Human Gas Exchange and Ventilation Processes

Essential Function of the Human Breathing System

• A breathing system is necessary to facilitate the intake of gases, which are subsequently distributed to body cells through the circulatory system.
• The human breathing system consists of various organs and structures collectively forming the respiratory tract and lungs.

Anatomy of the Respiratory Tract and Lungs

• Nasal Cavity (鼻腔):
  • Contains mucus-secreting cells that produce mucus to capture bacteria and dust, while also moistening the air.
  • Features ciliated epithelial cells with cilia that transport mucus toward the pharynx.
  • Richly supplied with blood capillaries to warm incoming air.
• Nostril (鼻孔):
  • Equipped with hairs that act as a filter for large dust particles.
• Pharynx (咽):
  • The intersection where the digestive system (alimentary canal) and respiratory tract meet.
• Epiglottis (會厭):
  • A structure that prevents food or liquid from entering the trachea during the process of swallowing.
• Larynx (喉):
  • Constructed from cartilages and houses vocal cords that produce sound through vibration as air passes through.
• Trachea (氣管):
  • Reinforced with C-shaped cartilages to prevent collapse while allowing the adjacent oesophagus to expand during swallowing.
  • Internal lining includes ciliated epithelial cells, mucus-secreting cells, and capillaries.
• Bronchus (支氣管):
  • Supported by circular rings of cartilage and lined with ciliated epithelial cells and mucus-secreting cells.
• Bronchiole (小支氣管):
  • Lacks cartilage support.
  • The smallest bronchioles may lack mucus-secreting cells but still possess ciliated epithelial cells and capillaries.
• Lungs (肺):
  • Composed primarily of air sacs and bronchioles.
  • Naturally elastic, allowing for self-recoil after expansion.
  • Each lung is enclosed by two pleural membranes.
• Pleural Cavity and Membranes:
  • Pleural membranes (M and O) surround the lungs and secrete pleural fluid.
  • The pleural cavity (N) holds this fluid, which functions as a lubricant to minimize friction between membranes during breathing.
• Diaphragm (橫膈膜):
  • A muscular sheet located at the base of the rib cage that facilitates ventilation through contraction and relaxation.

Alveoli and Gas Exchange Adaptations

• Air Sacs/Alveoli (氣囊):
  • The primary site for gas exchange.
  • Provide an exceptionally large surface area for the diffusion of gases.
  • Thin Epithelium: One-cell thick wall reduces the diffusion distance, allowing for rapid exchange.
  • Water Film: A thin layer of moisture on the interior surface allows gases to dissolve, facilitating their diffusion across the epithelium into the blood.
  • Capillary Network: Surrounds each air sac to transport oxygen away and bring carbon dioxide to the lungs, maintaining a steep concentration gradient.
  • Elasticity: Air sacs contain elastin fibers and are supported by smooth muscles to facilitate the elastic recoil of lung tissue.

Gas Treatment and Air Conditioning

• Warming Air: Blood in the capillaries of the nasal cavity and respiratory tract carries body core heat, which warms the air as it enters.
• Moistening Air: Mucus produced along the respiratory tract and nasal cavity provides moisture to the incoming air.
• Filtering Air:
  • Nasal hairs trap large debris.
  • Mucus traps bacteria and fine dust.
  • Ciliary action (beating) moves the trapped particles in mucus upward to the pharynx, where it is either swallowed or expelled through coughing.

Mechanisms of Gas Exchange and Transport

• Oxygen Uptake:
  1. Oxygen from inhaled air dissolves into the water film of the air sac.
  2. Since oxygen concentration is lower in the blood than the water film, oxygen diffuses across the air sac and capillary walls.
  3. Oxygen enters red blood cells (RBCs) and binds with haemoglobin to form oxyhaemoglobin.
  • Reaction: $\text{haemoglobin} + ​ ​ ​ ​ ​ ​ \text{oxygen} \rightarrow ​ ​ ​ ​ ​ ​ \text{oxyhaemoglobin}$.
  • This process changes blood color to bright red.
• Carbon Dioxide Removal:
  1. $CO_2$ concentration is higher in the blood capillaries than in the air sacs.
  2. $CO_2$ diffuses across the capillary and air sac walls.
  3. $CO_2$ is expelled via exhalation.

Transport of Respiratory Gases in the Blood

• Oxygen Transport:
  • Primarily carried by red blood cells.
  • Red Blood Cell Adaptations: Biconcave disc shape increases surface area-to-volume ratio; lack of nucleus provides more space for haemoglobin; flexibility allows passage through narrow capillaries.
  • In the body cells, oxyhaemoglobin breaks down: $\text{oxyhaemoglobin} \rightarrow ​ ​ ​ ​ ​ ​ \text{haemoglobin} + ​ ​ ​ ​ ​ \text{oxygen}$. The blood becomes dark red.
• Carbon Dioxide Transport:
  • Hydrogencarbonate Ions ($70\%$): $CO_2$ enters RBCs, reacts with water to form hydrogencarbonate ions (catalyzed by enzymes), and then diffuses into the plasma.
  • Reaction: $\text{carbon dioxide} + ​ ​ ​ ​ ​ \text{water} \rightarrow ​ ​ ​ ​ ​ \text{hydrogencarbonate ion}$.
  • Haemoglobin Binding ($23\%$): $CO_2$ combines with haemoglobin to form carbaminohemoglobin inside RBCs.
  • Plasma Dissolution ($7\%$): A small fraction dissolves directly in the plasma.
  • pH Regulation: High $CO_2$ levels increase the concentration of hydrogen ions, making blood more acidic. This change in pH affects enzyme function, triggering the body to increase breathing rate to remove excess $CO_2$.

Mechanics of Ventilation (Breathing)

• Inhalation (Inspiration):
  1. External intercostal muscles contract, moving the rib cage upward and outward.
  2. Diaphragm muscles contract, causing the diaphragm to flatten.
  3. Thoracic cavity volume increases; air pressure within the cavity decreases.
  4. Lungs expand, causing lung pressure to drop below atmospheric pressure ($101.3\, ​ ​ ​ ​ ​ \text{kPa}$ at standard sea level).
  5. Air rushes into the lungs.
• Exhalation (Expiration):
  1. External intercostal muscles relax; the rib cage moves downward and inward.
  2. Diaphragm muscles relax, and the diaphragm returns to a dome shape.
  3. Thoracic cavity volume decreases; air pressure within the cavity increases.
  4. Lungs recoil, causing lung pressure to become higher than atmospheric pressure.
  5. Air is forced out of the lungs.

Composition of Air in Ventilation

• Oxygen: Decreases because it diffuses into the blood for cellular respiration.
• Carbon Dioxide: Increases because it is a byproduct of respiration diffusing from blood to air sacs.
• Water Vapour: Increases because air is moistened by the mucus and water film linings.
• Nitrogen: Remains unchanged as it is not utilized or produced by the body.

Impact of Cigarette Smoking on Gas Exchange

• Cigarette smoke contains over $4000$ chemicals.
• Tar:
  • Irritates respiratory linings, causing excessive mucus secretion and narrowing airways.
  • Damages and paralyzes cilia, leading to mucus accumulation and infections.
  • Deposits on air sac surfaces, increasing diffusion distance and reducing the rate of gas exchange.
  • Destroys air sac walls, decreasing the total surface area for exchange.
• Carbon Monoxide:
  • Possesses a higher affinity for haemoglobin than oxygen, significantly reducing the blood's oxygen-carrying capacity.
• Nicotine:
  • Acts as an addictive substance.
  • Causes the constriction of blood vessels, which is particularly harmful to the heart and developing fetuses.

Comparison of Gas Exchange in Humans and Plants

• Common Adaptive Principles:
  • Large Surface Area: Provided by numerous air sacs in human lungs and spongy mesophyll/numerous leaves in plants.
  • Moist Surface: Water film on air sacs in humans and moist surfaces of spongy mesophyll in plants to dissolve gases.
  • Short Diffusion Distance: One-cell thick walls in human lungs/capillaries and thin, flat leaves in plants.
• Efficiency of Human System:
  • Humans utilize active ventilation (breathing movements) to draw in and expel air.
  • The capillary network and blood flow actively transport gases away to maintain a steep concentration gradient.
  • In contrast, plants rely primarily on passive diffusion.