How organ systems keep the internal environment
- Concept
- Exam priority: medium
Cells live in a shared fluid. Lungs, gut, liver and kidneys keep refilling it with what cells need and clearing what they make.
Also called: Milieu intérieur, Extracellular fluid composition
See it in 3D: Left lower lobe · Liver · Kidneys · Stomach · Jejunum
What it is
Every cell bathes in extracellular fluid, which the circulation mixes so fast that its composition is nearly the same everywhere. Supply organs (lungs, gut, liver, muscle and fat stores) add O₂ and nutrients; removal organs (lungs, kidneys, liver) take out CO₂, acids, urea and other end products. Each key variable has a narrow reference range and an organ that corrects it; ranges also shift with age, sex and pregnancy.
Key facts
- Physiological variability is real: hemoglobin and creatinine run higher in men (more androgens, more muscle); heart and breathing rates are higher in infants; GFR falls with age.
- Pregnancy lowers PaCO₂ (≈ 30 mmHg, progesterone drives breathing), plasma osmolality and hemoglobin concentration: normal for pregnancy, abnormal outside it.
- Ancestry-based corrections are being removed from some formulas (e.g. race-free eGFR equations); measured muscle mass and diet explain much of the variation.
- Altitude lowers PaO₂ and raises hemoglobin in residents of high cities such as Bogotá (≈ 2600 m).
Reference values in adults (vary by laboratory)
| Parameter | Value | Note |
|---|---|---|
| Arterial pH | 7.35–7.45 | |
| PaCO₂ | 35–45 mmHg | |
| PaO₂ (sea level, breathing air) | ≈ 80–100 mmHg | lower at altitude and with age |
| Plasma Na⁺ | 135–145 mmol/L | |
| Plasma K⁺ | 3.5–5.0 mmol/L | |
| Plasma HCO₃⁻ | 22–28 mmol/L | |
| Total calcium | ≈ 8.5–10.5 mg/dL | ionized ≈ 4.5–5.3 mg/dL |
| Fasting glucose | 70–99 mg/dL | |
| Plasma osmolality | ≈ 275–295 mOsm/kg H₂O | |
| Core temperature | ≈ 36.5–37.5 °C | lowest early morning |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Who supplies, who removes
| Variable | Main supplier | Main remover or corrector | Time scale |
|---|---|---|---|
| O₂ | Lungs (alveolar gas exchange) | Used by all cells | Seconds |
| CO₂ (and pH) | Produced by cell metabolism | Lungs (ventilation); kidney adjusts HCO₃⁻ | Minutes (lung), days (kidney) |
| Glucose | Gut (absorption), liver (glycogen, gluconeogenesis) | Uptake by muscle, fat, brain under insulin | Minutes to hours |
| Amino acids and fats | Gut absorption, liver processing, fat and muscle stores | Liver (urea formation), tissues | Hours |
| Urea, creatinine, uric acid | Liver (urea), muscle (creatinine) | Kidneys | Hours |
| Na⁺, K⁺, water | Diet and drinking | Kidneys (aldosterone, ADH), with thirst | Hours to days |
| Heat | Metabolism, muscle | Skin (blood flow, sweat), lungs | Minutes |
Clinical notes
- Shock: falling perfusion means supply (O₂) and removal (CO₂, lactate) both fail at tissue level, so lactate and acidosis rise even when lungs and kidneys are healthy.
- Diabetes: glucose supply from the liver is no longer restrained and uptake into muscle and fat falls, so plasma glucose leaves its range.
- Heat stroke: heat production or gain exceeds what the skin can remove; core temperature above ≈ 40 °C damages proteins and organs.
Test yourself
Think of the answer, then tap the question to see it.
A patient hypoventilates and PaCO₂ rises acutely. Which organ corrects the pH first, and which over days?
Buffers act at once, the lung is the fast corrector once breathing improves, and the kidney compensates over days by retaining HCO₃⁻ and excreting H⁺.
Why is a PaCO₂ of 30 mmHg normal in late pregnancy but abnormal in a non-pregnant adult?
Progesterone stimulates breathing in pregnancy, so the expected PaCO₂ falls; reference ranges depend on the physiological state.
Which organ both supplies glucose and removes ammonia from the blood?
The liver: it releases glucose (glycogenolysis, gluconeogenesis) and converts ammonia to urea, which the kidneys excrete.
Why can plasma creatinine be normal in an elderly woman whose GFR is reduced?
Creatinine comes from muscle; with less muscle mass, less is made, so plasma creatinine stays low despite lower filtration.
Connections
See it in 3D5
The structures of Durgical's 3D anatomy where it happens.
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: Sodium Blood Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2025); Potassium Blood Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2025); Calcium Blood Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2025); Hypovolemia and Hypovolemic Shock (StatPearls (NCBI Bookshelf), 2025).
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Representative model, for educational purposes only.