Figure 18.16

Three examples of essential sulfur-containing nutrients.

Sodium is readily absorbed from foods by active transport. The blood concentration of this element is regulated by the kidneys under the influence of the adrenal cortical hormone aldosterone, which causes the kidneys to reabsorb sodium while expelling potassium.

Sodium makes a major contribution to the solute concentration of extracellular fluids and thus helps regulate water movement between cells and their surroundings. It is necessary for nerve impulse conduction and muscle fiber contraction and plays a role in moving substances, such as chloride ions, through cell membranes (see chapter 21, p. 865).

The usual human diet probably provides more than enough sodium to meet the body's needs. Sodium may be lost as a result of diarrhea, vomiting, kidney disorders, sweating, or using diuretics. Such losses may cause a variety of symptoms including nausea, muscular cramps, and convulsions.

The amount of sodium naturally present in foods varies greatly, and it is commonly added to foods in the form of table salt (sodium chloride). In some geographic regions, drinking water contains significant concentrations of sodium. Foods high in sodium include cured ham, sauerkraut, cheese, and graham crackers.

H In what compounds and tissues of the body is sulfur found?

Which hormone regulates the blood concentration of sodium?

9 What are the functions of sodium?

6. Chlorine. Chlorine (Cl) in the form of chloride ions is widely distributed throughout the body, although it is most highly concentrated in cerebrospinal fluid and in gastric juice. Together with sodium, chlorine helps to maintain the solute concentration of extracellular fluids, regulate pH, and maintain electrolyte balance. It is also essential for the formation of hydrochloric acid in gastric juice, and it functions in the transport of carbon dioxide by red blood cells.

Chlorine and sodium are usually obtained together in the form of table salt (sodium chloride), and as in the case of sodium, an ordinary diet usually provides considerably more chlorine than the body requires. Vomiting, diarrhea, kidney disorders, sweating, or using diuretics can deplete chlorine in the body.

7. Magnesium. Magnesium (Mg) is responsible for about 0.05% of body weight and is found in all cells. It is particularly abundant in bones in the form of phosphates and carbonates.

Magnesium is important in ATP-forming reactions in mitochondria, as well as in breaking down ATP to ADP. Therefore, it is important in providing energy for cellular processes.

Magnesium absorption in the intestinal tract adapts to dietary intake of the mineral. When the intake of magnesium is high, a smaller percentage is absorbed from the intestinal tract, and when the intake is low, a larger percentage is absorbed. Absorption increases as protein intake increases, and decreases as calcium and vitamin D intake increase. Bone tissue stores a reserve supply of magnesium, and excess is excreted in the urine.

The recommended daily allowance of magnesium is 300 mg for females and 350 mg for males. A typical diet usually provides only about 120 mg of magnesium for every 1,000 calories, barely meeting the body's needs. Good sources of magnesium include milk and dairy products (except butter), legumes, nuts, and leafy green vegetables. Table 18.8 summarizes the major minerals.

D Where are chloride ions most highly concentrated in the body?

3 Where is magnesium stored?

9 What factors influence the absorption of magnesium from the intestinal tract?

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