular filtration (filtration from the glomerulus into Bowman's space) (2) tubular reabsorption (reabsorption of the tubular fluid from the tubular lumen into the peritubular capillaries) (3) tubular secretion (secretion from the peritubular plasma or the tubular cells themselves into the tubular lumen)
What are the functions of the kidneys?
(1) regulate water, inorganic ion, and acid-base balance; (2) remove metabolic waste products from the blood and excrete them in the urine;. (3) remove foreign chemicals from the blood and excrete them in the urine;. (4) perform gluconeogenesis; and (5) secrete hormones.
List the different types of diuretics and briefly summarize their mechanisms of action.
1) Loop diuretics act on the ascending limb of the loop of Henle to inhibit the transport protein that mediates the first step in Na+ reabsorption across the luminal membrane in this segment. (2) K+-sparing diuretics either block the action of aldosterone, which inhibits Na+ reabsorption in the cortical collecting duct while simultaneously inhibiting K+ secretion there, or they block the epithelial Na+ channels in the cortical collecting duct, which blocks Na+ reabsorption. (3) Osmotic diuretics are filtered but not reabsorbed, thus they help retain water in the urine.
List the body's major buffer systems.
The major extracellular buffering system is the CO2/HCO3- system, and the major intracellular buffers are proteins and phosphates.
Describe the characteristics of the two limbs of the loop of Henle with regard to their transport of Na+, Cl-, and water.
Along the entire length of the ascending limb of the loop of Henle, Na+ and Cl- are reabsorbed into the medullary interstitial fluid. In the upper (thick) portion of the ascending limb, this reabsorption is achieved by transporters that actively cotransport Na+, Cl-, and K+. Such transporters are not present in the lower (thin) portion of the ascending limb, and the reabsorption there is a passive process. The ascending limb is relatively impermeable to water, and so little water follows the salt. The net result is that the interstitial fluid of the medulla becomes hyperosmotic compared to the fluid in the ascending limb. The descending limb, in contrast, does not reabsorb sodium chloride and is highly permeable to water. Therefore, there is a net diffusion of water out of the descending limb into the more concentrated interstitial fluid until the osmolarities inside this limb and in the interstitial fluid are again equal. The interstitial hyperosmolarity is retained during this equilibration because the ascending limb continues to pump sodium chloride to maintain the concentration difference between it and the interstitial fluid.
Fluid flows in sequence through what structures from the glomerulus to the bladder? Blood flows through what structures from the renal artery to the renal vein?
As blood flows through the glomerulus, a portion of the plasma filters into Bowman's space within Bowman's capsule. To enter the space, this fluid filters through the single-cell-thick capillary endothelium, a proteinaceous layer of basement membrane, and the foot processes of the podocytes that make up the epithelial lining of Bowman's capsule in this region. The renal tubule is continuous with Bowman's capsule. The segment of the tubule that drains Bowman's capsule is the proximal tubule. This leads to the descending limb, then to the ascending limb, of the loop of Henle. The ascending limb feeds into the distal convoluted tubule which leads into the collecting duct system, the first portion of which is the collecting tubule, followed by the cortical collecting duct and then the medullary collecting duct. At the level of the cortical collecting duct, collecting ducts from other nephrons merge together. From the medullary collecting ducts, fluid drains into the renal pelvis, which is continuous with the ureter draining the filtrate from the kidney into the bladder. Blood flows from the renal artery to the afferent arteriole, which supplies blood to the glomerulus. The blood cells, proteins, and the plasma that was not filtered at the glomerulus leave it by way of the efferent arteriole. This supplies another capillary bed, the peritubular capillaries, which surround the renal tubule all along its length. After supplying the tubules with blood, the peritubular capillaries join together to form the renal veins.
Contrast the mechanisms of reabsorption for glucose and urea. Which one shows a Tm?
In a normal person, all of the filtered load of glucose will be reabsorbed in the proximal tubule. Glucose reabsorption occurs by secondary active transport (cotransport) coupled to sodium ion. The glucose transport system has a finite limit, the transport maximum (Tm). In contrast, urea reabsorption in the proximal tubule is by diffusion, secondary to the removal of water as fluid flows through the tubule. As water leaves the tubule, urea becomes more concentrated within it and diffuses down its concentration gradient from the tubular lumen into the peritubular capillaries.
Which of the basic renal processes apply to potassium? Which of them is the controlled process, and which tubular segment performs it?
K+ is filtered, reabsorbed, and secreted. Secretion of K+ is the controlled process, and it occurs in the cortical collecting ducts.
What are the stimuli for thirst?
The most important stimulus for thirst under normal physiological conditions is increased plasma osmolarity. Others include decreased plasma volume, increased angiotensin II, and a dry mouth and throat.
List the sequence of events leading from increased renin secretion to increased aldosterone secretion.
Renin secreted by the juxtaglomerular cells of the kidneys catalyzes the removal of a small peptide, angiotensin I, from the large plasma protein angiotensinogen, which is secreted by the liver and is normally present in plasma in high quantities. Angiotensin I is converted to the active angiotensin II by angiotensin-converting enzyme, which is found in very high concentration on the luminal surface of capillary endothelial cells, particularly those in the lung. In addition to being a potent vasoconstrictor, angiotensin II stimulates the secretion of aldosterone by the adrenal cortex.
What are the sources of gain and loss of H+ in the body?
Sources of H+ gain include: (1) generation of H+ from CO2; (2) production of nonvolatile acids from the metabolism of proteins and other organic molecules; (3) gain of H+ due to loss of HCO3- in diarrhea or other nongastric gastrointestinal fluids; and (4) gain of H+ due to loss of HCO3- in the urine. Sources of H+ loss include: (1) utilization of H+ in the metabolism of various organic anions; (2) loss of H+ in vomitus; (3) loss of H+ in the urine; and (4) hyperventilation.
What is the relationship between body sodium and extracellular fluid volume?
That is, increased Na+ in the extracellular fluid will draw water out of the cells, increasing the extracellular volume. Decreased extracellular sodium would have the opposite effect. Since extracellular volume comprises plasma volume and interstitial volume, plasma volume is also positively related to total-body Na+.
Describe the forces that determine the magnitude of the GFR. What is a normal value of GFR?
The forces that determine glomerular filtration are the glomerular capillary blood pressure (PGC), which favors filtration, and the fluid pressure in Bowman's space (PBS) and the osmotic force due to plasma proteins (GC), which oppose filtration. The glomerular filtration rate (GFR) is determined by the net filtration pressure and also by the permeability of the corpuscular membranes and their surface area. In a 70-kg person, the GFR averages 180 L/day (125 ml/min).
What is the mechanism of water reabsorption, and how is it coupled to Na+ reabsorption?
The reabsorption of water is a passive process that occurs because the active reabsorption of Na+, Cl-, and other molecules coupled to Na+ reabsorption decreases the osmolarity (i.e., raises the water concentration) of the tubular fluid while it simultaneously raises the osmolarity (i.e., lowers the water concentration) in the interstitial fluid. The difference in water concentration between lumen and interstitial fluid causes net diffusion of water from the lumen across the tubular cells' plasma membranes and/or tight junctions into the interstitial fluid. From there, water moves into the peritubular capillaries by bulk flow. (These capillaries have a very low hydrostatic pressure.) Water movement across the tubular epithelium can occur only if the epithelium is permeable to water. Water permeability varies from tubular segment to segment and depends largely on the presence of water channels called aquaporins in the plasma membranes. The water permeability in the proximal tubules is always very high, and so water molecules are reabsorbed by this segment almost as rapidly as Na+. As a result, the proximal tubule always reabsorbs Na+ and water in the same proportions.
What are the two major controls of aldosterone secretion, and what are this hormone's major actions?
The two major controls of aldosterone secretion are plasma angiotensin II concentration and plasma K+ concentration. Aldosterone is a steroid, and acts by inducing the synthesis of proteins in its target cells. Among the target cells for aldosterone are the cells of the cortical collecting ducts. The proteins whose synthesis the hormone stimulates are Na+ and K+ channels in the luminal membrane of cortical collecting-duct cells, and the Na/K-ATPase pumps in the cells' basolateral membrane. Thus, the hormone acts to increase both the reabsorption of sodium in the cortical collecting ducts (and thus to decrease its excretion) and the secretion of potassium in the cortical collecting ducts (and thus to increase its excretion).
What is the mechanism of Na+ reabsorption, and how is the reabsorption of other solutes coupled to it?
Throughout the tubule (with the exception of the descending limb of the loop of Henle), sodium is reabsorbed by primary active transport out of the cells and into the interstitial fluid. This transport is achieved by Na/K-ATPase pumps in the basolateral membrane of the tubule cells. The active transport of sodium out of the cell keeps the intracellular concentration of sodium low compared to the luminal concentration, and so sodium moves "downhill" out of the lumen into the tubular epithelial cells. The precise mechanism of the downhill sodium movement across the luminal membrane into the cell varies from segment to segment of the tubule depending on which channels and/or transport proteins are present in their luminal membranes. In the proximal tubule, the luminal entry step is either by cotransport with a variety of organic molecules, such as glucose and amino acids, or by countertransport with H+. In this manner, in the proximal tubule sodium reabsorption drives the reabsorption of the cotransported substances and the secretion of H+. (Throughout the tubule, Cl-, the major extracellular anion, follows Na+ along its electrochemical gradient).
What two processes determine how much Na+ is excreted per unit time?
the amount of Na+ filtered per unit time (i.e., the GFR) and the amount of Na+ that is reabsorbed
vasopressin/antidiuretic hormone
triggers insertion of aquaporins into the luminal membranes of collecting ducts
a person who is unable to synthesize vasopression
will excrete a hypo tonic urine
