Human Physiology, 14th edition (2016)

694 Chapter 19

Figure 19.20 Scanning electron micrographs of bone. Normal bone (a) and bone from a person with osteoporosis (b) can
be compared, demonstrating the loss of bone matrix that occurs in osteoporosis.

(a) (b)

1,25-dihydroxyvitamin D 3 also, along with PTH, stimulates
the resorption of calcium and phosphate from bones when a per-
son has low blood Ca^2 1. Like PTH, 1,25-dihydroxyvitamin D 3
stimulates renal reabsorption of calcium. However, whereas

PTH promotes the renal excretion of phosphate, 1,25-

dihydroxyvitamin D 3 stimulates the renal reabsorption of phos-

phate. By simultaneously raising the blood concentrations of

Ca^2 1 and PO 4 3 2 , 1,25-dihydroxyvitamin D 3 promotes bone

HO HO

Skin

Sunlight

Vitamin D 3

HO

OH

Liver

Blood

HO

OH

OH

Kidney

Blood

(Bone, intestine,

kidneys)

Blood

7-Dehydrocholesterol

Target organs

25-Hydroxylase

25-Hydroxyvitamin D 3

1,25-Dihydroxyvitamin D 3

1 α-Hydroxylase

CH (^2) CH
2
CH 2
Figure 19.21 The production of 1,25-dihydroxyvitamin D 3.
This hormone is produced in the kidneys from the inactive precursor
25-hydroxyvitamin D 3 (formed in the liver). The latter molecule is produced
from vitamin D 3 secreted by the skin.