BloodCambridge IGCSE Biology: Revision notes

Blood vessels have three main types, each with distinct structural adaptations:

Arteries carry blood away from the heart at high pressure, so they have thick, muscular walls containing elastic fibres to withstand and recoil from pressure waves. Their narrow lumen maintains high pressure.

Veins return blood to the heart at low pressure. Their thin walls cannot support high pressure, but their wide lumen offers less resistance to flow. Valves prevent backflow of blood, which is crucial since veins rely on muscle contractions and breathing movements rather than heart pressure.

Capillaries are the smallest blood vessels, consisting of a single layer of endothelial cells. This allows efficient exchange of substances between blood and tissues.

Blood pressure is the force exerted by blood on vessel walls. It is highest in arteries and decreases as blood moves through the circulatory system.

In arteries:

• Thick, muscular walls contain elastic fibres that stretch when blood is ejected from the heart
• These elastic walls recoil (bounce back), helping to maintain pressure between heartbeats
• The narrow lumen concentrates the force of blood over a smaller area, keeping pressure high
• This structure allows arteries to withstand systolic and diastolic pressure changes

In veins:

• Thin walls cannot withstand high pressure, but they don't need to—pressure here is low
• The wide lumen spreads the force over a larger area, reducing pressure further
• Valves are essential because low pressure alone cannot push blood back to the heart; valves prevent backflow when muscles relax
• Veins rely on skeletal muscle contractions and breathing movements to push blood forward

The relationship between pressure and vessel structure is a key principle: high pressure = need for thick, elastic walls; low pressure = thin walls with valves needed for directional flow.

Capillaries are specialised for exchange of materials between blood and body tissues. Their structure is perfectly adapted for this function:

Structural features enabling exchange:

• Single-cell-thick walls: Only one layer of endothelial cells separates blood from tissue fluid, minimising diffusion distance
• Extremely narrow lumen: Red blood cells must squeeze through single-file, slowing blood flow to allow time for exchange
• Extensive branching networks: Large total surface area relative to their length increases opportunity for material exchange
• Permeable walls: Gaps between endothelial cells allow small molecules to move out, whilst large molecules like proteins and cells remain in the blood

Materials exchanged at capillaries:

• Oxygen and glucose diffuse OUT of capillaries into tissues (down concentration gradients)
• Carbon dioxide and other waste products diffuse IN from tissues into blood (down concentration gradients)
• Tissue fluid bathes cells, allowing indirect exchange between blood and individual cells

Without capillaries' unique structure, this vital exchange could not occur efficiently. The thin walls, narrow lumen, and large surface area work together to maximise diffusion rates.

Students must identify and locate these major blood vessels:

Heart and lungs circulation:

• Aorta: Largest artery, carries oxygenated blood away from the left ventricle to the body
• Vena cava (superior and inferior): Large vein bringing deoxygenated blood from the body back to the right atrium
• Pulmonary artery: Carries deoxygenated blood from the right ventricle to the lungs
• Pulmonary vein: Carries oxygenated blood from the lungs back to the left atrium

Kidney circulation:

• Renal artery: Carries blood (with urea and excess ions) into the kidney for filtration
• Renal vein: Carries filtered blood (with reabsorbed glucose and ions) away from the kidney

Liver circulation:

• Hepatic artery: Carries oxygenated blood from the aorta into the liver
• Hepatic vein: Carries blood away from the liver back to the vena cava
• Hepatic portal vein: Carries nutrient-rich blood directly from the small intestine to the liver (this is a vein but carries oxygenated blood—unusual!)

These vessels are the main routes through which blood circulates. Understanding their names and locations is essential for exam questions on circulation routes.

The hepatic portal system is a unique circulatory arrangement that connects two capillary networks (intestine and liver) without passing through the heart first.

Structure of the hepatic portal system:

• Blood leaves the small intestine capillaries via the hepatic portal vein
• This vein carries nutrient-rich, deoxygenated blood directly to the liver
• The liver has two blood supplies: the hepatic portal vein (75% of blood flow, nutrient-rich but low oxygen) and the hepatic artery (25% of blood flow, oxygenated)
• Blood leaves the liver via the hepatic vein into the vena cava

Why the hepatic portal system is important:

1. First-pass metabolism: The liver receives nutrients from the intestine before they enter general circulation, allowing processing and storage
2. Detoxification: Any harmful substances absorbed in the intestine are filtered by the liver before reaching the rest of the body
3. Regulation of blood glucose: The liver receives glucose-rich blood and can store excess as glycogen or release glucose when blood levels drop

This is a specialised adaptation that prioritises the liver's role in nutrient processing and homeostasis. No other major organ has this unique dual blood supply arrangement.