Maintaining Water Balance: Urine

Maintaining Water Balance

When you drink a large glass of water, the water gets absorbed into the blood and the following happens:

The absorbed water increases the amount of water filtered in the glomerulus.
The absorbed water in the blood reduces the Na concentration a little.
The reduced Na concentration lowers the amount of Na filtered in the glomerulus.
The nephron reabsorbs all of the reduced Na load and some of the accompanying water, leaving excess water in the filtrate.
The reduced Na concentration is sensed by the osmoreceptors.
The osmoreceptors do not secrete as much ADH.
Because the collecting ducts don't see as much ADH, they don't allow much water to be reabsorbed in response to the Na concentration gradient set up by the loop of Henle.
The excess water gets excreted in the urine.
When the excess water is excreted, the Na concentration of the blood returns to normal.

Typically, we don't drink water overnight when we sleep. So, our intestines aren't absorbing water:

Decreased water absorption by the intestine reduces the amount of water in the blood.
Decreased water in the blood reduces the amount of water filtered in the glomerulus.
Decreased water in the blood increases the Na concentration in the blood.
Increased Na concentration in the blood increases the amount of Na filtered in the glomerulus.
The nephron doesn't reabsorb all of the filtered Na, and some water remains with it in the filtrate.
The increased Na concentration in the blood is sensed by the osmoreceptors.
The osmoreceptors secrete ADH.
The collecting ducts see more ADH and allow water to be reabsorbed in response to the Na concentration gradient set up by the loop of Henle.
More water gets reabsorbed from the collecting ducts, producing a concentrated urine. A little water is lost in the urine because of the Na; we can't excrete solid urine.
The removal of Na and increased reabsorption of water help return the blood concentration of Na to normal.

So, the loop of Henle sets up the Na concentration gradient across the medulla, allowing for water to be reabsorbed from the collecting ducts, and ADH allows the water to pass through those collecting ducts.

Your blood maintains a constant concentration of hydrogen ion (pH) by a chemical mixture of hydrogen ions and sodium bicarbonate. The sodium bicarbonate is produced by the carbon dioxide (CO₂) formed in the cells as a byproduct of many chemical reactions. The CO₂ enters the blood in the capillaries, where red blood cells contain an enzyme called carbonic anhydrase that helps combine CO ₂ and water (H ₂O) to form carbonic acid (H ₂ CO₃ ) quickly. The carbonic acid formed then rapidly separates into hydrogen ions (H⁺ ) and bicarbonate ions (HCO₃^-). This reaction can also proceed in the reverse direction, whereby sodium bicarbonate plus hydrogen ion yields carbon dioxide and water.

Carbonic Anhydrase

CO ₂ + H ₂ O <---------> H ₂ CO₃ <---------> H⁺ + HCO ₃^-

The correct pH is maintained by keeping the ratio of hydrogen ion to bicarbonate in the blood constant. If you add acid (hydrogen ion) to the blood, then you will reduce the bicarbonate concentration and alter the pH of the blood. Similarly, if you reduce the hydrogen ion by adding alkali, you will increase the bicarbonate concentration and alter the pH of the blood.

Now, the acid/base balance of our blood changes in response to many things including:

Diet - diets rich in meats provide acids to the bloods when digested. In contrast, diets rich in fruits and vegetables make our blood alkaline because they are rich in bicarbonates.
Exercise - exercising muscles produce lactic acid that must be eliminated from the body or metabolized.
Breathing - high altitude causes rapid breathing that makes our blood alkaline. In contrast, certain lung diseases that block the diffusion of oxygen can cause the blood to be acidic.

In the next section, we'll take a look at how the kidneys regulate blood composition.