The human circulatory system

Made up of organs that work together to move blood and dissolved substances around the body.

Substances are exchanged between the blood and body tissues at capillaries.

The heart

The heart is a muscular organ that pumps blood in a double-circulatory system:

• Right side: Pumps deoxygenated blood to the lungs.

• Left side: Pumps oxygenated blood around the body.

Structure:

• Atria: Two small chambers where blood enters the heart.

• Ventricles: Two larger chambers that pump blood out of the heart.

Blood flow:

• Oxygenated blood:

  • From lungs → left atrium (via pulmonary vein).

  • Left atrium → left ventricle → rest of the body (via arteries).

• Deoxygenated blood:

  • From the body → right atrium (via vena cava).

  • Right atrium → right ventricle → lungs (via pulmonary artery).

The natural heart rate:

• Controlled by pacemaker cells in the right atrium.

• Artificial pacemakers are electrical devices used to correct irregular heart rates.

The human heart

Blood vessels

Arteries:

• Carry blood away from the heart.

• Thick walls made of muscle and elastic tissue to withstand high pressure.

Capillaries:

• Connect arteries and veins.

• Very thin walls (one cell thick) for quick diffusion of substances.

Veins:

• Carry blood back to the heart.

• Thin walls as blood is at lower pressure.

• Contain valves to prevent backflow.

The three types of blood vessel

Blood

Blood is a tissue made up of:

Plasma:

• Yellow liquid that carries blood cells, nutrients, hormones, waste products (e.g., urea), and carbon dioxide.

Red blood cells:

• Biconcave shape, no nucleus, and contain haemoglobin.

• Transport oxygen by binding to haemoglobin.

White blood cells:

• Larger than red blood cells, with a flexible shape.

• Defend the body by destroying pathogens.

Platelets:

• Cell fragments responsible for blood clotting.

Additional maths skills