Human Physiology: Aging, Endocrine Pancreas, Pituitary, and Thyroid Functions

A comprehensive vocabulary review covering physiological aging across body systems, pancreatic endocrine secretions, hypothalamic-pituitary axes, posterior pituitary storage hormones, and thyroid structure, synthesis, and pathological conditions.

Telomere Shortening

A mechanism of cellular aging where chromosomes progressively lose protective end caps during division, eventually halting cell division completely.

Osteoarthritis in Aging

An age-related degenerative joint disorder commonly affecting weight-bearing joints such as the knees and spine.

Presbyopia

Age-related impairment of visual accommodation typically occurring after age $40$, caused by lens hardening and ciliary muscle degeneration.

Islets of Langerhans

The endocrine clusters of the pancreas comprising Type A cells ($20\text{\%}$ glucagon), Type B cells ($60\text{\%}$ to $75\text{\%}$ insulin and amylin), Type D cells (somatostatin), and Type F cells (pancreatic polypeptide).

Proinsulin Processing

The cleavage process where preproinsulin loses its signal sequence to form proinsulin, followed by the removal of the connecting polypeptide (C-peptide) to yield mature active insulin.

C-Peptide

A connecting peptide cleaved from proinsulin during insulin maturation that serves as an essential clinical marker for endogenous insulin secretion.

GLUT2 Transporter

A glucose transporter located on pancreatic beta cells that facilitates glucose entry to initiate glycolysis, increase the $\text{ATP/ADP}$ ratio, close ATP-sensitive potassium channels, and trigger insulin exocytosis.

Glucagon

A $29$-amino-acid polypeptide hormone produced by pancreatic A cells and intestinal L cells that raises blood glucose levels via hepatic glycogenolysis, gluconeogenesis, lipolysis, and ketogenesis.

Amylin

A peptide hormone co-secreted with insulin from pancreatic beta cells that suppresses postprandial glucagon secretion, slows gastric emptying, and reduces food intake.

Hypothalamic-Pituitary Portal System

A specialized vascular network transporting releasing and inhibitory hormones directly from the median eminence of the hypothalamus to the anterior pituitary gland.

Anterior Pituitary Cell Types

The three histologic categories of anterior pituitary secretory cells: acidophils ($40\text{\%}$, producing GH and prolactin), basophils ($10\text{\%}$, producing ACTH, TSH, FSH, and LH), and chromophobes ($50\text{\%}$).

Pro-Opiomelanocortin (POMC)

A precursor prohormone cleaved to produce Adrenocorticotropic Hormone (ACTH), Melanocyte-Stimulating Hormones ($\alpha$, $\beta$, $\gamma$-MSH), $\beta$-lipotropin, and $\beta$-endorphin.

Adrenocorticotropic Hormone (ACTH)

A $39$-amino-acid polypeptide secreted by anterior pituitary basophils that stimulates growth, vascularity, and glucocorticoid secretion of the adrenal cortex, while exhibiting melanocyte-stimulating activity.

Anti-Diuretic Hormone (ADH)

A nonapeptide synthesized primarily in the supraoptic nucleus of the hypothalamus, stored in the posterior pituitary, which promotes renal water reabsorption via $V_2$ receptors and vasoconstriction via $V_{1A}$ receptors.

Oxytocin

A nonapeptide produced in the hypothalamus and released by the posterior pituitary that induces myoepithelial contraction for milk ejection during suckling and uterine smooth muscle contraction during labor.

Sheehan Syndrome

Postpartum pan-hypopituitarism caused by ischemic necrosis of the enlarged pituitary gland following severe obstetric hemorrhage and hypotension.

Thyroid Follicle

The structural and functional unit of the thyroid gland, consisting of a central colloid lumen surrounded by a single layer of cuboidal epithelial cells and basement membrane.

Thyroxine ($T_4$)

The primary circulating thyroid prohormone ($99.98\text{\%}$ protein-bound, half-life of $6$ to $7$ days) secreted by follicular cells, which undergoes peripheral deiodination to active $T_3$ or inactive reverse $T_3$.

Triiodothyronine ($T_3$)

The major biologically active thyroid hormone ($99.8\text{\%}$ protein-bound, half-life of $1.5$ days) that binds intracellular nuclear receptors to regulate basal metabolic rate, oxygen consumption, and tissue development.

Graves' Disease

An autoimmune form of primary hyperthyroidism caused by thyroid-stimulating IgG autoantibodies targeting TSH receptors, presenting with elevated $T_3$/$T_4$, suppressed TSH, exophthalmos, and lid lag.

Myxedema

Adult hypothyroidism characterized by low basal metabolic rate, cold intolerance, bradycardia, dry yellow skin (carotenemia), coarse hair, slow mentation, and hypercholesterolemia.

Cretinism

Congenital hypothyroidism stemming from maternal iodine deficiency or congenital thyroid aplasia, resulting in severe irreversible mental retardation, short stature (dwarfism), motor rigidity, and macroglossia.

Endemic Goiter

Thyroid gland enlargement resulting from dietary iodine deficiency, leading to inadequate $T_3$/$T_4$ synthesis, loss of negative feedback, and uninhibited TSH stimulation causing follicular hypertrophy.

Aging

The process of getting old, defined as the progressive accumulation of physiological changes in humans over time.

Telomere Shortening Theory

A cellular theory of aging stating that chromosomes progressively lose protective end caps (telomeres) during successive cell divisions until division eventually stops.

Glycation

An age-related chemical process in which glucose sugar binds to and inhibits DNA, proteins, and lipids.

Cataract

An age-related visual impairment caused by hardening and clouding of the eye lens that affects more than $50\%$ of individuals over age $80$.

Islets of Langerhans

The endocrine region of the pancreas containing Type A cells ($20\%$; secreting glucagon), Type B cells ($60\text{--}75\%$; secreting insulin and amylin), Type D cells (secreting somatostatin), and Type F cells (secreting pancreatic polypeptide).

Preproinsulin

The initial precursor polypeptide synthesized during insulin production, which undergoes signal sequence cleavage and disulfide bond formation to produce proinsulin.

C Peptide

The connecting polypeptide cleaved from proinsulin during hormone maturation that serves as a blood marker for endogenous insulin release.

Glucagon

A $29$-amino-acid polypeptide hormone produced by pancreatic Type A cells and upper GIT L cells that increases blood glucose through liver glycogenolysis, gluconeogenesis, lipolysis, and ketogenesis.

Amylin

A hypoglycemic hormone co-secreted with insulin by pancreatic beta cells that delays gastric emptying, suppresses glucagon release, and reduces food intake and body weight.

Somatostatin (Pancreatic)

A $14$- or $28$-amino-acid peptide hormone secreted by pancreatic delta cells that acts locally to inhibit both insulin and glucagon secretion.

Adenohypophysis Histology

The cellular breakdown of the anterior pituitary lobe, composed of acidophils ($40\%$; secreting growth hormone and prolactin), basophils ($10\%$; secreting ACTH, TSH, FSH, and LH), and chromophobes ($50\%$).

Hypothalamic-Pituitary Portal System

A specialized vascular pathway that carries hypothalamic releasing and inhibitory hormones directly from the median eminence to the anterior pituitary gland.

Adrenocorticotropic Hormone (ACTH)

A $39$-amino-acid polypeptide hormone derived from POMC and secreted by anterior pituitary basophils that exerts trophic effects on the adrenal cortex and exhibits skin-pigmenting melanocyte-stimulating activity.

Pro-opiomelanocortin (POMC)

The common precursor polypeptide from which ACTH, CLIP, \n\alpha\text{-MSH}, $\beta\text{-MSH}$, $\gamma\text{-MSH}$, $\beta\text{-endorphin}$, and lipotropins are cleaved.

Anti-Diuretic Hormone (ADH)

A $9$-amino-acid nonapeptide synthesized primarily in the supraoptic nucleus, stored in the posterior pituitary, and released to stimulate water retention via $V_2$ receptors in collecting ducts and vasoconstriction via $V_1A$ receptors.

Oxytocin

A $9$-amino-acid nonapeptide synthesized in the hypothalamus and stored in the posterior pituitary that contracts mammary myoepithelial cells for milk ejection and contracts the uterus during labor.

Sheehan Syndrome

A cause of pan-hypopituitarism resulting from ischemic necrosis of the pituitary gland following severe hemorrhage during childbirth.

Thyroid Follicle

The spherical functional unit of the thyroid gland, bounded by a single layer of cuboidal epithelioid cells and filled with thyroglobulin colloid.

Thyroxine ($T_4$)

The primary tetraiodothyronine hormone secreted by thyroid follicular cells, circulating largely bound to TBG ($99.98\%$ bound) with a circulatory half-life of $6\text{--}7\text{ days}$.

Triiodothyronine ($T_3$)

The most biologically active thyroid hormone, possessing a half-life of $1.5\text{ days}$ and produced in small amounts by follicular cells or derived via peripheral conversion of $T_4$ in target tissues.

Calcitonin

A peptide hormone secreted by thyroid parafollicular cells (C-cells) located between follicles that aids in calcium homeostasis.

Graves' Disease

An autoimmune form of primary hyperthyroidism caused by anti-TSH receptor IgG antibodies (thyroid-stimulating antibodies) that continually stimulate thyroid hormone release, leading to exophthalmos and lid lag.

Myxedema

Adult hypothyroidism characterized by low basal metabolic rate, cold intolerance, dry yellowish skin (carotenemia), sparse coarse hair, bradycardia, slow mentation, and elevated cholesterol.

Cretinism

Congenital childhood hypothyroidism resulting from maternal iodine deficiency or thyroid aplasia, presenting with severe mental retardation, short stature (dwarfism), and an enlarged tongue.

Goiter

An enlargement or swelling of the thyroid gland caused by excessive TSH stimulation, commonly due to dietary iodine deficiency (endemic goiter) or a lack of negative feedback inhibition.