Physiology at the Bedside

A senior-clinician teaching guide for doctors in their early years of practice Source companion: introductory Physiology notes Educational material only. Local protocols, current guidelines, and specialist advice take precedence in patient care.

A Word From Your Senior

After decades in clinical rooms, wards, and emergency departments, one lesson becomes difficult to ignore: physiology is not a preclinical subject that you leave behind after an examination. It is the language in which a deteriorating patient speaks before the diagnosis is certain.

When a patient becomes confused, tachypnoeic, oliguric, or hypotensive, do not begin by asking, "Which rare disease is this?" Begin with four quieter questions:

1. What variable is the body trying to defend?
2. What disturbance is threatening it?
3. Which compensatory response is still working?
4. Which response is beginning to fail?

The junior doctor who learns to read compensation will often recognise danger before the laboratory report does.

1. Homeostasis Is Dynamic, Not Motionless

Homeostasis is the regulated maintenance of the internal environment within a viable range. It is not perfect constancy. Temperature, glucose, blood pressure, osmolality, and pH fluctuate with time, posture, meals, sleep, exercise, pregnancy, and illness.

A useful control loop contains:

• a regulated variable, such as arterial pressure or plasma osmolality;
• a sensor, such as a baroreceptor or hypothalamic osmoreceptor;
• an integrating system, which need not always be the brain;
• an effector, such as the heart, vessels, kidneys, skeletal muscle, or an endocrine gland;
• a feedback signal that modifies the response.

Negative feedback opposes a disturbance. Positive feedback amplifies a process and therefore requires a terminating event; examples include the rising oxytocin-contraction cycle during labour and parts of coagulation.

Bedside translation

A blood pressure of 92/58 mmHg is not interpretable in isolation. Ask whether the patient is warm or cold, alert or confused, passing urine, tachycardic, bleeding, febrile, taking beta-blockers, pregnant, or normally hypertensive. The number matters, but the adequacy of perfusion matters more.

Senior pause: Compensation is not proof of stability. A young patient can maintain blood pressure through tachycardia and vasoconstriction until shortly before collapse.

2. Membranes, Gradients, and Why Fluids Can Harm

Cell membranes are selectively permeable lipid bilayers containing channels, carriers, pumps, receptors, and enzymes. Transport is driven by chemical and electrical gradients:

• simple diffusion moves suitable molecules down a concentration gradient;
• facilitated diffusion uses a carrier or channel but no direct metabolic energy;
• primary active transport directly uses energy, as in the Na+/K+-ATPase;
• secondary active transport uses a gradient established by another energy-dependent process;
• osmosis is net water movement driven by differences in water activity across a semipermeable membrane.

Tonicity is the clinically useful idea: it predicts sustained cell-volume change and depends mainly on effective osmoles that do not freely cross the relevant membrane. Osmolality and tonicity are related but are not interchangeable.

Bedside translation

Rapid correction of chronic hyponatraemia can produce osmotic demyelination; excessively rapid correction of chronic hypernatraemia can promote cerebral oedema. The exact correction target depends on acuity, symptoms, cause, and the current guideline or local protocol. Never treat the sodium value without establishing the timeline and monitoring the trajectory.

Do not carry forward this oversimplification: Water does not move merely "toward salt." It moves according to an osmotic gradient, and only solutes acting as effective osmoles sustain tonicity.

3. Excitable Tissue: Signals Have Context

The neuronal action potential results from time-dependent changes in membrane conductance, commonly involving voltage-gated sodium, potassium, and sometimes calcium channels. Conduction velocity varies widely with axon diameter and myelination; it is not uniformly greater than 100 m/s.

At synapses:

• chemical synapses release neurotransmitter across a synaptic cleft;
• electrical synapses transmit current through gap junctions;
• neurotransmission can be excitatory, inhibitory, or modulatory depending on receptor and cell context.

The withdrawal reflex can begin before conscious pain perception because spinal circuits generate the motor response while ascending signals continue toward the brain.

Bedside translation

Hyperkalaemia, hypokalaemia, hypocalcaemia, hypomagnesaemia, ischaemia, toxins, and channel-blocking drugs can all alter excitability. When weakness, paraesthesia, seizure, or arrhythmia appears, physiology should make you review electrolytes, acid-base state, medication exposure, renal function, and the ECG together.

Senior habit: Before labelling altered behaviour as "psychiatric," first exclude hypoglycaemia, hypoxia, hypercapnia, sepsis, drug effects, and major electrolyte disturbance.

4. Muscle: Force, Energy, and Failure

In skeletal muscle, calcium binds troponin, allowing actin-myosin cross-bridge cycling. ATP is required both for cross-bridge detachment and for restoring ionic gradients, including calcium reuptake. Rigor mortis occurs after death when ATP depletion prevents cross-bridge detachment, but its timing and progression are influenced by temperature, exertion, body composition, and cause of death.

Cardiac and smooth muscle use different regulatory arrangements. Do not project the skeletal-muscle troponin model onto every muscle type.

Bedside translation

Weakness may reflect failure at different levels: central motor pathways, peripheral nerve, neuromuscular junction, muscle membrane, contractile apparatus, energy supply, or electrolyte balance. The pattern on history and examination should localise the failure before tests are scattered widely.

5. Circulation: Flow Is the Clinical Question

Arteries carry blood away from the heart and veins carry blood toward the heart. Oxygen content is not the defining feature: pulmonary arteries carry deoxygenated blood, while pulmonary veins carry oxygenated blood.

The key relationships are:

These equations are conceptual guides, not complete descriptions of a pulsatile, compliant circulation. Tissue oxygen delivery also depends on haemoglobin concentration and arterial oxygen saturation, not blood pressure alone.

Bedside translation

A patient can have a normal oxygen saturation and still have poor oxygen delivery because of severe anaemia or low cardiac output. Likewise, a normal blood pressure does not guarantee adequate microcirculatory perfusion.

Assess the whole perfusion picture:

• mental state and skin perfusion;
• pulse character and heart rate;
• capillary refill in clinical context;
• urine output trend;
• lactate trend when indicated;
• likely preload, pump function, vascular tone, and obstruction.

Red flag: In chest pain, dyspnoea, syncope, or shock, prioritise immediately dangerous causes before elegant physiology discussions delay treatment.

6. Ventilation, Gas Exchange, and Oxygen Delivery

Ventilation moves gas between atmosphere and alveoli. Diffusion transfers oxygen and carbon dioxide across the alveolar-capillary membrane. Perfusion delivers blood to ventilated alveoli. Failure may arise from hypoventilation, ventilation-perfusion mismatch, shunt, diffusion limitation, or low inspired oxygen.

In many healthy people, rising carbon dioxide and the resulting pH change are major ventilatory stimuli. This should not become the blanket statement that oxygen is unimportant. Hypoxaemia contributes to ventilatory drive, and the balance changes with altitude, sleep, lung disease, metabolic state, and chronic hypercapnia.

Bedside translation

Pulse oximetry estimates oxygen saturation; it does not measure ventilation. A patient receiving oxygen may have a reassuring saturation while retaining dangerous levels of carbon dioxide. If hypoventilation is plausible, assess clinically and obtain blood-gas information when indicated.

Senior habit: Treat hypoxaemia promptly, prescribe an appropriate target range, and investigate the mechanism. Do not withhold necessary oxygen because of an imprecise fear of "removing hypoxic drive."

7. The Kidney: Filtration Is Only the Beginning

The kidneys regulate extracellular volume, osmolality, electrolytes, acid-base balance, blood pressure, erythropoietin production, and vitamin D metabolism while excreting metabolic waste and xenobiotics.

In a typical healthy adult, glomerular filtration rate is often approximated near 125 mL/min, producing roughly 180 L/day of filtrate, most of which is reabsorbed. These are teaching estimates, not universal normal values; filtration varies with age, body size, pregnancy, haemodynamics, and disease.

The source note's statement that the kidneys clean the entire blood supply about 300 times daily is not a sound clinical description. Renal plasma flow, glomerular filtration, tubular reabsorption, secretion, and final excretion are distinct processes.

Bedside translation

Creatinine is a delayed and imperfect marker. Acute kidney injury can be evolving while creatinine remains initially unchanged. Follow urine output, haemodynamics, fluid balance, medications, obstruction risk, and serial laboratory values.

Red flag: Oliguria is a sign requiring explanation, not an automatic instruction to give fluid. Consider hypovolaemia, pump failure, vasodilation, intrinsic renal injury, obstruction, and measurement error.

8. Endocrine Signalling: Receptors Decide the Response

Hormones circulate widely, but response depends on receptor expression, intracellular signalling, concentration, binding proteins, pulsatility, and feedback loops.

Insulin suppresses hepatic glucose output and promotes glucose uptake particularly in skeletal muscle and adipose tissue through GLUT4. It is inaccurate to say simply that insulin makes all cells absorb glucose; some tissues, including red blood cells and much of the brain, use insulin-independent glucose transporters.

Diabetes mellitus is also more complex than "too little insulin." Type 1 diabetes involves autoimmune beta-cell destruction and absolute insulin deficiency; type 2 diabetes combines insulin resistance with progressive beta-cell dysfunction. Other specific and gestational forms exist.

Bedside translation

When reviewing an endocrine result, ask:

1. Was the sample taken at the correct time and under the right conditions?
2. Is the target-gland hormone appropriate for the trophic hormone?
3. Could acute illness, pregnancy, medication, or assay interference alter the result?
4. Does the biochemical pattern fit the patient?

9. One Patient, Several Systems

A 72-year-old patient arrives with fever, tachypnoea, confusion, blood pressure 96/60 mmHg, warm peripheries, and declining urine output.

Do not reduce the case to "infection plus low BP." Read the physiology:

• Fever and inflammatory signalling increase metabolic demand.
• Tachypnoea may reflect hypoxaemia, metabolic acidosis, lung pathology, pain, or central drive.
• Warm peripheries with low pressure may suggest reduced vascular tone, though shock phenotypes evolve.
• Confusion may indicate impaired cerebral function from sepsis, hypoxaemia, hypotension, metabolic disturbance, medication, or another cause.
• Oliguria may reflect reduced renal perfusion, intrinsic injury, obstruction, or inaccurate measurement.

Your first responsibility is simultaneous: stabilise immediate threats, obtain focused data, start time-critical treatment under the applicable sepsis pathway, and repeatedly reassess response. Physiology is not a substitute for the protocol; it tells you what the protocol is trying to restore and when the expected response is absent.

10. Questions I Would Ask on a Ward Round

1. Which finding in this patient is compensation, and what would failure look like?
2. What mechanism could explain all the major abnormalities with the fewest assumptions?
3. Which dangerous alternative cannot be missed today?
4. What treatment could worsen the physiology if our assumption is wrong?
5. What measurable response do we expect, and by when will we reassess it?
6. Which part of our explanation is established fact, and which part is inference?

A compact answer framework

Use M-O-V-E-R:

• Mechanism: what process is disturbed?
• Oxygen delivery: is ventilation, saturation, haemoglobin, flow, or extraction failing?
• Volume and vascular tone: what is the effective circulating state?
• Electrolytes and endocrine control: what regulated variable is abnormal?
• Response: is compensation effective, and did treatment produce the predicted change?

Research and Learning Prompts

Use these to move from competent recall to mature clinical judgment:

• Compare static vital-sign thresholds with trajectory-based recognition of deterioration.
• Review how pregnancy, ageing, athletic conditioning, and medications alter expected physiology.
• Study why oxygen saturation can remain normal despite falling oxygen delivery.
• Compare osmolality with tonicity using hyponatraemia cases.
• Trace one patient with shock through preload, cardiac output, vascular resistance, microcirculation, and cellular oxygen use.
• Examine where creatinine and estimated GFR become unreliable in acute illness or unusual body composition.

Accuracy Notes on the Companion Source

The companion guide is useful for first intuition, but a doctor should refine these statements:

Trusted Starting References

• NCBI Bookshelf: Physiology (opens in a new tab)
• Merck Manual Professional: Biology of the cardiovascular system (opens in a new tab)
• Merck Manual Professional: Control of breathing (opens in a new tab)
• KDIGO clinical practice guidelines (opens in a new tab)
• World Health Organization: Patient safety (opens in a new tab)

References are starting points, not a claim that every page remains unchanged. Check publication dates, updates, local policy, and the primary literature before applying a number or recommendation.

Closing Counsel

You do not need to know every answer immediately to be a trustworthy doctor. You do need to recognise instability, make your assumptions visible, ask for help early, and return to see whether the patient responded as predicted.

Physiology gives you something more durable than a list: it gives you a way to remain useful when the diagnosis is still uncertain.