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Gas Exchange in Humans - Breathing and ventilation

Grade 12IGCSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The human gas exchange system includes the trachea, bronchi, bronchioles, and alveoli. The alveoli are the primary site of gas exchange between the air and the blood.

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Inhalation (Inspiration) occurs when the diaphragm and external intercostal muscles contract. This increases the thoracic volume, which decreases the internal pressure (PP) relative to atmospheric pressure, forcing air into the lungs.

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Exhalation (Expiration) occurs when the diaphragm and external intercostal muscles relax, and internal intercostal muscles contract (during forced breathing). This decreases thoracic volume and increases pressure, forcing air out.

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Alveoli are adapted for efficient gas exchange by having a very large surface area (AA), a moist lining, and walls that are only one cell thick (<1μm< 1 \mu m) to minimize diffusion distance.

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The concentration gradient for gas exchange is maintained by a dense network of capillaries and continuous ventilation. Oxygen (O2O_2) diffuses into the blood, while Carbon Dioxide (CO2CO_2) diffuses into the alveoli.

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The percentage of O2O_2 in inhaled air is approximately 21%21\%, which drops to 16%16\% in exhaled air. Conversely, CO2CO_2 increases from 0.04%0.04\% in inhaled air to approximately 4%4\% in exhaled air.

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Hemoglobin (HbHb) in red blood cells transports oxygen by forming oxyhemoglobin: Hb+4O2⇌Hb(O2)4Hb + 4O_2 \rightleftharpoons Hb(O_2)_4.

📐Formulae

Minute Ventilation (MV)=Tidal Volume (TV)×Breathing Rate (f)\text{Minute Ventilation (MV)} = \text{Tidal Volume (TV)} \times \text{Breathing Rate (f)}

Diffusion Rate∝Surface Area×Concentration GradientDiffusion Distance\text{Diffusion Rate} \propto \frac{\text{Surface Area} \times \text{Concentration Gradient}}{\text{Diffusion Distance}}

P∝1V (Boyle’s Law applied to thoracic cavity)P \propto \frac{1}{V} \text{ (Boyle's Law applied to thoracic cavity)}

Breathing Rate (breaths/min)=Total number of breathsTime in minutes\text{Breathing Rate (breaths/min)} = \frac{\text{Total number of breaths}}{\text{Time in minutes}}

💡Examples

Problem 1:

A student measures their Tidal Volume (VTV_T) as 500 cm3500\text{ cm}^3 and takes 1212 breaths per minute. Calculate their Minute Ventilation (VEV_E) in dm3 min−1\text{dm}^3\text{ min}^{-1}.

Solution:

VE=0.5 dm3×12 breaths/min=6.0 dm3 min−1V_E = 0.5\text{ dm}^3 \times 12\text{ breaths/min} = 6.0\text{ dm}^3\text{ min}^{-1}

Explanation:

First, convert 500 cm3500\text{ cm}^3 to dm3\text{dm}^3 by dividing by 10001000 (0.5 dm30.5\text{ dm}^3). Then, multiply the Tidal Volume by the breathing rate to find the total volume of air inhaled per minute.

Problem 2:

During exercise, the concentration of CO2CO_2 in the blood increases, leading to a decrease in blood pHpH. Explain the physiological response.

Solution:

CO2+H2O⇌H2CO3⇌H++HCO3−CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-

Explanation:

Increased CO2CO_2 reacts with water to form carbonic acid, which dissociates into H+H^+ ions, lowering the pHpH. This is detected by chemoreceptors in the medulla oblongata, which send impulses to the diaphragm and intercostal muscles to increase the breathing rate and depth to expel CO2CO_2 faster.

Breathing and ventilation - Revision Notes & Key Diagrams | IGCSE Grade 12 Biology