Heart rate and stroke volume rise, increasing cardiac output. Blood flow is redistributed towards active skeletal muscle to support its increased metabolic demand.

Creatinine reaches urine through both glomerular filtration and tubular secretion. The secreted component makes its clearance greater than filtration alone would predict.

Plasma volume expands more than red cell mass. This dilutes circulating red cells, producing the physiological haemodilution of pregnancy.

Rapid eye movements occur alongside marked suppression of skeletal muscle tone. This muscle atonia is a characteristic feature of normal REM sleep.

PTH promotes renal calcium retention, phosphate excretion and activation of vitamin D. Active vitamin D increases intestinal calcium absorption. PTH also promotes calcium mobilisation from bone.

The hypothalamic preoptic region integrates thermal information. Sweating increases evaporative cooling, while skin vasodilatation promotes transfer of heat towards the environment.

Iron balance is regulated mainly through absorption rather than controlled excretion. Iron is lost through processes such as cell shedding and bleeding, without an equivalent regulated excretory pathway.

Hypothalamic TRH stimulates pituitary TSH release. TSH stimulates thyroid hormone production. Circulating thyroid hormones inhibit further TRH and TSH secretion.

Glucagon stimulates hepatic glycogen breakdown and glucose synthesis. These actions increase glucose release from the liver when dietary glucose is unavailable.

Hypothalamic CRH stimulates pituitary ACTH, which drives adrenal cortisol secretion. Cortisol feeds back to restrain hypothalamic and pituitary stimulation.

Barometric pressure falls, lowering inspired oxygen partial pressure even though oxygen's proportion in air remains essentially unchanged. Hypoxia stimulates increased ventilation.

Angiotensin II preferentially constricts the efferent arteriole. This supports glomerular capillary pressure despite reduced renal blood flow.

Acetylcholinesterase breaks down acetylcholine in the synaptic cleft. Choline is taken back into the nerve terminal for reuse in acetylcholine synthesis.

ATP binding allows myosin to detach from actin. ATP hydrolysis then energises the head for another cross-bridge cycle.

Ventilation increases, lowering arterial carbon dioxide. This reduces the carbonic acid contribution and moves pH towards normal without removing the underlying metabolic disturbance.

The ductus venosus routes some oxygen-rich umbilical venous blood towards the inferior vena cava, bypassing hepatic circulation.