For each of the clinical descriptions (A–E) select the most appropriate options from the following list of circulatory changes

A. Option 1 Increased cerebral vascular resistance. Hyperventilation leads to constriction of cerebral vessels due to washout of carbon dioxide from the body. This leads to decreased cerebral capillary pressure and a reduction in cerebral interstitial fluid volume, thereby reducing the oedema generated by head injury.

B. Option 5 Increased splanchnic resistance. In circulatory failure, blood pressure is supported by increased peripheral vascular resistance induced by the baroreceptor reflex, particularly in the splanchnic circulation. Splanchnic vasoconstriction occurs early in the condition. The resultant stagnant hypoxia in the alimentary mucosa is thus a sensitive index of the early stages of circulatory failure before more severe effects such as hypotension are obvious.

C. Option 4 Decreased coronary resistance. Myocardial infarction results from complete or almost complete cessation of perfusion of a region of cardiac muscle due to blocking, often by thrombosis, of a coronary artery or arteries. This reduces flow by a massive increase in resistance and a considerable mass of myocardium is threatened by stagnant hypoxia due to poor flow (ischaemia). Activation of circulating plasminogen to plasmin allows breakdown of blood clot (thrombolysis) and decreases regional coronary resistance to a level which allows recovery of ischaemic areas.

D. Option 2 Decreased cerebral vascular resistance. Carbon dioxide is an important determinant of cerebral blood flow by its local vasodilator action (in underperfused areas carbon dioxide accumulates and this leads to vasodilation and restoration of normal perfusion). When there is a raised level of carbon dioxide in arterial blood there is generalized cerebral vasodilation and this leads to increased formation of tissue fluid (oedema). The resultant increased intracranial pressure leads to headaches and papilloedema, imitating the effects of an intracranial tumour or abscess.

E. Option 12 Decreased regional perfusion pressure. With persistent diarrhoea and vomiting, extracellular fluid volume can fall severely. The reduced plasma volume leads to hypotension, especially in the elderly whose compensatory mechanisms are blunted. Sitting and standing trap circulating fluid in the feet causing a severe fall in arterial blood pressure. This decreases cerebral perfusion pressure to a point at which a local decrease in vascular resistance cannot compensate and loss of consciousness (syncope) occurs from cerebral ischaemia (inadequate blood flow).