A. Respiratory pigments

1. Kinds of pigments

a. Iron containing--hemoglobin, hemerythrin, chlorocruorin

b. Copper containing--haemocyanin

2. Locations

a. In the blood/lymph fluid

b. In cells

3. Phylogenetic relationships--only a loose relationship. For example in the annelids, members of the same family may have different respiratory pigments, and often even in the same genus. Likewise earthworm possess hemoglobin as do most mammals, including humans, yet, we are far removed from earthworms in phylogeny.

B. Function of respiratory pigments

1. Function as a carrier of oxygen, where molecules actually attach to the respiratory pigment at special sites.

2. Amount that attaches or unattaches is related to the partial pressure of the oxygen at the site of the gills and at the tissues.

3. A curve, the dissociation curves, shows this relationship. The x-axis represents the partial pressure of oxygen and the y, the percent saturation of the molecule. The curve is sigmoidal. It can be shifted right or left dependent upon the pH, temperature or pressure associated with the system. There is a strong correlation with the environment and the type of dissociation curve, with some curves favoring loading at the intake and other favoring unloading at the tissue site. Loading is often more important than unloading because unloading at the tissues will always occur if cells are "using up" their oxygen supply when actively metabolizing.

C. Some physical structures

1. Organisms that are small do not need a special system as the diffusion distance, the one millimeter distance, functions to allow gases to diffuse in and out of the organism.

2. Other structures

a. gills--a variety of shapes, many feather-like, other sheet-like.

b. lungs--hollow sacs

c. book gills--sheet-like gills with many sheets

d. tracheal systems-tubes that penetrate the body

D. The role of a counter current mechanism

1. Counter current mechanisms are present in situations where flow in two vessels is opposite each other. This maintains a constant diffusion gradient in one direction. This can be done by having a capillary in the gill flow in a direction opposite the flow of the medium over the gill.