Integrated respiratory, circulatory and cellular physiology
Respiratory physiology separates ventilation, perfusion, diffusion and oxygen transport. A shunt is perfusion of inadequately ventilated regions, whereas physiological dead space includes ventilation that does not effectively exchange carbon dioxide with perfusing blood. Hypoxic pulmonary vasoconstriction can improve regional matching by diverting perfusion away from poorly ventilated lung. Functional residual capacity describes the gas remaining after a normal tidal expiration. Reduced reserve together with increased oxygen consumption helps explain why some patients desaturate rapidly during apnea, even if a previous saturation reading was reassuring.
Oxygen saturation is not oxygen content. Hemoglobin concentration, saturation and dissolved oxygen contribute differently to content, while delivery also depends on blood flow. Changes in temperature, hydrogen-ion concentration, carbon dioxide and red-cell metabolites affect hemoglobin affinity. A rightward dissociation-curve shift promotes unloading at a given oxygen tension; it does not independently prove that whole-body oxygen delivery has improved. Carbon dioxide travels predominantly as bicarbonate, with hemoglobin and other buffering systems contributing to the transport process.
Cardiac output depends on heart rate and stroke volume, with stroke volume influenced by preload, afterload and contractility. Frank-Starling reasoning applies within physiological limits and does not mean unlimited volume always improves output. Left ventricular coronary perfusion occurs predominantly during diastole, so tachycardia can reduce perfusion time while increasing demand. Vascular resistance, venous return and ventricular interaction must be considered when interpreting a pressure change. Autoregulation is a physiological concept with patient-dependent limits, not a guarantee that every organ remains protected throughout one fixed textbook pressure range.
Renal reasoning integrates perfusion, filtration, tubular handling and endocrine control. Vasopressin promotes water reabsorption through collecting-duct aquaporin regulation, while aldosterone influences sodium and potassium handling. Osmolality and tonicity are related but distinct: the effective osmotic action across a relevant membrane determines water movement. Acid-base analysis starts with pH, carbon dioxide and bicarbonate, then tests whether the compensation is appropriate and whether an additional disorder is present. A low bicarbonate with a low carbon dioxide does not by itself exclude a mixed disturbance. At the cellular level, distinguish fast sodium-dependent depolarization in working ventricular myocytes from the calcium-dependent upstroke in nodal pacemaker tissue. The anatomical tissue and the phase of its action potential must be specified before choosing the relevant ion current.