The Mammalian Gas Exchange System

Membranes and Transport

What are the main features of an effective gas exchange system?

A large surface area, thin exchange surface, rich blood supply, moist surface and permeable surface.

Why is a large surface area important for gas exchange?

It allows more gas molecules to diffuse across at the same time, increasing the rate of diffusion.

Why must a gas exchange surface be thin?

It creates a short diffusion distance, increasing the rate of diffusion.

Why is a rich blood supply important at a gas exchange surface?

It transports gases to and from the exchange surface and maintains steep concentration gradients.

Why must gas exchange surfaces be moist?

Respiratory gases dissolve in the moisture before diffusing across the surface.

Why must gas exchange surfaces be permeable?

They must allow oxygen and carbon dioxide to pass freely across them.

Why do mammals need specialised gas exchange systems?

They are large organisms with a relatively small surface area to volume ratio and high metabolic demands, so diffusion across the body surface alone is insufficient.

Where does most gas exchange occur in the mammalian lungs?

In the alveoli.

Define an alveolus.

A tiny air sac in the lungs where gas exchange occurs.

What is an alveolus made from?

A single layer of flattened squamous epithelial cells.

How thick is the wall of an alveolus?

One cell thick.

How thick is the wall of a capillary surrounding an alveolus?

One cell thick.

Why are the walls of alveoli and capillaries only one cell thick?

They provide a very short diffusion distance, increasing the rate of gas exchange.

What is the approximate diffusion distance between alveolar air and blood?

About 0.5–1.5 μm.

What is lung surfactant?

A special phospholipid that coats the inner surface of the alveoli.

What is the function of lung surfactant?

It prevents the alveoli from collapsing and makes breathing easier.

Why would alveoli tend to collapse without surfactant?

Their elastic tissue creates a natural tendency for them to collapse.

What is elastic recoil of the lungs?

The tendency of stretched elastic tissue in the lungs to return to its original shape, helping force air out during exhalation.

How does gas exchange occur between the alveoli and blood?

By simple diffusion.

In which direction does oxygen diffuse at the alveoli?

From the alveolar air into the blood.

Why does oxygen diffuse from the alveoli into the blood?

The oxygen concentration is higher in the alveoli than in the deoxygenated blood, so oxygen diffuses down its concentration gradient.

In which direction does carbon dioxide diffuse at the alveoli?

From the blood into the alveolar air.

Why does carbon dioxide diffuse from the blood into the alveoli?

The carbon dioxide concentration is higher in the blood than in the alveolar air, so it diffuses down its concentration gradient.

State Fick's Law of Diffusion.

Rate of diffusion = (surface area × concentration difference) ÷ thickness of the exchange surface.

According to Fick's Law, how does increasing surface area affect diffusion?

It increases the rate of diffusion.

According to Fick's Law, how does increasing the concentration difference affect diffusion?

It increases the rate of diffusion.

According to Fick's Law, how does decreasing the thickness of the exchange surface affect diffusion?

It increases the rate of diffusion by reducing the diffusion distance.

How are alveoli adapted to provide a large surface area?

The lungs contain around 480–500 million alveoli, producing an enormous total surface area.

Approximately what is the total gas exchange surface area of the adult human lungs?

About 40–75 m².

How does the large number of alveoli increase gas exchange?

It provides a very large surface area, allowing more gas molecules to diffuse simultaneously.

How is a steep oxygen concentration gradient maintained at the alveoli?

Ventilation continually brings oxygen-rich air into the alveoli while blood flow carries oxygen away.

How is a steep carbon dioxide concentration gradient maintained at the alveoli?

Blood continually brings carbon dioxide to the alveoli while ventilation removes carbon dioxide-rich air.

Why is continuous blood flow through alveolar capillaries important?

It maintains steep concentration gradients by carrying oxygen away and bringing carbon dioxide to the alveoli.

Why is ventilation important for gas exchange?

It continually replaces alveolar air, maintaining steep concentration gradients for oxygen and carbon dioxide.

Define ventilation.

The movement of air into and out of the lungs caused by pressure changes in the chest.

What is another term for breathing?

Ventilation.

Define inhalation.

Breathing in.

Define exhalation.

Breathing out.

Is inhalation an active or passive process?

Active.

Why is inhalation an active process?

Muscles contract and use energy to increase the volume of the thorax.

What happens to the diaphragm during inhalation?

It contracts, flattens and moves downwards.

What happens to the external intercostal muscles during inhalation?

They contract.

What happens to the ribcage during inhalation?

It moves upwards and outwards.

What happens to the volume of the thorax during inhalation?

It increases.

What happens to pressure inside the lungs during inhalation?

It decreases below atmospheric pressure.

Why does air enter the lungs during inhalation?

Pressure inside the lungs becomes lower than atmospheric pressure, so air moves into the lungs down the pressure gradient.

Give the sequence of events during inhalation.

Diaphragm contracts and flattens → external intercostal muscles contract → ribs move up and out → thoracic volume increases → pressure decreases → air enters.

Is normal exhalation active or passive?

Passive.

What happens to the diaphragm during normal exhalation?

It relaxes and becomes dome-shaped.

What happens to the external intercostal muscles during normal exhalation?

They relax.

What happens to the ribcage during normal exhalation?

It moves downwards and inwards.

What happens to the volume of the thorax during exhalation?

It decreases.

What happens to pressure inside the lungs during exhalation?

It increases above atmospheric pressure.

Why does air leave the lungs during exhalation?

Pressure inside the lungs becomes higher than atmospheric pressure, so air moves out down the pressure gradient.

What role does elastic recoil play in normal exhalation?

Elastic fibres in the lungs recoil, helping decrease thoracic volume and force air out.

Give the sequence of events during normal exhalation.

Diaphragm relaxes → external intercostal muscles relax → ribs move down and in → elastic recoil occurs → thoracic volume decreases → pressure increases → air leaves.

What happens during forced exhalation?

The internal intercostal muscles contract, pulling the ribs down and in, while abdominal muscles can force the diaphragm upwards.

What is the role of the internal intercostal muscles during forced exhalation?

They contract and pull the ribcage downwards and inwards.

Why does forced exhalation move air out faster than normal exhalation?

Stronger muscle contractions decrease thoracic volume more rapidly, producing a larger pressure difference.

Why is coughing an example of forced exhalation?

It uses strong muscular contractions to rapidly force air from the respiratory system.

Why does ventilation increase the rate of gas exchange?

It maintains steep concentration differences between alveolar air and blood.

What percentage of oxygen is present in inspired air?

About 20.7%.

What percentage of oxygen is present in alveolar air?

About 13.2%.

What percentage of oxygen is present in expired air?

About 14.5%.

Why does expired air contain less oxygen than inspired air?

Some oxygen diffuses from the alveoli into the blood.

Why does expired air contain slightly more oxygen than alveolar air?

Expired air mixes with air from the airways where little or no gas exchange occurs.

What percentage of carbon dioxide is present in inspired air?

About 0.04%.

What percentage of carbon dioxide is present in alveolar air?

About 5.0%.

What percentage of carbon dioxide is present in expired air?

About 3.9%.

Why does expired air contain more carbon dioxide than inspired air?

Carbon dioxide produced by respiration diffuses from the blood into the alveoli and is exhaled.

Why does expired air contain less carbon dioxide than alveolar air?

Alveolar air mixes with air from the airways where little or no gas exchange occurs.

Why is nitrogen concentration relatively similar in inspired and expired air?

Nitrogen is not significantly exchanged during respiration.

Why does alveolar air contain more water vapour than inspired air?

Air becomes humidified as it passes through the moist respiratory surfaces.

How does the respiratory system protect the lungs from particles and pathogens?

Mucus traps particles and pathogens, and cilia move the mucus away from the lungs.

What are pathogens?

Microorganisms that cause disease.

What produces mucus in the respiratory tract?

Goblet cells.

What is the function of mucus in the respiratory system?

It traps dust, pollen, microorganisms and other particles.

Where are ciliated epithelial cells found in the respiratory system?

In the lining of the trachea and bronchi.

What is the function of cilia in the respiratory system?

They beat to move mucus containing trapped particles and pathogens towards the back of the throat.

What usually happens to mucus moved to the back of the throat?

It is swallowed.

What happens to swallowed pathogens trapped in respiratory mucus?

Stomach acid and digestive enzymes help destroy them.

What are macrophages?

Phagocytic white blood cells that engulf pathogens and particles.

What is the role of macrophages in the alveoli?

They engulf bacteria and other particles, helping keep the alveoli clear.

Why is breathing through the nose generally more protective than breathing through the mouth?

The nasal cavity filters, warms and moistens incoming air before it reaches the lungs.

What is the function of the nasal cavity?

It filters, warms and moistens incoming air.

What is the function of the epiglottis?

It closes over the glottis during swallowing to prevent food entering the respiratory system.

What is the function of the larynx?

It is the voice box and uses airflow to produce sound.

What is the function of the trachea?

It carries air towards the bronchi.

Why does the trachea contain incomplete rings of cartilage?

They prevent the trachea from collapsing while allowing the oesophagus to expand when food is swallowed.

What are the bronchi?

Two main airways that branch from the trachea and enter the lungs.

What are bronchioles?

Smaller branching airways within the lungs that lead to the alveoli.

What is the function of the alveoli?

They are the main gas exchange surfaces of the lungs.

What is the function of the pleural membranes?

They surround the lungs and help form a sealed chest cavity.

What is the function of pleural fluid?

It lubricates the pleural membranes, reducing friction during breathing movements.

What is the function of the diaphragm?

It changes the volume of the thorax during ventilation.

What is the function of the intercostal muscles?

They move the ribs to change thoracic volume during ventilation.