Human Biology
The Human Respiratory System
5th Year · 6th Year (Leaving Cert)
- ✓By the end of this lesson students will be able to identify and describe the main structures of the human respiratory system.
- ✓By the end of this lesson students will be able to explain the mechanics of inspiration and expiration, including the roles of the diaphragm and intercostal muscles.
- ✓By the end of this lesson students will be able to describe the process of gas exchange in the alveoli and body tissues, including the role of partial pressures (HL).
- ✓By the end of this lesson students will be able to outline how oxygen and carbon dioxide are transported in the blood.
Key concepts
The respiratory system is responsible for gas exchange between the body and the external environment. Air enters through the nasal cavity (or mouth), where it is warmed, moistened, and filtered. It then passes through the pharynx (throat) and larynx (voice box) into the trachea (windpipe). The trachea is supported by C-shaped rings of cartilage and branches into two bronchi, one for each lung. Each bronchus further divides into smaller bronchioles, which terminate in tiny air sacs called alveoli. The lungs are spongy organs located in the thoracic cavity, protected by the rib cage. The diaphragm is a dome-shaped muscle located at the base of the thoracic cavity, and the intercostal muscles are found between the ribs; both are crucial for breathing.
Inspiration (inhalation) is an active process. The diaphragm contracts and flattens, moving downwards. Simultaneously, the external intercostal muscles contract, pulling the rib cage upwards and outwards. These actions increase the volume of the thoracic cavity. As volume increases, the pressure inside the lungs (intrapulmonary pressure) decreases below atmospheric pressure. Air then flows from the higher atmospheric pressure outside the body into the lungs until the pressures equalise.
Expiration (exhalation) at rest is typically a passive process. The diaphragm relaxes and moves upwards, becoming dome-shaped. The external intercostal muscles relax, allowing the rib cage to move downwards and inwards. These actions decrease the volume of the thoracic cavity. As volume decreases, the pressure inside the lungs (intrapulmonary pressure) increases above atmospheric pressure. Air then flows from the higher pressure inside the lungs to the lower atmospheric pressure outside the body. During forced expiration (e.g., during exercise), the internal intercostal muscles and abdominal muscles contract to further decrease thoracic volume and expel more air.
Gas exchange occurs by diffusion across the thin walls of the alveoli and the surrounding capillaries. The driving force for diffusion is the difference in partial pressures of the gases. In the alveoli, the partial pressure of oxygen (PO2) is high (approx. 104 mmHg) and the partial pressure of carbon dioxide (PCO2) is low (approx. 40 mmHg). In the deoxygenated blood arriving at the lungs, PO2 is low (approx. 40 mmHg) and PCO2 is high (approx. 45 mmHg). Due to these partial pressure gradients, oxygen diffuses from the alveoli into the blood, and carbon dioxide diffuses from the blood into the alveoli to be exhaled.
At the body tissues, gas exchange also occurs by diffusion. In the oxygenated blood arriving at the tissues, PO2 is high (approx. 95 mmHg) and PCO2 is low (approx. 40 mmHg). In the metabolically active tissue cells, PO2 is low (approx. 40 mmHg or less) due to cellular respiration, and PCO2 is high (approx. 45 mmHg or more) as a waste product. Due to these partial pressure gradients, oxygen diffuses from the blood into the tissue cells, and carbon dioxide diffuses from the tissue cells into the blood.
Oxygen is primarily transported by haemoglobin within red blood cells, forming oxyhaemoglobin. A small amount is dissolved in the plasma. Carbon dioxide is transported in three main ways: as bicarbonate ions (HCO3-) in the plasma (approx. 70%), bound to haemoglobin as carbaminohaemoglobin (approx. 23%), and dissolved in the plasma (approx. 7%). The conversion to bicarbonate ions is facilitated by the enzyme carbonic anhydrase in red blood cells.
Key facts to remember
- 1The trachea is supported by C-shaped cartilage rings to prevent collapse.
- 2The lungs are housed within the thoracic cavity and protected by the rib cage.
- 3The diaphragm and intercostal muscles are the primary muscles of respiration.
- 4Inspiration is an active process, while quiet expiration is passive.
- 5Gas exchange occurs in the alveoli by diffusion, driven by partial pressure differences.
- 6Oxygen is mainly transported by haemoglobin in red blood cells.
- 7Carbon dioxide is primarily transported as bicarbonate ions in the plasma.
- 8The respiratory centre in the brain (medulla oblongata) controls breathing rate.
Worked examples
Example 1
Describe the structural features of the alveoli that make them efficient for gas exchange.
Answer
The alveoli are highly adapted for efficient gas exchange due to several structural features: 1. **Thin Walls:** The alveolar walls are extremely thin (one cell thick), as are the capillary walls, forming a very short diffusion distance for gases. 2. **Large Surface Area:** There are millions of alveoli in the lungs, providing an enormous total surface area (approx. 70-100 m²) for gas exchange. 3. **Rich Blood Supply:** Each alveolus is surrounded by a dense network of capillaries, ensuring a constant supply of deoxygenated blood and removal of oxygenated blood, maintaining steep concentration gradients. 4. **Moist Surface:** The inner surface of the alveoli is covered by a thin film of moisture, allowing oxygen to dissolve before diffusing across the membrane.
Example 2
Explain the changes in pressure and muscle activity that lead to inspiration.
Answer
Inspiration is an active process driven by muscle contraction: 1. The **diaphragm** contracts and flattens, moving downwards into the abdominal cavity. 2. The **external intercostal muscles** contract, pulling the rib cage upwards and outwards. 3. These combined actions significantly **increase the volume of the thoracic cavity**. 4. According to Boyle's Law, as the volume of the thoracic cavity increases, the **intrapulmonary pressure (pressure within the lungs) decreases** to a level below atmospheric pressure. 5. This pressure gradient causes **air to rush into the lungs** from the higher atmospheric pressure outside, until the pressures inside and outside the lungs are equalised.
Example 3
Compare the partial pressures of oxygen and carbon dioxide in the alveoli and in the deoxygenated blood arriving at the lungs, and explain the direction of gas movement (HL).
Answer
In the alveoli: * Partial pressure of oxygen (PO2) is approximately 104 mmHg. * Partial pressure of carbon dioxide (PCO2) is approximately 40 mmHg. In the deoxygenated blood arriving at the lungs: * Partial pressure of oxygen (PO2) is approximately 40 mmHg. * Partial pressure of carbon dioxide (PCO2) is approximately 45 mmHg. **Direction of Gas Movement:** * **Oxygen:** Since the PO2 in the alveoli (104 mmHg) is significantly higher than the PO2 in the deoxygenated blood (40 mmHg), oxygen diffuses from the alveoli into the blood. * **Carbon Dioxide:** Since the PCO2 in the deoxygenated blood (45 mmHg) is higher than the PCO2 in the alveoli (40 mmHg), carbon dioxide diffuses from the blood into the alveoli to be exhaled. This movement occurs passively down the respective partial pressure gradients, ensuring efficient gas exchange.
Common mistakes
- ✗Confusing the roles of the internal and external intercostal muscles, especially during forced expiration.
- ✗Not understanding that gas exchange is driven by partial pressure gradients, not just 'concentration' (especially for HL).
- ✗Incorrectly stating that the lungs themselves contract; the lungs are passive and expand/recoil due to changes in thoracic volume.
- ✗Forgetting to mention the moist surface of the alveoli as an adaptation for gas exchange.
- ✗Mixing up the direction of gas movement in the alveoli versus the tissues.
Exam tips
- ★When describing the mechanics of breathing, always link muscle action to changes in thoracic volume, then to changes in pressure, and finally to the flow of air.
- ★For gas exchange questions, explicitly state the partial pressures (HL) and explain how the gradients determine the direction of diffusion.
- ★Use clear, labelled diagrams to illustrate the structures of the respiratory system; this can earn marks and clarify your understanding.
- ★Practise explaining the transport of both oxygen and carbon dioxide, detailing the different forms each gas takes in the blood.
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