Breathing and Gas Exchange Notes

Respiration and Gaseous Exchange

Definitions

• Respiration: The process of releasing energy from the breakdown of glucose by combining it with oxygen inside living cells.
• Gaseous Exchange: The movement of gases (oxygen and carbon dioxide) into and out of an organism.

The Gaseous Exchange System in Humans

The lungs are highly adapted organs in mammals that facilitate the exchange of gases between the environment and the blood. This process is also referred to as the breathing system or the respiratory system.

Structure of the Thorax

The thorax contains several key components:

• Trachea: A tube with incomplete rings of cartilage that carries air to the lungs. It is lined with cells that produce mucus and cilia to move the mucus away.
• Bronchi: Carry air from the trachea to the lungs.
• Bronchioles: Carry air from the bronchi to the alveoli.
• Alveoli: Tiny air sacs adapted for gas exchange.
• Diaphragm: A sheet of muscle that is domed and helps in breathing movements, separating the thorax from the abdomen.
• Ribs: Bones that protect and ventilate the lungs.
• Intercostal Muscles: Muscles that move the ribs for ventilation (breathing).
• Pleural Membranes: Thin, moist membranes forming an airtight seal around the lungs, separating the inside of the thorax from the lungs.

Ventilation (Breathing)

• Ventilation involves moving air in and out of the lungs.
• This process requires a difference in air pressure and relies on the thorax being an airtight cavity.
• Movements of the intercostal muscles and diaphragm change the volume inside the cavity, thereby affecting the air pressure.
• These changes in pressure cause the air to move in or out.

Inhalation

1. Intercostal muscles contract.
2. Ribs move up and out.
3. Diaphragm contracts and flattens.
4. Volume of the thorax increases.
5. Pressure decreases, causing air to move into the lungs.

Exhalation

1. Intercostal muscles relax.
2. Ribs move down and in.
3. Diaphragm relaxes and returns to a dome shape.
4. Volume of the thorax decreases.
5. Pressure increases, forcing air out of the lungs.

Alveoli

• The lungs contain approximately 700,000,000 tiny air sacs called alveoli.
• These alveoli are surrounded by a network of blood capillaries.
• Gases diffuse into and out of the blood based on concentration gradients.
• Oxygenated blood travels back to the heart to be pumped around the body.

Gas Exchange Process in Alveoli

1. Deoxygenated blood flows from the tissues, rich in carbon dioxide.
2. Carbon dioxide diffuses out of the bloodstream into the alveoli and then into the bronchiole.
3. Oxygen diffuses from the bronchiole into the alveoli and then into the red blood cells.

Features of the Alveoli

1. Large, moist surface area.
2. Excellent capillary blood supply.
3. Very thin cell membrane separating blood and lung.

• Each alveolus is surrounded by blood capillaries, which carry deoxygenated blood.
• Carbon dioxide in the blood diffuses into the alveoli to be exhaled.
• Oxygen diffuses from the alveoli into the blood and binds to the red blood cells.

Alveoli Adaptations for Efficient Gas Exchange

• Large surface area for diffusion.
• Moist surface to dissolve gases and increase the diffusion rate.
• Rich blood supply to maintain a steep diffusion gradient between the alveoli and the blood.
• Thin walls (one cell thick) for a short diffusion distance between the air and the blood.
• Permeable walls.

Ventilation

• Inhalation: Taking air into the lungs.
• Exhalation: Removing air from the lungs.

Composition of Inhaled and Exhaled Air

Breathing In (Inhalation)

Rib cage moves upwards and outwards, the diaphragm moves downwards and becomes flatter. This increases the volume of the thorax, decreasing the pressure, so air is drawn IN.

Breathing Out (Exhalation)

Rib cage moves downwards and inwards, the diaphragm moves upwards and becomes dome-shaped. This decreases the volume of the thorax, increasing the pressure, so air is pushed OUT.

Practical Investigation: Effect of Exercise on Breathing Rate

1. Determine resting breathing rate by counting inhalations per minute, repeat three times, and take an average.
2. Calculate breathing rate during exercise by jogging on the spot for one minute, counting breaths, repeat three times, and take the average.
3. Repeat with other people.

• Breathing rate increases with exercise to provide muscles with more oxygen for aerobic respiration and to remove extra carbon dioxide.

Controlled Variables

• Same time spent exercising.
• Same type of exercise.
• Same age/sex of person.
• Same temperature of room.
• Same food and drink before the experiment.

Investigating the Release of Carbon Dioxide from Breathing

• Use an indicator (lime water or hydrogen-carbonate indicator) to detect carbon dioxide levels in inhaled and exhaled air.
• Flask A (inhaled air): No change to the indicator.
• Flask B (exhaled air): Lime water goes cloudy or hydrogen carbonate goes yellow.

Effects of Smoking on the Gaseous Exchange System

• Smoking contributes to:
  • Lung Cancer
  • Chronic Bronchitis
  • Emphysema

Lung Cancer

• Diseased lung showing a large tumor and smaller tumors, often blackened by tar from cigarettes.

Chronic Bronchitis

• In healthy lungs, ciliated epithelial cells have cilia that waft mucus, secreted by goblet cells, up to the throat to be swallowed and destroyed in stomach acid.
• Tar in cigarettes damages and paralyzes cilia, leading to bacteria buildup and chest infections.
• The tar also irritates the lining, encouraging mucus production that cannot be cleared, causing a smoker’s cough and chronic bronchitis.

Emphysema

• Smoke damages alveoli walls, causing them to burst and fuse together, reducing the surface area for gaseous exchange.
• Sufferers struggle to perform basic tasks due to lack of oxygen.
• There is no cure.

Effects of Smoking on the Circulatory System

• Smoking damages the circulatory system and increases the risk of developing coronary heart disease (CHD).

Coronary Heart Disease (CHD)

• Fatty deposits build up in the walls of the coronary arteries, which normally supply the heart with oxygen.
• Eventually, these arteries can become blocked entirely by a clot, preventing oxygen from reaching the heart, leading to a heart attack.

Additional Effects

• Carbon monoxide in smoke binds to haemoglobin in the blood instead of oxygen, forcing the heart to work harder.
• Nicotine makes blood cells stickier and narrows blood vessels (vasoconstriction).
• All these factors increase the effects of CHD and make a heart attack more likely.

Concept Check – True or False

• An increase in pressure inside the thorax causes inhalation (False)
• Gas exchange occurs in the bronchi (False)
• The intercostal muscles contract for inhalation (True)
• The diaphragm contracts for exhalation (False)
• Exercise increases breathing rate (True)
• Oxygen is absorbed into the blood by active transport (False)
• Carbon dioxide diffuses into the blood from the alveoli (False)
• Lung cancer is caused by nicotine (False)
• Emphysema is when your alveoli burst and fuse (True)
• Smoking increases your heart rate (True)

Exam Style Question

Q: Explain what happens in the lungs for the process of inhalation to occur (4 Marks)

• Intercostal muscles contract
• Ribs move up and out
• Diaphragm contracts/flattens
• Volume inside thorax increases
• Pressure decreases
• Air is drawn in