6.4 ~ Gas exchange

Lung structure

• Air enters the ventilation system through the nose or mouth and travels down the trachea

• Trachea has rings of cartilage in its wall to prevent it from collapsing when there is low air pressure

• Trachea branches into two bronchi

  • Each bronchus leads to a lung

• Bronchi branch into bronchioles

  • These continue to branch extensively throughout the lung, getting smaller and smaller in diamter

• Some of these bronchioles have smooth muscle in their walks, allowing them to contract or expand

• At the end of a bronchiole is a group of alveoli

  • Site of gas exchange

Gas exchange

• Is the process of absorbing one gas from the environment and releasing a different one

• In terrestrial organisms, gases are exchanged with the air

  • In humans, gas exchange occurs in small structures in the lungs called alveoli

• Gas exchange is a passive process that takes place by diffusion

  • Oxygen and carbon dioxide diffuse between the alveoli and the blood in the capillaries adjacent to the alveoli

Diffusion

• Takes place because of the concentration gradients of oxygen and carbon dioxide between the alveoli and the blood

  • To maintain these concentration gradients, stale air in the lungs needs to be replaced by fresh air (ventilation) and blood continually

Ventilation

• Occurs due to pressure changes in the lungs

  • These are caused by muscle contractions that changes the size of the thoracic cavity

• Pressure and volume are inversely related: when volume is large, pressure is low: when volume is small and the pressure is high

  • Gases will move from areas of high pressure to areas of low pressure

• The intercostal muscles lie between the ribs

  • There are external and internal intercostal muscles

  • The contraction of these muscles causes the volume of the thorax to change

Intercostal muscles

1. The external intercostal muscles are located on the outside of the rib cage

2. The internal intercostal muscles are located on the inside of the rib cage

Breathing - Inhale

1. External intercostal muscles contract

2. Internal intercostal muscles relax

3. Rib cage moves up and out

4. Diaphragm contracts & flattens

5. Volume inside thorax increases

6. Pressure inside thorax drops to below atmospheric pressure

7. Air moves into the lungs to equalise the pressure

8. Lungs inflate

Breathing - exhale

1. External intercostal muscles relax

2. Internal intercostal muscles contract

3. Rib cage moves down and in

4. The diaphragm relaxes & moves up

5. Volume inside the thorax decreases

6. Pressure inside thorax increases to above atmospheric pressure

7. Air moves out of the lungs to equalise the pressure

8. Lungs dilate

Contracting muscles

• To elicit movement, muscles work in antagonistic pairs: as one contracts, the other relaxes and vice versa

• Muscles are attached to bones around a joint, so contraction of a muscles causes movement around that joint

• When muscles contract, they shorten

  • This is an active process that requires energy

• When muscles relax, they elongate

  • Passive process: they are usually being pulled into an elongated state by another muscle contracting

• Inhalation and exhalation are caused by two antagonistic pairs of muscles: the internal and external intercostal muscles and the diaphragm and abdominal muscles

Type 1 pneumocytes

• Extremely thin alveolar cells that are adapted to carry out gas exchanges

• Their extreme thinness reduces diffusion distances

• Lungs contain millions of alveoli to ensure a huge surface area for diffusion (95%)

• Each alveolus is extremely thin: the wall is a single layer of cells called the epithelium

  • Most of which are type 1 pneumocytes

  • Flattened cells approx. 0.15um thick so diffusion distance is minimal

• Wall of adjacent capillaries is also extremely thin

  • Overall, distance between the inside of alveoli and inside of capillary is less than 0.5um

• Distance that O2 and CO2 have to travel between the blood and the air is very short

  • An adaptation to increase the rate of gas exchange

Type 2 pneumocytes

• Secrete a solution containing surface t

• Creates a moist surface inside the alveoli to prevent the sides of the alveoli adhering to each other by reducing surface tensions

• More rounded cells that occupy around 5% of the alveolar surface area

• These cells secrete a fluid

  • Creates a film of moisture on the inside of the alveolus

• Jobs of the fluid

  • O2 dissolves into this fluid so it can diffuse into the blood

  • CO2 evaporates from this fluid so that it can be exhaled

  • It contains pulmonary surfactant

  • Made of phospholipids and proteins and is essential to prevent the alveoli from collapsing by reducing surface tensions

Adaptations for efficient gas exhange

1. High branching of airways and blood vessels

   1. Millions of capillaries and alveoli

   2. Huge Surface area for gas exchange

2. Both wall of alveol and capillary are very thin

   1. Short diffusion distance

3. Constant ventilation and blood flow

   1. Maintenance of concentration gradient

Causes and consequences of emphysema

• In emphysema, walls of the alveoli are destroyed

  • Instead of lots of small air sacs, there are fewer, much larger air sacs where gas exchange can take place

• Massively reduces the surface area available for gas exchange

  • Walls of alveoli become thickened which increases the diffusion distance

  • Reduces efficiency of gas exchange in lungs

• Smoking is the biggest risk factor for emphysema

  • Number of phagocytes in the lungs are increased in smoker as the immune system response to the inhalation of tobacco smoke

  • These phagocytes release proteases that destroy the bacteria

  • Can have off-target effects on lung tissue

• Chronic inflammation, as occurs in smokers, leads to continual release of these proteases which, over time, destroy the walls of the alveoli

• Alpha-1-antitrypsin (a1-AT) is a protest inhibitor that protects the lungs from degradation by proteases released by phagocytes

  • However, in smokers, there is such an excess of protease real ease that a1-AT cannot protect agaisnt alveolar destruction

• There are people with genetic deficiencies in a1-AT: these people are at high risk of developing emphysema, with or without smoking 

Causes and consequences of lung cancer

• 87% of lung cancer cases are caused by cigarette smoking

  • Tobacco contains many mutagenic chemicals that cause mutations in the cells in the lungs

  • Risk is linked with exposure, so risk of developing lung cancer increases with number of cigarettes smoked per day and number of years smoking

• A small number (3%) of cases are caused by second-hand / passive smoke: this is where someone is inhaling cigarette smoke from the environment around them where someone else is smoking

• Other causes are air pollution, in particular from burning wood, coal or biomass or diesel exhaust fumes: radon gas: and asbestos, silica and other chronic particulate inhalation

• Mortality is high: only 15% of lung cancer patients survive more than 5 years

• Symptoms include shortness of breath, persistent cough, coughing up blood, chest pain, weight loss, fatigue

• Treatment usually includes partial or complete removal of the affected lung

  • This has long-term consequences, even in cancer is treated

Ventilation rate

• The measure of how many breaths are taken per minute

  • This can be counted as the number of inhalations or number of exhalations in one minute

• Ventilation rate will change depending on the body’s need for O2 or need to expel CO2

• Volume of air drawn in and then expelled is called the tidal volume

• You can measure changes in ventilation rate or tidal volume at rest and during exercises

Methods to measure ventilation rate

• Counting the number of breaths or using a ventilation belt

• Tidal volume can be measured using a spirometer or using a setup similar to that shown on the right