260 HUMAN PHYSIOLOGY 



When it is relaxed, the lumen of the vessels dilates, and the 

 resistance to outflow is diminished. This muscular tissue 

 of the arterioles acts as a stop-cock to the flow of blood 

 from the arteries to the capillaries. It is of great import- 

 ance 



1st. In maintaining the uniform pressure in the arteries. 



2nd. In regulating the flow of blood into the capillaries. 



During the functional activity of a part, a free supply of 

 blood in its capillaries is required. This is brought about 

 by a relaxation of the muscular coats of the arterioles leading 

 to the part. When the part returns to rest, the free flow of 

 blood is checked by the contraction of the muscular walls of 

 the arterioles. 



The action of the arterioles is well seen under the influence 

 of certain drugs (vaso-dilators and vaso-constrictors). If, 

 while a tracing of the arterial pressure is being taken, nitrite 

 of amyl is administered to the animal, it will be seen that the 

 skin and mucous membranes become red and engorged with 

 blood, while at the same time the arterial pressure falls. 

 Nitrites cause the muscular coat of the arterioles to relax, 

 and thus, by diminishing peripheral resistance, permit blood 

 to flow freely from the arteries into the capillaries. 



Salts of barium have precisely the opposite effect, causing 

 the skin to become pale from imperfect filling of the capil- 

 laries, and producing a marked rise in the arterial pressure. 

 Contraction of the muscles of the arterioles is produced, and 

 the flow of blood from arteries to capillaries is retarded. 



Not only is the state of the arterioles influenced thus by 

 drugs, but it is also affected by the internal secretions (see 

 p. 284) from certain organs. In all probability a vaso-dilator 

 substance is formed in the thyroid, while a powerful vaso- 

 constrictor is certainly produced in the medulla of the supra- 

 renals. 



The condition of the arterioles may be studied in many 

 different ways 



1st. By direct observation. 1. With the naked eye. A red 

 engorged appearance of any part of the body may be due to 

 dilatation of the arteriole leading to it. It may, however, be 

 due to some obstruction to the outflow of blood from the part. 

 2. With the microscope. In certain transparent structures, 



