Human Transport » Circulation System Overview

What you'll learn this session

Study time: 30 minutes

The structure and function of the human circulatory system
The components of blood and their roles
How the heart works as a double pump
The differences between arteries, veins and capillaries
The pulmonary and systemic circulation pathways
Common disorders of the circulatory system

Introduction to the Human Circulatory System

Your body contains about 5 litres of blood that's constantly on the move. But how does it get around? That's where your circulatory system comes in! This amazing network of the heart, blood vessels and blood works tirelessly to keep you alive by delivering oxygen and nutrients to your cells while removing waste products.

Key Definitions:

Circulatory system: The body system that moves blood around the body, consisting of the heart, blood vessels and blood.
Double circulation: The system in mammals where blood passes through the heart twice in one complete circuit of the body.
Cardiac cycle: The sequence of events that occur when the heart beats.

❤️ Why Do We Need Circulation?

Your body's cells need a constant supply of oxygen and nutrients to stay alive and function properly. They also produce waste products like carbon dioxide that need to be removed. The circulatory system solves both problems by:

Delivering oxygen from the lungs to all body cells
Transporting nutrients from the digestive system to cells
Removing carbon dioxide and other waste products
Distributing hormones throughout the body
Helping regulate body temperature
Protecting against disease through white blood cells

💧 Blood Composition

Blood is a specialized fluid tissue made up of:

Plasma (55%): Yellowish liquid that carries blood cells, nutrients, hormones and waste products
Red blood cells (erythrocytes): Carry oxygen using haemoglobin
White blood cells (leukocytes): Fight infection and disease
Platelets: Cell fragments that help blood clot

The Heart: A Powerful Double Pump

Your heart is an amazing muscular organ about the size of your fist. It beats around 100,000 times per day, pumping blood throughout your entire body. The heart functions as a double pump, with the right side pumping deoxygenated blood to the lungs and the left side pumping oxygenated blood to the rest of the body.

Structure of the Heart

The heart has four chambers and various valves that ensure blood flows in the correct direction:

🟥 Chambers

Right atrium: Receives deoxygenated blood from the body
Right ventricle: Pumps blood to the lungs
Left atrium: Receives oxygenated blood from the lungs
Left ventricle: Pumps blood to the body (has the thickest walls)

🟣 Valves

Tricuspid valve: Between right atrium and ventricle
Pulmonary valve: Between right ventricle and pulmonary artery
Bicuspid valve: Between left atrium and ventricle
Aortic valve: Between left ventricle and aorta

🟢 Blood Vessels

Vena cava: Brings deoxygenated blood to the right atrium
Pulmonary artery: Carries deoxygenated blood to the lungs
Pulmonary vein: Returns oxygenated blood to the left atrium
Aorta: Carries oxygenated blood from the left ventricle to the body

The Cardiac Cycle

The cardiac cycle describes the sequence of events that occur during one heartbeat. It consists of:

Atrial systole: Both atria contract, pushing blood into the ventricles
Ventricular systole: Both ventricles contract, pushing blood out of the heart
Diastole: All chambers relax and the atria fill with blood

The sound of your heartbeat ("lub-dub") comes from the closing of heart valves during the cardiac cycle.

Case Study Focus: Measuring Heart Rate

Your heart rate is the number of times your heart beats per minute (bpm). A normal resting heart rate for adults is 60-100 bpm, but athletes often have lower resting heart rates (40-60 bpm) because their hearts are more efficient. You can measure your heart rate by feeling your pulse at your wrist (radial artery) or neck (carotid artery). Count the beats for 15 seconds and multiply by 4 to get your heart rate per minute.

Try measuring your heart rate at rest, then after 1 minute of exercise (like jumping jacks). How much did it increase? Your heart rate increases during exercise because your muscles need more oxygen, so your heart pumps faster to deliver it.

Blood Vessels: The Transport Network

Blood vessels form an extensive network that carries blood throughout your body. There are three main types of blood vessels, each with a structure suited to its function:

🟠 Arteries

Structure: Thick, muscular, elastic walls

Function: Carry blood away from the heart under high pressure

Features:

Small lumen (inner space)
No valves
Pulse can be felt

🟦 Veins

Structure: Thinner walls, less muscle and elastic tissue

Function: Carry blood towards the heart under low pressure

Features:

Large lumen
Contain valves to prevent backflow
No pulse

🟧 Capillaries

Structure: Very thin walls (one cell thick)

Function: Allow exchange of substances between blood and tissues

Features:

Microscopic in size
Form extensive networks
Slow blood flow for efficient exchange

Double Circulation

Humans have a double circulatory system, which means blood passes through the heart twice in one complete circuit of the body. This system is more efficient than a single circulation because it maintains high blood pressure in the systemic circuit, ensuring efficient delivery of oxygen to body tissues.

💚 Pulmonary Circulation

This is the shorter loop that carries blood between the heart and lungs:

Deoxygenated blood leaves the right ventricle through the pulmonary artery
Blood travels to the lungs where it picks up oxygen and releases carbon dioxide
Oxygenated blood returns to the left atrium via the pulmonary veins

💙 Systemic Circulation

This is the longer loop that carries blood between the heart and the rest of the body:

Oxygenated blood leaves the left ventricle through the aorta
Blood travels to all body tissues, delivering oxygen and nutrients
Deoxygenated blood returns to the right atrium via the vena cava

Common Circulatory Disorders

Several conditions can affect the circulatory system:

Coronary heart disease: Narrowing of the coronary arteries that supply the heart muscle with blood, often due to fatty deposits (atherosclerosis)
Heart attack (myocardial infarction): Occurs when blood flow to part of the heart muscle is blocked, causing tissue damage
Stroke: Disruption of blood supply to the brain, causing brain cells to die
Hypertension (high blood pressure): Persistent high pressure in the arteries, which can damage blood vessels and organs
Anaemia: Condition where there are too few red blood cells or not enough haemoglobin, reducing oxygen transport

Many circulatory disorders can be prevented or managed through a healthy lifestyle, including regular exercise, a balanced diet low in saturated fats and salt, not smoking and limiting alcohol consumption.

Summary

The human circulatory system is a complex network that transports essential substances throughout the body. The heart acts as a double pump, sending deoxygenated blood to the lungs and oxygenated blood to the body. Different types of blood vessels arteries, veins and capillaries have structures suited to their specific functions. Understanding how this system works helps us appreciate its importance and how to keep it healthy.

🧠 Test Your Knowledge!