Gaseous exchange 2

• Terrestrial: Refers to plants and animals living on land.

• Aquatic: Refers to plants and animals living in water.

• Gaseous Exchange: The physical exchange of oxygen ($O_2$) and carbon dioxide ($CO_2$) across a respiratory surface. In mammals, this exchange occurs at two distinct sites:

  • At a gaseous exchange surface between the lungs and the blood (external gaseous exchange).

  • Between the blood and body cells at the tissue level (internal gaseous exchange).

• Breathing / Ventilation: The mechanical process of inhalation and exhalation through which air moves into and out of the respiratory organs, enabling the uptake of oxygen ($O_2$) and the removal of carbon dioxide ($CO_2$).

• Diffusion: The passive movement of molecules from a region of high concentration to a region of low concentration until equilibrium is reached.

• Catabolism: The metabolic breaking down of complex molecules into simple molecules to release energy.

• Aerobic: Process or reaction occurring in the presence of oxygen ($O_2$).

• Anaerobic: Process or reaction occurring in the absence of oxygen ($O_2$).

• Cellular Respiration: The metabolic breakdown of organic compounds (such as glucose/sugar) within the mitochondria of cells into inorganic products ($CO_2$ and $H_2O$) with the release of cellular energy in the form of adenosine triphosphate ($\text{ATP}$). It can occur aerobically or anaerobically.

• Erythrocytes: Red blood cells responsible for carrying oxygen throughout the body.

• Haemoglobin: The oxygen-carrying protein pigment located within red blood cells.

• Iron (Fe): The chemical element located within the haemoglobin molecule to which oxygen atoms directly bind.

• Altitude: The vertical height or elevation of a place above sea level, measured in metres ($\text{m}$).

Breathing Mechanisms and Visual Models

• Bell Jar Breathing Model: A physical model constructed to demonstrate the mechanical mechanism of breathing in mammals:

  • Glass Tube: Represents the trachea.

  • Balloon: Represents a lung.

  • Bell Jar: Represents the chest / thoracic cavity.

  • Rubber Sheet: Represents the diaphragm muscle.

  • Rubber Stopper: Seals the top opening of the bell jar around the glass tube to keep the system airtight.

• Structural and Functional Limitations of the Model:

  • The bell jar is completely rigid and immobile, whereas the human chest cavity and rib cage actively expand and contract during breathing.

  • The model demonstrates only a single balloon representing one lung, whereas humans naturally possess two lungs.

Lung Volumes, Capacities, and Spirometry

• Spirometer: An instrument designed to measure the volume of air entering and leaving human lungs during inhalation and exhalation.

• Key Lung Volumes and Capacities (Scale from $0\,\text{mL}$ to $6,000\,\text{mL}$):

  • Total Lung Capacity: The total absolute amount of air that the lungs can hold when fully inflated (reaching $6,000\,\text{mL}$).

  • Tidal Volume: The specific amount of air inhaled or exhaled during a normal, unforced breath at rest (typically fluctuating between $2,500\,\text{mL}$ and $3,000\,\text{mL}$, yielding a standard tidal volume of approximately $500\,\text{mL}$).

  • Forced Inhalation Volume: The additional volume of air taken in during a maximal, forced inhalation effort beyond the normal tidal volume (reaching up to the peak capacity of $6,000\,\text{mL}$).

  • Forced Exhalation Volume: The volume of air expelled during a maximal, forced exhalation effort beyond normal tidal volume (expelling air down to the $1,000\,\text{mL}$ mark).

  • Vital Capacity: The maximum volume of air that a person can exhale after taking the deepest possible breath. It represents the numerical difference between maximum forced inhalation ($6,000\,\text{mL}$) and residual volume ($1,000\,\text{mL}$), equaling approximately $5,000\,\text{mL}$.

  • Residual Volume: The static volume of air remaining inside the lungs even after a maximal, forced exhalation (approximately $1,000\,\text{mL}$).

External Gaseous Exchange (Alveolar Level)

• Location and Environment: Occurs at the surface of the pulmonary alveoli, between the alveolar air spaces and the surrounding capillary network.

• Concentration Gradients Across the Respiratory Surface:

  • Inhaled air entering the alveoli contains a high concentration of oxygen ($O_2$) relative to the low oxygen concentration in the deoxygenated blood arriving from the pulmonary artery.

  • Inhaled air contains a lower concentration of carbon dioxide ($CO_2$) relative to the high carbon dioxide concentration in the deoxygenated blood arriving from the pulmonary artery.

• Mechanisms of Gas Diffusion:

  • Oxygen ($O_2$) diffuses down its concentration gradient from the air inside the alveoli across the respiratory membranes into the capillary blood, where it is taken up by red blood cells.

  • Carbon dioxide ($CO_2$) diffuses down its concentration gradient from the blood in the capillaries across the membrane into the alveolar space to be exhaled.

• Anatomical Structural Adaptations:

  • Alveolus Wall: Consists of a single layer of flattened squamous epithelium, measuring exactly one cell thick to minimize diffusion distance.

  • Capillary Wall / Endothelium: Consists of a single layer of cells, measuring exactly one cell thick.

• Blood Flow Circuit:

  • Deoxygenated blood high in $CO_2$ is pumped from the heart via the pulmonary artery to the pulmonary capillaries.

  • Red blood cells collect $O_2$ to become oxygenated red blood cells.

  • Oxygenated blood high in $O_2$ leaves the alveoli through the pulmonary vein to return to the heart for systemic distribution.

  • Exhaled air departing the alveoli carries a significantly higher concentration of $CO_2$ than inhaled air.

Internal Gaseous Exchange (Tissue Level) and Gas Transport

• Mechanisms of Oxygen ($O_2$) Transport:

  • Oxygen is transported through the body primarily by red blood cells (erythrocytes).

  • The majority of oxygen chemically combines with haemoglobin inside erythrocytes to form oxyhaemoglobin.

  • Oxyhaemoglobin is carried through the systemic arterial circulatory system to reach all tissue cells.

• Mechanisms of Carbon Dioxide ($CO_2$) Transport:

  • The majority of carbon dioxide is transported through the vascular system dissolved in blood plasma in the form of bicarbonate ions.

• Diffusion at the Tissue Level:

  • Oxygenated blood arrives from the heart into the microscopic capillary network adjacent to tissue cells.

  • Oxygen ($O_2$) diffuses out of the blood across the one-cell thick endothelium wall into adjacent cells down its concentration gradient.

  • Nutrients and food substances simultaneously pass from the capillary blood into the cells.

  • Continuous cellular respiration inside cells generates high internal concentrations of $CO_2$

  • Carbon dioxide ($CO_2$) diffuses out of tissue cells, into surrounding tissue fluid, and across the endothelium into the blood.

  • Deoxygenated blood containing high $CO_2$ levels travels back through veins to the heart, which directs it to the lungs for gaseous exchange and exhalation.

• Role of Tissue Fluid:

  • All active tissue cells are bathed in tissue fluid, which provides essential moisture necessary for efficient gas dissolution and diffusion.

• Summary Pathway of Respiration:

  • Atmospheric $O_2 \rightarrow$ Lungs (Inhalation/Breathing) $\rightarrow$ Blood Transport ($O_2$) $\rightarrow$ Body Cells (Cellular Respiration)

  • Body Cells ($CO_2$) $\rightarrow$ Blood Transport (Bicarbonate Ions) $\rightarrow$ Lungs (Exhalation/Breathing) $\rightarrow$ Atmospheric $CO_2$

Homeostatic Control of Carbon Dioxide and Breathing Regulation

• Negative Feedback Mechanism:

  • Negative feedback mechanisms function to stabilize biological systems by reversing deviations and returning parameters to normal physiological set points.

  • At rest, humans display a rhythmic baseline rate and depth of breathing, controlled by the concentration of carbon dioxide ($CO_2$) in the blood.

• Physiological Response to Exercise:

  • Physical activity increases energy demands in working skeletal muscles, elevating cellular respiration rates.

  • Increased cellular respiration produces higher amounts of $CO_2$, raising blood carbon dioxide concentrations above normal levels.

• Step-by-Step Homeostatic Regulation Cascade:

  1. Carbon dioxide ($CO_2$) levels in the blood rise above the normal homeostatic threshold.

  2. Specialized receptor cells located in the carotid artery in the neck detect the elevation in $CO_2$ and become stimulated.

  3. Receptor cells generate and transmit nerve impulses along neural pathways to the medulla oblongata in the brain.

  4. The respiratory centre within the medulla oblongata processes the signals and sends nerve impulses to the breathing muscles and the heart.

  5. Effector Organs Response:

  • Breathing Muscles (Intercostal Muscles and Diaphragm): Contract and relax more actively, significantly increasing both the rate and depth of breathing.

  • Heart: Beats faster, increasing overall heart rate and accelerating systemic blood flow speed for rapid gas transport.

  1. Gas Exchange Outcome: Accelerated blood circulation rapidly transports gases to and from tissues, allowing greater quantities of $O_2$ to be inhaled and larger amounts of $CO_2$ to be carried to the lungs and exhaled.

  2. Homeostatic Restoration: Blood carbon dioxide ($CO_2$) concentrations fall back down and return to normal resting levels.

Environmental Effects on Gaseous Exchange

• Altitude: Defined as the vertical height or elevation of a location measured in metres ($\text{m}$) above sea level.

• Effect of High Altitude on Gas Exchange:

  • At high altitudes, lower ambient oxygen availability results in less oxygen being absorbed by red blood cells in the lungs.

• Compensatory Mechanism:

  • To counteract decreased oxygen uptake, the human body adapts over time by producing a higher quantity of red blood cells (erythrocytes) to increase overall oxygen-carrying capacity.