3.Capillaries and Exchange of Materials
Learning Outcomes
Capillaries allow exchange of substances with tissues.
Pressure filtration of fluids through capillary walls. Similarity of tissue
fluid and blood plasma with the exception of plasma proteins. Tissue
fluid supplies cells with glucose, oxygen and other substances. Carbon
dioxide and other metabolic wastes diffuse out of the cells and into the
tissue fluid to be excreted
Much of the tissue fluid returns to the blood. Lymphatic vessels absorb
excess tissue fluid and return the lymph fluid to the circulatory system.
Capillaries
Blood is transported from arterioles to venules by passing through a
dense network of blood vessels called capillaries
All exchanges of substances between blood and living tissue takes place
through capillary walls
Capillary walls are composed of endothelium and are only one cell thick
Tissues
CO2
O2
CO2
O2
CO2
Artery
From
the
lungs
O2
To the
lungs
CO2
1. Oxygen
Capillary Network
CO2
O2
O2
2. Glucose
Vein
CO2
O2
1. Carbon Dioxide
2. Waste
Tissues
capillaries
Plasma and Tissue fluid
Plasma is a watery yellow fluid containing dissolved
substances such as glucose, amino acids, blood cells,
platelets and plasma proteins
Blood arriving at the arteriole end of a capillary bed is at a
higher pressure than blood in the capillaries
As blood is forced into the narrow capillaries, it undergoes
pressure filtration and much of the plasma is squeezed out
through the thin walls
This liquid is called tissue fluid
The only difference between plasma and tissue fluid is
that plasma has proteins
every living cell is bathed in tissue fluid . . .
Tissue fluid contains a high concentration of dissolved food, oxygen,
useful ions etc.
These diffuse, down a concentration gradient, into the surrounding cells
Carbon dioxide and other metabolic wastes diffuse out of the cells,
down a concentration gradient, into the tissue fluid to be excreted
Tissue
Tissuefluid
Fluid
Tissue fluid and Lymph
Much of the tissue fluid returns to the blood in the
capillaries at the venule end of the capillary bed
This is brought about by osmosis
Tissue fluid lacks plasma proteins so it has a higher water
concentration than the blood plasma
Some of the tissue fluid does not return to the blood in
the capillaries
This excess tissue fluid is absorbed by thin-walled
lymphatic vessels
When the tissue fluid is in the lymphatic vessel it is called
lymph
Lymphatic System
Tiny lymph vessels unite to form larger vessels
Flow of lymph is brought about by the vessels being compressed when
muscles contract during breathing, movement etc.
Larger lymph vessels have valves to prevent backflow
Lymph vessels return their contents to the blood via two lymphatic
ducts
These enter the veins coming from the arms
Try these questions . . .
1. What is tissue fluid?
2. Name a substance that passes from body cells into tissue fluid
3. Describe two methods by which tissue fluid returns to the
bloodstream
4. Describe the means by which lymph is forced along through the
lymphatic system
5. Which structures prevent the backflow of lymph in the lymphatic
system?
6. Which structures enable lymph to return to the blood circulatory
system?
Answers . . .
1. It is the liquid consisting of plasma and small, dissolved molecules
that is squeezed out of capillaries during pressure filtration
2. Carbon dioxide
3. It is returned to blood capillaries by osmosis and is absorbed into
the lymphatic system via thin-walled lymphatic vessels
4. When the vessels are pressed during muscular contraction, the
lymph is pushed along the lymph vessels
5. Valves
6. Two lymphatic ducts