ABSTRACT
The relationship between haematological variables and body weight (W) is studied in the American plaice. Haematocrit, haemoglobin concentration, and cell volume and numbers are directly correlated with W, indicating that small fish have low blood oxygen solubility 
in spite of a high weight specific oxygen consumption 
. Examination of analytical models of the branchial apparatus suggests that, in small fish, 
is lowered to minimize blood viscosity and that a high 
is maintained by increasing the arteriovenous oxygen partial pressure difference. Mean cell haemoglobin content is positively correlated while mean cell surface area per unit haemoglobin tends to be negatively correlated with W. Mean erythrocyte residence time in the secondary lamellae is shown analytically to increase with W, possibly accounting for these relationships.
