Summary

The Chemical Properties and Distributions of the Respiratory Pigments

• The four chemical classes of respiratory pigments are all metalloproteins. They bind reversibly with O₂ at specific O₂-binding sites associated with the metal atoms in their molecular structures.
• In hemoglobins, the unit molecule consists of heme bonded with protein (globin). The heme structure—an iron (ferrous) porphyrin—is identical in all hemoglobins. The globin, however, varies widely among species and among different molecular forms of hemoglobin within any single species.
• Hemoglobins are the most common and widespread respiratory pigments, occurring in at least nine phyla. Virtually all vertebrates have blood hemoglobin. The blood-hemoglobin molecules of vertebrates are usually tetramers consisting (in adults) of two α-globin and two β-globin unit molecules; they always occur in red blood cells. Although many invertebrates also have hemoglobins in blood cells, some invertebrates have hemoglobins dissolved in their blood plasma.
• Hemocyanins are the second most common of the respiratory pigments in animals. They contain copper and turn bright blue when oxygenated. There are two types of hemocyanins, which are of separate evolutionary origin: arthropod hemocyanins (occurring in crabs, lobsters, crayfish, horseshoe crabs, spiders, and some other arthropods) and mollusc hemocyanins (occurring in squids, octopuses, many snails, and some other molluscs). Hemocyanins are always dissolved in the blood plasma.
• Chlorocruorins, which are similar to hemoglobins, occur in only four families of marine annelid worms, and are always dissolved in the blood plasma.
• Hemerythrins are non-heme, iron-containing respiratory pigments that have a limited and scattered distribution, occurring in three or four different invertebrate phyla.

The O₂-Binding Characteristics of Respiratory Pigments

• The oxygen equilibrium curve of a respiratory pigment, which shows the relation between the extent of O₂ binding by the pigment and the O₂ partial pressure, is a key tool for interpreting respiratory-pigment function. The shape of the oxygen equilibrium curve depends on the degree of cooperativity among O₂-binding sites on respiratory-pigment molecules. When there is no cooperativity—as is the case when each molecule has only a single O₂-binding site—the oxygen equilibrium curve is hyperbolic. The curve is sigmoid when molecules have multiple O₂-binding sites that exhibit positive cooperativity. Hyperbolic curves are the norm for myoglobins; sigmoid curves are the norm for blood pigments.
• The Bohr effect is a reduction in O₂ affinity caused by a decrease in pH and/or an increase in CO₂ partial pressure. The Bohr effect typically enhances O₂ delivery because it promotes O₂ unloading in systemic tissues while promoting loading in the breathing organs.
• The Root effect, which occurs only rarely, is a substantial reduction of the oxygen-carrying capacity of a respiratory pigment caused by a decrease in pH and/or an increase in CO₂ partial pressure. In teleost fish it helps inflate the swim bladder and oxygenate the retina.
• Elevated blood temperatures often decrease the O₂ affinity of respiratory pigments.
• Organic molecules and inorganic ions frequently serve as allosteric modulators of respiratory-pigment function. 2,3-DPG (2,3-BPG) in the red blood cells of mammals, for example, chronically decreases the O₂ affinity of the hemoglobin in the cells.

The Functions of Respiratory Pigments in Animals

• Respiratory pigments are diverse in their functional properties. The functions they can potentially perform include O₂ transport, facilitation of CO₂ transport, transport of substances other than respiratory gases, blood buffering, facilitation of Ov diffusion through the cells of solid tissues such as muscle, and O₂ storage in blood or solid tissues.
• Blood respiratory pigments typically become well oxygenated in the breathing organs, and when animals are at rest, the respiratory pigments typically release only a modest fraction of their O₂ to the systemic tissues (25% in humans). During exercise, O₂ delivery is enhanced by increases in both the extent of pigment unloading and the rate of blood flow.
• The O₂ affinities of respiratory pigments are often critical for pigment function. When O₂ is transferred from one respiratory pigment to another in an individual animal—as when blood hemoglobin donates O₂ to myoglobin—it is usual for the pigment receiving the O₂ to have a higher O₂ affinity. Comparing related species, those with long evolutionary histories in O₂-poor environments often have evolved blood respiratory pigments with particularly high O₂ affinities.
• Respiratory-pigment physiology undergoes acclimation, as by changes in pigment amounts, synthesis of new molecular forms, or modulation of preexisting forms.

Carbon Dioxide Transport

• The carbon dioxide equilibrium curve, which shows the relation between the total carbon dioxide concentration of blood and the CO₂ partial pressure, is a key tool for analyzing carbon dioxide transport. In water breathers, the CO₂ partial pressures of both systemic arterial blood and systemic venous blood are typically low and on the steep portion of the carbon dioxide equilibrium curve. In air breathers, blood CO₂ partial pressures tend to be far higher and therefore on the flatter portion of the carbon dioxide equilibrium curve.
• Most carbon dioxide carried in blood is typically in the form of bicarbonate, HCO₃^–. The extent of HCO₃^– formation depends on blood buffers and determines the shape of the carbon dioxide equilibrium curve. Because respiratory pigments are major blood buffers, they play major roles in carbon dioxide transport.
• The Haldane effect, which is in part the necessary converse of the Bohr effect, is an increase in the total carbon dioxide concentration of the blood caused by deoxygenation of the respiratory pigment. The Haldane effect aids carbon dioxide transport by promoting CO₂ uptake by the blood in the systemic tissues and CO₂ loss from the blood in the breathing organs.
• Rapid uptake of CO₂ by the blood or loss of CO₂ from the blood requires the action of carbonic anhydrase, an enzyme localized to certain places (e.g., red blood cells).

Acid–Base Physiology

• The neutral pH varies with temperature, being higher at low temperatures than at high ones. In animals with variable body temperatures, the normal blood pH often varies in parallel with the neutral pH, being displaced in the alkaline direction to a constant extent (constant relative alkalinity).
• Acidosis and alkalosis are categories of acid–base disturbance. They occur, respectively, when the blood pH is to the acid or alkaline side of an animal’s normal pH for the prevailing body temperature. Either sort of disturbance can be respiratory (originating because of changes in CO₂ loss by breathing) or metabolic (originating because of changes in the blood bicarbonate concentration).
• Within their range of acid–base regulation, animals correct chronic acid–base disturbances by modulating the elimination of CO₂, H⁺, and HCO₃^– in regulatory ways.