Physiology, Blood Volume

Blood volume refers to the total amount of fluid circulating within the arteries, capillaries, veins, venules, and chambers of the heart at any time. The components that add volume to blood include red blood cells (erythrocytes), white blood cells (leukocytes), platelets, and plasma. Plasma accounts for about 60% of total blood volume, while erythrocytes make up roughly 40%, along with leukocytes and platelets. The amount of blood circulating within an individual depends on their size and weight, but the average human adult has nearly 5 liters of circulating blood. Women tend to have a lower blood volume than men. However, a woman's blood volume increases by roughly 50% during pregnancy.Blood volume is tightly regulated and related to multiple organ systems. Furthermore, it is closely associated with sodium content and hydration status. The maintenance of blood volume is crucial to normal function as it is necessary for the constant perfusion of body tissues. Blood volume can be increased or decreased by systemic dysfunction. Changes in blood volume can result in various clinical scenarios such as hypovolemic shock or edema. Two equations exist for estimating blood volume given the gender, height (H), and weight (W) of the patient. The Nadler equation is built upon the work of Dr. Allen in 1962, while the Lemmens-Bernstein-Brodsky equation is more accurate for higher ranges of body weight and body mass indices in patients without critical illnesses. Nadler Equation : Men: Blood Volume = (0.3669 × H^3) + (0.03219 × W) + 0.6041 Women: Blood Volume = (0.3561 × H^3) + (0.03308 × W) +0.1833 Lemmens-Bernstein-Brodsky Equation : Blood Volume = 70/[sqrt(body mass index/22)]