E. Oxygen (O2) in Blood

Hemoglobin

Hemoglobin (Hb) is a complex protein molecule. It consists of four separate polypeptide chains called protein subunits. It is made up of two identical alpha-chains and two identical beta-chains. Each chain forms a 3-dimensional molecule with iron (Fe) intertwined in the center. Each Fe group is capable of combining loosely with one molecule of O2. Therefore each Hb molecule can attach to four O2 molecules.

The importance of Hb is that when O2 is bound to Hb, the O2 is no longer in solution and does not effect partial pressure. Hb increases the carrying capacity of the blood for oxygen by 15 to 25 times. Carbon monoxide (CO) also binds with Hb. Blood fully saturated with CO can carry only 1/20th of the normal amount of O2. This is why CO can be quickly lethal for most animals. Fish at very cold temperatures (< 5o C) can survive CO poisoning [ha!], in fact antarctic icefish have no erythrocytes or hemoglobin in their blood (they have white gills) and live quite well moving oxygen in simple solution in their blood plasma.

Dissociation Curves

P50 = pressure (mm Hg) at which 50% of Hb is saturated with O2

An oxygen dissociation curve for Hb describes the relationship between partial pressure and the amount of oxygen bound on the Hb. At higher partial pressure more oxygen is forced into binding with Hb, and at lower partial pressure less oxygen is bound. Dissociation curves are not linear, but sigmoid. The defining attribute of a given dissociation curve is its P50. The P50 of Hb differs among species.

P50's
Humans:
25 - 30 mm Hg
Birds:
40 - 50 mm Hg
Trout:
20 - 40 mm Hg
Carp:
5 mm Hg

There is adaptive significance to each species' P50. There are advantages to a high P50 and advantages to a low P50 it all depends on the animal's environment and way of life. Consider the difference between the P50 of a trout and a carp.

A trout in water with low oxygen (5 - 20 mmHg partial pressure) can't load its Hb, therefore, its tissues will suffocate and it will die. But carp Hb has a higher affinity for O2. Therefore, a carp can load oxygen at a lower PO2 and live in a lower O2 environment, however, both the trout and the carp Hb will hold the same amount of oxygen. The dissociation curve of carp indicates that their Hb can fully load at a PO2 of about 20 mmHg (about 1 mg/L of DO), while it requires about 50 mmHg (3 mg/L) to fully load a trout's Hb. Why does a trout's Hb have a low affinity if that limits the amount of oxygen necessary for it to live? The advantage is that a higher P50 results in the Hb unloading the oxygen at the tissue level at a higher pressure. Therefore, a trout has a higher PO2 in its tissue than does a carp. Higher PO2 in the trout's tissue means that more oxygen is available for metabolism and the trout can maintain a higher level activity than a carp. It is a trade-off, and the advantage of each approach depends on the fish's environment and the way it lives.

Dissociation curves are determined in vitro and in vivo. PO2's in the water need to be higher than the P50 of the hemoglobin to support fish life because a certain amount of pressure differential is required to move O2 from the water across the epithelial cells of the gill into the plasma, then into the red blood cells, and finally into the hemoglobin. In life, trout tend to suffer from hypoxia in water less than about 4-5 mg/L.

Root and Bohr Effects

An increase in the partial pressure of carbon dioxide in the blood decreases the Hb's capacity for oxygen. This is termed the Root effect or shift. At high PCO2 the Hb's capacity to bind oxygen can be decreased as much as 50%.

A decrease in pH has a different effect on Hb, it causes a decreased affinity for oxygen. This is termed the Bohr effect or shift. At low pH, the Hb's affinity for O2 can decrease 2-3 times. To help remember the difference, associate the the two o's in Root with the two O's in CO2 and the h in bohr with the H in pH.

While they can be described as separate effects, the Root and Bohr shifts usually occur together.

The Root and Bohr shifts are important in maintaining a higher PO2 at the tissues than would occur without the effects. It works like this: at the gill, little CO2 and acid are present in the blood and the Hb dissociation curves are unshifted. As the blood circulates deep into the tissues carbon dioxide and lactic acid produced by metabolism cause the combined Root/Bohr effect. The reduced capacity and decreased affinity causes the Hb to "dump" oxygen increasing the PO2. As the blood circulates between the gill and tissues and back, the combined shifts occur back and forth again and again. The result is a higher PO2 at the tissues than would occur in the absence of the effects.

This works well for the fish in habitats with high oxygen content and low carbon dioxide. In habitats with low oxygen and high carbon dioxide (e.g. swamps) it can be counter-productive because carbon dioxide prevents a shift back at the gill (CO2 causes the Bohr as well as the Root effect because it reacts with water to form a weak acid). Fishes such as bullheads, carp, bowfin, and lungfishes, which are adapted to swamp life and other slow water habitats where low oxygen and high carbon dioxide content are normal, have blood with weak Root/Bohr effects. Fish that normally engage in vigorous burst swimming have some Hb that lacks the Root/Bohr effects because during periods of strenuous exercise blood acidosis may occur preventing shift back at the gill. By having Hb's with and without the effects they "hedge their bets" physiologically. [An Example]

Temperature Effect

With each degree centigrade increase in blood temperature there is a decrease in affinity equal to an increase in the P50 of 1 mmHg. While this effect is slight compared to the Root and Bohr effects, it is adaptive since active muscles tend to be warmer than the water at the gill.

Assignment IIE

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