Blood and Blood Vessels - White Blood Cells and Platelets (GCSE Biology)

Blood: White Blood Cells and Platelets

White Blood Cells (WBCs)

Function

• WBCs are the protective armies of the body, helping to protect against infection and foreign pathogens.
• Some WBCs (phagocytes) engulf and eat pathogens in a process called phagocytosis.
• Other WBCs (lymphocytes) produce antibodies or antitoxins to fight against pathogens.

Structure

• WBCs have flexible cell membranes, which can form extensions called pseudopodia, to help engulf foreign pathogens.
• WBCs also have many lysozyme enzymes in their cytoplasm, which are useful for breaking up and digesting foreign pathogens.
• Sometimes WBCs have a lobed nucleus (see diagram).

Platelets

Function

• Platelets are fragments of cells that are used to clot the blood. Without platelets, there would be excessive bleeding every time you cut yourself and had a wound. By clotting, platelets reduce blood flow around the wound. It also prevents pathogens from entering your wound.
• If you have too many platelets, then you are at risk of a blood clot (leading to things like stroke), but if you have too few platelets, you can bruise and bleed very easily.

Structure

• Like RBCs, platelets have no nucleus.
• Platelets are fragments (small parts of) cells which look very small in diagrams.
• Platelets have cell surface proteins, which enable them to clump together and close up a broken blood vessel.
• Platelets also have stored proteins, which they release to initiate a sequence of reactions called the clotting cascade. This causes something called fibrinogen to be converted to fibrin to form a mesh which causes the clotting of blood.

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Rate of Blood Flow: Calculations

Rate of blood flow is the volume of blood passing a certain point per unit time. The equation is as follows:

Practice Question: Jesse is investigating 3 models, each representing an artery, vein and capillary. The results are shown in the table below. Work out the rate of blood flow (in cm³ per second) for each model. And then suggest which model is likely to be the artery.

1. Make sure everything is in the correct unit form.
   The question wants us to give the answer in cm3 per second. Blood passed is in the correct units, but we need to convert the time elapsed to 120 seconds.
2. Work out rate for each model.
   Rate = volume of blood / number of minutes
   A. Rate = 100 / 120 = 0.83 cm³ / second
   B. Rate = 143 / 120 = 1.19 cm³ / second
   C. Rate = 127 / 120 = 1.06 cm³ / second
3. Final answer
   Arteries have blood travelling at the highest pressure and highest pace, therefore there blood flow will tend to be greatest in an artery. So we can confidently say that the artery model is likely to be Model B.