Summary

In this lesson, you explored the following questions:

• How does the nephron function in maintaining the composition of blood plasma?
• How do the kidneys function in excreting metabolic wastes and expelling them into the environment?

Three processes occurring in successive portions of the nephron accomplish the function of urine formation:

1. Filtration of water and dissolved substances out of the blood in the glomeruli and into the Bowman's capsule by arteriole blood pressure and passive transport. 
   
2. Reabsorption of water and dissolved substances from the filtrate out of the kidney tubules (proximal, loop of Henle, distal, and collecting duct) back into the blood occurs through passive and active transport. This process prevents substances needed by the body from being lost in the urine. The proximal tubule actively reabsorbs nutrients such as glucose and positively charged ions (Na⁺), while passively reabsorbing water and negatively charged ions (Cl^-). The main function of the descending loop of Henle is to reabsorb water through osmosis. The main function of the ascending loop of Henle is to reabsorb sodium ions. Active reabsorption of sodium ions occurs in the distal tubule, while negative ions and water are passively reabsorbed. 
   
3. The secretion of hydrogen ions (H⁺), potassium ions (K⁺), urea, and certain drugs out of the blood and into the kidney tubules occurs through passive and active transport. The proximal and distal tubules actively transport hydrogen ions from the blood, while urea and potassium ions move passively. These molecules are eventually eliminated in the urine.

Lesson Glossary

Bowman's capsule: a caplike formation at the top of each nephron that serves as a filtration structure; surrounds the glomerulus

distal tubule: the tubular portion of the nephron that lies between the loop of Henle and the proximal tubule; main function is the reabsorption of water and solutes and the secretion of various substances

glomerulus:  a fine network of capillaries within Bowman's capsule arising from the renal artery

loop of Henle:  a tubular portion of the nephron; an extension of the proximal tubule that loops into the medulla of the kidney; main function is the reabsorption of water and ions

proximal tubule: the tubule connected to Bowman's capsule. Its main function is the reabsorption of water and solutes as well as the secretion of hydrogen ions.