Endocrine System Overview

Endocrine System

• The endocrine system is responsible for regulating bodily functions.
• Dr. Yasmine Gamal Sabry presented this information.

Parathyroid Gland

• The parathyroid glands are typically four small glands located on the posterior aspect of the thyroid gland.
• These glands contain chief cells that secrete parathyroid hormone (PTH).
• PTH's primary function is regulating calcium concentration in extracellular fluid (ECF).
• PTH is essential for life.

PTH Actions on Target Organs

• PTH increases blood calcium and decreases blood phosphate levels by acting on three target organs:
  • Skeleton: Directly increases bone resorption and mobilizes ionic calcium and phosphate into the bloodstream. This includes immediate effects that begin within minutes and continue for hours, stimulating calcium pumps to move calcium from bone fluid to plasma. There are also delayed effects that take days to weeks, causing osteoclasts to break down calcium crystals.
  • Kidneys: directly stimulates calcium reabsorption and phosphate excretion in the kidneys. It also increases the formation of the active form of vitamin D3 (calcitriol).
  • Intestine: PTH indirectly increases calcium absorption via the formation of calcitriol by the kidneys, in turn increasing calcium absorption throughout the small intestine through increased calcium binding protein (calbindin D).

Regulation of PTH Levels

• PTH levels are primarily regulated by blood calcium levels, not by other hormones.
• A calcium sensing receptor on parathyroid cells and other tissues receives feedback on blood calcium
• High blood calcium inhibits PTH secretion.
• Low blood calcium increases PTH secretion.

Relationship between Calcium and Phosphorus

• Plasma phosphorus concentration is inversely related to calcium concentration. Their product is constant (solubility product)
• Maintaining a constant product of calcium and phosphorus is important.

Hyperparathyroidism

• Cause: Typically due to a parathyroid gland tumor (adenoma) or compensatory hypertrophy (e.g., vitamin D deficiency, renal failure).
• Manifestations: bone disease (bone softening and deforming due to demineralization and the presence of bone cysts), kidney stones due to excess calcium in the urine, neuromuscular effects including weakness, reduced alertness, and short-term memory problems. Also, possible digestive issues including abdominal pain, nausea, vomiting and constipation.

Hypoparathyroidism

• Cause: Accidental removal or injury to the parathyroid glands during thyroid surgery (thyroidectomy).
• Characteristics: Low calcium and high phosphate levels.
• Tetany: A state of muscle spasms due to increased neuromuscular excitability arising from low plasma calcium levels. Can be latent or manifest depending on severity.

Hypocalcemia Causes

• Hypoparathyroidism
• Renal failure
• Alkalemia (decreased ionized calcium)
• Vitamin D deficiency

Hypocalcemic Tetany Symptoms

• Manifest: Symptoms appear when blood calcium levels fall below 7 mg%, characterized by muscle twitching, tonic-clonic seizures, and generalized convulsions.
• Latent: Symptoms are minimal and absent at rest, but can be elicited with specific tests (e.g., Trousseau's sign, Chvostek's sign). These tests may induce mild symptoms when blood calcium is still slightly within the range of normal.

Hypocalcemic Tetany Treatment

• Manifest tetany (acute): Calcium gluconate administered intravenously.
• Latent tetany (chronic): Dietary supplements that are rich in calcium and possibly active vitamin D (1,25-dihydroxycholecalciferol/calcitriol).

Calcium Homeostasis

• Key organs involved in calcium metabolism: Bone, small intestine, and kidneys.
• Calcium is absorbed through the small intestine, taken up by blood, and incorporated into bone via calcium deposition and release.
• Calcium can be lost in urine or stool in excretion.

Role of Calcium in Physiology

• Bone and teeth formation:
• Neuromuscular excitability:
• Muscle contraction:
• Synaptic transmission:
• Hormone secretion:
• Intracellular communication:
• Blood clotting mechanism:
• Maintenance of tight junctions between cells.

Hormonal Control of Calcium

• PTH: Increases blood calcium levels.
• Calcitonin: Decreases blood calcium levels.
• Vitamin D(active): Increases calcium absorption and promotes bone mineralization.

Vitamin D3

• Vitamin D3 can be obtained from dietary sources (e.g., fatty fish, egg yolks) or produced in the skin through exposure to sunlight.
• Cholecalciferol is activated in the liver into 25-hydroxyvitamin D3 then in the kidneys into 1,25-dihydroxyvitamin D3 (calcitriol).
• Vitamin D3 primarily increases calcium levels in blood throughout the intestinal tract, kidney, and bone.

Calcitonin

• A polypeptide hormone released from parafollicular cells in the thyroid gland.
• It's antagonistic to PTH in its effect on calcium but similar in its effect on phosphate.
• It lowers blood calcium levels by inhibiting bone resorption, increasing calcium excretion in the kidneys and inhibiting calcium absorption in the intestines.

Test Your Knowledge (True/False)

• This section presents true/false questions regarding the information learned about calcium homeostasis and related hormones.

Explore the functions of the endocrine system, focusing on the parathyroid gland and its crucial role in calcium regulation. This quiz covers the actions of parathyroid hormone (PTH) on various target organs, highlighting its importance for maintaining bodily functions and overall health.