Circulatory System
The circulatory system is the heart, blood vessels, and blood that transport oxygen, nutrients, wastes, and hormones throughout the body.
Definition
The circulatory system is your body's delivery service: the heart pumps blood through tubes called blood vessels, carrying oxygen and food to every cell and carrying waste away. The cardiovascular system consists of the heart, the arteries, veins, and capillaries, and the blood, and it transports oxygen from the lungs and nutrients from the digestive system to body cells, removes carbon dioxide and other wastes, distributes hormones and heat, and moves immune cells; the four-chambered human heart drives two loops, pulmonary to the lungs and systemic to the body. It is a closed, double-circuit network driven by a myogenic pump whose rhythm originates in the sinoatrial node. Flow obeys $Q = \Delta P / R$, with resistance dominated by arteriole radius (resistance scales with $1/r^4$ by Poiseuille's law), so small changes in vessel diameter produce large changes in regional blood flow. Exchange occurs across capillary walls by diffusion and by bulk flow governed by hydrostatic and osmotic pressures (Starling forces), and oxygen transport depends on hemoglobin's cooperative binding, described by a sigmoidal dissociation curve that shifts with pH, temperature, and $CO_2$.
Example
Put your fingers on your wrist and feel your pulse: each beat is your heart pushing blood out through your arteries, about $70$ times a minute at rest. An adult has about $5\ \text{L}$ of blood, and the resting heart pumps about $5\ \text{L}$ per minute, so a red blood cell completes a full circuit in roughly one minute; laid end to end, the blood vessels would stretch tens of thousands of kilometers, most of it capillaries. Cardiac output equals heart rate times stroke volume, $70\ \text{beats/min} \times 70\ \text{mL} \approx 4.9\ \text{L/min}$ at rest, rising to $20\ \text{L/min}$ or more in a trained athlete during exercise, and halving an arteriole's radius increases its resistance $16$-fold.
Key Insight
Blood is the only thing in your body that visits every cell, so the circulatory system connects all the other systems together. Arteries carry blood away from the heart and veins carry it back, but which one carries oxygen-rich blood depends on the loop: the pulmonary artery carries oxygen-poor blood to the lungs. Blood pressure is a physics problem, since pressure, flow, and resistance are related the same way as voltage, current, and resistance in a circuit; hypertension is high resistance, and the heart is the pump that must work against it.