(a) What are intestinal glands? Describe the roles of intestinal juices in digestion and absorption of food. 20 (b) Define…

(a) Intestinal glands and intestinal juices. Intestinal glands are the simple tubular crypts of Lieberkühn lying in the mucosa of the small intestine, between villi. They are mucosal glands, distinct from the submucosal Brunner’s glands of the duodenum, which secrete alkaline mucus to protect the duodenal wall. The crypts contain enterocytes and goblet cells, and secrete succus entericus. The secreted juice is watery and contains mucus, electrolytes, bicarbonate and some lipase and nucleosidases; the peptidases and saccharases that complete digestion are largely integral brush-border enzymes of enterocytes, not free soluble juice. Enterokinase, a duodenal brush-border enzyme, activates trypsinogen. Brush-border disaccharidases such as maltase, sucrase, lactase and isomaltase hydrolyse maltose, sucrose, lactose and α-limit dextrins to monosaccharides, while peptidases convert di- and tripeptides to amino acids. Intestinal lipase acts mainly on monoglycerides, complementing pancreatic lipase, which has already hydrolysed triglycerides to fatty acids and monoglycerides. Absorption depends on villi and microvilli, which enlarge surface area. Glucose and galactose enter enterocytes apically by Na+-coupled SGLT1; fructose enters by facilitated diffusion through GLUT5. All monosaccharides leave basolaterally by GLUT2 into blood. Amino acids use separate Na+-dependent and Na+-independent amino-acid transporters, not SGLT1/GLUT2. Fatty acids and monoglycerides are reassembled into chylomicrons and enter lacteals, while water, electrolytes and small molecules move through paracellular and transcellular routes.

(b) Thermoregulation. Thermoregulation is the maintenance of thermal homeostasis by balancing heat production and heat loss around a hypothalamic set point, using negative feedback. Poikilotherms largely allow body temperature to vary with environment; homeotherms maintain a narrow internal range; heterotherms alternate between homeothermic and poikilothermic states, as in some hibernators. In cold climates, homeotherms conserve heat by cutaneous vasoconstriction, piloerection to trap air, reduced surface-area exposure, shivering thermogenesis and non-shivering thermogenesis in brown adipose tissue, where uncoupling protein 1 dissipates the proton gradient as heat; Himalayan tahr and other high-altitude mammals use thick coats, dense fur and metabolic heat production. In hot climates, they lose heat by cutaneous vasodilation, sweating, panting, radiation from large ears and nasal passages, and postural or behavioural adjustments. Indian examples include desert foxes of the Thar using burrows and nocturnal activity, and blackbucks reducing midday activity and seeking shade. The hypothalamus integrates cutaneous and core temperature signals, activating sympathetic, endocrine and behavioural responses to restore the set point.

(c) Retina and vision. In a drawn transverse section, the mammalian retina is inverted and lies between the choroid and vitreous humour. From choroid outward, the layers are retinal pigment epithelium, photoreceptor layer of rods and cones, external limiting membrane, outer nuclear layer, outer plexiform layer, inner nuclear layer, inner plexiform layer, ganglion cell layer, nerve fibre layer, and internal limiting membrane. The fovea centralis is a thin, cone-rich pit for sharp vision; the optic disc is the blind spot where ganglion axons leave. Light passes through cornea, pupil, aqueous humour, lens and vitreous humour and is focused on the retina. In rods, a photon is absorbed by 11-cis retinal in rhodopsin, converting it to all-trans retinal and activating metarhodopsin II. This activates transducin, which activates phosphodiesterase; cGMP falls, cGMP-gated sodium channels close, and the photoreceptor hyperpolarises. It releases less glutamate onto bipolar cells, changing their sign and driving ganglion cells to fire action potentials. Cones use iodopsins for colour and high-acuity vision. Signals pass through bipolar cells, amacrine cells and ganglion cells, whose axons form the optic nerve to the brain. Dark adaptation is slow because rhodopsin must be regenerated; light adaptation is rapid because excess light bleaches rhodopsin, reduces sensitivity and shifts vision toward cones. Thus, intestinal digestion, hypothalamic thermoregulation and retinal vision show how epithelial, endocrine and neural control maintain internal order.