GAS EXHANGE Flashcards
Gaseous exchange
This process involves the passage of gases such as oxygen into and carbon dioxide out of cells or a transport system. First, air needs to be in contact with the gaseous exchange surface.
breathing system of human
Gaseous exchange relies on diffusion. To be efficient, the gaseous exchange surface must:
- be thin – a short distance for gases to diffuse;
- have a large surface area – for gases to diffuse over;
- have good ventilation with air – this creates and maintains a concentration gradient;
- have a good blood supply – to transport oxygen to respiring tissues and bring carbon dioxide from those tissues.
alveoli in lungs
blood supply of alveoli
The composition of inspired and expired air
Testing for carbon dioxide
Limewater can be used to test for carbon dioxide – it changes colour from colourless to milky when the gas is bubbled through.
Expired air makes limewater change colour more quickly than inspired air because there is more carbon dioxide present in expired air.
Effects of physical activity on breathing
Both breathing rate and depth increase during exercise. The volume of air breathed in and out during normal, relaxed breathing is about 0.5 litres. This is the tidal volume. Breathing rate at rest is about 12 breaths per minute.
During exercise, the volume inhaled (depth) increases to about 5 litres (depending on the age, sex, size and fitness of the person). The maximum amount of air breathed in or out in one breath is the vital capacity. Breathing rate can increase to over 20 breaths per minute.
inhaling
The movement of the ribcage is brought about by the contraction of two sets of intercostal muscles that are attached to the ribs. The external intercostal muscles are attached to the external surface of the ribs; the internal intercostal muscles are attached to the internal surface. When the external intercostal muscles contract, they move the ribcage upwards and outwards, increasing the volume of the thorax.
The diaphragm is a tough, fibrous sheet at the base of the thorax, with muscle around its edge. When the diaphragm muscle contracts, the diaphragm moves down, again increasing the volume of the thorax. This increase in volume reduces the air pressure in the thoracic cavity. As the air pressure outside the body is higher, air rushes into the lungs through the mouth or nose, so ventilation is achieved.
exhaling
The opposite happens when breathing out; during forced exhalation, the internal intercostal muscles contract and the diaphragm muscles relax. Thoracic volume decreases, so air pressure becomes greater than outside the body. Air rushes out of the lungs to equalise the pressure. Again, ventilation is achieved.
Physical activity and the rate and depth of breathing
It has already been stated that the rate and depth of breathing increase during exercise. For limbs to move faster, aerobic respiration in the skeletal muscles increases. Carbon dioxide is a waste product of aerobic respiration (see Chapter 12). As a result, carbon dioxide builds up in the muscle cells and diffuses into the plasma in the bloodstream more rapidly. The brain detects increases in carbon dioxide concentration in the blood and stimulates the breathing mechanism to speed up, increasing the rate of expiration of the gas.
Protection of the gas exchange system from pathogens and particles
Pathogens, such as bacteria, and dust particles are present in the air we breathe in and are potentially dangerous if not actively removed.
Two types of cells (Figure 11.5) provide mechanisms to help achieve this.
Goblet cells are found in the epithelial lining of the trachea, bronchi and some bronchioles of the respiratory tract. Their role is to secrete mucus. The mucus forms a thin film over the internal lining. This sticky liquid traps pathogens and small particles, preventing them from entering the alveoli where they could cause infection or physical damage.
Ciliated cells are also present in the epithelial lining of the respiratory tract (see ‘Levels of organisation’ in Chapter 2). They are in a continually flicking motion to move the mucus, secreted by the goblet cells, upwards and away from the lungs. When the mucus reaches the top of the trachea, it passes down the gullet during normal swallowing.
