Human Anatomy and Physiology – York College of Pennsylvania

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Human Anatomy and Physiology
Urinary Physiology--Refer to Text Chapter 26 and other chapters as needed
The following are review questions that can be used to review the basic concepts of urinary
physiology. Please do not limit your knowledge to these questions--use your text and your
laboratory studies as well as the interactive physiology in BA209 (this is required). Here are my
"lecture notes." I hope this helps those of you who are still struggling to understand this system.

   I.      Glomerular Filtration

           1. Filtration of the blood occurs through the glomerulus w/in Bowman's capsule.
              Remember, the structure of this filtration membrane is specifically structured for
              filtration (see page 1005).

               What makes the glomerulus (glomerular capillaries) specially structured for
               filtration? (1) large surface area (2) filtration membrane = thin & porous/leaky
               w/fenestrations (3)high glomerular capillary blood hydrostatic pressure

           2. Describe the principles behind glomerular filtration. Blood in the glomerulus is
              under pressure (blood hydrostatic pressure) which forces blood through the
              capillary membrane (filtration pressure).

           3. Compare the relative diameters of the afferent and efferent arterioles and explain
              the significance in this size differential. The afferent arteriole is wider than the
              efferent arteriole which means that blood enters the glomerulus through a wider
              opening than the blood exiting the glomerulus, thus creating an increased "back
              pressure" (=hydrostatic filtration pressure). They hydrostatic pressure is higher in
              the glomerulus than in other capillaries. By varying the size of the afferent and/or
              arterioles, the glomerular filtration rate (GFR) may be increased or decreased.

           4. What percentage of water and solutes of the blood becomes filtrate (primitive
              urine)? 20% of the water in the blood is filtered out to become part of the filtrate.
              This produces 180 liters of water (potential urine) a day. All solutes present in the
              blood (excluding proteins, especially larger proteins such as albumin) including
              ions (such as Na+, Cl-, HCO3-, etc.), glucose, amion acids, creatine, and uric acid
              are also filtered out of the blood to become part of the filtrate (primitive urine).
              Remember that most of these solutes will be completely or at least partially
              reabsorbed later on in through the renal tubules. Most of the water will be
              reabsorbed too--all but about 1-2 liters, which will be excreted as urine. (Study
              normal and abnormal constituents of urine-Table 26.6).

           5. What is NOT filtered out through glomerular filtration due to their large size?
              Proteins, especially larger proteins such as albumin, fibrinogen and globulins.
              RBC's and other large molecules are not filtered out either.
              Thinking about proteins: Glomerularnephritis (inflammation of the glomerulus)
              increases the permeability of the glomerular filtration membrane. Now proteins
              leak through the membrane and are filtered out into the filtrate (primitive urine)
              with no way to be returned to the blood later on. This then increases the capsular
              osmotic pressure (see Fig. 26.9) as well as the osmotic pressure within the renal

Revised Spring 2006                              1
Human Anatomy and Physiology
Urinary Physiology--Refer to Text Chapter 26 and other chapters as needed
             tubules while decreasing the osmotic pressure within the capillaries and within the
             entire vascular system. Why would this condition cause edema in other areas of
             the body--refer back to cardiovascular system, "Edema, where did I go wrong?"

          6. Where are the juxtaglomerular cells located? The juxtaglomerular cells (modified
             smooth muscle fibers) are located within the walls of the afferent arteriole and are
             closely situated next to the macula densa cells in the end of the ascending limb of
             the loop of Henle just before it enters the DCT. (see Fig. 26.6) The
             juxtaglomerlular cells w/ the macula densa make up the justaglomerular apparatus

          7. What is the function of the juxtaglomerular cells? (See Renal Autoregulation of
             GFR, Table 26.2) The juxtaglomerular apparatus (JGA) helps regulate the arterial
             blood flow w/in the kidney which then determines the rate of blood filtration by
             the glomerulus (GFR). These mechanisms work to maintain a constant GFR at a
             localized level (= renal autoregulation)

             A. Juxtaglomerular cells: As blood flow increases through the afferent arteriole
                (a.a), the juxtaglomerular cells detect stretching due to increased blood
                pressure; they contract in response, which narrows the lumen of the a.a.,
                which results in decreased blood flow to the glomerulus (=GFR). If there is
                less blood flow through the a.a. there is less stretching, the a.a. relaxes and
                dilates, allowing more blood flow into the glomerulus (= GFR).

             B. Macula densa cells are sensitive to increased levels of Na+, Cl- and H20 in the
                renal tubule as a result of GFR (due to high systemic blood pressure). Their
                response is to inhibit the release of NO. NO normally causes vasodilation of
                arterioles as a localized response. Inhibiting NO therefore causes
                vasoconstriction of the a.a., which decreases blood flow through the
                glomerulus. This brings the GFR back down to normal (=GFR). If blood
                pressure drops in the nephron, then more NO will be produced, resulting in
                vasodilation of the a.a.; which then increases the GFR back to normal.

                 Remember, these are localized responses to maintain a steady glomerular
                 filtration rate (GFR). See Table 26.2 p. 1008 for regulation of GFR.

          8. What is the function of renin? (See Hormonal Regulation of GFR, Table 26.2 p.
             1008 and review Fig. 18.16, p. 643) Renin is produced by the JGC in response to
             lowered arterial blood pressure (perhaps due to hemorrhaging). Renin initiates the
             Renin-Angiotensin-Aldosterone Pathway. Eventually, Angiotensin II causes
             constriction of the afferent arteriole (and also efferent arteriole) resulting in
             decreased blood flow to the glomerulus decreased glomerular filtration rate
             (=GFR). This means decreased urine output (UO) so you conserve fluid and
             increase your blood pressure. Note: This response is a systemic response to
             lowered blood pressure. This response may also be stimulated by lowered levels
             of Na+ in the blood and therefore, in the filtrate.

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Human Anatomy and Physiology
Urinary Physiology--Refer to Text Chapter 26 and other chapters as needed
           9. Specialized cells, principal cells, in the last part of the distal convoluted tubule (=
              DCT) and throughout the collecting duct (CD) are sensitive to decreased levels of
              Na+ in the filtrate. Decreased Na+ levels in the filtrate stimulate the release of
              renin, which then activates the renin-angiotensin-aldosterone pathway. As a
              result, Angiotensin II, a powerful vasoconstrictor, constricts the afferent (and to
              some extent the efferent) arteriole, resulting in ________________ (increased,
              decreased) GFR.

           10. What happens to the pressure (glomerular filtration pressure) if the diameter of
               the afferent arteriole increases? If the a.a. dilates then more blood flows into the
               glomerulus which results in increased GFP (=GFR) and potentially more urine
               output (UO).

           11. A decrease in the level of renin results in  Angiotensin II, vasoconstriction of
               a.a. (=vasodialtion/diameter of a.a.) and therefore =GFR.

           12. How are afferent arteriole diameter, blood flow, and pressure in the glomerulus
               interrelated? (See Neural Regulation of GFR, Table 26.2. Note that the afferent
               arteriole and the efferent arteriole can be regulated independently. What happens
               if a sympathetic response causes constriction of the afferent arteriole? Think
               about what happens if the efferent arteriole is dilated/or constricted.  diameter of
               the a.a. =  blood flow into glomerulus and  glomerular filtration pressure and
               GFR and UO.

           13. What effect does ANP (atrial natriuretic peptide) have on GFR? (See Hormonal
               Regulation of GFR Table 26.2 p. 1008) What is the stimulus? When the atria of
               the heart are stretched due to increased blood pressure (increased venous return)
               the hormone ANP is released. It targets specialized cells in the glomerulus. The
               response is to increase capillary surface area which then increases the GFR
               resulting in UO and  blood pressure due to decreased blood volume.

We have been focusing on the localized, hormonal, and neural effects on GFR. Some of these
mechanisms are also involved in tubular reabsorption and secretion. We will discuss this in a bit
more detail with Tubular Reabsorption.

   II.     Tubular Reabsorption is the movement of substances from the filtrate (urine) in the
           renal tubules back into the blood via the peritubular capillaries and the vasa recti. It is
           a very discriminating process which involves specialized cells in specific regions of
           the nephron (Table 26.1). Only specific substances and certain amounts of those
           substances will be reabsorbed in specific regions of the renal tubules.

           1. What percent of tubular filtrate is excreted as urine? Only 1%. We make about
              150-180 liters of filtrate/day (= about 45 gallons), but only about 1% of the
              original filtrate is actually excreted as urine. We produce about 1-2 liters of
              urine/day (average = 1 liter), depending on fluid intake and other personal

Revised Spring 2006                               3
Human Anatomy and Physiology
Urinary Physiology--Refer to Text Chapter 26 and other chapters as needed
          2. What happens to the remaining 99% of the water and dissolved substances in the
             filtrate? It is reabsorbed back into the blood.
          3. What are the two basic processes of tubular reabsorption? Contrast the two.
             (1)   Passive transport
             (2)   Active transport
          4. List and describe three examples of passive transport. (does not require energy)
             (1)    Osmosis (reabsorption of water)
             (2)    Diffusion (& facilitated diffusion)
             (3)    Electrochemical attraction (where a negatively charged ion (Cl-) is
                    attracted to and follows a positively charged ion (Na+) and vice versa.
          5. Active transport involves __carrier molecules___ to move the transported
             molecules through the membrane. Active transport requires the expenditure of
             energy (ATP) or the use of antiporters (opposite direction) or symporters (same
             direction). Note: each type of transporter has a limited capacity to transport its
             product within a given amount of time (= tubular maximum, Tmax, or renal
             plasma threshold). When the Tubular Maximum is surpassed, we say the product
             "spills over into the urine." An interesting example is when a person with
             uncontrolled diabetes mellitus has hyperglycemia and exceeds his/her Tmax for
             glucose (>200mg/ml). The result is glucosuria. Because of the resulting increased
             osmotic pressure of the filtrate there will be less reabsorption of water, resulting
             in UO and, therefore, dehydration. Remember the "3 polys" associated with
             diabetes mellitus (polydypsia, polyuria, polyphagia).
          6. Give examples of ions and molecules reabsorbed (via passive and/or active
             processes) from the kidney tubules into the renal intersititial spaces and on into
             the peritubular capillaries. (Table 26.3) H2O, glucose, amino acids, uric acid,
             urea, Na+, K+, Ca2+, Cl-, HCO3-, HPO42-. Note: large proteins such as albumin are
             not included in this list. Explain. What happens with glomerular nephritis? What
             is the Tmax for proteins?
          7. How much water is reabsorbed due to passive and active transport? Osmosis =
             85-90% in the PCT, LH, DCT and Active Transport = 5-10% in the latter portion
             of the DCT and CD (principle cells) under the influence of ADH.
          8. Where does the majority of tubular reabsorption occur? PCT
             Note: About 70% of the Na+ is reabsorbed in the PCT. Angiotensin II stimulates
             the reabsorption of Na+ and Cl- and water follows by osmosis while aldosterone
             stimulates the reabsorption of Na+ and Cl- followed by water in the CD.
          9. By the time the filtrate reaches the DCT what major changes have occurred?
             (1) +/- 80% of the filtered was has been reabsorbed (PCT). More or less water
             will be reabsorbed at the end of the DCT and in the CD depending on ADH level.
             (2) glucose has been totally reabsorbed into the blood (PCT)
             (3) the majority of the electrolytes, minerals and nutrients have been reabsorbed.
             (4) there is an increased concentration of waste products

Revised Spring 2006                            4
Human Anatomy and Physiology
Urinary Physiology--Refer to Text Chapter 26 and other chapters as needed
          10. What are some of the waste products found in the filtrate as it approaches the
              DCT? These would normally be found in the urine.
              (1) urea
              (2) creatinine
              (3) uric acid
              (4) ammonia

          11. Fluid and electrolyte balance of blood and acid base balance of the body is
             primarily regulated in the DCT and CD. They are regulated by hormones which
             increase/decrease tubular reabsorption and/or tubular secretion. The body has the
             ability to secrete a dilute or a concentrated urine, depending on the needs of the
             body. Explain how this works. While the pH of the blood is maintained within
             very narrow limits of 7.35-7.45 (average =7.4), the urine is much more flexible
             with a broader range between 4.6 and 8 (average = 6) depending on the diet and
             other conditions/needs of the body. The urinary system helps maintain the proper
             pH of the blood by secreting either H+ or HCO3- in the urine--the urine is much
             more flexible in its levels and acts as the dumping ground for many excesses.

   III.   Regulation by Hormonal Influence
          1. What three hormones regulate the amount of fluid and electrolytes conserved or
             excreted by the kidneys?
             a. Antidiuretic Hormone (ADH)
             b. Renin-Angiotensin-Aldosterone Pathway (Aldosterone)
             c. Atrial Natriuretic Peptide (ANP)

          2. ADH (antidiuretic hormone) is produced by the __hypothalamus__ and is stored
             in the __posterior___ ___pituitary gland ___.

          3. What is the stimulus for the release of ADH? Osmoreceptors in the hypothalamus
             detect decrease in blood volume, increased osmolarity of plasma/extracellular
             fluid (=dehydration).

          4. What is the target of ADH? Principal cells of the last part of the DCT and
             throughout the CD (especially the CD).

          5. What effect does ADH have on DCT and CD? Increases the permeability of the
             DCT and CD by inserting water channel proteins in the membrane of the principal
             cells. This then causes increased reabsorption of water into the blood and
             increases hydration (decreases osmolarity of body fluids).

          6. Does increased ADH levels cause you to excrete more or less urine? LESS

          7. What is the negative feedback mechanism involved in "turning off" ADH
             secretion? As the person increases hydration (osmotic concentration of body
             fluids (blood) decreases due to increased blood volume), osmoreceptors are no
             longer stimulated and the level of ADH decreases.

Revised Spring 2006                            5
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