Exchange between Organisms - Human Gas Exchange Flashcards
Why do gases need to be exchanged?
- All aerobic organisms require a constant supply of oxygen to release energy in the form of ATP during respiration.
- The carbon dioxide produced in the process needs to be removed as its build-up could be harmful to the body.
Why is the volume of oxygen that has to be absorbed and the volume of carbon dioxide that must be removed large in animals?
- they are relatively large organisms with a large volume of living cells
- they maintain a high body temperature which is related to them having high metabolic and respiratory rates
What specialised surfaces have mammals evolved?
Mammals have evolved specialised surfaces, called lungs, to ensure efficient gas exchange between the air and their blood.
Why are lungs located inside the body?
The lungs are the site of gas exchange in mammals. They are located inside the body because:
- air is not dense enough to support and protect these delicate structures
- the body as a whole would otherwise lose a great deal of water and dry out
What is the ribcage?
The lungs are supported and protected by a bony box called the ribcage. The ribs can be moved by the muscles between them. A lubricating substance is secreted to prevent friction between the rib cage and lungs during inflation and deflation.
How are the lungs ventilated?
The lungs are ventilated by a tidal stream of air, thereby ensuring that the air within them is constantly replenished.
What are the lungs?
The lungs are a pair of lobed structures made up of a series of highly branched tubules, called bronchioles, which end in tiny air sacs called alveoli.
What is the trachea?
The trachea is a flexible airway that is supported by rings of cartilage. The cartilage prevents kinking and the trachea collapsing as the air pressure inside falls when breathing in. The tracheal walls are made up of smooth muscle and elastic tissue which allows for constriction and recoil, lined with ciliated epithelium and goblet cells.
The rings of cartilage are incomplete in the trachea to allow passage of food down the oesophagus behind the trachea.
What are the bronchi?
The bronchi are two divisions of the trachea, each leading to one lung. They are similar in structure to the trachea but are narrower and, like the trachea, they also produce mucus to trap dirt particles and have cilia that move the dirt-laden mucus towards the throat. The larger bronchi are supported by cartilage, although the amount of cartilage is reduced as the bronchi get smaller.
What are the bronchioles?
The bronchioles are a series of branching subdivisions of the bronchi. They have no cartilage (except the larger ones) or ciliated epithelial cells. Their walls are made of smooth muscle lined with epithelial cells. This muscle allows them to constrict so that they can control the flow of air in and out of the alveoli. The smallest of bronchioles have alveoli clusters at the ends.
What are the alveoli?
The alveoli are minute air-sacs at the end of the bronchioles. Between the alveoli there are some collagen and elastic fibres. The alveoli are lined with squamous epithelium. The elastic fibres allow the alveoli to stretch as they fill with air when breathing in. They then spring back during breathing out in order to expel the carbon dioxide-rich air. The alveolar membrane is the gas-exchange surface.
What is ventilation?
To maintain diffusion of gases across the alveolar epithelium, air is constantly moved in and out of the lungs (thoracic cavity). The thorax, or chest, is made up of a rib cage which houses the lungs and the heart. This process is called breathing, or ventilation.
What is inspiration?
When the air pressure of the atmosphere is greater than the air pressure inside the lungs, air is forced into the lungs. This is called inspiration (inhalation).
What is expiration?
When the air pressure in the lungs is greater than that of the atmosphere, air is forced out of the lungs. This is called expiration (exhalation).
What are the three sets of muscles that pressure changes within the lungs are brought about by the movement of?
- the diaphragm
- the internal intercostal muscles
- the external intercostal muscles
What is the diaphragm?
A sheet of muscle that separates the thorax from the abdomen.
What are the intercostal muscles?
There are two sets of intercostal muscles that lie between the ribs. They contract to raise and lower the ribcage respectively.
- the internal intercostal muscles, whose contraction leads to expiration
- the external intercostal muscles, whose contraction leads to inspiration
What are the two basic physical laws of the movement of air?
Within a closed container, as the volume of a gas increases, its pressure decreases. Similarly, as the volume of a gas decreases so the pressure increases.
Gases move from a region where their pressure is higher to a region where their pressure is lower.
How does inspiration (inhalation) take place?
Breathing in is an active process (it uses energy) and occurs as follows:
- The external intercostal muscles contract, while the internal intercostal muscles relax.
- The ribs are pulled upwards and outwards, increasing the volume of the thorax.
- The diaphragm muscles contract, causing it to flatten, which also increases the volume of the thorax.
- The increased volume of the thorax results in reduction of pressure in the lungs.
- Atmospheric pressure is now greater than pulmonary pressure, and so air is forced into the lungs.
How does expiration take place?
Breathing out is a largely passive process (it does not require much energy) and occurs as follows:
- The internal intercostal muscles contract, while the external intercostal muscles relax.
- The ribs move downwards and inwards, decreasing the volume of the thorax.
- The diaphragm muscles relax and so it is pushed up again by the contents of the abdomen that were compressed during inspiration. The volume of the thorax is therefore further decreased.
- The decreased volume of the thorax increases the pressure in the lungs.
- The pulmonary pressure is now greater than that of the atmosphere, and so air is forced out of the lungs.
What’s the difference between normal, quiet breathing and breathing under strenuous conditions?
During normal quiet breathing, the recoil of the elastic tissue in the lungs is the main cause of air being forced out (like air being expelled from a partly inflated balloon). Only under more strenuous conditions such as exercise do the various muscles play a major part.
What is the pulmonary ventilation rate?
pulmonary ventilation rate = tidal volume x breathing rate
What is the site of gas exchange in mammals?
The site of gas exchange in mammals is the epithelium of the alveoli. These alveoli are minute air sacs and are situated in the lungs. To ensure a constant supply of oxygen to the body, a diffusion gradient must be maintained at the alveolar surface. To maintain a diffusion gradient, there has to be movement of both the environmental medium and the internal medium.
Aside from diffusion, what else is needed to maintain adequate transfer of gases?
Being thin, these specialised exchange surfaces are easily damaged and therefore are often located inside an organism for protection. Where an exchange surface, such as the lungs, is located inside the body, the organism has some means of moving the external medium over the surface, for example, a means of ventilating the lungs in a mammal. This is because diffusion alone is not fast enough to maintain adequate transfer of oxygen and carbon dioxide along the trachea, bronchi and bronchioles.
How do alveoli maintain efficient gas exchange?
- There are about 300 million alveoli in each human lung.
- Their total surface area is around 70m2.
- Each alveolus is lined with epithelial cells only 0.05um to 0.3um thick.
- Around each alveolus is a network of pulmonary capillaries, so narrow (7-10um) that red blood cells are flattened against the thin capillary walls in order to squeeze through.
- These capillaries have walls that are only a single layer of cells thick (0.04-0.2um).
Why is the diffusion pathway short?
The diffusion pathway is short because the alveoli have only a single layer of epithelial cells and the blood capillaries have only a single layer of endothelial cells. So the diffusion pathway is 2 cells thick.
Why will the diffusion of gases between the alveoli and the blood be very rapid? What are the adaptations of alveoli?
The function of alveoli is at the site of gas exchange, and hence they have specialised structural features to help fulfil this role.
- Red blood cells are slowed as they pass through pulmonary capillaries, allowing more time for diffusion.
- The distance between the alveolar air and red blood cells is reduced as the red blood cells are flattened against the capillary walls.
- The walls of both alveoli and capillaries are very thin (one cell thick) and therefore the distance over which diffusion takes place is very short.
- Alveoli and pulmonary capillaries have a very large total surface area.
- Breathing movement constantly ventilate the lungs, and the action of the heart constantly circulates blood around the alveoli. the capillaries constantly supply blood. Together, these ensure that a steep concentration gradient of the gases to be exchanged is maintained.
- Blood flow through the pulmonary capillaries maintains a concentration gradient.
- The lining of the alveolus is composed of very thin flattened epithelial cells- minimise diffusion distances.
- Surrounded by collagen and elastic tissue - allows for stretching and recoil during breathing.
- Surrounded by a dense capillary network, which increase the capacity for gas exchange with the blood.
- Their internal surface is covered with a layer of fluid, as dissolved gases are better able to diffuse into the bloodstream.
Which direction do oxygen and carbon dioxide move?
Oxygen moves by diffusion from the alveolar air down the concentration gradient into red blood cells. Carbon dioxide moves down its concentration gradient in the opposite direction.
What is the route the gases take?
atmospheric air nasal chamber or mouth trachea bronchi bronchioles alveoli blood
What is the cartilage made of?
The inside surface of the cartilage is a layer of glandular and connective tissue, elastic fibres, smooth muscle and blood vessels. This is referred to as the ‘loose tissue’. The inner lining is an epithelial layer composed of ciliated epithelium and goblet cells.
What is the cartilage?
Involved in supporting the trachea and bronchi, plays an important role in preventing the lungs from collapsing in the event of pressure drop during exhalation (so provides flexibility).
What is the ciliated epithelium?
Present in bronchi and trachea, involved in moving mucus along to prevent lung infection by moving it towards the throat where it can be swallowed.
Bronchioles have simple epithelium cells, and alveoli have squamous epithelium cells.
What are the goblet cells?
Cells present in the trachea, bronchi and bronchioles involved in mucus secretion to trap bacteria and dust to reduce the risk of infection with the help of lysozymes which digest bacteria.
