Blood, tissue fluid and lymph

• Functions of the blood:

  Transport of:

  • oxygen to, and carbon dioxide from respiring cells
  • digested food from the small intestine
  • nitrogenous waste products from the cells to the excretory organs
  • chemical messages (hormones)
  • food molecules from storage compounds to the cells that need them
  • platelets to damaged areas
  • cells and antibodies involved in the immune response
• oncotic pressure= the tendency of water to move into the blood by osmosis
• hydrostatic pressure= as blood flows through the arterioles into the capillaries it is still under pressure from the surge of blood that occurs every time the heart contracts
• At the arterial end of the capillary, the hydrostatic pressure forcing fluid out of the capillaries is relatively high.
• Lymph= tissue fluid that does not return to the capillaries and instead drains into a system of blind-ended tubes called lymph capillaries.
• Lymph is similar in composition to plasma and tissue fluid but has less oxygen and fewer nutrients. It also contains fatty acids, which have been absorbed into the lymph from the villi of the small intestine.
• Along the lymph vessels are the lymph nodes.
• The lymph capillaries join up to form larger vessels. The fluid is transported through them by the squeezing of body muscles.
• Eventually lymph returns to the blood, flowing into the right and left subclavian veins.
• Lymphocytes build up in the lymph node when necessary and produce antibodies, which are then passed into the blood.
• Lymph nodes also intercept bacteria and other debris from the lymph, which are ingested by phagocytes found in the nodes.
• The lymphatic system plays a major role in the defence mechanisms of the body.
• Hydrostatic pressure = pressure exerted by a liquid
• Oncotic pressure= the tendency of water to move into the blood via osmosis
• As blood enters the capillaries from the arterioles, the smaller diameter results in high hydrostatic pressure. This pressure forces water, amino acids, fatty acids, ions and oxygen out of the capillaries at the arterial end. The solution that has been forced out is tissue fluid.
• Large molecules (e.g. plasma proteins) remain in the capillaries and therefore lower the water potential of the blood. This will result in high oncotic pressure.
• Tissue fluid does not contain erythrocytes as erythrocytes are too large and cannot change shape to fit between the small gaps between endothelium cells.
• Role of haemoglobin in transporting oxygen:

  • haemoglobin has a high affinity for oxygen
  • oxygen binds to haemoglobin in the lungs
  • this forms oxyhaemoglobin
  • oxygen is released in tissues where it is needed
• How hydrogencarbonate ions are produced in erythrocytes:

  • carbon dioxide diffuses into erythrocytes
  • carbon dioxide combines with water to form carbonic acid
  • carbonic anhydrase catalyses this reaction
  • carbonic acid dissociates to form hydrogencarbonate ions
• The Bohr effect:

  • reduces haemoglobin's affinity for oxygen
  • so more oxygen is released into tissue
• Blood is made from cells (red blood cells, white blood cells and platelets) and blood plasma (glucose, amino acids, mineral ions, oxygen and plasma proteins).
• Tissue fluid = the fluid that passes out of the blood in the capillaries and bathes the tissue cells.
• Tissue fluid- transfers molecules like oxygen and glucose to the tissue cells. Waste molecules from the tissue cells (like carbon dioxide) pass into the tissue fluid which is reabsorbed by the blood in the capillaries at the venous end.
• Hydrostatic pressure forces tissue fluid out of the blood at the arterial end.
• Oncotic pressure = the tendency of water to move back into the blood by osmosis
• Plasma proteins are hydrophilic so lower the water potential of the blood plasma.
• At the arterial end, hydrostatic pressure is greater than oncotic pressure so tissue fluid is forced out of the capillary through the gaps between endothelial cells (ultrafiltration).
• At the venous end, hydrostatic pressure is less than oncotic pressure. Hydrostatic pressure is low because a large amount of water has left the blood. However, oncotic pressure is still high due to the plasma proteins in the blood plasma. This causes water to move back into the blood via osmosis.
• Most of the tissue fluid is reabsorbed into the blood. The rest drains into lymph capillaries and then into lymph vessels.