Human Physiology for Engineers

The body's systems through an engineer's eyes. Learn the cardiovascular system (heart as a pump, cardiac cycle, pressure–volume loop, Windkessel model), respiratory system (volumes, gas exchange, compliance), nervous and musculoskeletal systems, renal filtration, and homeostasis as feedback control — with a Python Windkessel model of arterial pressure.

You cannot design a heart valve without understanding the cardiac cycle, or a ventilator without knowing lung mechanics. Engineers study physiology quantitatively: pressures, flows, volumes, time constants and control loops.

Many physiological systems map neatly onto engineering models — lumped-parameter circuits, pumps and valves, diffusion and feedback control — which is what makes simulation, device design and diagnostics possible.

An analogy

The cardiovascular system is a plumbing network with a two-sided pulsating pump (the heart), elastic pipes that smooth the pulses (arteries — like a pressure accumulator), and millions of tiny resistors (capillaries) where the real exchange happens.

Cardiovascular system

Two pumps in series: the right heart sends blood to the lungs (low pressure, ~25/10 mmHg), the left heart to the body (~120/80 mmHg). Four valves keep flow one-way. The cardiac cycle has isovolumic contraction, ejection, isovolumic relaxation and filling; the pressure–volume loop's area is the stroke work.

Respiratory and renal systems

Lungs move ~0.5 L per breath at 12–16 breaths/min; gas exchange happens by diffusion across a ~70 m² alveolar membrane only ~0.5 µm thick. Lung compliance and airway resistance form an RC system — the basis of ventilator design. Kidneys filter ~180 L/day of plasma (GFR ≈ 125 mL/min) and reabsorb 99%: a precise separation plant, replaced in failure by dialysis.

Nervous and musculoskeletal systems

Neurons transmit electrical action potentials (up to ~120 m/s in myelinated fibres) and communicate chemically at synapses. Skeletal muscles are actuators controlled by motor units; force grows with recruitment and firing rate. Bones are levers, joints are bearings with remarkably low friction (μ ≈ 0.001–0.01 in cartilage).

Homeostasis as control

The body maintains its internal environment with feedback loops: baroreflex (blood pressure), thermoregulation, glucose–insulin (the target of artificial pancreas systems), and respiratory chemoreceptors (CO₂). These loops have gains, delays and set-points; disease often appears as a loop failure, and closed-loop medical devices (insulin pumps, anaesthesia delivery, ventilators) must interact safely with them.

Formulas

Worked example: Ejection fraction

Given: EDV 120 mL, ESV 50 mL

Find: SV and EF.

Answer: 58% (normal)

Worked example: Fick cardiac output

Given: V̇O₂ 250 mL/min, CaO₂ 200 mL/L, CvO₂ 150 mL/L

Find: CO.

Answer: 5 L/min

Worked example: Minute ventilation

Given: V_T 500 mL, 14 breaths/min

Find: V̇_E.

Answer: 7 L/min

Windkessel model of arterial pressure

# Windkessel model: arterial pressure from heart ejection (2-element)
import math
R, C = 1.0, 1.5            # mmHg*s/mL, mL/mmHg
T, Ts, SV = 0.8, 0.3, 70.0 # period, systole duration, stroke volume
dt, P = 0.0005, 80.0
Pmax = Pmin = None
for beat in range(20):
    Pmx, Pmn = -1e9, 1e9
    for n in range(int(T / dt)):
        t = n * dt
        Q = (SV * math.pi / (2 * Ts)) * math.sin(math.pi * t / Ts) if t < Ts else 0.0   # mL/s
        P += dt * (Q - P / R) / C
        Pmx, Pmn = max(Pmx, P), min(Pmn, P)
    Pmax, Pmin = Pmx, Pmn
print(f"steady-state pressure: {Pmax:.0f}/{Pmin:.0f} mmHg, mean flow {SV/T*60/1000:.2f} L/min")
# steady-state pressure: 103/73 mmHg, mean flow 5.25 L/min

The compliant arteries store blood during systole and release it in diastole, turning the heart's pulsatile ejection into a steady capillary flow — exactly like an RC low-pass filter. Stiffer arteries (smaller C, e.g. with age) raise systolic and pulse pressure.

Common mistakes

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Key takeaways

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