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

(a) Nephron and 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 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. 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. 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. In India, CKD hotspots in Andhra Pradesh/Telangana make this physiology clinically important; loss of nephrons reduces concentrating ability and GFR.

(b) Hormones and steroidogenesis 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. 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-hydroxyprogesterone to dehydroepiandrosterone and androstenedione; 5α-reductase 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) 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. 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. Thus, renal, endocrine and developmental physiology are linked by precise transport, enzymatic and hormonal controls.