Blood and lymph

• Blood composition


  • 55% plasma - water, proteins, very little solutes
  • 44% erythrocytes
  • <1% leukocytes and platelets
• Blood functions


  • maintains a steady body temperature
  • maintains a steady pH (a buffer)
  • transporting - carbon dioxide, oxygen, hormones, digested food, food molecules from storage compounds, nitrogenous waste, platelets, leukocytes, antibodies
• Tissue fluid


  • fluid leaves the blood from the arterial end
  • large molecules remain in the blood (plasma proteins, erythrocytes)
  • tissue fluid re-enters the blood stream at the veins end
• Oncotic pressure 


  tendency of water to move into the blood stream by osmosis as a result of the plasma proteins conc
• Hydrostatic pressure


  Pressure caused by fluid within an enclosed system
• Tissue fluid


  • at arteriole end, high hydrostatic pressure forces fluid into the tissue
  • it is greater than oncotic pressure that is high anyway as the blood has more solutes compared to tissue fluid
  • at venous end, hydrostatic pressure is reduced as there is less water and oncotic pressure is still high
  • oncotic pressure is greater than hydrostatic pressure, water moves into blood by osmosis
• Tissue fluid


  • 90% of tissue fluid reenters the blood
  • 10% enters the lymphatic system
  • Substances dissolved in fluid - glucose, amino acids, ions, oxygen
• Lymphatic system


  • network of vessels, nodes and ducts that collect excess fluid from the blood
  • e.g. lymph nodes, tonsils, thymus gland and spleen
  • contains more leukocytes, waste and FEWER nutrients compared to tissue fluid
• Role of lymph


  • drains excess fluid from tissues
  • takes up cell debris, large particles and phagocytosed bacteria
  • lymphocyte storage in lymph nodes - for defence
• Problems with excess lymph

  • fewer amino acids for protein synthesis
  • less plasma proteins, lowered conc. , lower oncotic pressure, less reabsorption of tissue fluid