719
CHAPTER EIGHTEEN
Nutrition and Metabolism
helps regulate water movement between cells and
their surroundings. It is necessary for nerve impulse
conduction and helps to move substances, such as
chloride ions, through cell membranes (see chapter 21,
p. 818).
The usual human diet probably provides more
than enough sodium to meet the body’s requirements.
Sodium toxicity, which shrinks cells, including those
of the brain, requires unusual ingestion of additional
sodium, such as drinking ocean water or accidentally
using table salt instead of sugar for feeding infants.
Sodium may be lost as a result of diarrhea, vomiting,
kidney disorders, sweating, or using diuretics. Sodium
loss may cause a variety of symptoms, including
nausea, 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 signiF
cant
concentrations of sodium. ±oods high in sodium include
cured ham, sauerkraut, and cheese.
PRACTICE
62
In which compounds and tissues of the body is sulfur found?
63
Which hormone regulates the blood concentration of sodium?
64
What are the functions of sodium?
6.
Chlorine.
Chlorine (Cl) in the form of chloride ions
is found throughout the body and is most highly
concentrated in cerebrospinal fl uid and in gastric juice.
outside cells. The ratio of potassium to sodium in a cell is
10:1, whereas the ratio outside the cell is 1:28.
Potassium helps maintain intracellular osmotic
pressure and pH. It is a cofactor for enzymes that
catalyze reactions of carbohydrate and protein
metabolism and is vital in establishing the membrane
potential in nerve impulse conduction.
Nutritionists recommend a daily adult intake of
2.5 grams (2,500 mg) of potassium. This mineral is in
many foods, so a typical adult diet provides between 2
and 6 grams each day. Excess potassium in the blood is
uncommon because of the uptake of potassium by body
cells and the excretion of potassium in urine. Potassium
deF ciency due to diet is rare, but it may occur for other
reasons. ±or example, when a person has diarrhea, the
intestinal contents may pass through the digestive tract
so rapidly that potassium absorption is greatly reduced.
Vomiting or using diuretic drugs may also deplete
potassium. Such losses may cause muscular weakness,
cardiac abnormalities, and edema.
±oods rich in potassium are avocados, dried
apricots, meats, milk, peanut butter, potatoes, and
bananas. Citrus fruits, apples, carrots, and tomatoes
provide lesser amounts.
PRACTICE
59
How is potassium distributed in the body?
60
What is the function of potassium?
61
Which foods are good sources of potassium?
4.
Sulfur.
Sulfur (S) is responsible for about 0.25% of body
weight and is widely distributed throughout tissues. It is
abundant in skin, hair, and nails. Most sulfur is part of
the amino acids
methionine
and
cysteine.
Other sulfur-
containing compounds include thiamine, insulin, and
biotin
(f g. 18.18)
. In addition, sulfur is a constituent of
mucopolysaccharides in cartilage, tendons, and bones
and of sulfolipids in the liver, kidneys, salivary glands,
and brain.
No daily requirement for sulfur has been
established. It is thought, however, that a diet providing
adequate amounts of protein will also meet the body’s
sulfur requirement. Good food sources of this mineral
include meats, milk, eggs, and legumes.
5.
Sodium.
About 0.15% of adult body weight is sodium
(Na), which is widely distributed throughout the body.
Only about 10% of this mineral is inside cells, and
about 40% is in the extracellular fl
uids. The remainder
is bound to the inorganic salts of bone.
Active transport readily absorbs sodium from foods.
The kidneys regulate the blood concentration of sodium
under the infl
uence 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 fl
uids and thus
NH
2
H
Methionine
Thiamine hydrochloride
(vitamin B
1
)
Biotin
COOH
CH
2
CH
3
CH
2
C
C
C
O
NH
HN
C
C
S
S
S
COOH
CH
2
CH
2
CH
2
CH
2
H
H
H
H
HC
H
3
C
NH
2
HCl
N
N
C
CH
2
CH
2
OH
CC
CH
C
H
CH
3
C
H
C
Cl
H
C
N
FIGURE 18.18
Three essential sulfur-containing nutrients.
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