(a) Explain the structure of nephron and its role in urine formation. Add a note on hormonal regulation of urine formation…

(a)(i) Structure of the Nephron and Its Role in Urine Formation

The kidney forms urine through the nephron proper—renal corpuscle, proximal convoluted tubule (PCT), loop of Henle and distal convoluted tubule (DCT); the collecting duct system, though not part of the nephron, receives DCT output and completes final modification. The renal corpuscle contains a glomerular capillary tuft enclosed by Bowman’s capsule. Blood pressure forces plasma water and small solutes into Bowman’s space, forming an ultrafiltrate free of cells and most proteins; the filtration barrier is the fenestrated capillary endothelium, the glomerular basement membrane and the filtration slits between podocyte foot processes. About 180 litres of filtrate are formed each day (GFR about 125 mL/min), of which only about 1.5 litres leave as urine. Most nephrons are cortical, with short loops, while about 15% are juxtamedullary, with long loops reaching deep into the medulla. The juxtaglomerular apparatus senses tubular NaCl and helps regulate glomerular filtration rate. In the PCT, epithelial cells reabsorb about 65% of filtered water passively/osmotically, while glucose, amino acids, bicarbonate and most sodium are recovered by active or secondary-active transport; tubular epithelial cells also secrete H+, K+, NH4+, creatinine and many xenobiotics into the lumen. The descending limb is permeable to water, concentrating tubular fluid, whereas the thick ascending limb actively reabsorbs NaCl without water, diluting the fluid and generating the medullary osmotic gradient (from about 300 to 1,200 mOsm/kg) by counter-current multiplication, which the vasa recta preserve by counter-current exchange and urea recycling reinforces. The DCT fine-tunes Na+ and Ca2+ reabsorption. In the collecting duct, principal and intercalated cells adjust water, Na+, K+ and acid-base balance, producing concentrated or dilute urine.

(a)(ii) Hormonal Regulation of Urine Formation

Hormonal regulation is central: ADH/vasopressin increases aquaporin-2 channels in collecting ducts, enhancing water reabsorption and concentrating urine; aldosterone acts on DCT and collecting ducts to increase ENaC-mediated Na+ reabsorption and K+/H+ secretion, conserving sodium and controlling potassium; ANP opposes this by promoting natriuresis, inhibiting renin-angiotensin-aldosterone and reducing ADH release, thereby lowering blood volume and pressure. The renin–angiotensin–aldosterone system links these: a fall in renal perfusion or tubular NaCl makes juxtaglomerular cells release renin, and the resulting angiotensin II constricts efferent arterioles, stimulates aldosterone and ADH release and thirst. Parathyroid hormone increases Ca²⁺ reabsorption in the DCT and inhibits phosphate reabsorption in the PCT. In India, CKD hotspots in Andhra Pradesh/Telangana make this physiology clinically important; loss of nephrons reduces concentrating ability and GFR.

(b) Classification of Hormones and Steroid Hormone Biosynthesis

Hormones are classified by chemical nature as peptide/protein hormones (insulin, growth hormone), steroid hormones (cortisol, testosterone, oestradiol) and amino-acid derivatives (catecholamines, thyroid hormones). By source, they include hypothalamic releasing/inhibiting hormones, anterior and posterior pituitary hormones, and peripheral hormones from adrenal cortex, gonads, thyroid, pancreas, kidney and others. By mechanism of action, lipophilic hormones (steroids, thyroid hormones) bind intracellular receptors that act as transcription factors, whereas hydrophilic hormones (peptides, catecholamines) bind cell-surface receptors and act through second messengers such as cAMP, IP₃/DAG and Ca²⁺. Steroids are synthesised in zones of the adrenal cortex (aldosterone in the zona glomerulosa by aldosterone synthase, CYP11B2; cortisol in the zona fasciculata; androgens in the zona reticularis), in the gonads and in the placenta. Steroid biosynthesis begins with cholesterol, derived from LDL or de novo synthesis, which is transferred by StAR protein from the outer to the inner mitochondrial membrane. CYP11A1 then cleaves the side chain to form pregnenolone, the rate-limiting step. 3β-hydroxysteroid dehydrogenase converts pregnenolone to progesterone. For cortisol, CYP17A1 first 17α-hydroxylates progesterone to 17-hydroxyprogesterone; 21-hydroxylase then forms 11-deoxycortisol, and 11β-hydroxylase produces cortisol. Without 17α-hydroxylation, the pathway yields corticosterone. CYP17A1 also has 17,20-lyase activity, converting 17-hydroxypregnenolone to dehydroepiandrosterone and 17-hydroxyprogesterone to androstenedione; 17β-hydroxysteroid dehydrogenase then forms testosterone. Aromatase (CYP19A1) converts androgens to oestrone and oestradiol. Tissue-specific enzymes determine which steroid is made, explaining steroid therapeutics and enzyme-defect diseases such as congenital adrenal hyperplasia.

(c)(i) Meaning of Teratogenesis

Teratogenesis is the induction of structural or functional defects during prenatal development, when an insult disrupts normal cell division, migration, differentiation or organ formation. Following Wilson's principles, susceptibility depends on the genotype of the embryo, the developmental stage at exposure, the dose and the specific mechanism of the agent; the embryo is most vulnerable during organogenesis (about weeks 3–8 in humans), while exposure in the first two weeks usually causes either death or no effect ("all or none").

(c)(ii) Genetic and Induced Teratogenesis

Genetic teratogenesis arises from inherited or de novo chromosomal and single-gene abnormalities. Chromosomal aberrations include trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), trisomy 13 (Patau syndrome) and monosomy X (Turner syndrome), usually due to meiotic nondisjunction; single-gene mutations can cause cleft palate, neural-tube defects, thalassaemia or metabolic disorders. Induced teratogenesis results from environmental teratogens acting during critical periods, especially organogenesis in the first trimester, with effects depending on dose, timing and susceptibility. Thalidomide causes phocomelia by disrupting limb-bud angiogenesis; isotretinoin causes craniofacial, cardiac and CNS defects; alcohol causes fetal alcohol syndrome; rubella and radiation can cause microcephaly, cataracts, cardiac defects and growth restriction. Endocrine disruptors are a special class: diethylstilbestrol (DES) causes vaginal adenosis, and maternal thyroid deficiency can cause cretinism. Prevention therefore combines folic acid supplementation, genetic counselling, infection control, and avoidance of teratogens such as alcohol, smoking and unregulated drugs. Other important teratogens include the TORCH infections (toxoplasma, rubella, cytomegalovirus, herpes), Zika virus (microcephaly), anticonvulsants such as valproate (neural-tube defects), and maternal diabetes (caudal regression and heart defects).