Biology 20 · Circulation
A double circuit, two loops
Blood makes two journeys, not one: a short loop to the lungs and a long one to everything else, passing through the heart between them. Four chambers are what make that possible without mixing.
The words, first
The idea: Artery and vein are defined by direction, not by what the blood is carrying — and the pulmonary vessels prove it.
| Word | What it means |
|---|---|
| Artery | Carries blood away from the heart. Thick muscular walls; carries the highest pressure. |
| Vein | Carries blood towards the heart. Thin walls, low pressure, valves to stop backflow. |
| Capillary | One cell thick, where all exchange happens. |
| Atrium | A receiving chamber. |
| Ventricle | A pumping chamber. The left one is thickest because it supplies the whole body. |
| Pulmonary circuit | Right ventricle → lungs → left atrium. |
| Systemic circuit | Left ventricle → body → right atrium. |
| SA node | The pacemaker, in the right atrium. It fires on its own; nerves and hormones only adjust the rate. |
| Plasma | The liquid part of blood, about 55% by volume, carrying dissolved proteins, nutrients and wastes. |
| Hemoglobin | The protein in red blood cells that binds oxygen — four molecules per hemoglobin. |
| Alveolus | A tiny air sac in the lung; the site of gas exchange. |
| Partial pressure | The pressure one gas contributes in a mixture. Gases diffuse from high to low partial pressure. |
The heart and the two circuits
The idea: Deoxygenated blood goes right, oxygenated goes left, and the two never mix in a healthy heart.
The path. Body → vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta → body.
The exceptions everyone gets wrong. The pulmonary artery carries deoxygenated blood, and the pulmonary vein carries oxygenated blood — because the names describe direction, not content.
Why the left ventricle is thickest. It pushes blood around the entire body; the right only has to reach the lungs. Both move the same volume per beat — the difference is pressure, not quantity.
Valves are one-way flaps that close when pressure would push blood backwards. The sound of a heartbeat is valves closing.
The pacemaker. The SA node depolarizes on its own, which is why a heart keeps beating when removed from the body. The signal spreads across the atria to the AV node, then down to the ventricles.
Blood
The idea: Just over half liquid, just under half cells. Each component has one job, and questions usually ask which.
| Component | Job |
|---|---|
| Plasma | Carries dissolved nutrients, wastes, hormones, proteins and most of the CO₂ |
| Red blood cells | Carry oxygen on hemoglobin. No nucleus, biconcave — both increase capacity and surface area |
| White blood cells | Immune defence |
| Platelets | Clotting |
Why hemoglobin is necessary. Oxygen dissolves poorly in water, so plasma alone could carry nowhere near enough. Hemoglobin raises the blood's capacity many times over.
Gas exchange
The idea: Diffusion, down partial pressure gradients, across a membrane one cell thick. No energy is spent on the exchange itself.
At the alveolus. Oxygen is at higher partial pressure in the air sac than in the blood, so it diffuses in; CO₂ is higher in the blood, so it diffuses out. The alveoli give an enormous area — roughly the size of a tennis court — and the barrier is two cells thick in total.
How CO₂ travels. About 70% is converted to bicarbonate by carbonic anhydrase inside red blood cells and carried in the plasma; around 20% rides on hemoglobin; only about 10% stays as dissolved gas.
The Bohr effect. Active muscle produces acid and heat, both of which make hemoglobin release oxygen more readily. The tissue working hardest changes its own local conditions and gets a bigger share — a feedback mechanism with no nerves or hormones involved.
Breathing is mechanical. The diaphragm contracts and flattens, the chest cavity enlarges, pressure inside drops below atmospheric, and air flows in. Air is pushed in by the outside, not pulled in by the lungs.
What costs marks
The idea: Definitions and one mechanism.
- Defining arteries as carrying oxygenated blood. Direction, not content.
- Saying the left ventricle pumps more blood. It pumps at higher pressure.
- Saying most CO₂ is carried on hemoglobin. Most is bicarbonate in the plasma.
- Saying the lungs pull air in. Pressure differences push it in.