(a) Describe the role of chemoreceptors in regulation of respiration in avians. (15 marks) (b) Describe the different events of…

(a) Role of Chemoreceptors in Avian Respiration Avian respiration is uniquely regulated by chemoreceptors due to the unidirectional airflow through their rigid lung system, which lacks the tidal volume changes seen in mammals. The primary regulatory mechanism involves peripheral chemoreceptors located in the carotid bodies, which are highly sensitive to arterial partial pressure of oxygen (P_aO₂) and carbon dioxide (P_aCO₂). In domestic fowl, hypoxia triggers a significant hyperventilatory response mediated by these arterial chemoreceptors, increasing minute ventilation to maintain oxygen saturation. Conversely, hypercapnia stimulates central chemoreceptors in the medulla oblongata, which detect changes in cerebrospinal fluid pH resulting from CO₂ diffusion. Additionally, intrapulmonary chemoreceptors (IPC) in the lungs, which are inhibited by rising CO₂, provide breath-by-breath feedback on gas exchange. Together these receptors ensure that the high metabolic demands of flight are met by precise adjustments in respiratory rate and depth, maintaining homeostasis despite the structural constraints of the avian respiratory tract.

(b) Events of the Cardiac Cycle The cardiac cycle is a sequential process of contraction and relaxation that ensures efficient blood flow. It begins with atrial systole, where atrial contraction pushes blood into the ventricles, accounting for the final 20-30% of ventricular filling. This is followed by isovolumetric contraction, a phase that begins with closure of the AV valves (producing the first heart sound), when ventricular pressure rises sharply while all valves remain closed. Once ventricular pressure exceeds arterial pressure, the semilunar valves open, initiating rapid ejection, the phase of maximum blood flow into the aorta and pulmonary artery. As ventricular pressure drops below arterial pressure, ejection slows during reduced ejection. Subsequently, as the ventricles relax, ventricular pressure falls well below arterial pressure, causing the semilunar valves to snap shut (producing the second heart sound) and initiating isovolumetric relaxation. Finally, the AV valves open, allowing passive rapid filling of the ventricles, followed by reduced filling as the flow rate diminishes. These phases are defined by specific pressure-volume relationships and valve dynamics that prevent backflow and ensure unidirectional circulation.

(c) Physiological Functions of Digestive Organs in Sheep The sheep, as a ruminant, possesses a complex digestive system adapted for cellulose fermentation. The process begins in the mouth, where teeth mechanically break down feed, and salivary glands secrete saliva rich in bicarbonate and phosphate buffers to neutralize acids; ruminant saliva has little or no amylase (salivary lipase is present in young lambs). The esophagus transports the bolus to the forestomach. The reticulum acts as a "honeycomb" structure that traps foreign bodies and facilitates the regurgitation of cud for re-chewing. The rumen is the primary site of microbial fermentation, where bacteria and protozoa break down cellulose into volatile fatty acids (VFAs) like acetate, propionate, and butyrate, which are absorbed directly through the rumen wall to provide up to 70% of the animal's energy. Eructation releases excess gas produced during this process. The omasum functions as a "sponge," absorbing water, electrolytes and VFAs, and grinding small particles of feed, thereby reducing the volume of material entering the abomasum. The abomasum is the true stomach, secreting hydrochloric acid and pepsinogen (converted to pepsin) to initiate proteolysis, breaking down microbial and dietary proteins into peptides. The liver produces bile, which emulsifies fats, and the pancreas secretes digestive enzymes (lipase, amylase, proteases) and bicarbonate into the small intestine. Here, the final digestion of peptides into absorbable amino acids occurs via intestinal peptidases, alongside the absorption of nutrients. The caecum and colon carry out some secondary microbial fermentation, and the large intestine absorbs remaining water and electrolytes, forming the pelleted faeces typical of sheep.