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
- 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:
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%
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).