The respiratory system of mammals consists of several parts that work together to enable gas exchange. These include the nostrils, nasal cavity, pharynx, trachea, bronchi, bronchioles, alveoli, ribs, intercostal muscles, and diaphragm.

Each part has a specific function:

• Nostrils and nasal cavity — filter, warm, and moisten incoming air
• Trachea (windpipe) — carries air to the bronchi
• Bronchi and bronchioles — tube-like passages that direct air into the lungs
• Alveoli — tiny air sacs where actual gas exchange takes place
• Ribs and intercostal muscles — protect lungs and help with breathing movements
• Diaphragm — a dome-shaped muscle that separates chest from abdomen and controls breathing

The alveoli are particularly important because they provide a large surface area for gas exchange. They have thin membranes, a moist surface, and are surrounded by dense networks of capillaries.

Gas exchange occurs through two main processes: inhalation (breathing in) and exhalation (breathing out).

Inhalation

During inhalation:

1. The diaphragm muscles contract and flatten
2. The external intercostal muscles contract, pulling the ribs upward and outward
3. The volume of the chest cavity increases
4. Air pressure inside the chest decreases
5. Air from the outside is drawn into the lungs through the nostrils

Exhalation

During exhalation:

1. The diaphragm muscles relax and returns to its dome shape
2. The internal intercostal muscles contract, pulling the ribs downward and inward
3. The volume of the chest cavity decreases
4. Air pressure inside the chest increases
5. Air is forced out of the lungs

The composition of inhaled and exhaled air differs significantly. Inhaled air contains about 21% oxygen and 0.03% carbon dioxide, while exhaled air contains about 16% oxygen and 4% carbon dioxide.

The actual exchange of gases happens in the alveoli. When we inhale, oxygen concentration is higher in the alveoli than in the blood capillaries. Therefore, oxygen diffuses from the alveoli into the blood, where it combines with haemoglobin to form oxyhaemoglobin.

At the same time, carbon dioxide concentration is higher in the blood capillaries than in the alveoli. Thus, carbon dioxide diffuses from the blood into the alveoli. This carbon dioxide is then breathed out during exhalation.

Several factors influence how quickly gas exchange occurs:

• Physical activity — during exercise, the body needs more oxygen, so breathing rate and depth increase
• Haemoglobin concentration — low haemoglobin (as in anaemia) reduces oxygen transport, increasing the rate of gas exchange to compensate
• Carbon dioxide concentration — high carbon dioxide levels in the blood trigger faster breathing
• Altitude — at high altitudes, oxygen pressure is lower, so breathing rate increases
• Health status — diseases like asthma narrow the airways and reduce gas exchange efficiency

Aerobic respiration is the process of breaking down glucose using oxygen to release energy, carbon dioxide, and water. It can be written as:

$Glucose + Oxygen \rightarrow Carbon\ dioxide + Water + Energy$

Aerobic respiration occurs in two main stages: glycolysis (in the cytoplasm) and the Krebs cycle (in the mitochondria). It produces a large amount of energy in the form of ATP.

Anaerobic respiration occurs when there is not enough oxygen for aerobic respiration. It releases less energy because glucose is not completely broken down.

In animals (including humans), anaerobic respiration produces lactic acid:

$Glucose \rightarrow Lactic\ acid + Energy$

During vigorous exercise, muscles may not receive enough oxygen, leading to lactic acid buildup. This creates an oxygen debt — the amount of oxygen needed to convert lactic acid back to carbon dioxide and water. After exercise, breathing remains rapid until the oxygen debt is repaid.

In plants and yeast, anaerobic respiration (fermentation) produces ethanol and carbon dioxide instead of lactic acid.

Understanding gas exchange and respiration is important for maintaining healthy lungs. In Tanzania, respiratory infections like pneumonia and tuberculosis are common, especially among children. Knowing how breathing works helps students understand why doctors advise avoiding smoke, getting vaccinated, and seeking early treatment for cough and breathing difficulties. Athletes in Tanzania also apply knowledge of anaerobic respiration and oxygen debt when training to improve their endurance.