Bone Physiology: Calcium & Phosphate Metabolism

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Questions and Answers

What is the general formula for hydroxyapatites in bone?

Ca10(PO4)6(OH)2 (correct)

Ca7(PO4)4(OH)3

Ca5(PO4)3(OH)

Ca8(PO4)5(OH)

What percentage of bone in the body is made up of trabecular bone?

20% (correct)

30%

10%

40%

What is the function of canaliculi in compact bone?

To receive nutrients (correct)

To facilitate bone resorption

To provide structural support

To allow for bone growth

What is the total blood flow to bone in adult humans?

200–400 mL/min Signup and view all the answers

What is the purpose of bone remodeling?

To respond to stresses and strains Signup and view all the answers

What is the function of compact bone?

To provide structural support Signup and view all the answers

What percentage of bone mass is being remodeled by bone-remodeling units in the human skeleton at any one time?

5% Signup and view all the answers

What is the primary function of free, ionized calcium in the body?

Cell signaling, nerve function, muscle contraction, and blood coagulation Signup and view all the answers

Where is the majority of calcium stored in the body?

Bones Signup and view all the answers

What is the role of calcitonin in calcium regulation?

Lowering calcium levels in the blood Signup and view all the answers

What is the rate of renewal for compact bone?

4% per year Signup and view all the answers

Which of the following is NOT a way in which phosphate levels are regulated?

Hormonal regulation of vitamin D levels Signup and view all the answers

What is the primary mechanism of nutrient supply in compact bone?

Haversian canals containing blood vessels Signup and view all the answers

What is the composition of collagen in bone matrix?

Over 90% type I collagen Signup and view all the answers

How many collagen genes have been identified so far?

Over 40 Signup and view all the answers

What is the process called when cartilage is transformed into bone?

Enchondral bone formation Signup and view all the answers

Which bones are formed directly from mesenchymal cells?

Clavicles, mandibles, and certain skull bones Signup and view all the answers

What is the arrangement of collagen around each haversian canal?

Concentric layers forming cylinders Signup and view all the answers

What is the primary action of parathyroid hormone (PTH) on calcium levels?

Mobilize calcium from bone and increase urinary phosphate excretion Signup and view all the answers

What is the role of 1,25-Dihydroxycholecalciferol in calcium regulation?

Increasing calcium absorption in the intestine Signup and view all the answers

What is the function of the kidneys in calcium homeostasis?

To excrete excess calcium Signup and view all the answers

What is the significance of calcium in the body?

It is an essential intracellular signaling molecule Signup and view all the answers

What is the main target of parathyroid hormone (PTH) in the body?

The bones Signup and view all the answers

What is the primary function of the endocrine system in calcium regulation?

To regulate calcium levels in the blood Signup and view all the answers

What percentage of total plasma calcium is ionized?

10% Signup and view all the answers

What is the result of a decrease in extracellular Ca2+?

Hypocalcemic tetany Signup and view all the answers

What is the effect of hyperventilation on the symptoms of tetany?

It increases the symptoms of tetany Signup and view all the answers

What is the importance of knowing the plasma protein level when evaluating the total plasma calcium?

It affects the binding of Ca2+ by plasma proteins Signup and view all the answers

What is the role of Ca2+ in blood clotting?

It is essential for blood clotting Signup and view all the answers

What is the effect of laryngospasm on the airway?

It obstructs the airway Signup and view all the answers

What happens to plasma proteins when the pH is high?

They become more ionized, providing more protein anions to bind with Ca2+ Signup and view all the answers

What is the approximate amount of Ca2+ moved into and out of the readily exchangeable pool in the bone per day?

500 mmol Signup and view all the answers

What regulates the overall transportation process of Ca2+ across the brush border of intestinal epithelial cells to the bloodstream?

1,25-dihydroxycholecalciferol Signup and view all the answers

What percentage of filtered Ca2+ is reabsorbed in the kidneys?

98-99% Signup and view all the answers

Where does about 60% of Ca2+ reabsorption occur?

Proximal tubules Signup and view all the answers

What regulates distal tubular reabsorption of Ca2+?

PTH Signup and view all the answers

What is the approximate plasma level of 1,25-dihydroxycholecalciferol?

0.03 ng/mL Signup and view all the answers

What is the effect of high plasma Ca2+ levels on the production of 1,25-dihydroxycholecalciferol?

It stimulates the production of 24,25-dihydroxycholecalciferol instead Signup and view all the answers

What is facilitated by 1,25-dihydroxycholecalciferol in the kidneys?

Ca2+ reabsorption Signup and view all the answers

What is the consequence of 1,25-dihydroxycholecalciferol stimulating osteoblasts?

An increase in osteoclast activity Signup and view all the answers

What is the role of renal 1α-hydroxylase in the regulation of calcium metabolism?

It catalyzes the formation of 1,25-dihydroxycholecalciferol Signup and view all the answers

What is the primary function of 1,25-dihydroxycholecalciferol in the intestine?

It increases Ca2+ absorption Signup and view all the answers

What percentage of total body phosphorus is found in the skeleton?

85-90% Signup and view all the answers

What is the approximate amount of phosphorus that normally enters bone per kilogram of body weight per day?

3 mg (97 μmol) Signup and view all the answers

What percentage of filtered Pi is reabsorbed in the proximal tubule?

85-90% Signup and view all the answers

What is the effect of PTH on NaPi-Iia?

It inhibits NaPi-Iia Signup and view all the answers

What is the form of phosphorus found in plasma?

Two-thirds organic compounds and one-third inorganic phosphorus Signup and view all the answers

What stimulates phosphate absorption in the duodenum and small intestine?

1,25-dihydroxycholecalciferol and other stimuli that increase Ca2+ absorption Signup and view all the answers

What stimulates calcitonin secretion in the thyroid gland?

Plasma calcium level above 9.5 mg/dL Signup and view all the answers

Which hormone does gastrin stimulate the secretion of?

Calcitonin Signup and view all the answers

What is the half-life of calcitonin in humans?

Less than 10 minutes Signup and view all the answers

In which organs are receptors for calcitonin found?

Bones and kidneys Signup and view all the answers

What is the effect of β-adrenergic agonists on calcitonin secretion?

Stimulation Signup and view all the answers

What is the consequence of elevated plasma calcium levels on calcitonin secretion?

Stimulation Signup and view all the answers

What is the primary function of bone in the body?

To protect vital organs and support locomotion Signup and view all the answers

What type of bone has a high surface-to-volume ratio and many cells sitting on the surface of the plates?

Trabecular bone Signup and view all the answers

What is the approximate percentage of bone in the body that is compact bone?

80% Signup and view all the answers

What is the total blood flow to bone in adult humans?

200-400 mL/min Signup and view all the answers

What is the formula for hydroxyapatites in bone?

Ca10(PO4)6(OH)2 Signup and view all the answers

What is the process of bone resorption and formation permitting?

Bone remodeling Signup and view all the answers

What is the primary effect of high plasma Ca2+ levels on the production of 1,25-dihydroxycholecalciferol?

Decreased production of 1,25-dihydroxycholecalciferol Signup and view all the answers

What is the consequence of 1,25-dihydroxycholecalciferol facilitating Ca2+ reabsorption in the kidneys?

Decreased excretion of Ca2+ in the urine Signup and view all the answers

What is the role of 24,25-dihydroxycholecalciferol in calcium regulation?

It is the less active metabolite of vitamin D Signup and view all the answers

What is the effect of 1,25-dihydroxycholecalciferol on osteoblasts?

It stimulates the activity of osteoblasts Signup and view all the answers

What is the normal plasma level of 25-hydroxycholecalciferol?

30 ng/mL Signup and view all the answers

What is the role of renal 1α-hydroxylase in the regulation of calcium metabolism?

It converts 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol Signup and view all the answers

What is the consequence of having a high pH on plasma proteins?

They become more ionized Signup and view all the answers

What is the approximate amount of Ca2+ moved into and out of the readily exchangeable pool in the bone per day?

500 mmol Signup and view all the answers

What percentage of filtered Ca2+ is reabsorbed in the kidneys?

98-99% Signup and view all the answers

Which system regulates the overall transportation process of Ca2+ across the brush border of intestinal epithelial cells to the bloodstream?

1,25-dihydroxycholecalciferol system Signup and view all the answers

What is the primary location of Ca2+ reabsorption in the kidneys?

Proximal tubules and distal tubules Signup and view all the answers

What regulates distal tubular reabsorption of Ca2+?

Parathyroid hormone (PTH) Signup and view all the answers

What is the primary factor that determines the width of the epiphysial plate?

Pituitary growth hormone and insulin-like growth factor-I Signup and view all the answers

What is the outcome when the epiphyses unite with the shaft of the bone?

Linear bone growth ceases Signup and view all the answers

What is the purpose of radiographing the skeleton in young individuals?

To determine bone age Signup and view all the answers

What is the function of the periosteum in bones?

To cover the surface of bones Signup and view all the answers

What is the sequence of epiphysial closure?

Orderly temporal Signup and view all the answers

What is the consequence of cartilage cells becoming hypertrophic?

Secretion of vascular endothelial growth factor Signup and view all the answers

Study Notes

Bone Structure and Composition

Bone is a special form of connective tissue with a collagen framework impregnated with Ca2+ and PO4(3–) salts, particularly hydroxyapatites.
Collagen, which is 90% type I, provides strength and structure to bone, and is made up of a triple helix of three polypeptides bound tightly together.
Two types of bone: compact or cortical bone (80% of bone in the body) and trabecular or spongy bone (20% of bone in the body).

Bone Physiology

Bone is a living tissue that is well vascularized and has a total blood flow of 200–400 mL/min in adult humans.
Old bone is constantly being resorbed and new bone formed, permitting remodeling that allows it to respond to the stresses and strains that are put upon it.
Bone is involved in overall Ca2+ and PO4(3–) homeostasis, and protects vital organs, and provides support for locomotion and gravity.

Calcium Metabolism

The free, ionized calcium (Ca2+) in the body fluids is a vital second messenger and is necessary for blood coagulation, muscle contraction, and nerve function.
Decrease in extracellular Ca2+ exerts a net excitatory effect on nerve and muscle cells, leading to hypocalcemic tetany.
Total plasma calcium is composed of diffusible and nondiffusible components, with the ionized calcium being the most important.
Calcium is regulated by the kidneys, which filter and reabsorb Ca2+, and the intestine, which absorbs Ca2+.

Parathyroid Hormone (PTH)

PTH is secreted by the parathyroid glands and mobilizes calcium from bone and increases urinary phosphate excretion.
PTH regulates the reabsorption of Ca2+ in the proximal tubules and the distal tubule.

Vitamin D

Vitamin D is a steroid hormone that is formed from vitamin D by successive hydroxylations in the liver and kidneys.
1,25-Dihydroxycholecalciferol is the active form of vitamin D, which facilitates Ca2+ absorption from the intestine, reabsorption in the kidneys, and bone formation.
The formation of 1,25-dihydroxycholecalciferol is regulated by plasma Ca2+ and PO43+, and is increased by PTH.

Calcitonin

Calcitonin is a Ca2+-lowering hormone produced by the parafollicular cells of the thyroid gland.
Calcitonin secretion is increased by high plasma calcium levels, β-adrenergic agonists, dopamine, and estrogens.
Calcitonin receptors are found in bones and the kidneys, and its actions are short-lived.

Bone Remodeling

Bone remodeling is a continuous process of bone resorption and deposition, which allows the bone to respond to the stresses and strains that are put upon it.
Two independent but interacting homeostatic systems affect the calcium in bone: one that regulates plasma Ca2+ and the other that involves bone remodeling.

Bone Growth

Bone growth occurs at the epiphyses, which are separated from the shaft of the bone by a plate of actively proliferating cartilage.
The width of the epiphysial plate is proportional to the rate of growth, and is affected by several hormones, including pituitary growth hormone and IGF-I.
Linear bone growth ceases after the epiphyses unite with the shaft (epiphysial closure).

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Description

This quiz covers the physiology of bones, including their composition and role in calcium and phosphate metabolism. It also touches on the endocrine system's regulation of these processes.