Fluids of Physiology

The circulatory system is a closed system — blood circulates continuously, driven by pressure differences the heart generates.
Systole (heart contracts) raises arterial pressure and pushes blood forward; diastole (heart relaxes) lets chambers refill as pressure drops.
The respiratory system moves air via thoracic cavity pressure changes: inhalation lowers lung pressure below atmospheric, exhalation raises it above.
The two systems are physically interlinked: inhalation's negative thoracic pressure both draws in air and helps pull venous blood back to the heart.
Both systems ultimately run on the same fluid dynamics/pressure principles as the rest of the chapter — density, pressure, Pascal's/Archimedes' Principles, flow.

Phase	What Happens
Systole	Heart contracts → high-pressure wave pushes blood through arteries
Diastole	Heart relaxes → pressure drops → chambers refill with blood

Phase	What Happens
Inhalation	Diaphragm/intercostals contract → thoracic cavity expands → lung pressure drops below atmospheric → air flows in
Exhalation	Muscles relax → thoracic cavity shrinks → lung pressure rises above atmospheric → air is pushed out

Inhalation's negative thoracic pressure doesn't just pull air in — it also helps draw blood into the large veins returning to the heart, physically linking respiration and circulation.
Both circulation and breathing are pressure-gradient-driven flow: fluid (blood or air) always moves from higher to lower pressure.

Don't treat circulation and respiration as separate topics on the MCAT — a passage may test that inhalation's pressure drop assists venous return, not just lung filling.
Air flows into the lungs because lung pressure drops below atmospheric — it's not 'sucked in' by muscle force directly acting on the air.

How does inhalation help venous blood return to the heart?

What causes blood to flow through arteries during systole?

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