Gas exchange and transport Flashcards
What is gas exchange?
All organisms absorb one gas from the environment and release another one. This is gas exchange. Plants absorb carbon dioxide for use in photosynthesis, and release oxygen produced in the process. Humans absorb oxygen for cell respiration and release the carbon dioxide produced.
Unicellular and other small organisms have a large surface area to volume ratio. They can therefore use their outer surface for gas exchange. In larger organisms, the surface area to volume ratio is smaller, so the outer surface of the organism cannot carry out gas exchange rapidly enough. A specialised gas exchange surface is required that is much larger than the outer surface, for example, alveoli in lungs or the spongy mesophyll in leaves.
Features of gas exchange surfaces
Gas exchange happens at a surface where the cells of the organisms are exposed to the environment. In terrestrial organisms the gas exchange surface is where cells are exposed to air as in the lungs of a mammal. In aquatic organisms, it is where the cells are exposed to water as in the gills of a fish.
Gases are exchanged by diffusion across the surface. Because the molecules of oxygen and carbon dioxide move randomly, diffusion is a relatively slow process. To ensure that exchange is rapid enough for an organisms needs, its gas exchange surfaces must have these properties:
- permeable- oxygen and carbon dioxide can diffuse across freely
- large- the total surface are is large in relation to the volume of the organism
- moist- the surface is covered by a film of moisture in terrestrial organisms so gases can dissolve
- thin- the gases must diffuse only a short distance, in most cases though a single layer of cells
The importance of concentration gradients in gas exchange
Gases and other substances diffuse is there is a concentration gradient. For example, carbon dioxide diffuses from the air into photosynthesising leaf cells because the carbon dioxide concentration of the cells is lower. Diffusion tends to reduce concentration gradients, which could decrease the rate and eventually stop gas exchange if the concentrations become equal. For gases to continue to diffuse across exchange surfaces, concentration gradients must be maintained. In small, aerobically respiring organisms, cell respiration maintains concentration gradients. Oxygen is continuously used and carbon dioxide is produced, so the oxygen concentration within the organism remains lower than outside and the carbon dioxide concentration remains higher.
How do large multicellular animals maintain concentration gradients?
In large multicellular animals with a specialised organ for gas exchange (lungs or gills, pumping is required to maintain concentration gradients.
- blood is pumped through the dense capillary networks close to the gas exchange surface. Due to aerobic respiration in the animal, blood arriving at the surface has a low concentration of oxygen and a high concentration of carbon dioxide
- Air or water adjacent to the gas exchange surface is replaced by the process of ventilation. Mammals pump air in and out of the lungs to maintain high enough concentrations of carbon dioxide. Fish pump fresh water over their gills and then out through the gill slits. This one way flow of water combined with blood flow in the opposite direction ensures that the oxygen concentration in the water adjacent to the gills remains high and the carbon dioxide concentration remains low.
Lung structures and ventilation in mammals
The lungs are in the thorax. Air can only get into or out of the thorax through the airways. The airways used to ventilate the lungs consist of the nose, mouth, trachea, bronchi and bronchioles.
If gas is free to move, it will always flow from regions of higher pressure to regions of lower pressure. During ventilation, muscle contractions cause pressure changes inside the thorax that pull extra air into the alveoli and then push it out again. The muscles causing this are:
- the diaphragm that divides the thorax and abdomen
- muscle in the front wall of the abdomen
- intercostal muscle between the ribs, in two layers (internal and external) that are antagonistic.
How does inhalation (inspiration) work?
- the external intercostal muscles contract, moving the ribcage up and out
- the diaphragm contracts, becoming flatter and moving down
- these muscle movements increase the volume of the thorax
- the pressure inside the thorax therefore drops below atmospheric pressure
- air flows into the lungs from outside the body until the pressure inside the lungs rises to atmospheric pressure
How does exhalation (expiration) work
- the internal intercostal muscles contract, moving the ribcage down and in
- the abdominal muscles contract, pushing the diaphragm up into a dome shape
- these muscle movements decrease the volume of the thorax
- the pressure inside the thorax therefore rises above atmospheric pressure
- air flows out from the lungs to outside the body until the pressure inside the lungs falls to atmospheric pressure
How are the lungs adapted for efficient gas exchange?
- airways for ventilation of each lung, consisting of branching bronchioles, ending in alveolar ducts, each of which leads to a group of five or six alveoli
- large surface area for gas exchange- provided by having about 300 million alveoli in a pair of adult lungs. One alveolus is only 0.2-0.5 mm in diameter so only provides a small surface area for gas exchange, but because there are so many of them, the total area is very large: about 40 times greater than the outer surface of the body.
- extensive capillary beds- the surface area of the basket-like networks of blood capillaries around the alveoli is almost as large as that of the alveoli
- short distance for diffusion- both the alveolus wall and adjacent capillary walls are single layers of extremely thin cells. Air and blood are therefore a very short distance apart. The distance apart. The distance for diffusion of O2 and CO2 is less than a micrometre
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moist surface with surfactant- a fluid is secreted by cells in the alveolus wall that keeps the lining of the alveolus moist, allowing oxygen to dissolve. The fluid contains a pulmonary surfactant, that reduces the surface tension and prevents the water from causing the sides of the alveoli to stick together when air is exhaled from the lungs. This helps prevent collapse of the lung`
(Type II pneumocytes secrete surfactant) - Type I pneumocytes allow O2 and CO2 exchange from alveolar space to the bloodq
Measure of lung volumes by spirometry
Lung volumes are measured as part of tests for general health and to help diagnose conditions such as asthma, COPD or cystic fibrosis.
- ventilation rate is the number of times that air is drawn in or expelled per minute
- tidal volume is the volume of fresh air inhaled or the volume of stale air exhaled with each ventilation
- vital capacity is the total volume of air that can be exhaled after a maximum inhalation
- inspiratory reserve volume is the amount of air a person can inhale forcefully after normal tidal inhalation
- expiratory reserve volume is the amount of air a person can exhale forcefully after normal tidal exhalation
A spirometer is a device used to measure lung volumes. A simple version can be constructed using a bell jar and a tube. It is not safe to use this apparatus for repeatedly inhaling air as the carbon dioxide concentration will rise too high.
Doctors use specially designed electronic spirometers that measure flow rate into and out of the lungs and then use data logging software to deduce lung volumes. There are many different designs
Tidal volume is measured by breathing into the spirometer, three or more times, to check the readings are consistent
Vital capacity is measured by breathing in deeply and as fast as possible and then breathing out as fast and as forcefully as possible until the lungs are empty.
How are leaves adapted for gas exchange?
Chloroplasts need a supply of carbon dioxide for photosynthesis. The oxygen produced during the process of photosynthesis must be removed. A large area of moist surface is required over which carbon dioxide can be absorbed and oxygen oxygen excreted, without excessive water loss.
The following leaf adaptations provide this moist surface area:
- Waxy cuticle- the upper and lower surface of leaves is covered in a layer of waterproof wax, secreted by the epidermis cells. It reduces water loss but also prevents movement of carbon dioxide and oxygen.
- Guard cells- there are pairs of guard cells in the epidermis, which can change their shape either to open up a pore or close it. The pore is called a stoma and it allows carbon dioxide and oxygen to pass through. The guard cells usually close the stomata at night when photosynthesis is not occuring and gas exchange is not required. Stomata also close during water stress when plants might die from dehydration.
- Air spaces- the stomata connect the air outside to a network of air spaces in the spongy mesophyll of the leaf. Carbon dioxide and oxygen can diffuse through these air spaces
- Spongy mesophyll- the inner tissue of the leaf with extensive air spaces. It provides a very large total surface area of permanently moist cell walls for gas exchange. Carbon dioxide in the air spaces dissolves and diffuses into the cells. Oxygen diffuses from the cells to the air. Photosynthesis maintains the concentration gradients
- Veins- inevitably, there is some loss of water by evaporation from the moist spongy mesophyll cell walls and diffusion out through the stomata. This is replaced by water supplied by the xylem vessels, located in the leaf veins
What is transpiration?
Water molecules evaporate when hydrogen bonds between them break. The molecules separate from each other and become water vapour molecules in air. The opposite process is condensation, where water vapour molecules join others to become liquid water. If air is very humid and the number of water molecules evaporating is equal to the number condensing, the air is saturated with water vapour.
Air spaces inside the leaf are usually saturated (or close to). Water vapour molecules diffuse out of the leaf through the stomata unless the stomata are closed or the air outside the leaf is already saturated. This causes the humidity of the air spaces to drop below the saturation point, so more water evaporates from the permanently moist spongy mesophyll cell walls. Loss of water vapour from the leaves and stems is transpiration. Transpiration rates are affected by environmental factors.
What environmental factors affect transpiration rate?
- Temperature (positive correlation): at higher temperatures there is more energy available to break hydrogen bonds between water molecules , so the evaporation rate is higher and air hold more water vapour molecules before becoming saturated.
- Humidity (negative correlation): the higher the humidity of the air, the smaller the concentration gradient of water vapour between air spaces inside the leaf and the air outside, so the lower the rate of diffusion. There is no transpiration if the air outside the leaf is saturated with water vapour.
- Wind: in still conditions, transpiration is restricted by formation of pockets of saturated air near the stomata, even if the air further away is drier. Air movements prevent this, so increase transpiration, though stomata close in strong winds, so transpiration rate drops.
- Light intensity: when there is a higher light intensity, the rate of transpiration is higher, this is because more photosynthesis can occur, so more stomata are open, increasing the rate of transpiration
Difference between adult and foetal blood?
Humans produce foetal haemoglobin before birth and adult haemoglobin afterwards. During pregnancy a foetus obtains oxygen via the placenta. Oxygen dissociates from haemoglobin in maternal blood in the placenta and binds to haemoglobin in foetal blood. This can only happen because foetal haemoglobin has a stronger affinity for oxygen than adult haemoglobin at any partial pressure of oxygen. This means foetal haemoglobin is more saturated with oxygen than adult haemoglobin. At birth, a baby still has red blood cells with foetal haemoglobin. It takes several months for all red blood cells carrying foetal haemoglobin to be replaced with cells carrying adult haemoglobin.
What is Bohr shift?
Increases in carbon dioxide (as a result of it being produced at respiring tissues) concentration reduce the affinity of haemoglobin for oxygen by two mechanisms:
- there is a positive correlation between pH and affinity of haemoglobin for oxygen. CO2 reduces pH (CO2 + H2O →H+ + HCO3-) so decreases affinity for oxygen, as a lower pH alters the tertiary structure of haemoglobin into one with a lower affinity for oxygen.
Therefore haemoglobin dissociates oxygen into respiring tissues - CO2 binds reversibly to the polypeptides in haemoglobin, producing carbaminohaemoglobin, which has a lower affinity for oxygen than haemoglobin
Reduced affinity of haemoglobin for oxygen in high CO2 concentrations shifts the oxygen dissociation curve to the right
(Bohr shift)
The Bohr shift promotes release of oxygen in actively respiring tissues, such as contracting muscle, where high blood CO2 concentration causes low pH and haemoglobin to converted to carbaminohaemoglobin.
The Bohr shift allows blood to be fully oxygenated in the lungs where blood CO2 concentrations are low, so pH is high and carbohaemoglobin is converted back to haemoglobin
What happens to haemoglobin in the lungs/ areas of low oxygen levels (sea or altitude)
There is an uptake of oxygen
- Haemoglobin needs to have a high affinity for oxygen
- Curve shifts left (Bohr shift)
- So haemoglobin can become saturated at lower partial pressures of oxygen
- So can get enough oxygen into blood and to cells/ tissues for respiration
What happens to haemoglobin in respiring tissues?
Oxygen dissociates
- Haemoglobin needs to have a low affinity for oxygen
- Curve shifts right (Bohr shift)
- So haemoglobin can dissociate (unload) at higher partial pressures of oxygen
- So more oxygen is released faster to tissues that are respiring more
How do oxygen dissociation curves work?
- each subunit in haemoglobin has a haem group to which one oxygen molecule can bind reversibly, so one haemoglobin molecule can transport up to four oxygens
- Binding is cooperative, because binding of oxygen to any haem group causes conformational changes that increase the affinity for oxygen in the other haem groups. The two most probable states for haemoglobin are with four oxygen molecules bound, or none.
- Blood in which all haemoglobin molecules are carrying four oxygens is 100% saturated. If no oxygen is bound to any of the haemoglobin molecules, it is 0% saturated. Any saturation level from 0→100% is possible
- Oxygen concentration is measured in partial pressures with kPa as the pressure units. There is a positive correlation between partial pressure of oxygen and % saturation of haemoglobin. In human adults, haemoglobin reaches 100% saturation when partial pressures of oxygen reaches 10kPa. This happens as blood flows through capillaries around the alveoli
- 100% oxygenated blood leaving the lungs is carried to all other organs of the body, where due to aerobic respiration the partial pressures of oxygen is below 10kPa, so oxygen dissociates from haemoglobin and diffuses into the tissues
- because of cooperative binding, oxygen saturation of haemoglobin is not directly proportional to oxygen concentration. Instead, it changes from fully saturated to unsaturated over a relatively narrow range of oxygen concentrations, ensuring rapid dissociation of oxygen tissues where it is needed for aerobic respiration
How are capillaries adapted for exchange processes?
- Large surface area:
Capillaries are the narrowest blood vessels with a diameter of around 10 micrometres, so blood cells have to pass through in single file, this means that the flow of blood slows and so there is more time for diffusion (and a short distance between RBC’s and cells, so faster diffusion). Capillaries branch and rejoin repeatedly to form a capillary network with a huge total length. This gives a very large total surface area for exchange processes - Thin walls with poresThe capillary wall consists of one layer of endothelium cells which are a very thin and permeable layer. The layer of cells is supported by a coating of extracellular fibrous proteins. The basement membrane acts as a filter that allows small or medium sized particles to pass through but not macromolecules (such as proteins). Fluid leaks out of capillaries through the basement membranes because blood pressure is higher than pressure in the surrounding tissue and because there are pores between epithelium cells. The fluid passing out (tissue fluid) contains oxygen, glucose and other substances in blood plasma, but not plasma proteins. The tissue fluid flows between cells, allowing them to absorb useful substances and excrete waste products. The fluid flows between cells, allowing them to absorb useful substances and excrete waste products. The fluid then re-enters capillaries where pressure inside them has dropped, near where blood is transported out of the tissue in veins (venous end).
- FenestrationsIn some tissues, there are many particularly large pores (fenestrations) in the capillary walls. Fenestrated capillaries allow larger volumes of tissue fluid to be produced , which speeds up exchange between the tissue cells and the blood. Fenestrated capillaries in the kidney allow production of large volumes of filtrate in the first stage of urine production.
