Gaseous Exchange

Gaseous Exchange Definitions

- Breathing: The mechanical process whereby air moves in and out of the lungs.

- Gaseous exchange: The exchange of oxygen (O2) and carbon dioxide (CO2) across a gaseous exchange surface.

- Cellular respiration: The gradual release of energy from organic compounds (glucose) in the presence of oxygen.

- Main function of respiratory system: To absorb oxygen from the atmosphere and make it available to the cells for respiration.

- CO2 Removal: Carbon dioxide is released by the cells during respiration and must continually be removed; increased CO2 concentration can dangerously lower the pH of body fluids.

Requirements for an Effective Gaseous Exchange Surface

- Large surface area: To maximise gaseous exchange.

- Thin surface: For quick and easy diffusion.

- Moist surface: Gases (O2 and CO2) must be in solution in order to diffuse through a membrane.

- Transport system: For efficient transport of gases.

- Well protected: Surface is very thin and fragile.

How Requirements Apply to Human Lungs

- Large surface area: Bronchioles divided into many alveoli.

- Thin surface: Alveoli have thin squamous epithelium layer.

- Moist surface: Mucus cells secrete a thin layer of moisture.

- Transport system: Many blood capillaries around the alveoli.

- Protection: Ribcage and sternum.

Structure of the Respiratory System

- Human Respiratory System Components: Air passages, lungs, respiratory muscles.

- Diffusion: Gaseous exchange occurs by diffusion (movement of molecules from a high concentration to a low concentration until equilibrium is reached).

- 2 Nostrils: Air inhaled through 2 nostrils.

- Nasal Cavities: Cavities are lined with epithelial and goblet cells.

- Function of Mucus from Goblet Cells: Together with cilia, traps dirt and sweeps it out of the nose; keeps cavity moist.

- Function of Blood Capillaries in Nose: Warm the incoming air.

- 3 Functions of Cilia: Trap dirt, sweep dirt out of nose, keep air moist.

Structures of the Lower Air Passages

- Trachea (Windpipe): Situated in front of the oesophagus.

- C-shaped Cartilage Rings: Protect trachea and keep it open for easy movement of air.

- Bronchus / Bronchi: Trachea branches into left and right bronchi that enter the upper lobes of the lungs; held open by O-shaped cartilage rings; lined with mucous membranes.

- Bronchiolus / Bronchioli: Each bronchus divides into many smaller branches; passages are smaller and narrower and do NOT have cartilage for reinforcement.

- Alveoli ("Bunch" of alveoli = Infundibulum): Bronchioli end in a collection of alveoli (air sacs) that are sites for gaseous exchange.

- Alveoli Features: Many alveoli increase surface area; thin walls of squamous epithelial cells allow easy diffusion; tissue fluid keeps walls moist; large blood capillary network surrounds alveoli.

- Pharynx: Connects nasal cavity with larynx; lined with mucus; leads to trachea and oesophagus.

Respiratory Muscles & Breathing Mechanism

- Diaphragm: Dome-shaped muscle plate; in relaxed state separates chest and abdomen to form an airtight thoracic cavity.

- Intercostal Muscles: Located between consecutive ribs; consist of external intercostal muscles and internal intercostal muscles.

- Ventilation (Mechanism of Breathing): Movement of air between atmosphere and lungs caused by air pressure difference between atmospheric air and air in thoracic cavity/lungs.

- Respiratory Muscle Movement: Changes volume of thoracic cavity, which causes the difference in pressure.

- Phases of Ventilation: Inhalation (air moves into lungs) and Exhalation (air moves out of lungs).

Inhalation (Active Process)

- Diaphragm contracts and becomes flatter.

- Thoracic cavity enlarges from top to bottom.

- External intercostal muscles contract, causing ribs to move upwards and outwards.

- Thoracic cavity enlarges from side to side and front to back.

- Abdominal muscles relax so abdominal cavity can accommodate viscera (internal organs) pushed down by diaphragm.

- Total volume of thoracic cavity is increased.

- Air pressure in thoracic cavity and lungs decreases.

- Elastic lungs expand.

- Atmospheric pressure is higher than pressure in thoracic cavity/lungs, so O2-rich air flows into lungs.

- Active Process Reason: Associated with contraction of diaphragm and external intercostal muscles.

Exhalation (Passive Process)

- Diaphragm relaxes and returns to original dome shape.

- Thoracic cavity becomes smaller from top to bottom.

- External intercostal muscles relax, causing ribs to move downward and inward.

- Thoracic cavity reduces in size from side to side and front to back.

- Total volume of thoracic cavity is reduced.

- Air pressure in thoracic cavity and lungs increases.

- Air pressure in chest cavity/lungs is higher than atmospheric pressure, so CO2-rich air flows out of lungs.

- Passive Process Reason: Associated with relaxation of diaphragm and external intercostal muscles.

Active Exhalation (Whistling, Sneezing, Coughing, Shouting)

- Forced Exhalation: Becomes an active process during whistling, sneezing, coughing, or shouting.

- Internal Intercostal Muscles: Contract, causing ribs to move even further inward.

- Abdominal Muscles: Contract and intestines are pushed up against diaphragm, forcing it further upward.

- Result: Volume of thoracic cavity further reduced; air pressure in chest cavity significantly increased; air forced from lungs.

Gaseous Exchange in Alveoli

- Exchange of O2: Inhaled air in alveoli has higher O2 concentration than blood in surrounding capillaries -> Diffusion gradient created -> O2 dissolves in thin layer of moisture lining alveoli -> Diffuses through thin squamous epithelium of alveoli and endothelial walls of capillaries into blood.

- Exchange of CO2: Blood reaching alveoli from tissues has higher CO2 concentration than air in alveoli -> Diffusion gradient created -> CO2 diffuses from blood in capillaries through endothelial walls and thin squamous epithelium into air in alveoli.

Gaseous Exchange in Tissues

- Exchange of O2: Oxygenated blood reaches tissues -> Blood in capillaries has higher O2 concentration than tissue cells -> Diffusion gradient created -> O2 diffuses through endothelial walls into tissue fluid and into cells.

- Exchange of CO2: Cells have higher CO2 concentration than blood in capillaries -> Diffusion gradient created -> CO2 diffuses from cells into tissue fluid and into blood in capillaries.

Transport of Gases (O2 & CO2)

- Oxygen Transport (2 Ways):

1. Most O2 (98.5%) combines with haemoglobin in red blood cells to form oxyhaemoglobin (O2 + haemoglobin -> oxyhaemoglobin).

2. A very small portion (1.5%) dissolves in blood plasma.

- O2 Route: Transported via the heart to all tissues in these two ways.

- Carbon Dioxide Transport (3 Ways):

1. Most CO2 (65%) combines with water to form carbonic acid (CO2 + H2O -> carbonic acid), which dissociates and is transported as bicarbonate ions (HCO3-).

2. A portion (30%) combines with haemoglobin to form carbaminohaemoglobin (CO2 + haemoglobin -> carbaminohaemoglobin).

3. Smallest portion (5%) dissolves in blood plasma.

- CO2 Route: Transported in these three ways via the heart to the lungs.

Homeostatic Control of Breathing

- Chemoreceptors Location: In the wall of the aorta and at the base of the carotid arteries in the neck; very sensitive to changes in CO2 concentration in the blood.

- Key Function: We regulate CO2 in blood vessels (carotid artery & aorta), NOT O2. Maintaining CO2 levels regulates blood pH (if pH is not maintained, enzymes denature).

- Step 1: Carbon dioxide (CO2) levels in the blood increase above normal levels (e.g., during/after exercise).

- Step 2: Chemoreceptors in the carotid artery (neck) and aorta are stimulated.

- Step 3: Chemoreceptor cells send nerve impulses to the medulla oblongata in the brain.

- Step 4: Medulla oblongata sends nerve impulses to breathing muscles (intercostal muscles and diaphragm) and heart (cardiovascular centre sends impulses to heart muscle & arterioles).

- Step 5: Breathing muscles contract more actively (rate and depth of breathing increases) and heart beats faster (arterioles contract, blood flows faster).

- Step 6: More CO2 is transported to and exhaled from the lungs.

- Step 7: CO2 levels in the blood lower and return to normal.