Physiology Review for Medical Education

A firm grasp of physiology is the cornerstone of clinical reasoning; it transforms a list of symptoms into a coherent story of dysfunction. This review will solidify your understanding of how major organ systems operate, how they communicate, and how they fail—knowledge essential for diagnosing disease and predicting patient responses to treatment.

Homeostasis and Feedback: The Core Governing Principle

Homeostasis is the maintenance of a stable internal environment despite external changes. This dynamic equilibrium is orchestrated primarily through feedback loops. A negative feedback loop is the body's primary regulatory mechanism, where a change in a variable triggers a response that opposes the initial change, returning the system to its set point. For example, a rise in blood pressure is sensed by baroreceptors, leading to vasodilation and decreased heart rate to lower pressure back to normal. In contrast, a positive feedback loop amplifies the initial change, driving a system away from its starting point until a separate event halts the process. While less common, it is crucial in processes like parturition (uterine contractions) and blood clotting. Understanding these loops is key to predicting how the body will react to illness or intervention.

Cardiorespiratory Physiology: Pressure, Flow, and Gas Exchange

Cardiovascular System

The cardiovascular system's primary function is to generate pressure to deliver oxygen and nutrients. Central to this is the Frank-Starling mechanism, which states that the stroke volume of the heart increases in response to an increase in the volume of blood filling the heart (end-diastolic volume). This intrinsic regulation ensures that output matches venous return. Cardiac output ($CO$), the volume of blood pumped per minute, is the product of heart rate ($HR$) and stroke volume ($SV$): $$CO=HR\times SV$$. Systemic vascular resistance is the opposition to blood flow offered by all the systemic blood vessels, primarily the arterioles. Mean arterial pressure ($MAP$) is determined by cardiac output and systemic vascular resistance: $$MAP=CO\times SVR$$. This equation is fundamental; hypertension, for instance, results from an increase in CO, SVR, or both. The autonomic nervous system modulates this system moment-to-moment: sympathetic activation increases HR, contractility, and SVR (via vasoconstriction), while parasympathetic activation (vagal tone) decreases HR.

Respiratory System

The respiratory system exists to facilitate the exchange of $O_2$ and $C{O}_2$. Ventilation is the mechanical movement of air, while diffusion is the passive movement of gases down their partial pressure gradients. A critical high-yield concept is the $V/Q$ ratio (ventilation-perfusion ratio). Ideal gas exchange occurs where alveolar ventilation and capillary blood flow are matched ($V/Q$ = 1). A low $V/Q$ ratio (e.g., pulmonary embolism blocking blood flow) results in wasted ventilation (dead space). A high $V/Q$ ratio (e.g., asthma obstructing airflow) results in shunting, where blood passes unoxygenated. Oxygen is primarily transported bound to hemoglobin, and its binding is depicted by the oxyhemoglobin dissociation curve. A rightward shift (e.g., caused by increased $PC{O}_2$, acidity, temperature, or 2,3-DPG) decreases hemoglobin's affinity for oxygen, promoting unloading to tissues. A leftward shift (e.g., in the lungs) increases affinity, facilitating loading.

Renal and Endocrine Physiology: Fluid, Hormone, and Metabolic Regulation

Renal System

The kidney regulates fluid volume, electrolyte balance, and acid-base status through three core processes: filtration, reabsorption, and secretion. The glomerular filtration rate ($GFR$) is the volume of fluid filtered from the glomerular capillaries into Bowman's capsule per unit time. It is regulated by the renin-angiotensin-aldosterone system ($RAAS$), which is activated by low blood pressure, low sodium, or sympathetic stimulation. Renin leads to angiotensin II formation, causing vasoconstriction and stimulating aldosterone release, which promotes sodium (and water) reabsorption in the distal nephron. The countercurrent multiplier system in the Loop of Henle establishes a medullary concentration gradient, allowing the production of concentrated urine under the influence of antidiuretic hormone ($ADH$). The kidneys also maintain acid-base balance by reabsorbing filtered bicarbonate and secreting $H^{+}$ ions as titratable acid and ammonium ($N{H}_4^{+}$).

Endocrine System

Endocrine glands secrete hormones—chemical messengers transported in the bloodstream to act on distant target cells. Hormone action is governed by feedback, most commonly negative feedback. A classic axis is the hypothalamic-pituitary-target gland axis (e.g., hypothalamus (CRH) → anterior pituitary (ACTH) → adrenal cortex (cortisol)). Cortisol then feeds back to inhibit both CRH and ACTH release. Insulin and glucagon are pancreatic hormones that regulate plasma glucose. Insulin, released in response to high blood glucose, promotes glucose uptake (into muscle and fat) and storage as glycogen. Glucagon, released during hypoglycemia, stimulates glycogenolysis and gluconeogenesis to raise blood glucose. Understanding these axes allows you to pinpoint the level of dysfunction in endocrine disorders.

Gastrointestinal and Neurophysiology: Motility, Absorption, and Neural Signaling

Gastrointestinal System

The GI system processes food via coordinated motility and secretion. Swallowing initiates peristalsis, the propulsive wave of smooth muscle contraction. In the stomach, parietal cells secrete hydrochloric acid and intrinsic factor, while chief cells secrete pepsinogen. Gastric motility and secretion are controlled by the vagus nerve and hormones like gastrin. The enteric nervous system is the "gut brain" and can function independently. Most absorption occurs in the small intestine. Carbohydrates are absorbed as monosaccharides, proteins as amino acids and small peptides, and fats as micelles after being emulsified by bile. The sodium-glucose cotransporter ($SGLT1$) on the apical surface of enterocytes is a key secondary active transport system that couples glucose absorption to sodium entry.

Neurophysiology

At the cellular level, neuronal function rests on resting membrane potential (maintained by the $N{a}^{+}/K^{+}$ ATPase pump), graded potentials, and action potentials. An action potential is a rapid, all-or-none depolarization triggered when a stimulus reaches threshold. It propagates unidirectionally due to the refractory period. At the synapse, an action potential causes $C{a}^{2+}$ influx, leading to vesicle fusion and neurotransmitter release into the synaptic cleft. Neurotransmitters bind to receptors on the postsynaptic cell, causing either excitatory postsynaptic potentials ($EPSPs$) or inhibitory postsynaptic potentials ($IPSPs$). Spatial and temporal summation of these potentials determines if the postsynaptic neuron will fire. The autonomic nervous system's sympathetic ("fight or flight") and parasympathetic ("rest and digest") divisions are a prime example of efferent neurophysiology with direct clinical relevance.

Common Pitfalls

1. Confusing Starling's Law of the Heart with Starling's Forces in Capillaries: The Frank-Starling mechanism (heart) relates preload to stroke volume. Starling's forces (capillaries) govern fluid movement: Hydrostatic pressure pushes fluid out, and oncotic pressure pulls fluid in. Net filtration pressure = (Capillary hydrostatic pressure - Interstitial hydrostatic pressure) - (Capillary oncotic pressure - Interstitial oncotic pressure).
2. Misapplying the Oxyhemoglobin Dissociation Curve: A rightward shift does not mean hemoglobin holds less oxygen at a given $P{O}_2$; it means it has a lower affinity, so it releases oxygen more readily. The percent saturation for a given $P{O}_2$ is lower, which facilitates unloading to tissues.
3. Equating GFR with Urine Output: GFR is the filtration rate (~180 L/day), while urine output is the final excreted volume (~1-2 L/day). The massive difference is due to reabsorption. A patient can have a severely decreased GFR but still produce normal urine output if reabsorption is also impaired (as in diuretic use or diabetes insipidus).
4. Overlooking Hormone Feedback Loops: When interpreting endocrine labs, always think in terms of the axis. A high level of a target gland hormone (e.g., cortisol) with a low pituitary hormone (e.g., ACTH) indicates a primary disorder (e.g., adrenal tumor). A high target hormone with a high pituitary hormone indicates a secondary disorder (e.g., pituitary tumor).

Summary

• Physiology is integrative: Every organ system relies on and influences others through shared variables like blood pressure, $pH$, and metabolite levels, all governed by homeostatic feedback loops.
• Master the governing equations: Key relationships like $CO=HR\times SV$, $MAP=CO\times SVR$, and the determinants of net filtration pressure are not just formulas but frameworks for clinical thought.
• Understand the "why" behind curves and graphs: The oxyhemoglobin curve, cardiac function curves, and acid-base nomograms are visual representations of core principles; interpreting them requires knowing what causes shifts along or of the curve.
• Link mechanism to clinical presentation: A physiological disturbance (e.g., low $V/Q$ mismatch, RAAS overactivation, insulin deficiency) directly creates the signs and symptoms you will see in patients.
• Focus on regulation: For any system, ask: What is being regulated? How is it sensed? What is the effector response? This approach reliably leads you to the underlying pathophysiology.