Subsections

• • Air and water
  • Gill physiology and function
  • Respiratory movements
  • Transport of gases in blood
  • Factors affecting the respiratory gas exchange:

Respiration

Air and water

All animals using oxygen for respiratory activity, i.e. aerobic decomposition of substances that are a source of energy, must extract the oxygen from their surroundings. Due to the very different chemical and physical properties of air and water, the respiratory systems adapted to these media are very different. The difference of the media manifests itself in the oxygen content, viscosity and density. Normal air contains approximately 21% oxygen, but the largest part is nitrogen or 79%. Water contains less oxygen per volume unit than air, but the oxygen concentration is in strong relation to the temperature of the water, pressure and the amount of dissolved salts.

• The concentration of oxygen decreases with increased temperature (not in linear proportion). This is especially disadvantageous for animals of cold blood as metabolic rate and the need for oxygen increase with increased temperature.
• The concentration of oxygen per volume decreases with decreased pressure. This applies to air and water, the "thin air" at high altitudes being known to most people when climbing mountains. Similarly oxygen content should increase at increased depth (other factors being constant).
• The solubility of oxygen in water is less at increased salinity. This means that seawater contains less oxygen then freshwater at the same temperature and pressure.

Many factors affect the oxygen content of water, such as vertical mixing of the water, decay or decomposition using up oxygen and photosynthesis of algae and plants forming oxygen. A few other factors are of vital importance to aquatic respiratory animals. The viscosity of water is more than 100 times greater than of air, which means that the respiratory movements require/cost a lot more energy than on land. The land-base vertebrates use only 1-2% of their energy for respiration movement, while fish use 20%. On the other hand the oxygen uptake efficiency is very much better, reducing the amount of water needed through the gills. Fish manage to extract 80% of the oxygen from the water flowing through the gills, but humans for example only take up 25% of the total oxygen content flowing through the lungs. The fish can also use part of the energy used to pass water over the gills for moving.

The density of water is also a problem for aquatic life forms, as rate of diffusion in water is only a tiny fraction of the diffusion rate in air (1 against 300.000). The density of water is 800 times greater than of air (1 kg/L for water, 1,3 grams/L for air).

The release of carbon dioxide is a much less problem for fish than the oxygen uptake, as carbon dioxide is much more soluble in the water than oxygen (20-30 times more). In fish, the respiration rate is controlled by the amount of oxygen in the blood. In land vertebrates it is the amount of carbon dioxide that is used for control.

Gill physiology and function

In salmonids as in other osteoicthyans (i.e. fish with bone as skeletal support as opposed to cartilaginous fish), there are four gill arches on each side of the pharynx. On the outside they are shielded with the opercula. On each gill arch there are two parallel rows of primary lamellae that project from the arch like the teeth of a comb. The surface area of each primary lamella is increased further by the formation of regular folds on their dorsal and ventral surface. These folds are the secondary lamellae. On each lamella there are many such thin secondary lamellae. It is mostly in the secondary lamellae that the chemical exchange between the blood and surrounding water takes place. The structure of the gills ensures a large surface area in a limited space.

The blood flows in the ventral aorta, from the heart into the gills. The blood vessels to the gill arches and are distributed into the primary and from there into the fine capillaries in the secondary lamellae. The blood flow is in opposite direction to the water flow through the gills, so oxygen deficient blood meets oxygen rich water, which again ensures diffusion of oxygen into the blood stream. The fish can in this way extract 80-90% of the available oxygen from the water that flows through the gills. The oxygenated blood then flows from the gills into the dorsal aorta.

Respiratory movements

When "inhaling" the fish opens its mouth increasing the volume of the mouth cavity, creating a suction effect into its mouth. The gill cavity also expands and finally due to the pressure difference, water flows from the mouth out through the gills. The gill cover (operculum) closes the gill cavity in such a way that the water is prevented from flowing in the wrong direction. When "exhaling" the volume of the mouth cavity is decreased, increasing the pressure, opening the gill cover and water flows out. The flowthrough is important as there is a constant renewal of oxygen-rich water flowing through the gills. The oxygen content of water is too little to ensure a sufficient oxygen uptake if the flow direction was in and back out through the same opening as in the lungs of land vertebrates.

It is thus mostly the pumping movement of the mouth cavity that keeps the water flowing through the gills. A sizable amount of the total energy use of the fish is due to this movement. Some fish have solved this differently, by moving themselves through the water mass, with mouth and gill cover open, the water is kept flowing through the gills.

Figure: Respiratory movements.

 

Transport of gases in blood

The oxygen content of water is all dissolved oxygen, but in blood the oxygen is mostly bound to hemoglobin. Carbon dioxide in water and blood is both in dissolved state but also found in other forms such as bicarbonate (HCO3^-). The gas concentration in water depends on its partial pressure and the solubility. Carbon dioxide is much more soluble than oxygen at the same partial pressure.

The gas exchange in the gills depend on three main factors:

1. the size of the gas exchange interface area,
2. the concentration difference (differential ) between the blood and the water and
3. the diffusion rate of the gas in question.

The gas exchange rate for O₂ and CO₂ is approximately the same. The concentration differential for CO₂ and O₂ is similar, but the pressure differential for CO2 out to the water is much less than for O₂ into the blood, as the solubility of these two gases is widely different.

The blood of fish has great affinity for O₂ reaching a 100% saturation at a relatively low oxygen content of the water or oxygen partial pressure. The affinity is much greater than in the blood of land mammals, which have easier access to oxygen. P₅₀ is a measure of the partial pressure where 50% of blood hemoglobin is saturated with oxygen. For trout P₅₀ at 5°C is around 38 mm Hg and for many other species the figure is lower. This means that partial pressure of oxygen in the blood of fish does not have to be high for the hemoglobin to be totally saturated. The rate of the hemoglobin oxygenation is greater than the diffusion rate of oxygen through the gills, so the diffusion is the limiting factor of oxygen partial pressure in the blood. The amount of oxygen flowing into the blood may be increased by larger gas exchange interface area, or by shortening the diffusion span/interval/length. The oxygen differential can be increased by increasing the partial pressure of oxygen in the water p(O₂), or by decreasing the oxygen partial pressure in the blood. The system of counter current flow in the gills keeps the oxygen differential high everywhere in the gills. This means also that the fish can manage with less gill area, which is important to minimize the loss of ions and water that inevitably takes place in the gills.

The greatest part of the oxygen transported in the blood is bound to hemoglobin. A little part dissolves in the blood and is transmitted that way.

Carbon dioxide is transported by blood in three ways: As bicarbonate, dissolved as free carbon dioxide, and bound to hemoglobin.

Factors affecting the respiratory gas exchange:

• Temperature. At increased temperature the fish needs more oxygen due to increased metabolic rate. Fish react to temperature by increasing the respiratory volume, increasing the output of the heart (volume and heartbeat rate) and decreasing the resistance in the vascular system. This increases the blood flow in the gills. The limiting factors become the rate of respiration, the blood flow to the gills, and the rate of oxygen diffusion over the gills.
• Oxygen deficiency. A low oxygen content in the water leads to greater respiration rate, slower heartbeat but greater volume of each beat, so heart output is not much affected. The water flow through the gills increases. The fish uses a larger amount of the oxygen from the venal blood, increasing the amount of lactic acid in the muscles. Thus oxygen is more easily released from the hemoglobin, but this chain reaction can continue until the fish chokes in a prolonged period of oxygen deficiency.
• Increased activity of the fish (e.g. swimming). The heart output and respiratory volume are increased, thereby increasing the total oxygen diffusion rate. At the change of flow, there may be changes of ionic balance ( in freshwater salts are released and water absorbed). The fish may counter this effect by changes in renal function (kidneys). The osmosis through the gills and the kidneys are mostly controlled by hormones/ hormonal system.