Organ System Integration and Homeostasis (3B) - MCAT Biological and Biochemical Foundations of Living Systems

$MAP = CO \times TPR$. This relationship shows blood pressure depends on cardiac output and vascular resistance, key to homeostatic control.

Parathyroid hormone (PTH). PTH mobilizes calcium from bone, enhances renal reabsorption, and activates vitamin D to elevate serum levels during hypocalcemia.

Atrial natriuretic peptide (ANP). ANP counters volume overload by inhibiting sodium reabsorption, promoting diuresis and natriuresis to reduce blood pressure.

Decreases plasma $Ca^{2+}$ (inhibits osteoclast activity). Calcitonin, secreted by thyroid C cells, reduces calcium by suppressing bone resorption during hypercalcemia.

Hyperventilation to decrease $P_{CO_2}$. Hyperventilation expels CO2, shifting the bicarbonate equilibrium to raise pH and compensate for acid accumulation.

Increase $H^+$ secretion and increase $HCO_3^-$ reabsorption. Kidneys enhance acid excretion and bicarbonate generation to buffer retained CO2 and restore acid-base balance.

Rapidly adjusts heart rate and vascular tone to stabilize pressure. Baroreceptors detect pressure changes and trigger autonomic adjustments to maintain cardiovascular homeostasis acutely.

Carotid sinus and aortic arch. These high-pressure sites sense arterial stretch, providing rapid feedback to the brainstem for blood pressure regulation.

Increased sympathetic and decreased parasympathetic outflow. Reduced baroreceptor signaling during hypotension activates sympathetic responses to increase heart rate and vasoconstriction.

Increase blood pressure and extracellular fluid volume. RAAS activation conserves sodium and water, thereby expanding plasma volume and elevating blood pressure.

Maintenance of a stable internal environment despite external change. Homeostasis ensures physiological variables remain constant through feedback mechanisms that counteract deviations from set points.

Negative opposes change; positive amplifies change. Negative feedback restores equilibrium by reversing deviations, while positive feedback escalates them for processes like childbirth.

Sensor (receptor), integrator (control center), effector. These components detect changes, process information, and execute responses to maintain homeostasis in physiological systems.

Target value of a regulated variable (the reference level). It represents the optimal level around which feedback mechanisms operate to minimize deviations in the variable.

Core body temperature. Core temperature is tightly regulated via negative feedback to support enzymatic activity and metabolic processes.

Hypothalamus (especially the preoptic area). This region integrates thermal sensory input and coordinates effector responses to maintain core body temperature.

Cutaneous vasodilation and sweating. These mechanisms enhance evaporative and convective heat loss to counteract elevated core temperature in hot environments.

Shivering and increased metabolic heat production. These responses generate internal heat to restore core temperature during cold exposure, preventing hypothermia.

Decreased renal perfusion pressure (low afferent arteriole stretch). Low pressure sensed by juxtaglomerular cells signals hypovolemia, prompting renin secretion to activate RAAS.

Converts angiotensinogen to angiotensin I. Renin cleaves angiotensinogen, initiating the cascade that leads to vasoconstriction and sodium retention.

Vasoconstriction, increasing total peripheral resistance. Angiotensin II constricts arterioles to elevate blood pressure, supporting perfusion during hypovolemic states.

Increase $Na^+$ reabsorption and $K^+$ secretion. Aldosterone promotes sodium retention and potassium excretion to increase extracellular fluid volume and maintain electrolyte balance.

Increases water reabsorption by inserting aquaporin-2 channels. ADH enhances permeability of collecting ducts to water, concentrating urine and conserving body water during dehydration.

Increased plasma osmolality (hyperosmolarity). Osmoreceptors in the hypothalamus detect high osmolality, triggering ADH to restore plasma tonicity via water reabsorption.