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What is Blood?

  • Blood is a suspension of cells in a pale liquid called plasma
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Blood Components

  • Platelets- Cell fragments that are involved in blood clotting and plugging breaks in blood vessels 
  • Cells
  • Plasma- Transports blood cells, glucose, amino acids and other products of digestion, ions, CO2, urea, heat and clotting factors etc.
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Blood Cells


  • Small diameter, no nucleus, biconcave disc shape and packed with haemoglobin
  • Adapted for carriage of oxygen
  • No nucleus- more space for haemoglobin
  • Biconcave shape increases SA over which gas exchange can occur

Polymorphs (Macrophages)

  • Cells with a multi lobed nucleus and granular cytoplasm- Most common WBC
  • Short-lived, numerous cells can squeeze between endothelial cells of capillaries at the site of infection and engulf bacteria by phagocytosis


  • Large cells with a kidney shaped nucleus- least common WBC
  • Develop into macrophages and are involved in phagocytosis- longer lived than polymorphs


  • Huge nucleus, little cytoplasm- present in large numbers
  • B- Cells: produce antibodies
  • T- Cells: Involved in cell-mediated immunity- destroy infected cells
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Tissue Fluid

  • Fluid that lies immediately outside the capillaries and surrounds the cells of tissues
  • It has many functions including osmoregulation of cells, and facilitating transport of substances between blood and body cells
  • Tissue fluid supplies O2, glucose, amino acids etc. to diffuse from the tissues back into the capillaries
  • As blood travels through arteries, arterioles and then into the arterial end of the capillary network, narrowing of vessels creates a hydrostatic pressure which is strong enough to force liquid and small molecules out of capillaries- plasma proteins and blood cells are too large to be filtered by ultrafiltration
  • Hydrostatic pressure is opposed by a lower water potential of the blood as plasma proteins pull tissue fluid back by osmosis
  • Hydrostatic pressure of tissue fluid opposes inward force of liquid from capillaries
  • At arterial end of capillary network, hydrostatic pressure of blood exceeds other two forces, and liquid rich O2 is filtered out of blood and into tissue fluid that bathes the cell
  • O2 and other materials then enter the cells by diffusion
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Blood Clotting

1. Damage to a blood vessel

2. Platelets activated and release clotting factors inc. thromboplastin (platelets can form a plug to seal minor damage)

3. Platlets catalyse the conversion of prothrombin to thrombin with the addition of calcium ions, vitamin K and clotting factors

4. Thrombin catalyses the conversion of fibrinogen to fibrin

5. Fibrin forms a mesh that traps RBCs to form a clot

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