vessel. In about 30 minutes the clot shrinks; thus pulling the end of the severed vessel
in to close the diameter of the vessel even further.
2-8. MECHANISMS OF BLOOD CLOTTING
The actual clotting mechanisms involve several steps--each step is essential to
STEP 1: The blood platelets release a substance that is known as
STEP 2: Thromboplastin reacts with calcium and another substance,
prothrombin, to form thrombin. Vitamin K is necessary for
the proper formation of prothrombin.
STEP 3: The thrombin formed acts as an enzyme to convert fibrinogen
to fibrin threads that eventually form the blood clot.
NOTE: For a more in-depth discussion of blood clotting you should locate and read a
physiology text that is appropriate to your level of understanding.
2-9. BLOOD PRESSURE
a. Introduction. Blood pressure is the force exerted by the blood as it is
pumped throughout the circulatory system. Blood pressure is needed by the body for
the perfusion and distribution of nutrients throughout the body. Blood pressure is
expressed in numerical values with the use of an instrument such as the
sphygmomanometer. Blood pressure is expressed as systolic blood pressure over
diastolic blood pressure (for example, 120/70). Systolic blood pressure is the pressure
of blood as it is being pumped from the heart. When the heart contracts, it is said to be
in systole. Diastolic pressure is the residual pressure of the blood because of the
elasticity of the blood vessels (when the heart is at rest).
b. Regulation. In order to regulate blood pressure to meet the immediate
needs of the body, the body is equipped with various systems that can change the
blood pressure both by a change in the size of the openings of the various blood
vessels and by a change in the volume of the blood (that is, blood plasma).
(1) Baroreceptors. Baroreceptors are located in the aortic arch of the aorta
and in the internal carotid arteries. Baroreceptors are really a series of specialized
neurons that function as rapidly acting blood pressure regulators. They sense changes
in blood pressure and act in a reflex manner to change both the rate and force of the
contraction of the heart and the size of the openings of the blood vessels.
(2) Chemoreceptors. Chemoreceptors are receptors which sense changes
in the oxygen content of the blood. They are located in high numbers in the aortic arch