Human Physiology, 14th edition (2016)

696 Chapter 19

Figure 19.23 The negative feedback control of
calcitonin secretion. The action of calcitonin is antagonistic to
that of parathyroid hormone.

−

Negative

Decreased feedback

plasma

Ca2+

Inhibits

dissolution of

CaPO 4

crystals

Stimulates

excretion of

Ca2+ and PO 4

in urine

Kidneys Bone

Calcitonin

Parafollicular

cells of thyroid

Increased

plasma

Ca2+

Sensor

Integrating center

Effector

3−

Marty uses a cream containing a vitamin D derivative as

a treatment for his psoriasis because the vitamin D helps

reduce skin inflammation, which causes psoriasis in peo-

ple genetically predisposed to it. Vitamin D also stimu-

lates the intestinal absorption of calcium and phosphate,

raising the blood levels of these minerals so they can be

deposited in the bone matrix as hydroxyapatite crystals.

A deficiency of vitamin D can cause bone softening, or

osteomalacia, due to inadequate mineralization of bone.

Marty has a BMI of 34, indicating obesity, and his fast-

ing plasma glucose level of 120 mg/dL indicates impaired

glucose tolerance, or a state of insulin resistance that pre-

cedes type 2 diabetes mellitus. If he develops this type of

diabetes, and keeps his central obesity, he may develop

hypertension and the dyslipidemia of metabolic syn-

drome. This greatly increases his risk of cardiovascular

disease, different cancers, and other problems. If he gets

diabetes, he would have to take metformin or an alterna-

tive, which reduce insulin resistance and improves insulin

secretion to lower the plasma glucose concentration.

See the additional chapter 19 Clinical Investigations

on Sustained Hyperglycemia and Hypocalcemia in the

Connect site for this text.

Clinical Investigation SUMMARY

| CHECKPOINT

14. Describe the mechanisms by which the secretion of
    parathyroid hormone and of calcitonin is regulated.
    15a. List the steps involved in the formation of
    1,25-dihydroxyvitamin D 3 and state how this
    formation is influenced by parathyroid hormone.
    15b. Describe the actions of parathyroid hormone,
    1,25-dihydroxyvitamin D 3 , and calcitonin on the
    intestine, skeletal system, and kidneys, and explain
    how these actions affect the blood levels of calcium.
    15c. Account for the different effects of
    1,25-dihydroxyvitamin D 3 on bones according to
    whether calcium intake is adequate or inadequate.

SUMMARY

2. The energy provided in food and the energy consumed

by the body are measured in units of kilocalories.

3. When the caloric intake is greater than the energy

expenditure over a period of time, the excess calories

are stored primarily as fat.

19.1 Nutritional Requirements 662

A. Food provides molecules used in cell respiration for energy.

1. The metabolic rate is influenced by physical activity,
   temperature, and eating. The basal metabolic rate is
   measured as the rate of oxygen consumption when such
   influences are standardized and minimal.