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) \[3\]

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.][1]* 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.](http://www.ncbi.nlm.nih.gov/pubmed/961879) Am. J. Physiol. 1976 Aug;231(2):292-5. PubMed PMID: 961879. 
3. Michael J. Gregory, Ph.D. *[The Circulatory System.][2]* 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)


  [1]: http://jap.physiology.org/content/92/5/2071
  [2]: http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20100/Bio%20100%20Lectures/Organ%20Systems/Circulatory%20System/Circulatory%20System.htm