2. Blood Vessels and their Functions

3. Questions:
State two factors that make it more likely that an organism will have a circulatory pump such as the heart.
What is the main advantage of the double circulation system found in mammals?

4. Learning Objectives:
What are the structures of arteries, arterioles and veins?
How is the structure of each of the blood vessels related to its function?
What is the structure of capillaries and how is it related to their function?

5. Blood Vessels Arteries: Carry blood away from the heart
Arterioles: Control blood flow from arteries to capillaries
Capillaries: Link arterioles to veins
Veins: Carry blood towards the heart

7. Structure of Blood Vessels
Tough outer layer – resists pressure
Muscle layer – can contract and control the flow of blood
Elastic layer – can stretch and recoil to maintain blood pressure
Endothelium – smooth layer to prevent friction
Lumen – not a layer – a cavity

8. Structure of Arteries Thick muscle layer – control the flow of blood
Thick elastic layer – smooth surges from the heart
No valves

9. Structure of Arterioles
Thicker muscle layer than arteries
Thinner elastic layer than arteries
No valves

10. Structure of Veins
Thin muscle layer
Thin elastic layer
Valves

11. Structure of Capillaries
No muscle
No elastic
No valves
Thin layer of cells only

12. Capillary Structure to Function
Thin layer of cells – short diffusion distance.
Numerous and highly branched – large SA for diffusion.
Narrow diameter – keep all cells close by.
Narrow lumen – bring RBC close to the cells = short diffusion distance.
Spaces between cells – allow WBC to escape.

13. Capillary – endothelium – large number – large surface area for exchange
Wall - one cell thick – short diffusion distance
Endothelium is continuous throughout circulatory system
Capillary
Artery
Vein
Narrow lumen; High pressure
Highly elastic – expand and recoil
Thick muscular wall – to withstand force; more elastic fibres (recoil)
No valves (except aortic and pulmonary semilunar at the start)
Oxygenated blood from heart – except pulmonary artery to lungs
Pulsatile blood flow (expansion + recoil)
Pulse can be felt – e.g. wrist
Wide lumen; Low pressure
Thin wall - less elastic and less muscular
Valves (semilunar) – prevent backflow
Deoxygenated blood to heart from tissues - except pulmonary vein from lungs
Non pulsatile – smooth flow of blood

14. Tissue Fluid

15. Tissue Fluid What is the role of tissue fluid?
It is the fluid which allows the exchange of substances between the blood and cells
What substances are found in tissue fluid?
glucose, amino acids, fatty acids, salts and oxygen = all delivered to the cells.
carbon dioxide and other waste substances = removed from the cells.

16. Hydrostatic Pressure
As the capillaries are narrower than the arterioles, a pressure builds up which forces tissue fluid out of the blood plasma = hydrostatic pressure.
This pressure is resisted by:
Pressure of the tissue fluid on the capillaries (from the outside)
The lower water potential of the blood (caused by plasma proteins – too large to leave the blood)
Overall, pressure pushes tissue fluid and small molecules out of the capillary, leaving cells and large proteins behind = ultrafiltration.

17. Return of tissue fluid
Most tissue fluid is returned to the blood plasma via the capillaries.
Hydrostatic pressure at the venule end of the capillary is higher outside the capillary and tissue fluid is forced back in.
Osmotic forces (resulting from the proteins in the plasma) pull water back into capillaries.
Remaining tissue fluid enters the lymph vessels – drain back into the veins close to the heart.

18. Hydrostatic pressure

19. Lymph System

20. Lymph Lymph is moved by: Hydrostatic pressure
Contraction of body muscles (aided by valves in the lymph vessels)