The respiratory system

The respiratory system supplies the blood with oxygen (O₂) and removes carbon dioxide (CO₂). Main components:

Upper airways: nose, pharynx, larynx. Lower airways: trachea, bronchi, bronchioles. Lungs: two with alveoli — ~300 million per lung, total surface area about 70 m². Diaphragm: main breathing muscle. On inhalation it lowers → lungs expand.

Gas exchange in the alveoli

Gas exchange in the alveoli occurs by diffusion:

O₂ moves from the alveoli into the blood of the lung capillaries (partial-pressure gradient).
CO₂ moves from blood into the alveoli → exhaled.

Driving force: partial-pressure differences (Dalton's law).

In the alveoli, typical partial pressures (sea level):

pO₂: about 100 mmHg.
pCO₂: about 40 mmHg.

In arterial blood after gas exchange: pO₂ ≈ 95 mmHg, pCO₂ ≈ 40 mmHg.

Breathing regulation

Breathing is regulated primarily by the CO₂ level in the blood (not primarily by O₂!):

CO₂ rise in the blood (hypercapnia) → respiratory centre stimulated → faster/deeper breathing.
CO₂ fall (hypocapnia, e.g. through hyperventilation) → respiratory centre suppressed → less drive.

Consequence for hypoxia: because the body does not directly measure O₂, it does not notice hypoxia at altitude — hence the danger of insidious hypoxia (see hypoxia lessons).

The circulatory system

The cardiovascular system transports:

O₂ and nutrients to all body cells,
CO₂ and metabolic waste back to lungs and kidneys,
Hormones, immune cells, heat.

Heart structure

Four chambers in two "halves":

Right side (pulmonary circuit):

Right atrium collects deoxygenated blood from the body.
Right ventricle pumps it to the lungs.

Left side (systemic circuit):

Left atrium collects oxygenated blood from the lungs.
Left ventricle pumps it into the body (highest pressure — aorta).

Resting heart rate: 60–80 beats/min. Cardiac output at rest: about 5 L/min, up to 20 L/min under load.

The two circuits

1. Pulmonary circuit (small):

Right ventricle → pulmonary artery → lungs (gas exchange) → pulmonary veins → left atrium.

2. Systemic circuit (large):

Left ventricle → aorta → arteries → capillaries → tissues (gas exchange) → veins → right atrium.

Blood

Components:

Plasma (55%) — fluid with proteins, salts, glucose.
Erythrocytes / red blood cells (45%) — carry haemoglobin, which binds O₂ and CO₂.
Leukocytes (white cells) — immune defence.
Thrombocytes (platelets) — clotting.

Haemoglobin and oxygen transport

Haemoglobin (Hb) binds O₂ in the lung and releases it in the tissue. Binding is partial-pressure dependent:

High pO₂ (lung) → almost 100% Hb saturation.
Low pO₂ (tissue) → Hb releases O₂.

Oxygen saturation at altitude:

Sea level: ~98%.
10 000 ft: ~87% — first hypoxia symptoms.
15 000 ft: ~80% — hypoxia clearly noticeable.
18 000 ft: ~70% — severe hypoxia, TUC only minutes.

Carbon monoxide (CO) binds Hb 200–250× more strongly than O₂ → CO poisoning displaces O₂. Even small CO levels in the cockpit (defective heater muff!) lead to hypoxia symptoms.

Effects for the pilot

At altitude ambient pO₂ drops → less O₂ transport → hypoxia.
Hyperventilation lowers pCO₂ → alkalosis → respiratory-centre suppression, chest tightness.
CO poisoning (heater): symptoms like hypoxia + headache, nausea → on suspicion heater off, fresh air, descend.
G load (positive g): blood is pushed into the lower extremities → reduced cerebral blood flow → grey-out, G-LOC.