(a) Explain the basic stimuli causing pancreatic secretion. Discuss the role of pancreas in major food digestion. 20 (b) Explain…

Pancreatic secretion and digestion. Pancreatic secretion is initiated when food enters the duodenum. In the neural phase, distension and vagal afferents excite vagal efferents to acinar cells, causing early enzyme-rich secretion. The vagal phase is brief and prepares the pancreas before food arrives, while hormonal secretion is the main phase during digestion. In the hormonal phase, acid and peptides in the duodenum stimulate S cells to release secretin, which acts on duct cells to secrete bicarbonate-rich fluid, neutralising gastric acid and providing an alkaline pH for enzymes. Fatty acids and amino acids stimulate I cells to release cholecystokinin, which mainly stimulates acinar cells to secrete digestive enzymes and also promotes gallbladder contraction. Acinar cells package amylase, lipase, trypsinogen, chymotrypsinogen and procarboxypeptidases; duct cells add bicarbonate and water. In digestion, pancreatic α-amylase hydrolyses internal α-1,4 glycosidic bonds of gelatinised starch, producing maltose, maltotriose and α-limit dextrins; it does not act effectively on raw starch granules. Proteolytic zymogens are activated by enterokinase: trypsinogen becomes trypsin, which activates chymotrypsinogen and procarboxypeptidases; trypsin, chymotrypsin and carboxypeptidases hydrolyse peptide bonds, releasing amino acids and small peptides. Pancreatic lipase, with colipase and bile salts, hydrolyses triglycerides to fatty acids and 2-monoglycerides, enabling micelle formation and absorption. Thus the pancreas provides the alkaline medium and enzymes for carbohydrate, protein and lipid digestion.

Oxygen transport and dissociation curve. Oxygen is carried in blood mainly as oxyhaemoglobin, about 98.5%, and only about 1.5% as dissolved plasma oxygen, the dissolved fraction determining blood pO2. Haemoglobin is a tetramer with four heme groups; binding of one O2 molecule increases affinity of the remaining sites, giving cooperative binding represented as Hb + 4O2 ⇌ Hb(O2)4. The oxygen-haemoglobin dissociation curve is sigmoidal, allowing efficient loading at pulmonary pO2 and unloading at tissue pO2. A right shift, caused by increased pCO2, decreased pH (Bohr effect), increased temperature and increased 2,3-BPG, lowers haemoglobin affinity and promotes O2 unloading in metabolically active tissues. During exercise, increased tissue pCO2, H+ and temperature shift the curve right, facilitating unloading. A left shift, caused by decreased pCO2, increased pH, lower temperature or lower 2,3-BPG, increases affinity and promotes loading in the lungs. 2,3-BPG binds deoxyhaemoglobin and stabilises the low-affinity state; fetal haemoglobin has reduced 2,3-BPG binding affinity because of γ-globin chains, not absent binding sites, so it retains higher O2 affinity for placental transfer. Thus the curve shifts according to tissue needs, matching oxygen supply to metabolism.

Cochlear anatomy and organ of Corti. A suitable diagram shows the cochlea as a bony spiral with three fluid-filled chambers: scala vestibuli and scala tympani containing perilymph, separated by the cochlear duct or scala media containing endolymph. The vestibular membrane separates scala vestibuli from scala media; the basilar membrane separates scala media from scala tympani. The helicotrema at the apex connects scala vestibuli and scala tympani. The round window at the base bulges to dissipate pressure. Sound vibrations pass from the stapes footplate to the oval window, generating pressure waves in perilymph that travel along the basilar membrane. The basilar membrane is stiff and narrow at the base and broad and flexible at the apex, so high frequencies produce maximal displacement near the base and low frequencies near the apex, providing frequency discrimination. On the basilar membrane lies the organ of Corti, with inner and outer hair cells, supporting cells and the overlying tectorial membrane. When the basilar membrane moves, stereocilia of hair cells shear against the tectorial membrane, opening mechanosensitive ion channels; depolarisation releases neurotransmitter onto auditory nerve fibres. Inner hair cells are principal sensory receptors encoding sound intensity and frequency, while outer hair cells amplify and sharpen the basilar membrane response. Together, the cochlea converts mechanical sound into neural signals and separates frequencies for hearing.