Sonoma State University
Department of Biology - Hanes
Animal Physiology

Chapter 13: Gas Exchange & Acid-Base Balance

Lecture

General Considerations
Diffusion
Respiratory Pigments
CO₂ Transport in Blood
The Vertebrate Lung
Bird Ventilation
Insect Ventilation
Fish Ventilation
Control of Ventilation

Application
Discussion
Reflection

Lecture
General Considerations

Gas transport at sufficient speed is required for aerobic metabolism. Oxygen is used by mitochondria as the final acceptor of electrons and H+ is removed from food sources. The supply of oxygen must be continuous and uninterrupted. CO₂ is the result of destruction of food chemicals. It it accumulates, fluids become too acid and this disrupts enzyme function. In larger animals, blood must carry sufficient oxygen and carbon dioxide and travel fast enough to keep the animal in equilibrium. Gasses must be carried in solution - some are not really very soluble in water. There are no known enzyme pumps for gasses in animals. (There are means of concentrating them however.) They move only by diffusion down concentration gradients. Concentrations in blood are high. In the case of oxygen, by binding oxygen to a protein and thus holding much more in the blood than a simple solution can hold. Carbon dioxide may also bind to protein and it can exist in water in a number of different forms which helps it to be in higher concentrations. Also to keep flow rates of gasses high, the surface areas of the gas exchange surfaces must be huge and the air or water at this surface must not be allowed to become stagnant.

Diffusion

All cells exchange gases with their environments by diffusion. Oxygen must diffuse in sufficient quantities to sustain life, and carbon dioxide must diffuse out rapidly enough to not poison the cell. More oxygen is needed in cells that are larger and have more metabolizing mass. Yet, the surface area (of a sphere) through which gases must travel does not increase as fast as the mass as spheres get larger. This may limit the size of cells. The amount of diffused gas varies directly with area of diffusion surface, diffusion coefficient of the medium, and the concentration (partial pressure) difference. The amount of diffused gas varies inversely with distance. This means that active cells would have diffusion advantages only if small and if their environment were replaced rapidly. Diffusion rates increase directly with difference in concentration, increased temperature, diffusion surface area, and inversely with the square root of the molecular weight of the gas and with the square of the distance diffusion must travel.

Respiratory Pigments

When speaking about gases, we need to keep prominantly in mind the units that we are using. When we speak of milliliters of oxygen, we always mean under standard temperature and pressure dry (0^o C, 760 torr). It is less confusing to speak of a gas in mg/L to express concentration or torr to indicate a direction of diffusion. pO₂ is expressed in torr and can indicate the direction of diffusion between two media such as water and air that have very different saturation levels.

Oxygen makes up about 20.9% of the atmosphere or 209 ml/L of air. It is not very soluble in water. Its solubility will be best in cold, fresh water. More will dissolve in water if exposed to a higher partial pressure of oxygen (the pressure that only oxygen contributes to atmospheric pressure - about 159 torr (20.9 X 760). So more oxygen will dissolve in water or body fluids if it is cold, exposed to a high partial pressure of oxygen and has little dissolved in it. Even so, a cold stream may be saturated with oxygen at 13 ml of oxygen per L of water. Water is heavier, more viscous, and diffusion is much slower in it than in air, so water breathers have a distinct disadvantage in obtaining oxygen.

While streams are in constant turmoil and expose water to the air, ponds and lakes have little churning. Water is most dense at 4^o C. It loses density nearer frezzing and at higher temperatures. In the winter of temperate climates, lakes and ponds have 4^{o C water at the bottom and decrease in temperasture up to the ice on top. Ice prevents much oxygen uptake and the water does not churn slowly reducing the oxygen content of the water through the winter. Fortunately for cold-blooded animals their metabolism is also reduced. A spring warms the water, all of the water will become 4o C. Then any wind will cause the water to turn over and the mineral rich water from the bottom will come to the top and be exposed to oxygen. The nutrient rich water will result in a bloom of algae in the spring. As water becomes warmer on top, a thermocline will develop. This is a layer of rapidly changing temperature that acts as a barrier to mixing of surface and deep water. Again the deeper water is cut off from oxygen until there is a fall turn over of the water as the temperatures become equal.}

^{Oxygen and carbon dioxide are similar in size and diffusion rates. Oxygen and carbon dioxide are also used or excreted at similar rates. The major difference between them is solubility in water and tissue fluid. Solubility of both decreases with temperature and salinity.}

Mammal blood plasma at body temperature will hold only 3 ml of O₂/L without pigment. Hemoglobin is the most popular of the blood pigments. With normal amounts of hemoglobin present, mammal blood holds about 200 ml O₂/L. One mole of Hb will bind to about 1 mole of Oxygen and there is 9 M of Hb in a liter of blood. So 9 X 22.4 = 202 ml O₂/ L blood. Each molecule of hemoglobin is composed of 4 polypeptide chains (two alpha and 2 beta) each of which is associated with a heme group (a non protein protoporphyrin ring). Each heme reversible associates with one molecule of O2 so that fully saturated Hb is often designated HbO8 and called oxyhemoglobin. There is Fe++ in the middle of the heme, but it is not oxidized when oxygen attaches. If the iron is oxidized to Fe+++, the molecule is called methemoglobin and it will not bind to O₂. Carbon monoxide has a stronger affinity for Hb than O₂ by 200:1. If it binds to the Hb, it is called carboxyhemoglobin. CO₂ also binds reversibly with free amino groups on Hb as it does with many proteins. and in this state is called carbaminohemoglobin.

The oxygen dissociation curve is used to describe the relationship between the percent saturation and the partial pressure of oxygen. There are a number of factors that can affect the affinity of O₂ to Hb. See p 531 to 533. Note that the curve is "S" shaped. This means that as one heme associates with oxygen, the other hemes increase their affinity for oxygen. This phenomenon is called a heme-heme interaction. Compare this to the myoglobin dissociation curve. Myoglobin is trapped within muscle cells and has only one heme. The greatest influence on the amount of Hb that will be attached to O₂ 100 torr. Air has 159 torr of O₂. The curve drops quickly between about 45 and 30 torr meaning that a lot of oxygen is pushed off into solution as the pO2 drops. The average pO2 around systemic capillaries may be about 35 (although around hard working muscle it can be 6) torr and around pulmonary capillaries 100 torr. The steepness of the curve is important as it indicates that between physiological values of pO2, there is a tremendous loading and unloading of hemoglobin.

Besides pO₂, acidity (Bohr effect) and pCO₂ also have an effect on oxygen loading. High values shift the whole oxygen dissociation curve to the right (facilitating the removal of O₂ from Hb). A higher temperature will also shift the curve to the right. An increase of blood diphosphoglycerate (DPG) will also shift the curve to the right. Low lung pO₂, high altitude, and pregnancy increase blood DPG. Fetal blood has decreased levels of DPG.

Fish have an unusual Bohr effect, in fact, exaggerated to the point it is called the Root effect. Fish make two different types of Hb. One has a normal Bohr effect and the other will completely repel 02 at low pH's. This is used by fish to force O2 out of the blood into the gas bladder. What would the dissociation curves of fish Hb types look like separately and together in an acid environment?.

Hemocyanin is a blood pigment in crustaceans and molluscs. It also produces a Bohr effect. It contains a true heme, but the metal is copper and there is one heme and coppermolecule per protein. The copper oxidizes to cupric oxide giving the blood a light blue color. Unoxygenated it is colorless. It is not contained in cells, but is a plasma protein.

Hemerythrin is a non-heme protein containing iron that oxidizes and reduces with oxygen uptake and release. It is found in a few groups of marine animals. It is found in a few obscure marine annelids

Chlorocruorin is found in a few marine annelids also. It is a heme associated protein. Heme groups are also found in the cytochromes of the mitochondria.

CO₂ Transport in Blood

CO₂ exists in many forms in blood all in equilibrium. These forms include CO₂, H₂CO₃, and HCO₃- in ratios of 1,000 : 1: 21,000. The reaction CO₂ + H₂O -> H₂CO₃ is relatively slow and requires about 3 sec to reach equilibrium. In RBC,s carbonic anhydrase speeds this reaction to nearly instantaneous. H₂CO₃ -> H+ + HCO₃- is very rapid without enzymes. Carbon dioxide also reacts reversibly with amine groups on dibasic amino acids (like histidine) forming NH₂CO₂- (carbamino compounds). Because hemoglobin has a large number of histidine moieties and it is blood's predominant protein, it carries most of the CO₂ of the carbamino type. Converting histidines to carbamino groups also slightly changes the character of hemoglobin and decreases its affinity for O2 accounting for much of the Bohr effect.

In systemic capillaries, CO₂ diffuses into the blood plasma and into RBC's. In the RBC's, it may for carboxyhemoglobin or carbonic anhydrase may convert it to H₂CO₃ which breaks down into HCO3- and H+. The H+ also reversibly attaches to Hb forming HHb (reduced hemoglobin) which also reduces its affinity for O₂ and adds to the Bohr effect. Because of the presence of the enzyme, this can all happen very quickly, long before the 0.1 sec it takes for blood to traverse the capillary.

HCO₃- diffuses out of the cell and is replaced by Cl- (the chloride shift) so the electrical voltage of the cell remains nearly constant. Fig 13-10 p 536.

The Vertebrate Lung & Ventilation p 546

The vertebrate lung is lined by delicate, living cells that are protected by their depth within the body. The air is water saturated and warmed before encountering these cells. Curled nasal bones mix the air and mucous cells add their liquid to it. Bronchi, bronchial branches and bronchioles are all lined with mucous cells. These branch further into cavities lined with squamous cells (type I) cells called alveolar sacs and alveoli. Even the strongest exhale can not empty the alveoli. The last transport of oxygen to the blood must take place by diffusion through alveolar spaces, type I cells and capillary endothelium to reach the blood plasma. A surfactant (dipalmitoyl lecithin) is secreted by rounded type II cells which prevents the cohesive and adhesive forces of water to collapse the lungs. Type II cells mature into type I cells.

A number of measurements can be made to indicate the efficiency of the lungs. You should know the meanings of the following: vital capacity, tidal volume, residual volume, anatomical dead space, physiological dead space, inspiratory reserve volume, expiratory reserve volume. Fig 13-23 p 548.

Bird Ventilation

Bird lungs are quite small for the size of the animals, and are stiff and inelastic. They are attached to a number of air sacs composed of very thin membranes that are not well vascularized, so that there is little gas exchange in them. Upon an inspiration, air passes mostly into the posterior air sacs and from there into the lungs. The next expiration pushes the air through the lungs. The next inspiration draws the air from the lungs into the anterior air sacs. The next expiration pushes the air from the anterior air sacs to the trachea and outside. The lung does not have blind alveoli, but well ventilated tubes of respiratory surface. These are called parabronchi which receive new air on both inspiration and expiration. Very efficient. Bird blood is well oxygenated even at the low pO₂'s of high altitude. Illustrations p 553.

Insect Ventilation

Insects do not have oxygen carrying blood pigments. Oxygen is carried nearly to the cell level in a gaseous state in tracheae. These are composed of chitinous tubes with stronger chitinous rings to resist water's capillary attraction. The finest tips of these tubes are filled with fluid (capillary attraction). When the animal is very active the fluid will retreat some. There is some pumping of the abdomen to move air in and out of the large tracheae, but from there diffusion is the major means of transport. Air enters and leaves via spiracles (openings in the skin) that can be opened or shut. Loss of water vapor is the reason for shutting them. p 558

There are some very interesting adaptions in some diving insects for carrying an air supply with them as they dive. Problems that exist are: 1) the deeper the insect goes the more pressure on a bubble and the more likely it will collapse; 2) the higher pressure on the bubble will increase the partial pressures of the gases inside so that they tend to diffuse out of the bubble and collapse it. On the other hand, a bubble can act as a respiratory surface; and if the pO2 is lower in the bubble than in the water, O2 will diffuse into the bubble. p 559.

Fish Ventilation

There is not much O₂ in water and water is very heavy and hard to move. Fish must be very efficient. A few fish (some sharks, tuna) swim constantly with their mouths open and ventilate their gills in this manner. Most fish expand their buccal cavity (mouth), draw water in, shut their mouth, and contract the buccal cavity to force water over the gills. Posterior to the gills is another cavity (the opercular cavity) that can expand drawing water across the gills. It can also open and contract to expel water from the cavity. These two pumps keep a flow of water across the gills that is nearly continuous. The respiratory surface of the gills is aligned with the water flow. Systemic blood from the heart enters the respiratory surface posteriorly and travels anteriorly thus flowing opposite in direction to water. This is a counter- current flow that allows the whole length of respiratory surface to pick up O2 and blood is exposed to the highest O2 concentration just as it leaves the gill surface. Be able to explain what would happen if water and blood flowed the same direction across the gill. Recently oxygenated fish blood may have pO2's near 160 torr - very, very efficient. p 561

Control of Ventilation

The same mechanisms that detect CO₂, O₂, and pH of blood for circulation control also provide information for respiratory control. There are stretch receptors in the lungs and intercostal muscles that prevent over inflation. Other respiratory centers are located in the pons and medulla of the brain. There are even separate centers for inspiration and expiration that are mutually inhibitory guaranteeing that breathing is a cyclic process.

Application

What are the advantages and disadvantages of a tracheal system as in insects compared to a blood circulation system for gasses?

Discussion

What are the mechanisms that assist uptake of O₂ onto Hb at high pO₂'s and removal of O₂ from Hb at low pO₂'s in the animal body.

Why is the muscle myoglobin thought not to be for O₂ storage?

What is the significance of the "S" shaped dissociation curve for Hb?

How is CO₂ carried by the blood?

What is the significance of carbonic anhydrase in the RBC?

Define the terms in bold type above.

Why is bird ventilation more efficient than mammalian?

Why is fish ventilation more efficient than mammalian?

Discuss respiration in diving beetles.

Reflection