Unit 2: Respiration

1. Definition of Respiration

Respiration is a multi-step process that involves the exchange of gases (O₂ and CO₂) between an organism and its environment, and the metabolic processes within the cell that use oxygen to produce ATP (energy) from glucose.

It can be divided into:

• External Respiration: Breathing; the exchange of gases between the lungs and the blood.
• Internal Respiration: The exchange of gases between the blood and the body tissues.
• Cellular Respiration: The metabolic reactions in the cell (mitochondria) that use O₂ to break down glucose and generate ATP.

2. Aerobic and Anaerobic Respiration

a) Aerobic Respiration

• Definition: The metabolic process that uses oxygen as the final electron acceptor in the electron transport chain.
• Process: Glycolysis → TCA Cycle → Oxidative Phosphorylation (ETC).
• Outcome: Highly efficient. Produces a large amount of ATP (~30-32 ATP per glucose).
• Equation: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ATP

b) Anaerobic Respiration

• Definition: The metabolic process that generates energy without oxygen.
• Process: Involves only glycolysis, followed by fermentation (e.g., lactic acid fermentation in human muscles during strenuous exercise).
• Outcome: Inefficient. Produces only 2 ATP per glucose.

3. Human Respiratory System

The system of organs responsible for taking in oxygen and expelling carbon dioxide.

[Image of the human respiratory system]

Pathway of Air:

1. Nose/Mouth: Air enters, is warmed, filtered (by hairs and mucus), and humidified.
2. Pharynx (Throat): Common passageway for air and food.
3. Larynx (Voice Box): Contains the vocal cords. The epiglottis is a flap that covers the larynx during swallowing to prevent food from entering the airway.
4. Trachea (Windpipe): A rigid tube supported by C-shaped cartilaginous rings. Lined with cilia and mucus to trap debris.
5. Bronchi: The trachea divides into two primary bronchi (one for each lung).
6. Bronchioles: The bronchi branch into progressively smaller tubes called bronchioles.
7. Alveoli: The bronchioles end in tiny, air-filled sacs called alveoli. This is the site of gas exchange.

4. Structure of Lungs

• Location: The two lungs are located in the thoracic (chest) cavity, protected by the rib cage.
• Pleura: Each lung is surrounded by a double-layered membrane called the pleura. The space between (pleural cavity) contains pleural fluid, which lubricates and reduces friction during breathing.
• Lobes: The right lung has three lobes; the left lung has two (to make space for the heart).
• Alveoli: The functional units of the lung. There are millions of alveoli, providing a massive surface area (about 70 m²) for efficient gas exchange. Each alveolus is wrapped in a dense network of blood capillaries.

[Image of the structure of alveoli and surrounding capillaries]

5. Mechanism of Breathing (Inspiration and Expiration)

Breathing (or ventilation) is a mechanical process driven by pressure changes in the thoracic cavity, controlled by the diaphragm and intercostal muscles (muscles between the ribs).

a) Inspiration (Inhalation)

This is an active process.

1. The diaphragm contracts and moves *down*.
2. The external intercostal muscles contract, pulling the rib cage *up and out*.
3. These actions increase the volume of the thoracic cavity.
4. As volume increases, the pressure inside the lungs (intrapulmonary pressure) *decreases* (becomes lower than atmospheric pressure).
5. Air rushes *into* the lungs to equalize the pressure.

b) Expiration (Exhalation)

This is normally a passive process.

1. The diaphragm relaxes and moves *up*.
2. The external intercostal muscles relax, and the rib cage moves *down and in*.
3. These actions decrease the volume of the thoracic cavity.
4. As volume decreases, the pressure inside the lungs *increases* (becomes higher than atmospheric pressure).
5. Air is forced *out of* the lungs.

6. Exchange of Gases

Gas exchange occurs at two sites via simple diffusion, driven by differences in partial pressures (P).

1. In the Lungs (External Respiration):
   • Blood arriving at the lungs is low in O₂ (PO₂ ≈ 40 mmHg) and high in CO₂ (PCO₂ ≈ 45 mmHg).
   • Air in the alveoli is high in O₂ (PO₂ ≈ 104 mmHg) and low in CO₂ (PCO₂ ≈ 40 mmHg).
   • Result: O₂ diffuses *from the alveoli into the blood*. CO₂ diffuses *from the blood into the alveoli*.
2. In the Tissues (Internal Respiration):
   • Blood arriving at the tissues is high in O₂ (PO₂ ≈ 100 mmHg) and low in CO₂ (PCO₂ ≈ 40 mmHg).
   • Body tissues (which are using O₂) are low in O₂ (PO₂ < 40 mmHg) and high in CO₂ (PCO₂ > 45 mmHg).
   • Result: O₂ diffuses *from the blood into the tissues*. CO₂ diffuses *from the tissues into the blood*.

[Image of gas exchange in lungs and tissues showing partial pressure gradients]

7. Transport of Oxygen (O₂)

Oxygen is not very soluble in water (plasma). Therefore, it is transported in the blood in two ways:

1. Dissolved in Plasma (1.5%): A very small amount.
2. Bound to Hemoglobin (98.5%): The vast majority of O₂ is transported by the protein hemoglobin (Hb) found in red blood cells. Hb can bind up to four O₂ molecules, forming oxyhemoglobin (HbO₂).
   Hb + 4 O₂ ⇌ Hb(O₂)₄

Oxygen-Hemoglobin Dissociation Curve

This S-shaped curve shows how the "stickiness" (saturation) of Hb for O₂ changes with the partial pressure of O₂.

• In the Lungs (High PO₂): Hb has a high affinity for O₂ and becomes fully saturated (it "loads" O₂).
• In the Tissues (Low PO₂): Hb's affinity for O₂ decreases, and it "unloads" O₂ to the tissues.

[Image of the oxygen-hemoglobin dissociation curve]

8. Transport of Carbon Dioxide (CO₂)

CO₂ is a waste product of cellular respiration. It is transported from the tissues to the lungs in three ways:

1. Dissolved in Plasma (7-10%): More soluble than O₂.
2. Bound to Hemoglobin (20-30%): CO₂ binds to the *amino groups* of Hb (not the heme), forming carbaminohemoglobin.
3. As Bicarbonate Ions (HCO₃⁻) (60-70%): This is the most important method.
   • CO₂ diffuses from tissues into red blood cells (RBCs).
   • Inside the RBC, the enzyme carbonic anhydrase rapidly converts CO₂ and water into carbonic acid (H₂CO₃).
     CO₂ + H₂O ⇌ H₂CO₃
   • Carbonic acid then dissociates into a hydrogen ion (H⁺) and a bicarbonate ion (HCO₃⁻).
     H₂CO₃ ⇌ H⁺ + HCO₃⁻
   • The H⁺ binds to hemoglobin (acting as a buffer).
   • The bicarbonate ion (HCO₃⁻) is transported out of the RBC into the blood plasma for transport to the lungs.

9. Chloride Shift

The Chloride Shift is an exchange process that maintains electrical neutrality in the red blood cell. As the negatively charged bicarbonate ion (HCO₃⁻) exits the RBC (at the tissues), a negatively charged chloride ion (Cl⁻) enters the RBC from the plasma to take its place.

This process is reversed in the lungs: Cl⁻ exits the RBC, HCO₃⁻ enters, is converted back to CO₂, and the CO₂ is exhaled.

[Image of the chloride shift in a red blood cell at the tissue level]