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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?

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    $\begingroup$ Did you also drink water or a protein shake around the time of the workout? $\endgroup$
    – Brinn Belyea
    Commented Aug 2, 2014 at 23:37
  • $\begingroup$ It happens even if you don't drink anything beforehand. $\endgroup$
    – CaptainCodeman
    Commented Aug 3, 2014 at 17:40

3 Answers 3

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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].

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.
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  • $\begingroup$ But veins also bulge and get bigger when you exercise; do you mean that if I exercise my arms it draws blood from the veins of my legs? $\endgroup$
    – CaptainCodeman
    Commented Aug 3, 2014 at 17:49
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    $\begingroup$ @CaptainCodeman the veins that bulge on the surface of the body part you are exercising are superficial veins. Because muscle contracts, profound veins are getting flat and the only way for blood to return to the heart is the superficial colateral venous system. And yes, it will draw also blood from your legs, from the abdominal veins etc. $\endgroup$
    – Cornelius
    Commented Aug 3, 2014 at 17:52
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How does extra blood come from to fill your muscles during exercise?

Blood pumps (blood) and sucks (lymph). There are many pumps in our body

  • thoracic pump
  • smooth musculature
  • respiratory pump

which work together to provide the blood to the peripheral circulation. These pumps provide us Pulse, Vasomotor tone and Respiratory waves which when act together can lead to local hyperemia or congestion; see my answer only about these waves here in the thread What does irregular heartbeat mean in simple language.

By Starling law, anything that goes inside the heart goes out from there. See this answer about elevated position on the venous return which is similar to the the exercise on the venous return. Smooth musculature accommodates blood to the venous return.

Why your muscle gets bigger during exercise?

There are many reasons which can be explained by Frank-Starling principle and Fick's law. They lead either to

  • hyperemia (increased local blood flow) or
  • congestion

because of

  • protein intake before the exercise
  • increased sympaticus (decreased parasympaticus; yes, sympathetic tone in digestive truct and some other systems is down-regulated) and blood flow through the muscle (see the above thread)
  • unstretched muscle can have congestion during exercise which is felt as hard muscle, also long after the training
  • too regular exercising without rest can also lead to congestion

in the spaces described in my earlier answers like here about What can cause the swelling in high protein diet of Whey proteins:

enter image description here

where explanations in my earlier answer.

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

From your body (See Cornelius' answer for specific locations)

  • veins 50-60%
  • spleen
  • liver

under physiologic fine regulation where heart is the main pump (positive gradient) while its sucking effect (negative gradient; lymph) keeps homeostasis stable for exercising muscles to maintain hyperemia supported by rapid adaptive smooth musculature, vasomotor tone and respiratory pumps (three types of waves).

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If you stand up too quickly, you get a head rush. One way to counteract the symptoms of the head rush is to contract your leg muscles really intensely. This forces blood out of your legs and into the rest of your circulation, including your head. This is actually a tip my family doctor told me that I've always remembered.

As mentioned in the links below working out a muscle group causes excess blood flow to these areas which could cause the blood vessels to dilate, causing the muscles in these areas to swell and appear larger. I now realize this has nothing to do with the head rush item I mentioned above, but still may be a useful piece of advice if you every get a head rush!

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1479005/

http://www.livestrong.com/article/75670-swell-after-workout/

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  • $\begingroup$ Can you add some references? $\endgroup$
    – Chris ♦
    Commented Aug 3, 2014 at 6:54
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    $\begingroup$ Why would pushing blood to your legs and away from your head help with the head rush(orthostatic hypotension)? The dizziness and other symptoms are caused by blood leaving your head. In the short term, flexing drives blood from the muscle group, and opens blood vessels to increase later blood supply to the muscle group.(see also Hick maneuver, a similar leg/torso flex to keep blood in the head for fighter pilots) $\endgroup$
    – Resonating
    Commented Aug 4, 2014 at 16:29
  • $\begingroup$ thank you I realized I said this wrong, fixing my answer now. $\endgroup$
    – JonHerman
    Commented Aug 5, 2014 at 0:26

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