Gas Exchange Notes

Gas Exchange

B3.1 Gas Exchange Overview

Gas exchange is a vital function in all organisms.
Key questions:
- How are multicellular organisms adapted to carry out gas exchange?
- What are the similarities and differences in gas exchange between a flowering plant and a mammal?

B3.1.1 Gas Exchange as a Vital Function

Challenges increase with organism size due to the decreasing surface area-to-volume ratio.
The distance from the center of an organism to its exterior increases with size, requiring adaptations for efficient gas exchange.

B3.1.2 Properties of Gas-Exchange Surfaces

Essential properties:
- Permeability: Allows gases to pass through.
- Thin tissue layer: Shortens the diffusion distance.
- Moisture: Gases dissolve in moisture for diffusion.
- Large surface area: Increases the rate of diffusion.

B3.1.3 Maintenance of Concentration Gradients in Animals

Mechanisms to maintain concentration gradients:
- Dense networks of blood vessels: Efficiently transport gases.
- Continuous blood flow: Replenishes and removes gases.
- Ventilation with air (lungs) or water (gills): Maintains a high concentration gradient.

B3.1.4 Adaptations of Mammalian Lungs

Adaptations of alveolar lungs:
- Surfactants: Reduce surface tension.
- Branched network of bronchioles: Increases surface area and distributes air.
- Extensive capillary beds: Enhance gas exchange.
- High surface area: Maximizes gas exchange.

B3.1.5 Ventilation of the Lungs

Role of muscles:
- Diaphragm: Changes the volume of the thoracic cavity.
- Intercostal muscles: Move the ribcage.
- Abdominal muscles: Assist in expiration.
- Ribs: Protect the lungs and aid in ventilation.

B3.1.6 Measurement of Lung Volumes

Measurements to determine:
- Tidal volume: Volume of air inhaled or exhaled during normal breathing.
- Vital capacity: Maximum amount of air that can be exhaled after a maximum inhalation.
- Inspiratory and expiratory reserves: Extra volume that can be inhaled or exhaled beyond tidal volume.

B3.1.7 Adaptations for Gas Exchange in Leaves

Leaf structure adaptations:
- Waxy cuticle: Reduces water loss.
- Epidermis: Protective outer layer.
- Air spaces: Facilitate gas diffusion.
- Spongy mesophyll: Allows gas movement.
- Stomatal guard cells: Control stomata opening and closing.
- Veins: Transport water and nutrients.

B3.1.8 Distribution of Tissues in a Leaf

Ability to draw and label a plan diagram showing tissue distribution in a transverse section of a dicotyledonous leaf.

B3.1.9 Transpiration as a Consequence of Gas Exchange

Awareness of factors affecting the rate of transpiration.

B3.1.10 Stomatal Density

Use micrographs or perform leaf casts to determine stomatal density.

Challenges of Gas Exchange

Efficiency decreases as organisms increase in size due to a reduction in the surface area-to-volume ratio.
Increased distance from the center to the exterior requires more efficient gas exchange adaptations.

Adaptations of Mammalian Lungs

Mammalian lungs differ from those of amphibians, reptiles, and birds.
Efficient air delivery from the outside to the center of the body and gas exchange over a large surface area.

Alveoli and Gas Exchange

Alveoli, capillary network, and thin membrane adaptations for breathing in large animals.
Type I and Type II pneumocytes make up the alveoli.
Type II cells secrete surfactant to reduce surface tension.
Surfactant is a phospholipid-rich secretion with hydrophilic and hydrophobic areas.

Function of Surfactant

Prevents the alveolus from collapsing during exhalation.
Creates a monolayer of phospholipids on the moist inner lining of the alveolus, preventing watery surfaces from adhering.

COPD and Gas Exchange

COPD is associated with smoking and air pollution.
COPD involves narrowed airways and a smaller number of larger air sacs.
Data-based questions related to COPD:
1. Quantify the differences in gas-exchange surface area between healthy and COPD-affected lung tissue using ruler measurements on micrographs to determine the number of times the edge of the ruler crosses a gas exchange surface.
2. Explain why people with COPD feel tired all the time (reduced gas exchange).
3. Explain why individuals with COPD often experience enlargement and strain on the right side of the heart (increased resistance in pulmonary circulation).

Properties of Gas-Exchange Surfaces

Gas exchange surfaces in animals and plants vary greatly.
Some organisms lack an open or closed circulatory system and exchange gases directly with the environment.
Key properties:
- Large surface area: Provided by structures like alveoli.
- Short diffusion pathway: Thin alveolar and capillary membranes.
- Moist surfaces: Gases dissolve for diffusion.
- Permeability: Allows gases to pass through.

Factors Improving Gas Movement

Size of the surface area.
Short diffusion pathway.
Solubility of gases.
Permeability of gases.

Maintenance of Concentration Gradients

Diffusion happens if there are concentration gradients.
Oxygen diffuses from air in the alveoli to the capillaries.
CO $_{2}$ diffuses from the blood to the air in the alveoli.
Concentration of gases are given as partial pressure (PO ${2}$ or PCO ${2}$ , respectively).
Consistent movement of blood throughout the capillary bed ensures a low concentration of O $_{2}$ .
Formulas:
$PO2 = \text{partial pressure of oxygen}$ $PCO2 = \text{partial pressure of carbon dioxide}$

Concentration Gradients in Alveoli

Breathing in (inhalation) increases the concentration gradient of oxygen between alveoli and blood.
Breathing out (exhalation) removes CO ${2}$ (and unused O ${2}$ ), increasing the concentration gradient of CO $_{2}$ between blood and alveolus.

Maintenance of Concentration Gradients in Gills

Movement of water through the gills will ensure a high concentration of O ${2}$ and low concentration of CO ${2}$ outside the gills.

Concentrations Gradients Data-Based Questions:

Figure shows the typical composition of atmospheric air, air in the alveoli, and gases dissolved in air returning to the lungs in the pulmonary arteries.
1. Explain why the oxygen concentration in the alveoli is not as high as in fresh air that is inhaled.
2. Calculate the difference in oxygen concentration between air in the alveolus and blood arriving at the alveolus, and deduce the process caused by this concentration difference.
3. Calculate the difference in carbon dioxide concentration between air inhaled and air exhaled, and explain this difference.
4. Despite the high concentration of nitrogen in air in alveoli, little or none diffuses from the air to the blood. Suggest reasons for this.

Ventilation of the Lungs

Ventilation refers to the movement of air into and out of the lungs in two stages: inspiration and expiration.
These stages are carried out by the movement of the diaphragm, the ribcage, abdominal and intercostal muscles.

Muscles Involved in Ventilation

Inspiration and expiration involve many different muscles with opposite movements (antagonistic pairs).
Muscles work in two states: Contracting and relaxing.
Examples:
- Internal and external intercostal muscles
- Abdominal muscles
- Pectoral muscles
- Diaphragm

Intercostal Muscles

The intercostal muscles are a good example for an antagonistic pair.
While the external muscles contract to move the ribcage up, the internal muscles relax, and vice versa.

Mechanics of Breathing - Inhalation

The diaphragm moves downwards when it contracts, which lengthens the cavity within the thorax.
The contraction of the external intercostal muscles moves the rib cage upwards and outwards.
This causes the diameter and the volume of the thorax to increase.
The volume of the thorax increases, resulting in pressure in the lungs decreasing below the atmospheric pressure. Air enters lungs as the atmospheric pressure is greater, inflating the lungs.

Mechanics of Breathing - Exhalation

The diaphragm and the external intercostal muscles relax whilst the internal intercostal muscles contract.
The rib cage moves downwards and inwards.
The diaphragm relaxes and the abdominal muscles (contract) to move the diaphragm upwards. The diameter of the thorax decreases, resulting in an increase in pressure in the lungs compared to the atmospheric pressure. Air leaves the lungs – which deflate.

Summary of Inhalation vs. Exhalation

Movement of the diaphragm: Summarize the processes required for inhalation and exhalation.
Movement of the ribcage: Summarize the processes required for inhalation and exhalation.
Internal intercostal Muscles: Summarize the processes required for inhalation and exhalation.
External intercostal Muscles: Summarize the processes required for inhalation and exhalation.
Abdominal Muscles: Summarize the processes required for inhalation and exhalation.
Volume changes: Summarize the processes required for inhalation and exhalation.
Pressure changes: Summarize the processes required for inhalation and exhalation.
Air flow: Summarize the processes required for inhalation and exhalation.

Investigation of Exercise on Ventilation

An investigation can be designed in which the effect of exercise on ventilation can be recorded.
- Independent variable: Intensity or type of exercise (running at different rates, on the treadmill, rowing machine…etc).
- Dependent variable: Vital capacity, tidal volume, breathing (ventilation) rate.
- Vital capacity: is the maximum amount of air a person can expel from the lungs after a maximum inhalation.
- Tidal volume: Normal volume of air breathed in or out during a normal, resting breath.
- Breathing (Ventilation) rate: the number of breaths per minute.

Measurement of Lung Volumes

Spirometer tracing showing lung volumes and capacities.
- Tidal Volume (VT): ~500mL, normal volume of air breathed in or out during a normal, resting breath
- Inspiratory Reserve Volume (IRV): ~3000mL, maximum volume of air that can be inhaled beyond normal tidal volume during a maximal inhalation
- Expiratory Reserve Volume (ERV): ~1100mL, maximum volume of air that can be exhaled beyond normal tidal volume during a maximal exhalation
- Residual Volume (RV): ~1200mL, smallest volume of air remaining in the lungs after a maximal exhalation
Lung Volumes
- Males
  - Inspiratory Reserve Volume (IRV): 3000 mL
  - Tidal Volume (VT): 500 mL
  - Expiratory Reserve Volume (ERV): 1100 mL
  - Residual Volume: 1200 mL
- Females
  - Inspiratory Capacity 1900 mL
  - Tidal Volume 500mL
  - Expiratory Reserve volume 700mL
  - Functional Residual capacity 1100 mL
  - Residual Volume 1200 mL
Lung Capacities
- Total capacity: 5800mL, sum of all four lung volumes that is, VT + IRV + ERV + RV
  - Males:
    - 5800 mL.
  - Females
    - 4200 mL
- Vital capacity 4600 mL, sum of tidal volume and the inspiratory and expiratory reserve volumes that is, VT +IRV + ERV

Measuring Vital Capacity and Tidal Volume

Vital Capacity: maximum inhalation and then forceful exhalation.
Tidal Volume: volume of air inhaled (or exhaled).
Formulas for Vital Capacity:
- Male Vital Capacity (L) = $((27.63 - 0.112 ewline \times \text{Age in years}) \times \text{Height in cm}) / 1000$
- Female Vital Capacity (L) = $((21.78 - 0.101 \times \text{Age in years}) \times \text{Height in cm}) / 1000$

Measuring Ventilation Rate

Count the number of times air is exhaled or inhaled in a minute.
Use a spirometer and a datalogger.

Lung Volume Experiment

Vital capacity differs depending on Gender and Height.

Monitoring ventilation in humans

Spirometer

Adaptations for Gas Exchange in Leaves

The structure of a leaf and its specialized tissue is complex.
Annotate the structures to explain how:
- Waxy cuticle
- Epidermis
- Air spaces
- Spongy mesophyll
- Stomatal guard cells
- Veins are adapted to gas exchange.

Distribution of Tissues in a Leaf

Draw a simple diagram to represent the different layers
Draw a transversal (cross-) section of a leaf and annotate how a leaf is best adapted to gas exchange.

Sun and Shade Leaves

On many trees there are differences in structure between leaves on upper branches that are in full sunlight and leaves on the same tree that are lower down and shaded.
1. Draw plan diagrams of the tissues in a representative part of each of the Prunus leaves.
2. Compare and contrast the structure of the two leaves, including overall thickness, thickness of waxy cuticle and the structure of the palisade mesophyll and spongy mesophyll.
3. Deduce which leaf grew in the sun and which in the shade, and discuss reasons for the differences.

Transpiration as a Consequence of Gas Exchange in a Leaf

Plants have openings in their upper and lower epidermis, called stomata.
The exchange of CO ${2}$ and O ${2}$ occurs through the stomata of the leaf, alongside which water (H $_{2}$ O) is released in the form of water vapor.
The evaporation of water from the mesophyll cells of the leaves is driven by a concentration gradient of water molecules.

Water Evaporation in Leaves

The walls of the spongy mesophyll inside a leaf are kept moist for gas exchange.
Water evaporates from the surface when the energy is high enough to break the hydrogen bonds and turn the liquid water into a gas.

Function of Guard Cells

The opening of the stomata is controlled using guard cells, which are found in pairs on either side of a stoma.
By opening and closing the stoma, the amount of water lost through transpiration can be minimized.

Factors Affecting the Rate of Transpiration

Wind:
- Movement of air increases transpiration by sweeping away water vapor, enhancing the concentration difference.
Light:
- More light increases transpiration because stomata open to absorb CO $_{2}$ , and sunlight warms the leaf.
Temperature:
- Increased temperature increases transpiration as water molecules have more energy.
Humidity:
- Increased humidity decreases transpiration by reducing the concentration gradient.

Measuring Transpiration Rates Using Potometers

A potometer is a device used for measuring the rate of water uptake of a leafy shoot.
Water is drawn up the stem to replace the water transpired (loses water vapor to air).
Readings are taken of the movement of bubble along a ruler in a given time.

Determining Stomatal Density

Stomatal density is the number of stomata per unit area of leaf surface.
Count the number of stomata in a known area under the microscope.