6.4 Gas exchange
Essential idea: The lungs are actively ventilated to ensure that gas exchange can occur passively.
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6.4.U1 Ventilation maintains concentration gradients of oxygen and carbon dioxide between air in alveoli and blood flowing in adjacent capillaries.
6.4.U1 Ventilation maintains concentration gradients of oxygen and carbon dioxide between air in alveoli and blood flowing in adjacent capillaries.
The processes involved in physiological respiration are:
Ventilation: The exchange of air between the atmosphere and the lungs – achieved by the physical act of breathing
Gas Exchange: The exchange of oxygen and carbon dioxide between the alveoli and bloodstream (via passive diffusion)
Cell Respiration: The release of energy (ATP) from organic molecules – it is enhanced by the presence of oxygen (aerobic)
Purpose of Ventilation:
Because gas exchange is a passive process, a ventilation system is needed to maintain a concentration gradient in alveoli
Oxygen is consumed by cells during cellular respiration and carbon dioxide is produced as a waste product
This means O2 is constantly being removed from the alveoli into the bloodstream (and CO2 is continually being released)
The lungs function as a ventilation system by continually cycling fresh air into the alveoli from the atmosphere
This means O2 levels stay high in alveoli (and diffuse into the blood) and CO2 levels stay low (and diffuse from the blood)
The lungs are also structured to have a very large surface area, so as to increase the overall rate of gas exchange
6.4.U2 Type I pneumocytes are extremely thin alveolar cells that are adapted to carry out gas exchange.
6.4.U2 Type I pneumocytes are extremely thin alveolar cells that are adapted to carry out gas exchange.
Alveoli function as the site of gas exchange, and hence have specialised structural features to help fulfil this role. Pneumocytes (or alveolar cells) are the cells that line the alveoli and comprise of the majority of the inner surface of the lungs
The walls of the alveoli are predominately made from a single layer of epithelial cells called Type I pneumocytes
They are squamous (flattened) in shape and extremely thin (~ 0.15µm) – minimising diffusion distance for respiratory gases
Since the alveoli are surrounded by capillaries that are also only one cell thick, oxygen and carbon dioxide have a very short distance to diffuse into the blood from the alveoli and out of the blood into the alveoli respectively
This adaptation allows for a rapid rate of gas exchange
Connected by occluding junctions, which prevents the leakage of tissue fluid into the alveolar air space
Amitotic and unable to replicate, however type II cells can differentiate into type I cells if required.
6.4.U5 Air is carried to the lungs in the trachea and bronchi and then to the alveoli in bronchioles.
6.4.U5 Air is carried to the lungs in the trachea and bronchi and then to the alveoli in bronchioles.
Air enters the respiratory system through the nose or mouth and travels through the pharynx and then the trachea (made from rings of cartilage)
The trachea divides into two bronchi (left and right)
Inside each lung the bronchi divide into many smaller tubes called bronchioles
These numerous bronchioles form a tree root-like structure that spreads throughout the lungs
Each bronchiole ends in a cluster of air sacs called alveoli
6.4.U6 Muscle contractions cause the pressure changes inside the thorax that force air in and out of the lungs to ventilate them.
6.4.U6 Muscle contractions cause the pressure changes inside the thorax that force air in and out of the lungs to ventilate them.
Gases will move from a region of high pressure to a region of lower pressure (similar to movement via concentration gradient)
When the pressure in the chest is less than the atmospheric pressure, air will move into the lungs (inspiration)
When the pressure in the chest is greater than the atmospheric pressure, air will move out of the lungs (expiration)
6.2.U6 Different muscles are required for inspiration and expiration because muscles only do work when they contract
6.2.U6 Different muscles are required for inspiration and expiration because muscles only do work when they contract
Inspiration (inhaling) and expiration (exhaling) are controlled by two sets of antagonistic muscle groups. When different muscles work together to perform opposite movements, they do so in an antagonistic fashion; when one muscle contracts the other will relax
When muscles contract and shorten (do work), they exert a pulling force that causes movement
The antagonistic muscle will relax and lengthen because of the pulling force of the other muscle; therefore no work is done
For example, when one breaths in air, the external intercostal muscles contract, moving the ribcage up and out and the internal intercostal muscles relax (biceps and triceps work in similar fashion in our arms). The opposite occurs during expiration.
Muscles therefore only cause movement in one direction while contracting (antagonistic pair relaxes). Movement in the other direction occurs when the other muscle of the pair contracts and the first muscle relaxes
6.4.A1 Causes and consequences of lung cancer.
6.4.A1 Causes and consequences of lung cancer.
Be able to:
Outline the causes of lung cancer.
List symptoms of lung cancer.
Lung cancer describes the uncontrolled proliferation of lung cells, leading to the abnormal growth of lung tissue (tumour) The abnormal growth can impact on normal tissue function, leading to a variety of symptoms according to size and location The tumours can remain in place (benign) or spread to other regions of the body (malignant)
Smoking
is the number one cause of lung cancer
there is an extremely high correlation with the number of cigarettes an individual smokes in a day and the incidence of lung cancer. Marijuana smoking is also linked to lung disease.
Cigarettes contain a high number of carcinogens, such as polycyclic aromatic hydrocarbons and nitrosamines
Second-hand smoke can also be considered a cause of cancer in non-smokers
Air Pollution
Air pollution from exhaust fumes containing nitrogen oxides, fumes from diesel engines and smoke from burning carbon compounds such as coal are a minor cause of lung cancer. This depends on where in the world you live and the air quality.
Radon Gas
In some parts of the world, this radioactive gas can leak out of certain rocks such as granite, accumulating in poorly ventilated buildings
Asbestos
Construction sites, factories and mines can have dust particles in the air. If steps aren’t taken to properly protect the worker's, lung cancers can develop.
Common Risk Factors for Lung Cancer
Lung cancer is a very serious disease and the consequences can be severe, especially if the cancer is not recognized early on.
If the tumour is large when it is discovered, metastasis might have occurred (cancer spreads to other parts of the body and forms secondary tumours). In these cases, mortality rates are very high. If the tumour is found early on, parts of the affected lung with the tumour can be removed and chemotherapy can be used to help kill the rest of the cancer cells. Re-occurrence of the disease is quite common.
6.2.A2 Causes and consequences of emphysema
6.2.A2 Causes and consequences of emphysema
Emphysema is a lung condition whereby the walls of the alveoli lose their elasticity due to damage to the alveolar walls The loss of elasticity results in the abnormal enlargement of the alveoli, leading to a lower total surface area for gas exchange The degradation of the alveolar walls can cause holes to develop and alveoli to merge into huge air spaces (pulmonary bullae)
Healthy lungs
Emphysema is a lung condition whereby the walls of the alveoli lose their elasticity due to damage to the alveolar walls
Emphysema is another respiratory disease that is often linked to smoking both tobacco and marijuana
The loss of elasticity results in the abnormal enlargement of the alveoli, leading to a lower total surface area for gas exchange
The degradation of the alveolar walls can cause holes to develop and alveoli to merge into huge air spaces (pulmona
Phagocytes (white blood cells that engulf foreign bacteria) usually prevent lung infections and produce a hydrolytic enzyme called elastase
An enzyme inhibitor usually prevents elastase from digesting lung tissue
Smokers lungs generally contain a high number of these phagocytes/macrophages in their blood
Since there is a higher level of phagocytes, more elastase is produced;however, not enough of the inhibitor that prevents elastase from digesting lung tissue
This results in the destruction of elastic fibres of the alveolar walls by the enzyme elastase
The alveoli can become over-inflated and fail to recoil properly
Small holes can also develop in the walls of the alveoli
The alveoli can merge forming huge air spaces and a lower surface area.
This destruction cannot be reversed
6.4.A3 External and internal intercostal muscles, and diaphragm and abdominal muscles as examples of antagonistic muscle action.
6.4.A3 External and internal intercostal muscles, and diaphragm and abdominal muscles as examples of antagonistic muscle action.
Ventilation consists of inhalation (inspiration) and exhalation (expiration)
Inhalation
Inhalation
External intercostal muscles contract pulling the ribs upwards and outwards.
The diaphragm which is a flat sheet of muscle extending across the bottom of the rib cage contracts and flattens out.
These two actions enlarge the thoracic cavity surrounded the lungs, thereby increasing the volume of the lungs.
When the volume of the lungs increases, the pressure inside the lungs decreases and becomes lower than the pressure in the surrounding atmosphere.
Since gas moves from higher pressure to lower pressure, air rushes into the lungs from the surrounding atmosphere to equalize the pressure.
Exhalation
Exhalation
The external intercostal muscles relax and the diaphragm snaps back to its original shape (domed shape).
This moves the ribs back down and inwards and decreases the volume of the thoracic cavity and the lungs.
This decrease in volume increases the pressure inside the lungs.
Since the pressure inside the lungs is now greater than the atmospheric pressure, and gas moves from high pressure to low pressure, air rushes out of the lungs into the surrounding environment.
NOTE: If there is a forced exhalation (push and squeeze the air out of the lungs) the internal intercostal muscles will also contract along with the abdominal muscles to pull the rib cage down and squeeze the organs in the abdomen
6.4.S1 Monitoring of ventilation in humans at rest and after mild and vigorous exercise.
6.4.S1 Monitoring of ventilation in humans at rest and after mild and vigorous exercise.
Ventilation in humans changes in response to levels of physical activity, as the body’s energy demands are increased
ATP production (via cellular respiration) produces carbon dioxide as a waste product (and may consume oxygen aerobically)
Changes in blood CO2 levels are detected by chemosensors in the walls of the arteries which send signals to the brainstem
As exercise intensity increases, so does the demand for gas exchange, leading to an increase in levels of ventilation
Trends in Tidal Volume and Breathing Frequency
Exercise will influence ventilation in two main ways:
Increase ventilation rate (a greater frequency of breaths allows for a more continuous exchange of gases)
Increase tidal volume (increasing the volume of air taken in and out per breath allows for more air in the lungs to be exchanged)
Measuring Ventilation
Measuring Ventilation
Ventilation in humans can be monitored in a number of ways:
Via simple observation (counting number of breaths per minute)
Chest belt and pressure meter (recording the rise and fall of the chest)
Spirometer (recording the volume of gas expelled per breath)
Spirometry involves measuring the amount (volume) and / or speed (flow) at which air can be inhaled or exhaled
A spirometer is a device that detects the changes in ventilation and presents the data on a digital display
A more simplistic method involves breathing into a balloon and measuring the volume of air in a single breath
The volume of air can be determined by submerging the balloon in water and measuring the volume displaced (1ml = 1cm3)
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