How much blood is in a rat? - briefly
An adult laboratory rat weighing roughly 250–300 g contains about 20–25 ml of blood, which is approximately 7–8 % of its body mass. This estimate applies to most common rat strains.
How much blood is in a rat? - in detail
A typical adult laboratory rat (Rattus norvegicus) contains approximately 6–7 % of its body mass as circulating blood. For a common adult weight of 250–300 g, the blood volume ranges from 15 to 22 ml. Smaller juveniles (50–100 g) have proportionally similar percentages, resulting in 3–7 ml of blood. Larger breeds used in research, such as the Sprague‑Dawley or Wistar, may weigh up to 500 g, yielding 30–35 ml of plasma.
Key determinants of the exact volume include:
- Body weight: Direct linear relationship; each gram of body mass contributes roughly 0.06–0.07 ml of blood.
- Strain and genetics: Certain strains exhibit modest deviations due to metabolic or cardiovascular differences.
- Sex: Males often have slightly higher absolute volumes because of greater average body mass.
- Age: Neonates possess a higher proportion of blood relative to body weight, which declines to adult levels during growth.
- Physiological state: Hydration, anemia, or pathological conditions can alter circulating volume.
Measurement techniques commonly employed are:
- Evans blue dye dilution: Injected intravenously, the dye equilibrates with plasma; concentration in a blood sample determines volume.
- Radioisotope labeling (e.g., ^125I‑albumin): Provides precise quantification, especially in small animals.
- Direct cardiac puncture: Allows immediate collection of total blood but is terminal and used primarily for validation.
- Hemoglobin-based calculations: Uses known hemoglobin concentration and total hemoglobin mass to infer volume.
Blood composition in rats mirrors that of other mammals: plasma constitutes about 55 % of total volume, containing electrolytes, proteins (albumin ≈ 30 g/L), glucose, and clotting factors. Cellular components include red blood cells (hematocrit 40–45 %), white blood cells, and platelets.
Understanding these parameters is essential for dosing of intravenous drugs, designing hematological studies, and ensuring ethical blood sampling limits (typically no more than 10 % of total volume per week to avoid hypovolemia).