The phase of pulmonary ventilation in which air is expelled from the alveoli is:

A. The cerebral cortex is involved in higher brain functions like thinking and voluntary control of breathing but does not contain central chemoreceptors.
B. Central chemoreceptors are located in the medulla oblongata, a part of the brain stem. They detect changes in the pH of cerebrospinal fluid, which reflects CO₂ levels in the blood, and help regulate breathing accordingly.
C. The aorta contains peripheral chemoreceptors, not central ones. These aortic bodies monitor blood oxygen, carbon dioxide, and pH, but they are outside the central nervous system.
D. The cerebellum coordinates motor control and balance and does not play a role in chemoreception.

A. The phrenic nerve is responsible for stimulating the diaphragm to contract and initiate breathing. It originates from the cervical spinal nerves (C3-C5) and plays a critical role in respiration.
B. The pons is a part of the brainstem involved in regulating breathing patterns but is not a nerve itself.
C. The sciatic nerve is the largest nerve in the body, primarily responsible for motor and sensory functions in the lower limbs, not for breathing.
D. The accessory nerve (Cranial Nerve XI) is involved in motor functions of the neck and shoulders, not the diaphragm.