Physiology Calculators

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Physiology is the scientific study of the normal functions of living organisms and their parts — how cells, tissues, organs, and organ systems work together to maintain life. It encompasses the study of all body systems: cardiovascular, respiratory, nervous, endocrine, digestive, renal, reproductive, musculoskeletal, and immune. Physiological processes are governed by the principle of homeostasis — the body's ability to maintain a stable internal environment (temperature, pH, ion concentrations, blood pressure) through negative feedback mechanisms. Human physiology provides the scientific foundation for medicine, pharmacology, and exercise science.

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Homeostasis and Feedback Control

Homeostasis: the maintenance of stable internal conditions despite external changes. Regulated by feedback loops: Negative feedback: a change in a variable triggers a response that opposes the change and restores the set point (e.g., rising blood glucose triggers insulin release → glucose uptake → falling blood glucose → reduced insulin → set point restored). Most physiological regulation uses negative feedback. Positive feedback: a change triggers responses that amplify the change (e.g., uterine contractions during labor — contraction → oxytocin release → more contraction). Positive feedback is less common and typically terminates at a discrete endpoint.

Major Body Systems

  • Cardiovascular: Heart pumps blood through pulmonary (lungs) and systemic circuits; cardiac output = heart rate × stroke volume; regulated by autonomic nervous system, hormones
  • Respiratory: Gas exchange — O₂ in, CO₂ out; controlled by medullary respiratory centers responding to PCO₂ and pH
  • Renal: Filters blood, regulates water/electrolyte balance, acid-base balance, excretes waste; GFR ≈ 125 mL/min
  • Nervous system: Rapid electrical signaling via action potentials; central (brain + spinal cord) and peripheral (somatic + autonomic)
  • Endocrine: Hormonal regulation; slower but sustained effects; hypothalamus-pituitary-target organ axes

Key Physiological Parameters

  • Blood pressure: 120/80 mmHg (systolic/diastolic)
  • Heart rate: 60–100 bpm at rest; cardiac output ≈ 5 L/min
  • Blood pH: 7.35–7.45 (maintained by buffers, lungs, kidneys)
  • Blood glucose: 70–110 mg/dL (3.9–6.1 mmol/L) fasting
  • Body temperature: 36.5–37.5°C core

Glossary

Homeostasis
Maintenance of stable internal conditions (temperature, pH, blood pressure, glucose) through negative feedback loops; essential for normal cellular function.
Cardiac Output (CO)
Heart rate × stroke volume; normally ~5 L/min at rest; increases to 20–25 L/min during exercise; regulated by autonomic nervous system, preload, afterload, and contractility.
GFR (Glomerular Filtration Rate)
Volume of plasma filtered by the kidneys per minute (~125 mL/min normally); used to stage chronic kidney disease; estimated from serum creatinine using the CKD-EPI equation.

Frequently Asked Questions

Homeostasis is the maintenance of stable internal conditions (temperature, pH, ion concentrations, blood pressure, blood glucose) despite changing external conditions or internal demands. It is maintained primarily by negative feedback loops: a change in a variable is sensed by a receptor → information is relayed to a control center → the control center activates effectors that counteract the change → the variable is restored to its set point → reduced signal to effectors. Example: falling blood pressure → baroreceptors signal vasomotor center → increased sympathetic activity → vasoconstriction and increased heart rate → blood pressure rises back toward normal. Without homeostasis, physiological variables would drift to levels incompatible with life.

Cardiac output (CO) = heart rate (HR) × stroke volume (SV); normally ~5 L/min at rest. HR (60–100 bpm at rest) is regulated by: sympathetic nervous system (increases HR via norepinephrine on β₁ receptors); parasympathetic (vagus nerve decreases HR via acetylcholine on muscarinic receptors); circulating catecholamines (epinephrine). SV is determined by: preload (ventricular filling volume — Frank-Starling mechanism); afterload (resistance against which ventricle pumps); contractility (intrinsic pumping strength). During exercise, CO can reach 20–25 L/min in trained athletes — primarily through increased SV and HR.

GFR (normally ~125 mL/min = 180 L/day) is the volume of plasma filtered through the glomerular capillaries into the Bowman's capsule per unit time. Nearly all filtered volume is reabsorbed (~99%) — only ~1.5–2 L/day is excreted as urine. GFR reflects overall kidney function and is used clinically to stage chronic kidney disease (CKD): normal ≥ 60 mL/min/1.73 m²; CKD stages 1–5 (stage 5 = kidney failure < 15 mL/min). Estimated GFR (eGFR) is calculated from serum creatinine (a muscle breakdown product cleared by filtration): eGFR = 141 × min(Scr/κ,1)^α × max(Scr/κ,1)^(−1.209) × 0.993^Age × 1.018 (if female) × 1.159 (if African American) [CKD-EPI equation].

Blood pH = 7.35–7.45, maintained by: (1) Chemical buffers (bicarbonate most important: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻); phosphate; protein. (2) Pulmonary regulation (minutes): changing ventilation rate changes PCO₂ → changes H₂CO₃ → changes pH. Respiratory acidosis (↑PCO₂): hypoventilate. Respiratory alkalosis (↓PCO₂): hyperventilate. (3) Renal regulation (hours–days): kidneys excrete H⁺ (in phosphate and ammonium buffers) or retain/excrete HCO₃⁻ based on pH. Metabolic acidosis → renal H⁺ excretion and HCO₃⁻ retention + respiratory compensation (hyperventilation). Together, these mechanisms maintain pH within narrow limits essential for enzyme function and cellular homeostasis.