tissue fluid

• What is tissue fluid?

  a watery substance containing glucose, amino acids, oxygen and other nutrients

  it supplies these to the cells, while also removing any waste materials
• What types of pressure influence formation of tissue fluid?

  hydrostatic pressure and oncotic pressure
• hydrostatic pressure?

  pressure exerted by a fluid at rest

  higher at the arterial end of capillary than venous
• oncotic pressure?

  changing water potential of the capillaries as water moves out, induced by proteins in the plasma
• How is tissue fluid formed?

  as blood is pumped through increasingly small vessels, hydrostatic pressure is greater than oncotic pressure, so fluid moves out of the capillaries

  it then exchanges substances with the cells
• How does tissue fluid differ from blood and lymph?

  tissue fluid is formed from blood, but does not contain red blood cells, platelets and various other solutes usually present in blood

  after tissue fluid has bathed cells it becomes lymph, and thus contains less oxygen and nutrients and more waste products
• Tissue fluid formation p1:

  1. at the start of the capillary bed, nearest the arteries the hydrostatic pressure inside the capillaries is greater than the hydrostatic pressure in the tissue fluid -> this difference in hydrostatic pressure forces fluid out of the capillaries and into the spaces around the cells, forming tissue fluid
  2. as fluid leaves, the hydrostatic pressure reduces in the capillaries, so the hydrostatic pressure is much lower at the end of the capillary bed that's nearest to the venules
• Tissue fluid formation p2:

  • 3) at the venule end of the capillary bed, the water potential in the capillaries is lower than the water potential in the tissue fluid due to the fluid loss from the capillaries and the high oncotic pressure
  • this means some water re-enters the capillaries from the tissue fluid at the venule end by osmosis
• What happens to excess tissue fluid?

  this extra fluid eventually gets returned to the blood through the lymphatic system

  1. the smallest lymph vessels are the lymph capillaries
  2. excess tissue fluid passes into lymph vessels, once it's inside it's called lymph
  3. valves in the lymph vessels stop the lymph going backwards
  4. lymph gradually moves towards the main lymph vessels in the thorax (chest cavity) - here it's returned to the blood, near the heart
• What does tissue fluid contain?

  water

  dissolved solutes

  very few white blood cells

  very few proteins
• What does tissue fluid not contain?

  red blood cells

  platelets
• What does lymph contain?

  white blood cells

  water

  dissolved solutes

  antibodies
• What does lymph not contain?

  red blood cells

  platelets

  proteins (except antibodies)
• What does blood contain?

  red blood cells

  white blood cells

  platelets

  proteins

  water

  dissolved solutes
• What is plasma?

  a straw-coloured liquid that constitutes around 55% of the blood

  plasma is composed of 95% water

  water is a good solvent thus many substances can dissolve in it, allowing the, to be transported around the body
• As blood passes through the capillaries what happens?

  some plasma leaks out through gaps in the walls of the capillary to surround the cells of the body

  this results in the formation of tissue fluid
• How does the composition of plasma and tissue fluid differ?

  fluid contain far fewer proteins as they’re too large to fit through gaps in the capillary walls and so remain in the blood
• What occurs in tissue fluid?

  Exchange of nutrients, gases, and waste products between cells and the blood

  for example, carbon dioxide produced in aerobic respiration will leave a cell, dissolve into the tissue fluid surrounding it, and then move into the capillary
• What does the volume of liquid that leaves the plasma to form tissue fluid depend on?

  hydrostatic and oncotic pressure
• What is hydrostatic pressure?

  the pressure exerted by a fluid e.g blood

  the hydrostatic pressure in this example is the blood pressure generated by the construction of the heart muscle
• What is oncotic pressure?

  the osmotic pressure exerted by plasma proteins within a blood vessel
• What do plasma proteins do?

  lower the water potential within the blood vessel, causing water to move into the blood vessel by osmosis
• What is happening at the arterial end?

  when blood is at the arterial end of a capillary the hydrostatic pressure is great enough to force fluid out of the capillary

  proteins remain in the blood as they are too large to pass through the pores in the capillary wall

  the increased protein content creates a water potential gradient between the capillary and tissue fluid

  the hydrostatic pressure is greater than the osmotic pressure so water moves out of capillaries to the tissue fluid
• What is happening at the venous end?

  the hydrostatic pressure within the capillary is reduced due to increased distance from the heart and the slowing of blood as it passes through capillaries

  the water potential gradient between the capillary and the tissue fluid remains the same as the arterial end

  at the venous end the osmotic pressure is greater than the hydrostatic pressure and water begins to flow back into the capillary from tissue fluid
• Roughly 90% of the fluid lost at the arterial end of the capillary is reabsorbed at the venous end

  The other 10% remains as tissue fluid and is eventually collected by lymph vessels and returned to the circulatory system
• What occurs if blood pressure is high (hypertension)?

  the pressure at the arterial end is even greater

  this pushes more fluid out of the capillary and fluid becomes to accumulate around the tissues

  this is called oedema
• What would happen if plasma proteins were not removed from tissue fluid?


  they could lower the potential of the tissue fluid and prevent the re absorption of water into the blood in the capillaries