Tissue Fluid
small artery
small vein
cells
arteriole
venule
lymphatic
capillaries
tissue fluid
Tissue fluid is:
• constantly being formed at the arteriole end of capillary beds
• essential for the efficient exchange of materials between the blood and the cells
• is constantly being drained away from the cells by lymph vessels
Capillaries and Tissue Fluid Formation
The cells of the body are bathed in a fluid called tissue fluid
Tissue fluid is essential for the efficient exchange of materials
between the blood and the cells
Tissue fluid is formed at the arteriole end of the capillaries
Arteriole end
of capillary
Capillary
Tissue fluid
Body
Cells
Venule end
of capillary
Capillaries and Tissue Fluid Formation
Blood entering the capillaries from the arterioles has a relatively high
hydrostatic pressure (blood pressure) due to the pumping action of the ventricles
This hydrostatic pressure (4.3 kPa) tends to force water, ions and other small
molecules out of the capillary by filtration
Opposing this outward pressure is a tendency for water in the
less concentrated tissue fluid to move into the more
concentrated blood plasma in the capillaries by osmosis
Arteriole end
Venule end
of capillary
of capillary
Hydrostatic
pressure (kPa)
Osmotic
potential (kPa)
4.3
Capillary
FILTRATION
Tissue fluid
Body
Cells
Capillaries and Tissue Fluid Formation
The osmotic or solute potential of the blood is more negative then that of the
tissue fluid due to the presence of plasma proteins that are too large to pass
from the capillaries to the tissue fluid
The osmotic pull of water into the capillaries from the tissue fluid has a
pressure value of 1.8 kPa (the difference in solute potential between
the blood and the tissue fluid)
Arteriole end
of capillary
Hydrostatic
pressure (kPa)
Osmotic
potential (kPa)
Venule end
of capillary
4.3
- 3.3
- 1.5
Body
Cells
Capillary
Tissue fluid
Capillaries and Tissue Fluid Formation
At the arteriole end of the capillary there is, therefore, an outward hydrostatic
pressure of 4.3 kPa and an inward osmotic pull of 1.8 kPa
The outward hydrostatic pressure exceeds the inward osmotic pull and so
blood is filtered and fluid leaves the blood to form tissue fluid
Tissue fluid is filtered plasma but lacking the plasma proteins that are
too large to pass through the capillary pores
Arteriole end
Venule end
of capillary
of capillary
Hydrostatic
pressure (kPa)
Osmotic
potential (kPa)
4.3
Capillary
1.8
Body
Cells
Tissue fluid
Capillaries and Tissue Fluid Formation
As the blood moves from the arteriole to the venule end of the capillary, there is
a fall in the hydrostatic pressure but the solute potentials remain unchanged
The fall in hydrostatic pressure is the result of:
• loss of fluid at the arteriole end of the capillary
• blood moving further away from the pumping action of the heart
Blood flow in the capillaries tends to slow down allowing
time for the exchange of materials between
Arteriole end
Venule end
the blood and the tissues
of capillary
of capillary
Hydrostatic
pressure (kPa)
Osmotic
potential (kPa)
Capillary
1.8
Body
Cells
1.6
4.3
Tissue fluid
1.8
Capillaries and Tissue Fluid Formation
At the venule end of the capillary, the inward osmotic pull now
exceeds the outward hydrostatic pressure
Some of the filtered water returns to the capillary from
the tissue fluid by osmosis
Arteriole end
of capillary
Hydrostatic
pressure (kPa)
Osmotic
potential (kPa)
Venule end
of capillary
Capillary
1.8
Body
Cells
1.6
4.3
Tissue fluid
1.8
Capillaries and Tissue Fluid Formation
Tissue fluid is constantly being formed and therefore needs to be replaced
Dipping into the tissue fluid there is a network of tiny vessels called lymphatics
whose function is to drain surplus tissue fluid away from the cells
Larger lymph vessels transport
the lymph into the great veins L
y
and then into the heart
m
p
h
Venule end
Arteriole end
of capillary
of capillary
Capillary
Tissue fluid
Body
Cells
lymphatic
v
e
s
s
e
l
Capillaries and Tissue Fluid Formation
Tissue fluid is constantly being formed and therefore needs to be replaced
Dipping into the tissue fluid there is a network of tiny vessels called lymphatics
whose function is to drain surplus tissue fluid away from the cells
The drained tissue fluid (lymph)
is returned to the bloodstream
Arteriole end
of capillary
Larger lymph vessels transport
the lymph into the great veins L
y
and then into the heart
m
p
h
Venule end
of capillary
Capillary
Tissue fluid
Body
Cells
lymphatic
v
e
s
s
e
l
Summary of Tissue Fluid Formation
At the arteriole end of the capillary, the outward hydrostatic pressure is
Greater than the inward osmotic pull
Water, ions and small molecules are filtered out of the blood into the
spaces between the cells - this is tissue fluid
The loss of fluid from the blood leads to a fall in hydrostatic pressure as
the blood approaches the venule end of the capillary
At the venule end of the capillary, the inward osmotic pull now exceeds
the outward hydrostatic pressure and some of the water re-enters the
capillary by osmosis
Tissue fluid is drained away from the cells by the lymphatic system and returned
to the circulation near the heart
Arteriole end
Venule end
of capillary
of capillary
4.3
1.6
Hydrostatic
Capillary
pressure
(kPa)
1.8
Body
Cells
Tissue fluid
1.8