Healthy blood flow

When observing the tail fin of a small fish with a microscope, the basis for judging arterioles, venules and capillaries is: arterioles are blood vessels flowing from the main body to branches, and the blood flows fastest; The blood vessels formed by the branches are venules, and the red blood cells pass through capillaries in a single row and are connected with the smallest arteriovenous. The characteristics of capillaries are: the lumen is the thinnest, and only red blood cells can pass through it in a single line; The tube wall is the thinnest and consists of only one layer of epithelial cells. The blood flow rate is the slowest; These characteristics are conducive to the exchange of substances between blood and tissues and cells. As can be seen from the figure, blood is ①→→→→ ③, so ① is arteriole, ② is capillary and ③ is venule.

(1)① artery; ② The displayed blood vessels are capillaries; ③ The blood vessels shown are venules.

(2)① The blood vessel velocity is the fastest; Red blood cells line up in blood vessels, which is indicated by symbol ②.

(3) The tail fin of small fish contains pigment, which is dark in color and has a dark field of vision. Therefore, it is necessary to observe the blood flow in the tail fin of small fish with a low-power microscope, which is brighter than a high-power microscope. Small fish live in water and get dissolved oxygen in the water through gill breathing. Therefore, in the experiment of observing the blood flow in the tail fin of small fish, it is necessary to wrap the gill cover and trunk of the head with soaked cotton wool in order to keep the fish breathing normally.

So the answer is:

( 1)③

(2)D？

(3) less pigment; ? Cover the gill cover and trunk of small fish with wet cotton.