Circulatory Systems, Heart & Blood Vessels

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Circulatory Systems, Heart & Blood Vessels

Your heart pumps blood through two separate loops — one to the lungs to pick up oxygen, one to your whole body to deliver it — and valves plus specially adapted blood vessels ensure blood flows the right way so your cells get exactly what they need.

Key Concepts at a Glance

Circulatory System

An organ system made of the heart (pump), blood vessels, and valves that transports blood around the body.

Double Circulation (Mammals)

Blood passes through the heart twice per circuit: once to the lungs (pulmonary), once to the body (systemic).

Heart Structure

Four chambers (two atria, two ventricles), a septum, coronary arteries, and two sets of valves control one-way flow.

Blood Vessels

Arteries (high pressure, away from heart), veins (low pressure, towards heart), capillaries (exchange with cells).

Blood Components

Red cells (oxygen transport), white cells (immunity), platelets (clotting), plasma (transport of nutrients, hormones, waste).

Coronary Heart Disease

Fatty plaques block coronary arteries. Partial blockage = angina. Complete blockage = heart attack.

1. The Circulatory System

What Is It?

The circulatory system is an organ system whose role is to transport blood around the body. It has three essential components:

Blood vessels — tubes that carry blood
A pump — the heart, which pushes blood through the vessels
Valves — one-way gates that ensure blood flows in the correct direction only

Single vs. Double Circulation

Fish have single circulation: A two-chambered heart pumps blood around the body, then blood passes through the heart once per circuit. The fish heart receives all deoxygenated blood, pumps it to the gills, and from there it goes directly to the body.

Mammals have double circulation: A four-chambered heart ensures blood passes through the heart twice per circuit. Here's why this matters:

First pass: Right side pumps deoxygenated blood to the lungs (pulmonary circulation) for gas exchange.
Second pass: Left side pumps oxygenated blood from the lungs around the entire body (systemic circulation).

💡 Key Advantage When blood passes through the tiny capillaries in the lungs, pressure drops. Double circulation brings that blood back to the heart, which re-pressurises it before sending it to the body. This means the body receives blood faster and more frequently than in single circulation.

Practice Q1

Explain one advantage of double circulation in mammals compared to single circulation in fish.

2. The Mammalian Heart — Structure & Function

The Four Chambers

The mammalian heart has two atria (thin-walled, receive blood) and two ventricles (thick-walled, pump blood out).

Chamber	Receives Blood From	Pumps Blood To	Wall Thickness
Right Atrium	Vena cava (deoxygenated from body)	Right ventricle	Thin
Right Ventricle	Right atrium

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Also in the full note

3. Heart Valves — Preventing Backflow
4. How the Heart Pumps Blood — The Cardiac Cycle
5. Heart Rate & Physical Activity
6. Blood Vessels — Three Types, Three Jobs
7. How Blood Circulates Around the Body
8. Blood — What's Inside?
9. Blood Clotting — Sealing the Leak
10. Coronary Heart Disease (CHD) — A Failing Pump

What's inside

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Circulatory Systems, Heart & Blood Vessels