Blood Vessels & Blood (AQA GCSE Biology: Combined Science)

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 vessels structure

• 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 (with arteries having a narrow lumen, and the veins a wider one)
  • The lumen of the capillaries is extremely narrow, at the smallest the width of a red blood cell!

The structure of arteries, capillaries and veins diagram

The blood vessels form a continuous network; the structure of each allows it to carry out its function

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 fibers
  • The elastic fibers 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 fibers, 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

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
• For example; if 2460 ml of blood flows through a blood vessel in 4 minutes, the rate of blood flow = volume of blood / number of minutes = 2460 / 4 = 615 ml/minute
• From this you may be asked to determine how much blood flows through the same vessel in one hour = rate of blood flow (ml/min) x 60 = 615 x 60 = 36 900 ml

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

 Structure of the blood diagram

The human blood is composed largely of plasma and red blood cells, with white blood cells and platelets making up a smaller proportion of total volume

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

• In an exam, you may be shown a photograph or diagram and be asked to identify the types of blood cell present
• Observing and drawing cells under the microscope is also an important skill you will need to develop

 

This is an image of how blood might look under a microscope. White blood cells are larger than red blood cells, with platelets being smaller again