Edexcel iGCSE Biology-3.19 Circulatory System: General Structure- Study Notes- New Syllabus

The Circulatory System – General Structure

📝 Introduction

• The circulatory system transports blood throughout the body.
• It carries oxygen, nutrients and hormones to body cells and removes carbon dioxide and other waste products.
• Humans have a double circulatory system, which means blood passes through the heart twice during one complete circulation.
• Different blood vessels connect the heart with important organs such as the lungs, liver and kidneys.
• Each blood vessel has a specific role in transporting blood to and from these organs.

❤️ Double Circulatory System

What is a Double Circulatory System?

A double circulatory system means that blood travels in two separate circuits:

• Pulmonary circulation – between the heart and lungs.
• Systemic circulation – between the heart and the rest of the body.

Why is it called “double”?

During one complete journey:

• Blood goes from the heart to the lungs and back to the heart.
• It is then pumped from the heart to the rest of the body and back again.

This means blood passes through the heart twice.

🫁 Pulmonary Circulation (Heart ↔ Lungs)

Function

• Carries deoxygenated blood from the heart to the lungs.
• Gas exchange takes place in the lungs.
• Returns oxygenated blood back to the heart.

Pathway

Right ventricle → Pulmonary artery → Lungs → Pulmonary vein → Left atrium

Blood Vessels Involved

Pulmonary Artery

• Carries deoxygenated blood from the right ventricle to the lungs.
• Carbon dioxide is transported to the lungs to be removed.

Important: It is the only artery that carries deoxygenated blood.

Gas Exchange in the Lungs

In the alveoli:

• Oxygen diffuses into the blood.
• Carbon dioxide diffuses out of the blood.

The blood becomes oxygenated.

Pulmonary Vein

• Carries oxygenated blood from the lungs to the left atrium.

Important: It is the only vein that carries oxygenated blood.

🌍 Systemic Circulation (Heart ↔ Body)

Function

• Delivers oxygen and nutrients to all body tissues.
• Returns deoxygenated blood back to the heart.

Pathway

Left ventricle → Aorta → Body organs → Vena cava → Right atrium

Blood Vessels Involved

Aorta

• The largest artery in the body.
• Carries oxygenated blood from the left ventricle.
• Distributes blood to all body organs.

Vena Cava

There are two vena cavae:

• Superior vena cava – returns blood from the head, neck and arms.
• Inferior vena cava – returns blood from the lower part of the body.

Function

• Carry deoxygenated blood back to the right atrium.

🟤 Blood Supply to the Liver (Hepatic Circulation)

The liver receives blood from two different blood vessels because it performs many important functions.

Hepatic Artery

Function

• Carries oxygenated blood from the aorta to the liver.
• Supplies oxygen needed by liver cells for respiration.

Hepatic Portal Vein

Function

• Carries nutrient-rich blood from the small intestine to the liver.
• The blood contains absorbed glucose, amino acids and other nutrients.

Why does blood go to the liver first?

The liver:

• Stores excess glucose as glycogen.
• Processes absorbed nutrients.
• Removes harmful substances.
• Regulates the amount of nutrients entering the bloodstream.

Hepatic Vein

Function

• Carries blood away from the liver.
• Returns blood to the vena cava.
• The blood then flows back to the heart.

🟢 Blood Supply to the Kidneys (Renal Circulation)

The kidneys receive a large blood supply because they continuously filter the blood.

Renal Artery

Function

• Carries oxygenated blood from the aorta to each kidney.
• Brings blood containing waste products such as urea.

What Happens in the Kidneys?

The kidneys:

• Filter the blood.
• Remove urea.
• Remove excess water.
• Remove excess mineral ions.
• Help maintain a stable internal environment.

Renal Vein

Function

• Carries filtered blood away from the kidneys.
• Returns the blood to the vena cava.
• The cleaned blood then returns to the heart.

🔄 Complete Blood Flow Through the Body

The overall pathway of blood circulation is:

Body → Vena cava → Right atrium → Right ventricle → Pulmonary artery → Lungs → Pulmonary vein → Left atrium → Left ventricle → Aorta → Body

This cycle repeats continuously throughout life.

📊 Blood Vessels Connected to Major Organs

📊 Important Blood Vessels

🌟 Why Different Organs Need Their Own Blood Supply

• Lungs need blood for gas exchange.
• Liver needs blood to process nutrients and remove harmful substances.
• Kidneys need blood to filter waste products.
• Body tissues need oxygen and nutrients for respiration and growth.

Each organ receives blood through specific arteries and returns blood through specific veins, ensuring efficient transport throughout the body.

⚡ Quick Recap
Double circulation: Pulmonary (heart ↔ lungs) + Systemic (heart ↔ body).
Pulmonary artery: only artery with deoxygenated blood.
Pulmonary vein: only vein with oxygenated blood.
Liver: gets blood via hepatic artery & portal vein → sends out via hepatic vein.
Kidneys: renal arteries in, renal veins out.