Blood Vessels

Since the blood in the veins are flowing at a slower flow rate (due to the loss of energy to overcome friction as it travels around the body) and at a lower blood pressure (due to the far distance from the pumping action of the left ventricle), and usually flowing upwards back to the heart against gravity, there is a high tendency for the blood to backflow ....
This is prevented by the presence of semi-lunar or venous valves (as shown in the longitudinal section of a vein above).
Note: Remember the skeletal muscular pump ... that would involve the contractions of skeletal muscles pressing aganist the walls of the valves to "squeeze" the blood up the vein towards the heart.

 Compare between cross-sections of artery and vein 

Can you differentiate the artery from the vein in the above diagram?

Artery:
- thicker wall (thicker elastic muscular layer)
- smaller (rounded) lumen
Vein:
- thinner wall (thinner elastic muscular layer)
- larger (irregularly-shaped) lumen

 Capillary 

Blood capillaries transport blood from the arterioles to the venules.
The lumens of blood capillaries are very tiny (really microscopic!), allowing them to come into close contact with many tissue cells (shorter diffusion distance).
The walls of the blood capillaries consists of only a one-cell thick endothelial layer (no outer fibrous layer ... no elastic muscular layer).
This an important structural feature since it effectively reduces the diffusion distance for the faster rate of exchange of substances (oxygen, nutrients and waste products such as carbon dioxide, urea) between the tissue cells and the blood.

Red blood cells flowing in blood capillaries

Notice from the above diagram that the lumens of the blood capillaries are so small that the red blood cells flow through in a single file ... and some of the red blood cells would need to change their biconcave shape into bell-shape (due to the flexibility & elasticity of their cell membranes .. to a certain extent that is!) in order to squeeze through more easily. Their bell-shape also allows a larger surface area for the exchange of substances by diffusion)

Note:
The branching network of blood capillaries (very large cross-sectional area) greatly reduces the velocity of blood flow through the blood capillaries, allowing more time for the exchange of substances between the tissue cells and blood.

One question ....
what directly surrounds the blood capillaries?
Tissue fluid!
Tissue fluid is water and other dissolved substances which are being forced out through the blood capillary walls due to high bllod pressure at the arteriole end of the blood capillary.

Another question ...
how is tissue fluid differ in composition from blood plasma ... same?
Nope! Tissue fluid does not contain plasma proteins.
how about tissue fluid and blood?
Tissue fluid does not have any red blood cells (too big to squeeze through the tiny pores of the blood capillary walls)