Chapter 7

For single celled organisms, diffusion alone is enough to efficiently supply the needs of a single celled organism, as the metabolic activity and oxygen demands are relativley low, and because the SA:V ratio is large. As organisms get larger they can be made of millions of cells arranged in tissue and organs, and their metabolic activity and oxygen demand is usually much higher, and the distances gases would have to travel to reach all the cells of the organism are too far for diffusion to effective, and the bigger the organism, the smaller the SA:V ratio. 

Large multicellular organisms have evolved specialised systmes for gas exchange, all effective exchange surfaces must have:

• Increased surface area: To overcome the limitations of the SA:V ratio a larger exchange surface is necessary.
• Thin layers: This is so the distances that substances have to travel are short making the process fast and efficient, eg the alveoli in the lungs.
• Good blood supply: The steeper the concentration gradient, the faster diffusion takes place, so having a good blood supply ensures that substances are constantly supplied and removed maintaining a steep concentration gradient for diffusion.
• Ventilation to maintain diffusion gradient: For gases a ventialtion system helps maintain a gradient and makes the process more efficient.

Mamals are quite big and have a small SA:V ratio and a high metabolic rate due to being warm blooded. As a result they need lots of oxygen for cellular respiration and they produce carbon dioxide, which needs to be removed. Key stuctures include:

Nasal cavitiy: The nasal cavirt has a large surface area with a good blood supply which warms the air to body temperature, a hairly lining that secretes mucus to trao dust and bacteria protecting the delicate lung tissue from irritation and infection.

Trachea: This the main airway carrying clean, warm, moist air to the chest, it is a wide tube supported by incomplete rings of cartilage for structure and so that  food can move down the oesophagus behind thr trachea. The trachea and its branches are lined with ciliated epithelium cells with goblet cells between them that secrete mucus to trap any debris that has escaped the nose lining. The cilia beat and move the mucus along away from the lungs. One of the effects of smoking is that it stops the cilia beating, leading to build up of mucus in the lungs and airways.

Bronchus: In the chest cavity and trachea divides to form the left bronchus and the right bronchus, and have similar structure to the trachea but smaller.

Bronchioles: In the lungs the bronchi divide to form many small bronchioles, the walls of which contain smooth muscle which allows them to conract and relax affecting the amount of air that reaches the lungs. They are also lined with layers of flattened epithelium making some gaseous exchage possible.

Alveoli: The alveoli are tiny air sacs which are the main gas exchange surface of the body and are unique to mammalian lungs, each alveolus contains a layer of thin flattened epithelial cells, some collagen and some elastin, which allows the alveoli to stretch as air is drawn in and help squeeze air out when the return to their resting size through elastic recoil. 

• Large surface area: there are 300-500 million alveoli per adult per lung, the alveolar surface for exchange is about 50-75 metres squared increasing the surface to area volume ratio.
• Thin layers: Both the alveoli and the capillaries that surround them have walls that are only a single epithelial cell thick, so the diffusion distances between the air, alveoli and blood are very short.
• Good blood supply: The millions of alveoli in each lung are supplied by a network of around 280 million capillaries, the constant flow of blood through these capillaries brings carbon dioxide and carries off oxygen, maintaining a steep concentration gradient for both gases.

The inner surface of the alveoli is covered by a thin layer of a solution of water and salts called lung surfactant, this is what makes it possible for the alveoli to remain inflated, oxygen dissolves in the water before diffusing into the blood, but water can also evapourate into the air, but the lungs have adaptations to reduce the loss of water.

Air is moved in and out of the lungs as a result of pressure changes in the thorax brought about by the breathing movements (ventilation). The rib cage provides a semi rigid case within which pressure can be lowered compared to the outside, the diaphragm is a domed sheet of muscle that forms the floor of the thorax. The external and internal intercostal muscles are found between the ribs. The thorax is lined by the pleural membranes which surround the lungs. The  pleural cavity is filled with a thin layer of lubricating fluid which allows the membranes to slide over each other as you breathe.

Inspiration: This is an energy using process, the dome shaped diaphragm contracts, flattening and lowering as the external intercostal muscles contract moving the ribs upwards and outwards. This increases the volume of the thorax so the pressure lowers to below that of the outside air, causing air to be drawn in to equalise the pressures.

Expiration: Normal expiration is passive, the diaphragm relaxes and so do the internal intercostal muscles so the ribs move inwards and downwards, the elastic fibres of the alveoli recoil, and the volume of the lungs decreases, increasing the pressure so it is greater than the outside, so air moves out of the lungs. You can exhale forcibly using energy where the internal intercostal muscles contract pulling the ribs down hard and fast forcing the diaphragm up rapidly.

Asthma affects 5.4 million people in the UK, and is where you have airways that are sensitive to every day triggers. During an asthma attack the cells lining the bronchioles release histamines that inflame the epithelial cells, causing them to swell, along with stimulating goblet cells to make excess mucus and the smooth muscle in the bronchiole walls to contract. As a result the airways narrow and fill with mucus, making it difficult to breathe. Asthma medicines have been developed to reduce and prevent attacks, relievers offer immmediate relief and contain chemicals simialr to adrenaline that attach to the active sites on the surface membranes of the smooth muscle of the bronchioles making them relax, dilating the airways. Preveneters are often steroids that are taken every day to reduce the sensitivity of the airways.

The first breath of a newborn baby needs a force 15-20 times greater than any normal inhalation to inflate the lungs, they are enourmously stretched as the elastic tissues stretch. This intake of breath is only possible due to the phospolipid lung surfactant that stops the alveoli from sticking together and collapsing as the baby exhales, without this the second breath would be just as hard as the first. If the cells of the lungs do not produce enough surfactant due to preature birth, as not enough is produced until the 30th week of pregnancy, then the baby ay struggle to breathe and die. In recent years artificial lung surfactants have been produced where a tiny amount is sprayed on the premature baby's lungs making breathing easier.

The volume of air that is drawn in and out can be measured in a variety of different ways:

• A peak flow meter is a simple device that measures the rate at which air can be expelled from the lungs and are often used to monitor people who have asthma.
• Vitalographs are more sophisticated versions of the peak flow, the patient being tested breathes out as quickly as possible through a mouthpiece and the instrument produces a graph of the amount of air they breathe out and how quickly this happens.
• A spirometer is commonly used to measure different aspects of the lung volume or to investigate breathing patterns.

• Tidal volume: The volume of air that moves in and out of the lungs with each resting breath, and is about 500cm3 amd used about 15% of the lungs.
• Vital capacity: Volume of air that can be breathed in when the strongest exhalation is followed by the strongest possible exhalation.
• Inspiratory reserve volume: The maximum amount of air you can breathe in over and above a normal inhalation.
• Expiratory reserve volume:The extra amount of air you can force out of your lungs over and above the normal tidal volume breathed out.
• Residual volume: The volume of air that is left in your lungs when you have the exhaled as hard as possible.
• Total lung capacity: The sum of the vital capicity plus the residual volume.

The breathing and volume patterns change as the demands of the body change, the ventilation rate is the total volume of air inhaled in one minute: ventilation rate = tidal volume x breathing rate (per minute). When the oxygen demands of the body increase, for example, the tidal volume can increase from 15% to 50% along with the breathing rate also increasing, in this way ventilation can be increased so the oxygen uptake during gaseous exchamge can meet the demands of the body. 

Many insects are very active during parts of their life cycles, and have relatively high oxygen demands, however they have a tough exoskeleton and don't usually have blood pigments that can carry oxygen, so have evolved a system that delivers oxygen directly to the cells. 

Along the thorax and abdomen of most insects are small openings knwon as spiracles, air enters and leaves through them, but water is also lost. therefore the spiracle sphincters are kept closed as much as possible to maximise the efficiency of gaseous exchange. When an insect is inactive or oxygen demands are low, the spiracles will be closed until carbon dioxide levels get too high. Leading away from the spircles are the tracheae, which are the largest tubes of the insect respiratory system and carry air into the body, they run borth along and into the body of the insect and are lined by spirals of chitin rhat keep them open when they are bent or pressed, but also makes them relatively impermeable to gases. The tracheae branch to form narrower tubes until they divide to form the tracheoles, each of which is a single, greatly elongated cell permeable to gases with no chitin lining. Becuase of their small size they can run along and in between induvidual cells, and is where exchange takes place between the air and the respiring cells. 

In most insects, for most of the time, air moves along the tracheae and tracheoles by diffusion alone, reaching all the tissues. The vast number of tiny tracheoles give a very large surface area for exchange. Oxygen dissolves in moisture on the walls of the tracheoles and diffuses into the surrounding cells. Towards the end of the tracheoles there is tracheal fluid, which limits the penetration of air for diffusion, but when oxygen demands build up, a lactic acid build up results in water moving out of the tracheoles by osmosis exposing more surface area for exchange. All of the oxygen needed by the cells of an insect is supplied by the tracheal system. The extent of gas exchange in most insects in ctonrolled by the opening and closing of the spiracles. Some insects also have alternative methods of increasing the level of gaseous exchange:

• Mechanical ventilation of the tracheal system: Air is actively oumped into the system by muscular pumping movements of the thorax/abdomen. These movements change the volume and therefore the pressure of the tracheae/tracheoles so air is drawn in or forced out as the presssure changes.
• Collapsible enlarged air sacs: These act as air reservoirs which are used to increase thr amount of air moved through the exchange system, and are usually inflated or deflated by the ventilating movements of the thorax and abdomen.

Discontinuous gas exchange cycles (DGC) have been found to be relatively common in many species of insect, and have three main states: closed, open, and flattering.

• When the spiracles are closed no gases move in or out of the insect. oxygen moves into the cells by diffusion from the tracheae and the carbon dioxide diffuses into the body fluids of the insect where it is held in a process called buffering.
• When the spircales flutter, they open and close rapidly. This moves fresh air into the tracheae to renew the oxygen supply whilst minimising water loss.
• When carbon dioxide levels build up really high in the nody fluids of the insect, the spiracles open widely and carbon dioxide diffuses out rapidly, there may also be pumping movements of the thorax and abdomen when the spiracles are open to maximise gaseous exchange.

Evidence suggests that thw advantage of DCG to help exchange in insects that spend a lot of their life in burrows, reducing entry of fungal spores and reducing water loss. 

Water is 1000 denser that oxygen and 100 times more viscous and has a much lowerr oxygen content. Because of these enviromental conditions fish have evolved a very specialised respiartory system different to mammals.

Gills: Bony fish such as trout and cod and relatively nig and active animals that have a high oxygen demand with a small SA:V ratio so diffusion alone is not enough to supply the inner cells with the oxygen they need. So fish have evolved to have gills that can take oxygen from the water and remove carbon dioxide. A one directional flow of water over the gills is maintained and gills have a large surface area, good blood supply and thin layers. IN bony fish they are contained in the gill cavity and covered by a protecticve operculum which is also active in maintaing flow of water over the gills. Gill lamellae are singular plates that arethe maint site of exchange, they occur in large stacks of gill filaments (gill plates) and need a constant supply of water to keep them apart, exposing the large surface area neded for gaseous exchange. To allow efficient exhcnage at all times, fish need to maintain a continuous flow of water over the gills even when they aren't moving.

When fish are swimming they can keep a current of water flowing over the gills simply by opening their mouth and operculum. However when the fish stops moving so does the flow of water, Cartilaginous fish like sharks rely on continuous movement  known as ram ventialtion. However bony fish have evolved a system which allows them to move water over the gills all the time: 

The mouth is opened and the floor of the buccal (mouth) cavity is lowered, increasing the volume of the volume so as a result the pressure lowers  and water moves into the buccal cavity. At the same time the opercular valve is shut and the opercular cavity containing the gills expands, lowering the pressure of this cavity whilst the buccal cavity floor starts to move up so water moves into the opercular cavity. The mouth closes and operculum opens and the sides of the opercular cavity moves inwards, all of thee actions increase the pressure in the opercular cavity and force water over the gills and out of the operculum. The floor of the buccal cavity is slowly moved up, maintaning the flow of water over the gills.

Gills have a large surface area for diffusion, rich blood supply to maintain a steep concentration gradient and thin layers for short diffusion distances. Gills also have two extra adaptations to ensure the most effective possible gaseous exchange occurs in water:

• The tips of adjacent gill filaments overlap, which increases the resistance to flow of water over the gill surfaces and slows down the movement of water, so there is more time for exchange to take place.
• The water movement over the gills and the blood in the gill lamellae flow in different conditions. A steep concentration gradient is needed for fast, efficient exchnage to take place, so if the blod and water flow in opposite directions, a counter current system is set up. This adaptaion ensures that steeper concentration gradients are maintained than if water and the blood flowed in the same direction, as the steep gradient is maintained all the waty along the gill rather than it stopping when equilibrium is reached. As a result of the countercurrent system mpre exchange can take place and can removed 80% of the oxygen from the water flowing over them.