Topic 6.4 - Gas Exchange

Spirometry

A process that involves measuring the amount (volume) and / or speed (flow) at which air can be inhaled or exhaled

They have a very thin epithelial layer (one cell thick) to minimise diffusion distances for respiratory gases. They are surrounded by a rich capillary network to increase the capacity for gas exchange with the blood. They are roughly spherical in shape, in order to maximise the available surface area for gas exchange. Their internal surface is covered with a layer of fluid, as dissolved gases are better able to diffuse into the bloodstream

Describe the features of alveoli that have adapted them to gas exchange

Mouth Trachea Bronchus (singular) / bronchi (plural) Bronchiole (singular) / bronchioles (plural) Alveolus (singular) / alveoli (plural)

Describe the path that air takes from the mouth to the alveoli.

Inspiration (inhaling) and expiration (exhaling) are controlled by two sets of antagonistic muscle groups. During expiration the diaphragm muscles contract, causing the diaphragm to flatten and increase the volume of the thoracic cavity. External intercostals contract, pulling ribs upwards and outwards (expanding chest). During inspiration the diaphragm muscles relax, causing the diaphragm to curve upwards and reduce the volume of the thoracic cavity. Internal intercostal muscles contract, pulling ribs inwards and downwards (reducing breadth of chest). Abdominal muscles contract and push the diaphragm upwards during forced exhalation. Additional muscle groups may help pull the ribs downwards (e.g. quadratas lumborum)

Describe the process of inspiration and expiration.

Breathing is the active movement of respiratory muscles that enables the passage of air into and out of the lungs. The contraction of respiratory muscles changes the volume of the thoracic cavity (i.e. the chest). 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)

Explain how muscle contractions cause the pressure changes inside the thorax that forces air in and out of the lungs?

Respiratory muscles contract to change the volume of the thoracic cavity and hence alter the pressure in the chest. Because muscles only do work via contraction, different groups of muscles are required to expand and contract chest volume. Changing chest volume creates a pressure differential between the chest and atmosphere - with air then moving to equalise. Muscles that increase the volume of the chest cause inspiration (as chest pressure is less than atmospheric pressure). Muscles the decrease the volume of the chest cause expiration (as chest pressure is greater than atmospheric pressure). Atmospheric pressure is lower at high altitudes, meaning a greater increase in chest volume is required before a pressure differential is formed - making it harder to breathe at high altitudes. The body will adapt mechanisms to improve oxygen uptake under these conditions, which is why athletes will often undertake high altitude training prior to competitions

Explain how the muscles contract to change the volume of the thoracic cavity.

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

Explain how ventilation in humans changes in response to levels of physical activity

Type I pneumocytes are involved in the process of gas exchange between the alveoli and the capillaries. They are squamous (flattened) in shape and extremely thin (~ 0.15µm) - minimising diffusion distance for respiratory gases. Type I pneumocytes are connected by occluding junctions, which prevents the leakage of tissue fluid into the alveolar air space. Type I pneumocytes are amitotic and unable to replicate, however type II cells can differentiate into type I cells if required

Explain the function of Type I Pneumocytes

Type II pneumocytes are responsible for the secretion of pulmonary surfactant, which reduces surface tension in the alveoli. They are cuboidal in shape and possess many granules (for storing surfactant components). Type II pneumocytes only comprise a fraction of the alveolar surface (~5%) but are relatively numerous (~60% of total cells)

Explain the function of Type II Pneumocytes

The major cause of emphysema is smoking, as the chemical irritants in cigarette smoke damage the alveolar walls. The damage to lung tissue leads to the recruitment of phagocytes to the region, which produce an enzyme called elastase. This elastase, released as part of an inflammatory response, breaks down the elastic fibres in the alveolar wall. Elastase activity can be blocked by an enzyme inhibitor (α-1-antitrypsin), but not when elastase concentrations are increased. A small proportion of emphysema cases are due to a hereditary deficiency in this enzyme inhibitor due to a gene mutation

Explain the major cause of emphysema

Muscles have two states: contracted and relaxed. Muscles can only cause an action when they contract. Muscle contraction requires ATP and is an active process. When muscles contract, they become shorter. When muscles relax they return to their original length (often being pulled by the contraction of other muscles). Ventilation involves opposite actions so it requires antagonistic pairs of muscles (muscles that can work against each other). When one of the pair contracts, the other is relaxed. The pairs of antagonistic muscles involved in ventilation are: External and internal intercostal muscles. The diaphragm and abdominal muscles. For example: The contraction of the external intercostal muscles causes the ribcage to be pushed up and out and the contraction of the internal intercostal muscles causes the ribcage to be pulled down and in.

Explain the need for antagonistic muscle pairs in ventilation.

Air enters the respiratory system through the nose or mouth and passes through the pharynx to the trachea. The air travels down the trachea until it divides into two bronchi (singular: bronchus) which connect to the lungs. The right lung is composed of three lobes, while the left lung is only comprised of two (smaller due to position of heart). Inside each lung, the bronchi divide into many smaller airways called bronchioles, greatly increasing surface area. Each bronchiole terminates with a cluster of air sacs called alveoli, where gas exchange with the bloodstream occurs

Explain the process of the respiratory system

The muscles responsible for expiration are the abdominal muscles and internal intercostals (plus some accessory muscles). Diaphragm muscles relax, causing the diaphragm to curve upwards and reduce the volume of the thoracic cavity. Internal intercostal muscles contract, pulling ribs inwards and downwards (reducing breadth of chest). Abdominal muscles contract and push the diaphragm upwards during forced exhalation. Additional muscle groups may help pull the ribs downwards (e.g. quadratas lumborum)

Explain which muscles responsible for expiration

The muscles responsible for inspiration are the diaphragm and external intercostals (plus some accessory muscles). Diaphragm muscles contract, causing the diaphragm to flatten and increase the volume of the thoracic cavity. External intercostals contract, pulling ribs upwards and outwards (expanding chest). Additional muscle groups may help pull the ribs up and out (e.g. sternocleidomastoid and pectoralis minor)

Explain which muscles responsible for inspiration

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

Give two reasons why we need a ventilation system.

The lungs share common features with other surfaces adapted to increase the rate of diffusion: There is a large surface area. Due to millions of alveoli. Alveoli are surrounded by dense networks of blood capillaries. Concentrations gradients are maintained. Due to ventilation. Due to the blood flow. There is a short distance for diffusion. The wall of each alveolus in one cell thick. The wall of each capillary is one cell thick.

How are lungs adapted for gas exchange?

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)

How can ventilation in humans can be monitored?

The exchange of oxygen and carbon dioxide between the alveoli and bloodstream (via passive diffusion)

Surface area is an important feature of the gas exchange surface of any organism. Define gas exchange

Pneumocytes

The cells that line the alveoli and comprise of the majority of the inner surface of the lungs

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)

Boyle's Law (pressure is inversely proportional to volume) (When the volume of the thoracic cavity increases, pressure in the thorax decreases. When the volume of the thoracic cavity decreases, pressure in the thorax increases)

The mechanism of breathing occurs according to which principle?

Cell Respiration

The release of energy (ATP) from organic molecules - it is enhanced by the presence of oxygen (aerobic)

A. The movement of air into and out of the lungs.

Ventilation can be best described as ... A. The movement of air into and out of the lungs. B. The movement of alveoli to help gas exchange. C. The movement of the trachea carrying air into the lungs. D. The movement of oxygen and carbon dioxide through the walls of the alveoli.

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)

W hen someone has emphysema, explain the effect of the the walls of the alveoli losing their elasticity.

The main causes of emphysema are smoking and air pollution. Smoking leads to regular chest infections (bronchitis) due to the accumulation of mucus in the lungs. Chest infections cause the number of phagocytes inside the lungs to greatly increase. The action of phagocytes not only kills microbes. They also secrete enzymes, which break down elastic fibers in the walls of the alveoli. Over time, the walls of the alveoli are severely damaged resulting in fewer, larger, less elastic structures. This greatly reduces the surface area for gas exchange and makes the lungs less elastic. The effects of emphysema on the body include: Shortness of breath (this becomes worse as the disease progresses). Wheezing. Persistent cough.

What are the causes of emphysema and what effect does it have on the body?

The causes of lung cancer have been established by epidemiological studies. A high incidence of lung cancer strongly correlates with the following factors: Smoking - tobacco smoke contains numerous carcinogenic compounds. Passive smoking - this involves inhaling the smoke exhaled from smokers. Asbestos fibers - fibers enter and persist in the lungs after exposure to asbestos dust. Radon gas - radon gas is released naturally from rocks and soil. It emits ionizing radiation. The effects of lung cancer on the body include: Persistent cough, shortness of breath, chest pain, feeling tired, weight loss, regular chest infections, coughing up blood, secondary tumors can form. More people die of lung cancer than any other type of cancer.

What are the causes of lung cancer and what effect does it have on the body?

Type I pneumocytes: The wall of each alveolus is made of a single layer of epithelial cells. The epithelial cells are mainly type I pneumocytes. Type I pneumocytes are very thin cells so they are adapted for gas exchange. The endothelial cells forming the wall of the capillary are also very thin so there is a very short distance (about 0.5 µm) for gas to diffuse from the alveolar air space into the bloodstream (or vice versa). Diffusion can occur rapidly over such short distances. Type II pneumocytes: Type II pneumocytes secrete fluid that lines the inside of the alveolus. The fluid contains a surfactant. Surfactants are amphipathic (they have hydrophobic and hydrophilic regions). They reduce surface tension and prevent the walls of the alveoli from sticking together.

What are the roles of type I and type II pneumocytes?

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)

What are the two main ways exercise influence ventilation?

B. It helps prevent alveoli from collapsing.

What is the main function of surfactant in the film of moisture on the surface of the alveoli? A. It lubricates the small tubes that branch off the bronchi. B. It helps prevent alveoli from collapsing. C. If creates a thin layer of flattened cells. D. It increases the movement of air into and out of the lungs.

C. They carry blood which transports oxygen and carbon dioxide.

What is the main function of the network of capillaries which surround the alveoli? A. They produce surfactant B. They carry blood which contains surfactant C. They carry blood which transports oxygen and carbon dioxide. D. To increase the movement of air into and out of the lungs.

B. They produce surfactant

What is the main function of type II pneumocytes in the alveoli? A. They carry blood which encourages diffusion of oxygen into leucocytes. B. They produce surfactant C. They provide a thin gas exchange surface D. To increase the movement of air into and out of the lungs

Cells in the body require oxygen for aerobic respiration. Carbon dioxide is a waste product. Blood becomes oxygenated as it flows through the lungs and carbon dioxide is excreted. Gas exchange occurs in the alveoli at the tips of the bronchioles. Alveoli are surrounded by a dense network of capillaries, through which blood is constantly moving. Oxygen moves by simple diffusion from the air spaces in the alveoli into capillaries. Carbon dioxide moves by simple diffusion from capillaries into the air spaces in the alveoli. Ventilation in humans is tidal; air is moved into and then out of the alveoli. Ventilation maintains concentration gradients for oxygen intake and carbon dioxide release.

What is the role of ventilation in gas exchange?

Common symptoms of emphysema include shortness of breath, phlegm production, expansion of the ribcage, cyanosis and an increased susceptibility to chest infections

What the common symptoms of emphysema

A. The diffusion of oxygen and carbon dioxide through the walls of the alveoli.

Which is the best description of Gas Exchange? A. The diffusion of oxygen and carbon dioxide through the walls of the alveoli. B. The movement of air into and out of the lungs. C. The movement of the bones which form the ribcage D. The diffusion of oxygen and carbon dioxide through the bronchi

D. The controlled release of energy from glucose; to make ATP in cells.

Which of the following best describes Cell Respiration? A. A thin layer of flattened cells. B. The movement of air into and out of the lungs. C. Covers the surface of alveoli and helps gas exchange. D. The controlled release of energy from glucose; to make ATP in cells.

Type II pneumocytes secrete a liquid known as pulmonary surfactant which reduces the surface tension in alveoli. As an alveoli expands with gas intake, the surfactant becomes more spread out across the moist alveolar lining. This increases surface tension and slows the rate of expansion, ensuring all alveoli inflate at roughly the same rate

Which type of pneumocytes secretes pulmonary surfactant and why?

Alveoli are lined by a layer of liquid in order to create a moist surface conducive to gas exchange with the capillaries. It is easier for oxygen to diffuse across the alveolar and capillary membranes when dissolved in liquid. While this moist lining assists with gas exchange, it also creates a tendency for the alveoli to collapse and resist inflation

Why are alveoli are lined by a layer of liquid?

Emphysema

a lung condition whereby the walls of the alveoli lose their elasticity due to damage to the alveolar walls

Surface tension

the elastic force created by a fluid surface that minimises the surface area (via cohesion of liquid molecules)

antagonistic muscle groups

working oppositely - when the inspiratory muscles contract, the expiratory muscles relax (and vice versa)