The nephrons of the kidneys process blood and create urine through a process of filtration, reabsorption, and secretion. Urine is about 95% water and 5% waste products. Nitrogenous wastes excreted in urine include urea, creatinine, ammonia, and uric acid.

Urine formation depends on three functions:

• Filtration is accomplished by the movement of fluids from the blood into the Bowman’s Capsule
• Reabsorption involves the transfer of essential solutes and water from the nephron back into the blood
• Secretion involves the movement of materials from the blood back into the nephron
• For a detailed diagram and summary, see Fig. 1 on p. 350 and Table 2 on p. 351

Filtration

• Blood running through the afferent arteriole into the glomerulus is under high pressure (65 mmHg compared to 25 mmHg normally found in capillary beds)
• Most dissolved solutes (see Table 1, p. 349 for a list) pass through the walls of the glomerulus into the Bowman’s capsule

Reabsorption

• On average, 600 mL of fluid flows through the kidneys every minute
• About 20% (or 120 mL) is filtered into the nephron
• If all of that fluid left in urine, dehydration would be a constant danger
• Fortunately, only 1 mL of urine is formed for every 120 mL, meaning that 119 mL of fluids and solutes are reabsorbed
• Selective reabsorption occurs by both active and passive transport
• Carrier molecules move Na+ ions across the cell membranes of the cells that line the nephron
• Negative ions (Cl- and HCO3-) follow the positive Na+ ions by charge attraction
• Many mitochondria supply energy needed for active transport
• Reabsorption occurs until the threshold level of a substance is reached
• Excess (like NaCl) remains in the nephron and is excreted with urine
• Other molecules are actively transported from the proximal tubule
• Glucose and amino acids attach to specific carrier molecules, which shuttle them out of the nephron and into the blood
• The amount of solute that can be reabsorbed is limited
• Ex – individuals with high blood glucose will excrete some in their urine
• The solutes that are actively transported out of the nephron create an osmotic gradient that draws water in from the nephron
• A second osmotic force, created by the proteins not filtered into the nephron, also help reabsorption
• The proteins remain in the blood stream and draw water from the interstitial fluid into the blood
• As water is reabsorbed from the nephron, the remaining solutes become more concentrated
• Molecules like urea and uric acid will diffuse from the nephron back into the blood

Secretion

• Secretion is the movement of wastes from the blood into the nephron
• Nitrogen containing wastes, excess H+ ions, and minerals like K+ ions are examples of substances secreted
• Drugs (like penicillin) can also be secreted
• Cells loaded with mitochondria line the distal tubule, providing energy for active transport