Functions of the Nephron – Filtration, Reabsorption, and Secretion
Answer all questions on a separate sheet of paper.
1. The volume of blood entering the kidney through the renal artery in one day is more than the volume leaving through the renal vein. What does this tell you about where urine comes from?
How the nephron cleanses the blood
Cleansing the blood of wastes, excess salts, and water can be compared to house cleaning. Cleaning out a desk drawer involves separating useful items from garbage, then keeping the useful items and discarding the garbage. You can choose one of two ways to do this. In one method, you would dump everything out of the drawer and then reclaim the useful items, leaving a pile of garbage. An alternative would be to sort through the materials in the drawer, removing the garbage and leaving the useful items in place.
The cleansing of blood in nephrons involves mostly the first method. Useful molecules as well as wastes and excess substances are filtered out of the blood. Then useful molecules are reclaimed back into the blood, leaving wastes (urine) to be excreted. A review of blood composition will help you to understand how it is cleansed.

2. List all the components of whole blood, beginning with the largest particles. Note that some components are waste molecules and some are useful. Some useful components occur in excess amounts.
Study the diagram of a nephron with its surrounding blood vessels that follows (text page
308). Remember that blood passes through two capillary beds as it flows around the nephron, and that exchange of components between blood and surrounding tissues can only occur through thin capillary walls.
3. Identify where useful molecules and wastes are filtered out of the blood and into the nephron. 4. Where are useful molecules reabsorbed from the nephron back into the blood?
5. Which structure transports the waste molecules (urine) left behind after reabsorption?
6. Which parts of the nephron are in the cortex of the kidney? Which parts penetrate deep into the medulla?
7. A common misconception is that “kidneys filter wastes out of the blood.” Use the words “filtration” and “reabsorption” to explain why this statement is false.

Urine formation occurs as blood pressure forces filtrate from the glomerulus into the capsule. This bulk flow of fluids into the capsules of the nephrons in both kidneys creates about
180 L of filtrate per day. All but about 1
L will be actively reabsorbed back into the blood, with great expenditure of ATP.

Glomerular Filtration, Reabsorption
8. How does bulk flow of filtrate into the capsule differ from diffusion?
9. In what ways is nephric filtration similar to formation of interstitial fluid (lymph) in other tissues in the body?
10. One of the effects of a drug overdose is a serious decrease in blood pressure.