Renal System Physiology

The kidneys are excretory and regulatory organs. By excreting water and solutes, the kidneys are responsible for ridding the body of waste products and excess water. The kidneys regulate 1) plasma osmolarity, or the concen-
tration of a solution expressed as osmoles of solute per liter of solvent; 2) plasma volume; 3) acid-base balance; 4) electrolyte balance; 5) excretion of metabolic wastes and foreign materials; and 6) the production and secretion of hormones that regulate osmolarity and electrolyte balance. All these activities are extremely im- portant to maintaining homeostasis in the body.

The kidneys are located between the posterior abdominal wall and the ab- dominal peritoneum. Although many textbooks depict the kidneys directly across from each other, the right kidney is actually slightly lower than the left. Each human kidney contains approximately 1.2 million nephrons, the functional units of the kidney. Each nephron is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries, called the glomeru- lus, which is enclosed by a fluid-filled capsule called Bowman’s capsule. An af- ferent arteriole supplies blood to the glomerulus. As blood flows through the glomerular capillaries, protein-free plasma filters into the Bowman’s capsule, a process called glomerular filtration. An efferent arteriole then drains the glomerulus of the remaining blood. The filtrate flows from Bowman’s capsule to the start of the renal tubule, called the proximal convoluted tubule, then on to the proximal straight tubule, followed by the loop of Henle, a U-shaped hairpin loop. The filtrate then flows into the distal convoluted tubule before reaching the con- necting tubule and the collecting duct, where urine collects. The distal tubule and collecting duct are composed of two cell types: principal cells and intercalated cells. Principal cells reabsorb Na+ and water and secrete K+. Intercalated cells secrete either H+ or HCO – and are, therefore, very important in the regulation
of the acid/base balance.

Glomerular Filtration
Let’s take a closer look at what happens during glomerular filtration. Blood en- ters the glomerulus from the afferent arteriole. Starling forces (hydrostatic and osmotic pressure gradients) drive protein-free plasma from the blood across the walls of the glomerular capillaries and into the Bowman’s capsule. The glomeru- lar filtration rate is an index of kidney function. In humans, the filtration rate

FIGURE 9.1 Opening screen of the Simulating Glomerular Filtration experiment.

ranges from 80 to 140 ml/min, so that in 24 hours as much as 180 liters of plasma is filtered by glomeruli. The filtrate formed is devoid of cellular debris and is essentially protein free. The concentration of salts and organic molecules are sim- ilar to that of blood. Normal urine output is 1–1.5 liters/24 hours. The difference is reabsorbed in the body. Normally, only about 20% of the blood that enters the nephron is filtered, due to the osmotic pressure of the blood (oncotic pressure) and the hydrostatic pressure from the fluids in Bowman’s capsule. The glomerular filtration rate can be altered by changing afferent arteriole resistance, efferent arteriole pressure, or the size of the filtration surface, or by a process called renal autoregula- tion.

Once the filtrate is formed, the nephron must reabsorb materials that the body needs and excrete unneeded materials from the body. While as much as 180 liters are filtered each day, less than 1% of the filtered water, sodium chloride, and other solutes are excreted in the urine. More than 67% of this reabsorption takes place in the proximal convoluted tubule. The distal convoluted tubule and collecting duct reabsorb ap- proximately 7% of the filtered NaCl, secrete a variable amount of K+ and H+, and reabsorb a variable amount of wa- ter. It is in this distal part of the nephron that hormones act to

reabsorb water and electrolytes. Aldosterone regulates NaCl reabsorption (and thus NaCl excretion as well). ADH (anti- diuretic hormone) causes the permeability of the distal tubule and collecting duct to increase, promoting the uptake of wa- ter from the filtrate. ADH is considered the body’s most im- portant hormone for regulating water balance.

In the first three activities you will be concentrating on how arterial diameter and pressure affect glomerular filtra- tion rate and urine volume. Follow the instructions for start- ing PhysioEx in the Getting Started section at the front of this manual. From the drop-down menu, choose Exercise 9: Re- nal System Physiology and click GO. Then click Simulat- ing Glomerular Filtration. You will see the screen shown in Figure 9.1.

Click Help at the top of the screen and then select Bal- loons On/Off. Now move your mouse around the simulated nephron in the yellow section of the screen. Labels will appear for the various parts of the nephron as you roll over them. Note in particular the glomerulus and the glomerulus capsule. Also note the “afferent tube” and “efferent tube” to the left of the glomerulus—these represent the afferent and efferent arterioles delivering and draining blood from the glomerulus. You may adjust the radius of either of these tubes

by clicking the (+) and (-) buttons next to the respective tubes. You may also adjust the blood pressure of the source beaker by clicking the (+) and (-) buttons next to the “Pres- sure (mmHg)” display.

Once you have identified all the equipment on screen,

8.Reduce the afferent arteriole radius to 0.30 mm, and click Start.

Under these conditions, does the fluid flow through the nephron?

click Help again and select Balloons On/Off (you cannot proceed with the experiment unless the labels are turned off).

At the bottom left of the screen are two beakers. The left beaker, which we call the “source beaker,” represents the blood supply being delivered to the nephron. When the Start button is clicked, blood will flow from the source beaker to

What is the glomerular filtration rate?

How does it compare to your baseline data, and why?

the afferent arteriole and then to the group of small tubes rep- resenting the glomerulus. As blood flows through the glomerulus, you will see ultrafiltration occur. (Ultrafiltration

means filtration from the plasma of everything except pro- teins and cells.) Blood will then be drained from the glomeru-

lus to the “drain beaker” next to the source beaker. At the end of the nephron tube, you will see the formation of urine in a small beaker at the lower right of the screen. To watch a test run of this process in action, click the Start button. At the end of the run, click Refill underneath the drain beaker before you begin the activities that follow.

9.Using the simulation, design and carry out an experi-

ment for testing the effects of increasing or decreasing the

efferent radius.

How did increasing the efferent radius affect glomerular filtration rate?

A C T I V I T Y 1

Effect of Arteriole Diameter on Glomerular Filtration
How did decreasing the efferent radius affect glomerular filtration rate?

In this activity you will investigate how the diameters of the afferent and efferent arterioles leading to and from the glomerulus can affect the glomerular filtration rate.

1.The afferent radius display should be set at 0.50 mm, and the efferent radius at 0.45 mm. If not, use the (+) or (-) but- tons next to the afferent and efferent radius displays to adjust accordingly.

Physiologically, what could be the cause of a change in affer- ent or efferent arteriole radius?

2.Be sure the left beaker is full. If not, click the Refill ■


3.The pressure gauge above the left beaker should read

90 mm Hg. If not, click the (+) or (-) buttons next to the pressure display to adjust accordingly.

4.Click the Start button. As the blood flows through the nephron, watch the displays for glomerular pressure and glomerular filtration rate at the top right of the screen, as well as the display for urine volume at the bottom right of the screen.

5.After the drain beaker has stopped filling with blood, click Record Data. This will be your baseline data for this activity.

6.Click the Refill button.

7.Increase the afferent radius by 0.05 mm and repeat steps 3–6, making sure to click Record Data at the end of each run. Keep all the other variables at their original settings. Continue repeating the activity until you have reached the maximum afferent radius of 0.60 mm.

Compare this data with your baseline data. How did increasing the afferent arteriole radius affect glomerular filtration rate?

A C T I V I T Y 2

Effect of Pressure on Glomerular Filtration
Next you will investigate the effect of blood pressure on glomerular filtration rate.

1.Under the Data Sets display, highlight Pressure. This will allow you to save data in a new data set window. You can always retrieve your data from the previous activity by high- lighting the Afferent data set.

2.Make sure that the source beaker is filled with blood, and that the drain beaker is empty. If not, click Refill.

3.Adjust the pressure gauge (on top of the source beaker) to 70 mm Hg. Set the afferent radius at 0.50 mm and the efferent radius at 0.45 mm.

4.Click the Start button. Watch the Glomerular Pressure and Glomerular Filtration Rate displays at the top right of the screen.

5.When the run has finished, click the Record Data but- ton. This is your baseline data.

What happened to the glomerular filtration rate and urine vol- ume after you reduced the pressure?

6.Increase the pressure by 5 mm Hg and repeat the experi- ment. Continue increasing the pressure by 5 mm and repeat-

ing the experiment until you have reached the maximum pressure of 100 mm Hg. Be sure to click Record Data and

Refill after each experimental run.

As pressure increased, what happened to the pressure in the glomerulus?

How could you adjust the afferent or efferent radius to com- pensate for the effect of the reduced pressure on glomerular filtration rate and urine volume? Use the simulation to deter- mine your answer.

What happened to the glomerular filtration rate?

Compare the urine volume in your baseline data with the urine volume as you increased the pressure.

How did the urine volume change?

8.Next, click the square valve button (currently reading “valve open”) above the collecting duct. The valve should now read “valve closed.”

9.Click Start. At the end of the run, click Record Data. What changes are seen in nephron function when the valve is closed?

How could increased urine volume be viewed as being bene- ficial to the body?

Why were these changes seen?

Combined Effects
In the first activity you looked at arteriole diameter and its

Is the kidney functional when the glomerular filtration rate is zero? Explain your answer.

role in glomerular filtration. Next you examined the effect of pressure on glomerular filtration. In the human body, both of

these effects are occurring simultaneously. In this activity you will investigate the combined effects of arteriole diame-

ter and pressure changes on glomerular filtration.

1.Under the Data Sets window, highlight Combined. This

What is the major “ingredient” that needs to be removed from

the blood?

will allow you to save data in a new data set window. You can always retrieve your data from previous activities by high-

lighting the Afferent data set or the Pressure data set.

2.Set the pressure at 90 mm Hg, the afferent arteriole at

0.50 mm, and the efferent arteriole at 0.45 mm.

Studies on aging have demonstrated that some nephrons may fail as we get older. Will this be a problem regarding urine formation?

3.Click the Start button and allow the run to complete. Then click Record Data. This is your baseline data.

Sample Solution