Blood Volume in animals
Blood Volume in animals makes up about 6 to 8% of the body weight. Blood volume measurement plays an important role when blood transfusion is attempted.
It is also important to interpret PCV, Hb, RBC, and hematological parameters. These values are altered when the blood volume changes e.g. hemoconcentration and hemodilution.
Blood volume is influenced by body type, body size, age, sex, breed, nutrition, pregnancy, and lactation, physical and metabolic activities. Males show higher blood volume than females.
Blood volume increases with pregnancy, muscular activity, stress, and high temperature, whereas starvation, haemorrhage, burns, dehydration, anemia and cold decrease the blood volume.
Average blood volume in various species
| Blood Volume (ml) | Cattle | Horse | Sheep | Goat | Pig | Dog | Cat | Poultry |
| Blood volume (ml) | 40,000 | 40,000 | 3,200 | 2,400 | 8,000 | 1,600 | 240 | 160 |
| % of body wt. | 8 | 10 | 8 | 6 | – | 7 | – | 6.5 |
Measurement of blood volume
The volume of any fluid compartment of the body can be measured by placing a substance in the compartment and allowing it to disperse through the fluid. The extent to which the substance gets diluted in the fluid is measured.
The substance used for determining blood volume must be capable of dispersing throughout the blood easily and must remain in the circulatory system for a longer period of time till the measurements are made.
Blood volume may be measured indirectly by two methods- Plasma volume method and Erythrocyte volume method.
Plasma volume method
Plasma volume can be measured by dye dilution or by radioisotope. Blood sample from the experimental animal is collected and the plasma of the pre injected anticoagulant-added blood serves as a blank.
Known quantity of Evan’s blue dye/ (T-1824)‚ or radioisotope 131I‚ when injected, combines with the plasma proteins and disperses throughout the circulatory system in about 10 minutes.
Blood samples are collected sequentially at 5 minutes interval for the next 15 to 30 minutes. These blood samples are centrifuged to get RBC free plasma. The concentrations of the dye or the radioactivity of isotope in the plasma samples can be measured spectrophotometrically at 620 nm or by the scintillation counter respectively.
Erythrocyte volume method
This can be determined by radio active 32P, 59Fe and 51Cr. Blood sample from the experimental animal is collected to get plasma free erythrocytes. Small quantity of 51Cr is mixed with these RBCs which are then incubated at 36 °C for 30 minutes to activate the binding process of 51Cr with the RBCs. Wash the erythrocytes with saline to remove the free 51Cr. These radioactive erythrocytes are then injected into the circulation of the experimental animal. After proper mixing of these RBCs in the circulation, blood samples are collected after 10 minutes.
RBCs from the blood samples are separated and the radioactivity of pre and post injected RBC samples are determined by scintillation counter.
The trapped plasma value may interfere with the PCV. Hence the correction factor for trapped plasma is introduced as 0.96 for blood volume determination. Starvation, hemorrhage, hot environment and water deprivation reduce the blood volume.