Blood Vessels & Blood (AQA GCSE Biology): Revision Note

Blood vessels

Types of blood vessels

• The body contains three different types of blood vessels:

  • Arteries: transport blood away from the heart (usually at high pressure)

  • Veins: transport blood to the heart (usually at low pressure)

  • Capillaries: links arteries to veins within the tissues of the body

Blood vessel structures

• The walls of each type of blood vessel have a structure that relates to the function of the vessel

• Blood flows through the lumen of a blood vessel

• The size of the lumen varies depending on the type of blood vessel: arteries have narrow lumens and the veins wider lumens

  • The lumen of the capillaries is extremely narrow-at their smallest just the width of a single red blood cell

The structure of arteries, capillaries and veins diagram

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How structure relates to function

• Arteries must withstand and maintain high pressures from the contracting and relaxing heart

  • Their thick walls contain collagen, smooth muscle and elastic fibres

  • The elastic fibres allow expansion and recoil, maintaining high blood pressure alongside a narrow lumen

• Veins receive low-pressure blood from capillaries and return it to the heart

  • They have thinner walls with fewer layers of collagen, smooth muscle and elastic fibres, but a much larger lumen

  • Veins contain valves to prevent backflow

• Capillary walls consist of a single layer of endothelial cells, minimising the diffusion distance for oxygen and carbon dioxide

  • These walls have pores that allow blood plasma to leak out and form tissue fluid

Rate calculations: Blood flow

Calculating the rate of blood flow

• The rate of blood flow can be calculated if the volume of blood flow and the time is known

Structure & function: Blood

What is blood?

• The role of blood in the body is to transport useful substances to every cell of the body, and to remove harmful waste substances

  • It also plays a vital role in transferring heat from “active” organs to cooler parts of the body (such as the extremities – hands and feet)

• Blood is a tissue consisting of the fluid plasma (which is largely water with dissolved substances in it)

• Red blood cells, white blood cells and platelets are suspended in blood plasma

 Components of the blood

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Red blood cells

• Red blood cells (RBCs) are cells with a distinctive biconcave disc shape

• This shape is a result of RBCs not having a nucleus

  • The biconcave shape gives RBCs a large surface to volume ratio; this is a key adaptation to maximise the efficiency of diffusion of gases into and out of the cell

• The cytoplasm of an RBC is packed with the protein haemoglobin

• Oxygen binds reversibly with haemoglobin, forming the red pigment oxyhaemoglobin:

  • oxygen + haemoglobin ⇌ oxyhaemoglobin

White blood cells

• White blood cells (WBCs) are part of the immune system, responsible for defending the body from infection by recognising and destroying pathogens

• WBCs defend the body in three particular ways:

  • Phagocytes engulf and digest pathogens, destroying them

  • Lymphocytes produce specific antibodies that help enhance phagocyte activity by sticking them together (clumping) or disabling pathogens

  • Some lymphocytes produce a type of antibody called an antitoxin which is able to bind to toxic substances produced by pathogens, neutralising them

• WBCs have a variety of adaptations:

  • Phagocytes have a lobed nucleus and are autonomous - they leave the blood and patrol the tissues

  • Lymphocytes have a large nucleus and can produce antibodies extremely quickly

Platelets

• Platelets are fragments of cells (they contain cytoplasm but no nucleus)

• When damage to a blood vessel occurs, the platelets are involved in forming a blood clot to prevent blood loss

• Individuals with insufficient platelets cannot clot their blood effectively – this can be life-threatening if excessive damage occurs

Recognising blood cells

• An exam question may include a photograph or diagram and ask the identity of the types of blood cell present

• Observing and drawing cells under the microscope is an important skill to develop

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