BLOOD VESSELS Blood vessels form a closed circuit of tubes that transport blood between the heart and body cells. The several   types   of   blood   vessels   include   arteries, arterioles, capillaries, venules, and veins. Blood Vessel Classifications The  blood  vessels  of  the  body  fall  into  three classifications: ·   Arteries and arterioles—distributors ·   Capillaries—exchangers ·   Veins and venules—collectors Arteries and Arterioles Arteries are elastic tubes constructed to withstand high pressure. They carry blood away from the heart to all  parts  of  the  body.  The  smallest  branches  of  the arteries are called arterioles. The walls of arteries and arterioles  consist  of  layers  of  endothelium,  smooth muscle, and connective tissue. The smooth muscles of arteries and arterioles constrict and dilate in response to  electrical  impulses  received  from  the  autonomic nervous system. Capillaries At the end of the arterioles is a system of minute vessels that vary in structure, but which are spoken of collectively as capillaries. It is from these capillaries that   the   tissues   of   the   body   are   fed.   There   are approximately 60,000 miles of capillaries in the body. As the blood passes through the capillaries, it releases oxygen  and  nutritive  substances  to  the  tissues  and takes up various waste products to be carried away by venules. Venules continue from capillaries and merge to form veins. Veins and Venules Veins  and  venules  form  the  venous  system.  The venous  system  is  comprised  of  vessels  that  collect blood from the capillaries and carry it back to the heart. Veins   begin   as   tiny   venules   formed   from   the capillaries.  Joining  together  as  tiny  rivulets,  veins connect and form a small stream. The force of muscles contracting  adjacent  to  veins  aids  in  the  forward propulsion of blood on its return to the heart. Valves, spaced frequently along the larger veins, prevent the backflow of blood. The walls of veins are similar to arteries,  but  are  thinner  and  contain  less  muscle  and elastic tissue. Arterial System Arterial   circulation   is   responsible   for   taking freshly oxygenated blood from the heart to the cells of the  body  (fig.  1-35).  To  take  this  oxygenated  blood from the heart to the entire body, the arterial system begins  with  the  contraction  of  blood  from  the  left ventricle into the aorta and its branches. AORTA.—The aorta, largest artery in the body, is a   large   tube-like   structure   arising   from   the   left ventricle  of  the  heart.  It  arches  upward  over  the  left lung and then down along the spinal column through the thorax and the abdomen, where it divides and sends arteries down both legs (fig. 1-35). KEY  BRANCHES  OF  THE  AORTA.—Key arterial   branches   of   the   aorta   are   the   coronary, innominate  (brachiocephalic),  left  common  carotid, and left subclavian. The coronary arteries are branches of what is called the  ascending aorta. The coronary arteries  supply  the  heart  with  blood.  There  are  three large arteries that arise from the aorta as it arches over the  left  lung.  First  is  the  innominate  artery,  which divides into the right subclavian artery to supply the right arm, and the right common carotid to supply the right  side  of  the  head.  The  second  branch  is  the  left common carotid, which supplies the left side of the head. The third branch is the  left subclavian, which supplies the left arm. ARTERIES  OF  THE  HEAD,  NECK,  AND BRAIN.—The  carotid  arteries  divide  into  internal and  external  branches,  the  external  supplying  the muscle and skin of the face and the internal supplying the brain and the eyes. ARTERIES   OF  THE   UPPER   EXTREM- ITIES.—The   subclavian   arteries   are   so   named because they run underneath the clavicle. They supply the upper extremities, branching off to the back, chest, neck, and brain through the spinal column (fig. 1-35). The  large  artery  going  to  the  arm  is  called  the axillary.  The  axillary  artery  becomes  the  brachial artery as it travels down the arm and divides into the ulnar  and  radial  arteries.  The  radial  artery  is  the artery at the wrist that you feel when you take the pulse of your patient (fig. 1-35). ARTERIES  OF  THE  ABDOMEN.—In   the abdomen, the aorta gives off branches to the abdominal viscera, including the stomach, liver, spleen, kidneys, 1-28