Reabsorption of Electrolytes

The osmolarity of body fluids is dependent upon the concentration of particular electrolytes within the fluids. Electrolytes are compounds and molecules that are able to ionize and, thus, carry an electrical current. The kidneys regulate electrolyte excretion and therefore help control blood composition.
     The Electrolytes The most common electrolytes in the plasma are sodium (Na⁺), potassium (K⁺), and bicarbonate ion (HCO₃^-). Na⁺ and K⁺ are termed cations because they are positively charged, and HCO₃^- is termed an anion because it is negatively charged.
     Sodium The movement of Na⁺ across an axon membrane, you will recall, is necessary to the formation of a nerve impulse and muscle contraction. The concentration of Na⁺ in the blood is also the best indicator of the blood’s osmolarity.
     Potassium The movement of K⁺ across an axon membrane is also necessary to the formation of a nerve impulse and muscle contraction. Abnormally low K⁺ concentrations in the blood, as might occur if diuretics are abused, can lead to cardiac arrest.
     Bicarbonate Ion HCO₃^- is the form in which carbon dioxide is carried in the blood. The bicarbonate ion has the very important function of helping maintain the pH of the blood.
     Other Ions The plasma contains many other ions. For example, calcium ions (Ca²+) and phosphate ions (HPO₄²-) are important to bone formation and cellular metabolism. Their absorption from the intestine and excretion by the kidneys is regulated by hormones.
     The Kidneys More than 99% of sodium (Na⁺) filtered at the glomerulus is returned to the blood. Most sodium (67%) is reabsorbed at the proximal convoluted tubule, and a sizable amount (25%) is actively transported from the tubule by the ascending limb of the loop of the nephron. The rest is reabsorbed from the distal convoluted tubule and collecting duct.
     Aldosterone Hormones control the reabsorption of sodium at the distal convoluted tubule. Aldosterone, a hormone secreted by the adrenal cortex, promotes the excretion of potassium ions (K⁺) and the reabsorption of sodium ions (Na⁺). The release of aldosterone is set in motion by the kidneys themselves. The juxtaglomerular apparatus is a region of contact between the afferent arteriole and the distal convoluted tubule (Fig. 16.8). When blood volume, and therefore blood pressure, is not sufficient to promote glomerular filtration, the juxtaglomerular apparatus secretes renin.
     Renin is an enzyme that changes angiotensinogen (a large plasma protein produced by the liver) into angiotensin I. Later, angiotensin I is converted to angiotensin II, a powerful vasoconstrictor that also stimulates the adrenal cortex to release aldosterone. The reabsorption of sodium ions is followed by the reabsorption of water. Therefore, blood volume and blood pressure increase.

Figure 16.8 Juxtaglomerular apparatus. This drawing shows that the afferent arteriole and the distal convoluted tubule usually lie next to each other. The juxtaglomerular apparatus occurs where they touch. The juxtaglomerular apparatus secretes renin, a substance that leads to the release of aldosterone by the adrenal cortex. Reabsorption of sodium ions followed by water then occurs. Therefore, blood volume and blood pressure increase.

Diuretic drugs developed to counteract high blood pressure inhibit active transport of Na⁺ at the loop of the nephron or at the distal convoluted tubule into the blood. A decrease in water reabsorption and a decrease in blood volume follow.

Acid-Base Balance

The pH scale, can be used to indicate the basicity (alkalinity) or the acidity of body fluids. A basic solution has a lesser hydrogen ion concentration [H⁺] than the neutral pH of 7.0. An acidic solution has a greater [H⁺] than neutral pH. The normal pH for body fluids is about 7.4. This is the pH at which our proteins, such as cellular enzymes, function properly. If the blood pH rises above 7.4, a person is said to have alkalosis, and if the blood pH decreases below 7.4, a person is said to have acidosis. Alkalosis and acidosis are abnormal conditions that may need medical attention. The foods we eat add basic or acidic substances to the blood, and so does metabolism. For example, cellular respiration adds carbon dioxide that combines with water to form carbonic acid, and fermentation adds lactic acid. The pH of body fluids stays at just about 7.4 via several mechanisms, primarily acid-base buffer systems, the respiratory center, and the kidneys.

Acid-Base Buffer Systems

The pH of the blood stays near 7.4 because the blood is buffered. A buffer is a chemical or a combination of chemicals that can take up excess hydrogen ions (H⁺) or excess hydroxide ions (OH^-). One of the most important buffers in the blood is a combination of carbonic acid (H₂CO₃) and bicarbonate ions (HCO₃^-). Carbonic acid is a weak acid that minimally dissociates and re-forms in the following manner:

When hydrogen ions (H⁺) are added to blood, the following reaction occurs:

When hydroxide ions (OH^-) are added to blood, this reaction occurs:

These reactions temporarily prevent any significant change in blood pH. A blood buffer, however, can be overwhelmed unless some more permanent adjustment is made. The next adjustment to keep the pH of the blood constant occurs at pulmonary capillaries.

Respiratory Center

The respiratory center in the medulla oblongata increases the breathing rate if the hydrogen ion concentration of the blood rises. Increasing the breathing rate rids the body of hydrogen ions because the following reaction takes place in pulmonary capillaries:

In other words, when carbon dioxide is exhaled, the amount of carbonic acid that dissociates to give hydrogen ions is reduced. It is important to have the correct proportion of carbonic acid and bicarbonate ions in the blood. Breathing readjusts this proportion so that this particular acid-base buffer system can continue to absorb both H⁺ and OH^- as needed.

Figure 16.9 Acid-base balance. In the kidneys, bicarbonate ions (HCO₃^-) are reabsorbed and hydrogen ions (H⁺) are excreted as needed to maintain the pH of the blood. Excess hydrogen ions are buffered, for example, by ammonia (NH₃), which becomes ammonium (NH₄⁺). Ammonia is produced in tubule cells by the deamination of amino acids.

The Kidneys

As powerful as the acid-base buffer and the respiratory center mechanisms are, only the kidneys can rid the body of a wide range of acidic and basic substances and otherwise adjust the pH. The kidneys are slower acting than the other two mechanisms, but they have a more powerful effect on pH. For the sake of simplicity, we can think of the kidneys as reabsorbing bicarbonate ions and excreting hydrogen ions as needed to maintain the normal pH of the blood (Fig. 16. 9). If the blood is acidic, hydrogen ions are excreted and bicarbonate ions are reabsorbed. If the blood is basic, hydrogen ions are not excreted and bicarbonate ions are not reabsorbed. Because the urine is usually acidic, it follows that an excess of hydrogen ions is usually excreted. Ammonia (NH₃) provides a means for buffering these hydrogen ions in urine: (NH₃ + H⁺-> NH₄⁺). Ammonia (whose presence is quite obvious in the diaper pail or kitty litter box) is produced in tubule cells by the deamination of amino acids. Phosphate provides another means of buffering hydrogen ions in urine. The importance of the kidneys’ ultimate control over the pH of the blood cannot be overemphasized. As mentioned, the enzymes of cells cannot continue to function if the internal environment does not have near-normal pH.