STUDENT INVESTIGATION 4.1

Background

The cells of the human body are surrounded by liquid that is remarkably constant in its properties. The continued regulation of the many dissolved compounds and ions in this internal environment is one aspect of what is referred to as homeostasis.

As you learned at the beginning of this chapter, the kidneys are extremely important in maintaining homeostasis. They function rapidly and sensitively in regulating blood composition and maintaining optimum concentration of dissolved substances. About 1440 liters of blood flow through the kidneys of an average person in a single day (which is about 1 liter/min). Of this amount, about 180 liters are filtered through the glomeruli into the nephrons, but only about 2-3 liters of this filtrate are excreted as urine. The amount of urine produced varies, depending on the individual's health, activity, water and salt intake, temperature, and many other factors. The amount of many of the various constituents of urine varies, as well, depending on the concentration of those constituents in the blood and depending on the hormone activity as the blood enters the kidney.

As the blood is filtered in the kidney, the concentration of two of the main electrolytes, sodium (Na⁺) and potassium (K⁺), is regulated primarily by the hormone aldosterone. This control is of vital importance to the body because Na+ is important in the regulation of blood volume and pressure, and K+ is required to maintain proper function of cardiac and skeletal muscles. Whereas the first step in the control and regulation of all blood electrolytes involves filtration out of the blood, the majority of electrolytes are subsequently reabsorbed back into the blood from the proximal tubule. Because of aldosterone, the control of potassium (K⁺) follows a different process. In the absence of aldosterone, all of the filtered K⁺ is reabsorbed from the proximal tubule back into the blood. In the absence of aldosterone, therefore, no K⁺ is excreted in the urine. In contrast, the presence of aldosterone stimulates the secretion of K⁺ from the peritubular blood into the distal tubule. Thus, secretion is the only means by which K⁺ can be eliminated in the urine. This information will be of importance in the interpretation of the data in Parts A and B that follow.

Part A-Blood vs. Urine

Procedure

Questions

Table 1. Comparison of materials present in the blood as it enters the kidney, and materials in the urine as it leaves the kidney

	% in Blood as it enters Kidney			% in Urine as it Leaves Kidney
	Water	91.50		Water	96.00
	Protein	7.00		Protein	0.00
	Glucose	0.10		Glucose	0.00
	Sodium	0.29		Sodium	0.29
	Potassium	0.02		Potassium	0.24
	Urea	0.03		Urea	2.70

Part B-Filtration Reabsorption, and Secretion

Procedure

Using a specialized test procedure called the micropuncture method, an experiment was done to study the six substances presented in Table 2. This procedure involves withdrawing samples of blood using a very fine pipette at four points within the kidney: where the blood enters the kidney, at a point within the kidney, where the blood exits the kidney, and in the urine collecting tubule. Study Table 2, which shows the data, collected using this technique. Use the data to answer the discussion questions.

Table 2. A comparison of substances that are found in fluid samples taken at four points within the kidney.*

		In blood entering kidney				Within kidney along tubule				In blood leaving kidney				In urine
	Water		100			30			99			1
	Protein		100			0			100			0
	Glucose		100			20			100			0
	Sodium		100			30			99			1
	Potassium		100			23			88			12
	Urea		100			50			10			90

1. For each of the substances listed in Table 2, state which of the following processes are involved in the movement of each substance: filtration, reabsorption, or secretion.
2. If you are in the movie theater and you were to eat some very salty popcorn and you did not drink a soda with it, which substance(s) would be affected? How would they be affected?
3. Explain the fact that 20% of glucose is filtered into the kidney but all of it is reabsorbed back into the blood.