Gas Exchange

Gas Exchange

Diffusion

• Definition: Diffusion is the process by which molecules move randomly from areas of higher concentration to areas of lower concentration.

• Slow Process: Due to random molecular movement, diffusion is relatively slow.

• Requirement for Efficient Gas Exchange: For gas exchange to rapidly occur, it must:

  • Take place over a large surface area.

  • Occur across a short distance.

• Surface Area to Volume Ratio (SA:V Ratio):

  • Significance: Larger SA:V ratios enhance the diffusion process, resulting in more effective gas exchange.

  • Example: Axolotls have large gill surface areas that facilitate underwater breathing due to high gas exchange rates.

Passive Diffusion

• Passive diffusion occurs from an area of higher concentration to an area of lower concentration, which is also known as movement towards the concentration gradient.

Maintenance of the Concentration Gradient

• Concentration Gradient: For diffusion to occur, a concentration gradient must be maintained—one area must have a higher concentration than another.

• Breath Holding Example: Holding breath leads to a leveling out of oxygen concentration in the lungs, eventually halting diffusion.

• Function of Breathing: Constant breathing is essential to maintain the concentration gradient.

• Ventilation Process: Ventilation involves taking in air (inhalation) and releasing air (exhalation).

  • Inhaled air has a higher oxygen concentration and exhaled air has a higher carbon dioxide concentration.

Respiration

Definition

• Respiration: The chemical process in which oxygen and glucose are used within cells to produce adenosine triphosphate (ATP).

Ventilation in Mammals

• Process: Mammals periodically expel air from their alveoli by exhaling and replace it by inhaling fresh air.

  • This process ensures oxygen concentration does not fall too low (to support a stable gradient) and carbon dioxide levels do not rise excessively.

• Adjustment of Ventilation Rate: The rate of ventilation adjusts according to carbon dioxide levels in the blood.

The Lungs and Alveoli

Structure

• General Function: All mammals, including dolphins and whales, use lungs for gas exchange.

• Air Passage:

  • Air enters through the trachea and splits into left and right bronchi.

  • Bronchi branch into numerous bronchioles ending in clusters of alveoli.

Alveolar Structure

• Alveolus: Each pulmonary alveolus has a diameter of approximately 200-500 µm, with walls only 0.2 µm thick.

• Capillary Network: Surrounded by a network of thin-walled capillaries that consists of a single cell layer, facilitating short diffusion distances between air and blood.

• Supplementary Cells:

  • Collagen and elastic fibres strengthen lung tissue and assist in passive exhalation.

Alveolar Cell Types

AT1 and AT2 Cells

• AT1 Cells: Form the capillary wall and play a role in diffusion.

• AT2 Cells: Secrete pulmonary surfactant, a lubricant that prevents alveolar collapse during breathing.

  • Surfactant Structure: Similar to phospholipids, forming a monolayer with hydrophilic heads inward and hydrophobic tails outward to prohibit sticking.

• Effective Gas Exchange: While individual alveoli provide limited surface area, approximately 300 million alveoli provide significant collective surface area, nearly 40 times greater than the body's exterior.

Ventilation Mechanics

Physics of Ventilation

• When gas particles disperse to occupy a larger volume, it results in decreased gas pressure and concentration. Conversely, gas compression leads to increased pressure.

• Inspiration Process: When intercostal muscles relax, thoracic pressure drops below atmospheric pressure, causing air to flow in until pressures equalize.

• Expiration Process: Contraction of different intercostal muscles raises thoracic pressure, pushing air out of the lungs.

Measurement of Lung Volume

• Tidal Volume: The volume of air inhaled and exhaled per ventilation cycle.

• Ventilation Rate: The frequency of air intake or expulsion per minute.

• Vital Capacity: Total volume exhaled after maximal inhalation, or total volume inhaled after maximal exhalation.

• Inspiratory Reserve Volume: Additional air that can be inhaled after normal tidal volume exhalation.

• Expiratory Reserve Volume: Additional air that can be exhaled after normal tidal volume inhalation.

Breathing Adaptations in Plants

Leaf Structure

• Waxy Cuticle: The outer layer of a leaf, secreted by epidermal cells, is a waterproof layer with low gas permeability.

• Guard Cells: Located in the epidermis, these cells can change shape to open or close stomata, allowing controlled gas exchange and transpiration.

Stomatal Function

• Stomatal Closure: Typically occurs at night when photosynthesis ceases or during dehydration.

• Location: Stomata are predominantly on the leaf's underside.

• Factors Influencing Transpiration:

  • Humidity

  • Temperature

  • Surface Area

  • Sunlight exposure

  • Water availability

• Potometers: Devices used to measure transpiration rates in plants.

Moisture Requirement

• Moistening of Palisade Mesophyll: Essential for transpiration and gas exchange; without moisture, these processes cannot function effectively.

• Stomatal Density: Defined as the quantity of stomata within a specified area or in the field of view under the microscope.

Hemoglobin and Oxygen Transport

Structure and Function of Hemoglobin

• Definition: Hemoglobin is the oxygen transport protein found in red blood cells.

• Hemoglobin Composition: Each molecule consists of four heme groups, providing four binding sites for oxygen molecules.

Cooperative Binding

• Mechanism: Binding of one oxygen molecule to hemoglobin induces a conformational change that increases the affinity of adjacent hemoglobin molecules for further oxygen binding.

• Adaptation in Low Oxygen Environments: This process allows humans to efficiently extract oxygen even at high altitudes or in oxygen-poor conditions.

Oxygen States

• R State: When a hemoglobin molecule is fully saturated (attached to four oxygen atoms).

• T State: When no oxygen is attached to a hemoglobin molecule, resulting in a lower affinity for oxygen.

• Blood Oxygen Saturation: Correlated positively with oxygen concentration, usually represented in terms of partial pressure, measured in kilopascals (kPa).

• Critical Threshold: If inhaled air has an oxygen concentration of 10 kPa or more, hemoglobin achieves 100% saturation.

Oxygen Transport Dynamics

• Oxygen Journey: Oxygenated blood exits the lungs (partial pressure range 10 kPa - 13 kPa) travels to organs where oxygen concentration is generally below 10 kPa, creating a concentration gradient that facilitates oxygen diffusion into tissues.

Oxygen Dissociation Curve

• Sigmoid Curve: Represents the relationship between oxygen concentration and hemoglobin saturation; they are not directly proportional but correlate positively.

Bohr Shift

Mechanism of Action

1. Increased aerobic respiration raises CO2 levels in blood, prompting oxygen dissociation from hemoglobin.

2. Chemical Reaction: CO2 and water convert into hydrogen ions and bicarbonate ions within red blood cells.

   • Equation: CO2 + H2O
     ightarrow H^+ + HCO_3^-

3. Physiological Effect: This reaction lowers blood pH, reducing hemoglobin's affinity for oxygen.

4. pH Differences:

   • In lungs, CO2 concentration is lower, with a pH around 7.4.

   • In active muscle, CO2 concentration is higher, with a pH around 7.2; this small difference enhances oxygen binding in lungs and facilitates dissociation in active tissues.

Interaction with CO2

• Each hemoglobin subunit reacts with CO2, transforming the amine group to form carbamate, resulting in carbaminohemoglobin formation.

  • Equilibrium Reaction: Hemoglobin + 4CO_2
    ightleftharpoons Carbaminohemoglobin

• Unbinding in Tissues: In tissue, carbaminohemoglobin has a reduced affinity for oxygen.

• Rebinding in Lungs: In the lungs, it reverts back to hemoglobin, regaining oxygen affinity.