biology chapter 7

• In microscopic organisms such as Amoeba, all of the oxygen needed by the organism, and the waste carbon dioxide produced, can be exchanged with the external environment by diffusion through the cell surface.
• The distances the substances have to travel are very small.
• The metabolic activity of a single-celled organism is usually low, so the oxygen demands and carbon dioxide production of the cell are relatively low.
• The surface area to volume (SA: V) ratio of the organism is large.
• As organisms get larger they can be made up of millions or even billions of cells arranged in tissues, organs, and organ systems.
• Their metabolic activity is usually much higher than most single-celled organisms.
• The amount of energy used in moving through the water means the oxygen demands of the muscle cells deep in the body will be high and they will produce lots of carbon dioxide.
• The distance between the cells where the oxygen is needed and the supply of oxygen is too far for effective diffusion to take place.
• Large, multicellular organisms have evolved specialised systems for the exchange of the substances they need and the substances they must remove.
• All effective exchange surfaces have certain features in common.
• Increased surface area provides the area needed for exchange and overcomes the limitations of the SA: V ratio of larger organisms.
• Thin layers mean the distances that substances have to diffuse are short, making the process fast and efficient.
• Good blood supply ensures substances are constantly delivered to and removed from the exchange surface, maintaining a steep concentration gradient for diffusion.
• Ventilation to maintain diffusion gradient is necessary for gases, a ventilation system also helps maintain concentration gradients and makes the process more efficient.
• Animals that live on the land face a continual conflict between the need for gaseous exchange and the need for water.
• Gaseous exchange surfaces are moist, so oxygen dissolves in the water before diffusing into the body tissues.
• Mammals have evolved complex systems that allow them to exchange gases efficiently but minimise the amount of water lost from the body.
• The bony fish, with their countercurrent systems, remove about 80% of the oxygen from the water flowing over them.
• Because the blood and water flow in opposite directions, a countercurrent exchange system is set up.
• This adaptation ensures that steeper concentration gradients are maintained than if blood and water flowed in the same direction, known as a parallel system.
• The cartilaginous fish have parallel systems and can only extract about 50% of the oxygen from the water flowing over them.
• A steep concentration gradient is needed for fast, efficient diffusion to take place.
• More gaseous exchange can take place in a countercurrent exchange system.
• The human gaseous exchange system includes the nasal cavity, which has a large surface area with a good blood supply, which warms the air to body temperature.
• Gills also need to carry out gaseous exchange as effectively as possible in water, a medium where diffusion is slower than in air.
• The gills of bony fish are their organs of gaseous exchange.
• In bony fish, gills are contained in a gill cavity and covered by a protective operculum, which is also active in maintaining a flow of water over the gills.
• The more primitive cartilaginous fish such as the sharks and rays often rely on continual movement to ventilate the gills.
• Gills have the large surface area, good blood supply, and thin layers needed for successful gaseous exchange.
• Bony fish have evolved a sophisticated system involving the operculum, which allows them to move water over their gills all the time.
• Bony fish such as trout and cod are relatively big, active animals that live almost exclusively in water.
• When the fish stops moving, the flow of water also stops.
• Gills have a large surface area for diffusion, a rich blood supply to maintain steep concentration gradients for diffusion, and thin layers so that diffusing substances have only short distances to travel.
• Bony fish have evolved a ventilatory system adapted to take oxygen from the water and get rid of carbon dioxide into the water.
• When fish are swimming, they can keep a current of water flowing over their gills simply by opening their mouth and operculum.
• The scaly outer covering of these fish does not allow gaseous exchange.
• The mouth is opened and the floor of the buccal cavity is lowered, increasing the volume of the buccal cavity and lowering the pressure there so water moves from the buccal cavity over the gills.
• The water moving over the gills and the blood in the gill filaments flow in different directions.
• Gills have many features in common with both mammalian and insect gaseous exchange surfaces.
• Fish need to maintain a continuous flow of water over the gills, even when they are not moving.