physiology - Where does extra blood come from to fill your muscles during exercise?

Let's say I go to the gym and lift some weights an hour. During this time my arms will grow due to the "pump" -- the extra blood rushing in to feed the muscles. For example, I've measured about 2-3 centimeters increase just in the diameter of the upper arm (bicep+tricep).

But where did this blood come from? Also, if my arms got bigger, since I'm the same weight that means some part of my body must have gotten smaller, right?

Answer

The blood comes from the body's reservoirs:

• spleen (mostly erythrocytes) [1]


• liver [2]


• veins (probably the most important blood resevoir as they contain 50-60 % of the volume) [3]

In pathological situations, if hypovolemia occurs, blood can also come from:

• splachnic vascular bed [5]

But what attracts the blood into the muscle? The phenomenon is called active hyperemia:

Active hyperemia is the increase in organ blood flow (hyperemia) that is associated with increased metabolic activity of an organ or tissue. An example of active hyperemia is the increase in blood flow that accompanies muscle contraction, which is also called exercise or functional hyperemia in skeletal muscle. Blood flow increases because the increased oxygen consumption of during muscle contraction stimulates the production of vasoactive substances that dilate the resistance vessels in the skeletal muscle [4].

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References:

1. The human spleen as an erythrocyte reservoir in diving-related interventions. Kurt Espersen, Hans Frandsen, Torben Lorentzen, Inge-Lis Kanstrup, Niels J. Christensen. Journal of Applied PhysiologyMay 2002,92(5)2071-2079;DOI: 10.1152/japplphysiol.00055.2001


2. Lautt WW, Greenway CV. Hepatic venous compliance and role of liver as a blood reservoir. Am. J. Physiol. 1976 Aug;231(2):292-5. PubMed PMID: 961879.


3. Michael J. Gregory, Ph.D. The Circulatory System. Licensed under CC-BY-NC-SA-3.0


4. Richard E. Klabunde, PhD. Cardiovascular Physiology Concepts. Active Hyperemia. Available from http://www.cvphysiology.com/Blood%20Flow/BF005.htm (accessed 03.08.2014)


5. Blaber AP, Hinghofer-Szalkay H, Goswami N. Blood volume redistribution during hypovolemia. Aviat Space Environ Med. 2013 Jan;84(1):59-64. PubMed PMID: 23305001.