Hind Leys Biology
F211
Transport in animals 5.6
Blood, tissue fluid and lymph
Objectives
 Explain the differences between blood, tissue fluid and lymph.
 Describe how tissue fluid is formed from plasma.
Blood and tissue fluid
Blood is the liquid held in the blood vessels, consisting of blood cells in a watery
fluid called plasma.
The plasma contains many dissolved substances, including;
 Oxygen
 Carbon dioxide
 Salts
 Glucose
 Fatty acids
 Amino acids
 Hormones
 Plasma proteins
Examiner tip
Give examples of the
nutrients carried in the
blood at least once in
your answer- don’t just
write ‘nutrients’.
Similarly, give examples
of waste products
produced by cells,
rather than just
‘wastes’.
The cells include;
 Red blood cells/erythrocytes
 White blood cells/leucocytes
 Fragments/platelets
Tissue fluid bathes the cells of individual tissues
Tissue fluid is similar to blood, but does not contain most of the cells found in blood, nor
any of the plasma proteins. The role of tissue fluid is to transport oxygen and nutrients
from the blood to the cells, and to carry carbon dioxide and other waste products back
to the blood.
Tissue fluid formation
In the tissues, arteries branch into smaller arterioles, then into a network of capillaries.
These eventually link up with venules to carry blood back to the veins. Blood flowing
within organs or tissues is contained within the capillaries (Figure 1).
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Hind Leys Biology
F211
Transport in animals 5.6
Figure 1 Artery linked to vein by a
capillary bed
At the arterial end of a capillary,
the blood is under high pressure
due to the contraction of the
heart muscle. This is known as
hydrostatic pressure. It will
tend to push the blood out of the
capillaries. The fluid can leave
through the tiny gaps in the
capillary wall.
The fluid that leaves the blood consists of plasma with dissolved nutrients and oxygen.
All the blood cells and plasma proteins remain in the blood as these are too large to pass
through the gaps.
This fluid that leaves the capillary is known as the tissue fluid. This bathes the tissue
cells allowing exchange of gases and nutrients to occur across the cell surface
membrane. This exchange occurs by diffusion and facilitated diffusion. Oxygen and
nutrients enter the cells; carbon dioxide and other wastes leave the cells.
Return of tissue fluid to the blood
Of course, if the carbon dioxide is to be exhaled in the lungs, it must enter the blood in
the tissues. The tissue fluid itself also has some hydrostatic pressure, which tends to
push it back into the capillaries. Also, both the blood and tissue fluid contain solutes,
giving them a negative water
potential. The water
potential of the tissue fluid
is less negative that that of
the blood. This means that
water tends to move back
into the blood from the
tissue fluid by osmosis.
Figure 2 Formation of tissue fluid and its return to the blood
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Hind Leys Biology
F211
Transport in animals 5.6
At the venous end of the capillary, the blood has a reduced hydrostatic pressure, but
has a more negative water potential with respect to the tissue fluid, due to the plasma
proteins within it. This combined effect results in the movement of fluid back into the
capillary by osmosis. Dissolved substances such as carbon dioxide are carried with it.
Worked example
Figure 3 A single capillary showing the relative
hydrostatic and osmotic forces (in kPa)
In tissue fluid:
HP = 1.1
At venous end:
HP in capillary = 1.6
SP = -1.3
SP in capillary = -3.3
Effective HP = 1.6 – 1.1 = 0.5
At arterial end:
Effective SP = -3.3 –(-1.3) = -2
HP in capillary = 4.3
Effective blood pressure = 0.5 – 2 = -1.5
SP in capillary = -3.3
Fluid pushed back into capillary
Effective HP = 4.3 – 1.1 = 3.2
Effective SP = -3.3 – (-1.3) = -2
HP = Hydrostatic pressure
Effective blood pressure = 3.2 – 2 = 1.2
SP = Solute potential (= water potential)
Fluid is pushed out of capillary
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Hind Leys Biology
F211
Transport in animals 5.6
Formation of lymph
Not all of the tissue fluid returns to the blood. Some is drained away into the lymphatic
system. This consists of a number of vessels that are similar to capillaries. Small vessels in
the tissues drain excess fluid into larger vessels, which eventually feed the fluid back into the
blood system in the chest cavity.
Lymph fluid is similar to tissue fluid and contains the same
solutes. There will be less oxygen and fewer nutrients, as
these have been absorbed by the body cells. There will also
be more waste products from cell metabolism, including
carbon dioxide. It also contains more fatty material that has
been absorbed from the intestine.
Figure 4 SEM of a lymphocyte
An important feature of lymph is its high numbers of white
blood cells or lymphocytes. These are produced in the
lymph nodes which are swellings found at intervals along the
lymphatic system. Any bacteria and foreign material is filtered
from the lymph in the lymph nodes. The lymphocytes then
engulf and destroy these, and as such form part of the
immune system protecting the body from infection.
http://www.youtube.com/watch?v=1rVsonBiBHk Formation of lymph
Feature
Cells
Blood
Erythrocytes,
Tissue fluid
Some phagocytic white
leucocytes, platelets
blood cells
Proteins Hormones, plasma
Some hormones, proteins
Lymph
Lymphocytes
Some proteins
proteins
secreted by cells
Some transported as
None
More than in blood
Glucose 80-120 mg/100 cm3
Less
Less
Amino
acids
Oxygen
More
Less
Less
More
Less
Less
Carbon
dioxide
Little
More
More
Fats
lipoproteins
Table 1 Comparison of the location, direction and cause of flow, and composition of blood, tissue fluid and
lymph.
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Hind Leys Biology
F211
Transport in animals 5.6
http://www.youtube.com/watch?v=qTXTDqvPnRk Lymphatic system
http://www.youtube.com/watch?v=o04FwZVRvU8&feature=related Lymphocytes
1. Explain why blood contains many proteins that are not found in the tissue fluid or
lymph.
2. What produces the hydrostatic pressure of the blood?
3. Describe how fluid can pass through the capillary wall from the plasma to the tissue
fluid.
4. By which two routes does tissue fluid return to the blood stream?
5. What effect would thinner muscle in the left ventricle have on tissue fluid?
This work can be reinforced using pages 77-79 of your textbook.
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