The Systolic and Diastolic Pressures
When a doctor measures a patient’s blood pressure, he learns how it rises when the heart ejects blood and how low it falls while the heart is filling for its next beat. The high point is called thesystolic pressure because this is read during the phase of the cardiac cycle calledsystole.The low point is called the diastolic pressure because this is read during diastole.
The various devices usually used to measure blood pressure are called sphygmomanometers. Because these measure blood pressure in terms of the force needed to raise a column of liquid mercury, blood pressure readings are expressed as millimeters (mm) of mercury (Hg). Normal pressure for a man aged 25 might be written as 120 mm Hg systolic and 80 mm Hg diastolic, or as 120/80 mm Hg. The difference between the systolic and diastolic pressure is the pulse rate.
Hypertension, or high blood pressure
Persistent elevations of 15 mm systolic and 8 mm diastolic above the average “normals” for a patient’s age are usually regarded as hypertension.However, although it is known that the heart and kidneys will be damaged if the blood pressure is maintained too long or too high a level, doctors have not yet found any completely satisfactory way of dividing normal from abnormally high pressures. What is dangerously high for one patient may cause no apparent harm in another.
Hypotension, or low blood pressure
A persistent reduction below 110 mm systolic in a man or 100 mm systolic in a woman is usually regarded as hypotension at any age. A sharp drop in blood pressure causes such symptoms as dizziness, nausea, and fainting. But, hypotension, when persistent and not associated with any disease or injury, is usually not a cause of concern. Such hypotension, because it makes heart and kidney disease less likely, may even prolong life.
The life of someone who has lost too much blood may be saved by giving him a transfusion of blood from someone else. But if this blood is too different from his own, it might do him harm than good and could kill him. To keep this from happening, samples of blood must be tested to see whether they are too different. One important test shows whether blood belongs to Group A, to Group B, to Group AB, or to Group O.
Every human being is born with blood of one of these groups. Which group a child has depends on what he inherited from his parents.
These four blood types differ because of antigens (agglutinogens) found in red cells and of antibodies (agglutinins) found in blood serum. If red cells from one person are added to serum from another, and if this serum contains an agglutinin that reacts against an agglutinogen present in the cells, the cells will stick together in an antigen-antibody reaction called agglutination. When this happens inside blood vessels, the red cells are soon destroyed (in a process called hemolysis), releasing substances that then act like poisons.
The two agglutinogens are named A and B. The agglutinin against A is named alpha.The agglutinin against B is named beta.The four blood groups are named to show which kind of agglutinogen their red cells contain.
A person having Group A blood cannot be given Group B or Group AB blood because the beta agglutinin in his serum would react against the B in the transfused red cells. Group O blood might be safe, however, because its red cells have no B and the amount of serum that would be transfused would have too little alpha agglutinin to do much harm.
Because the red cells of Group O have no A or B, they will not be agglutinated by alpha or beta. This is why a person having Group O blood may be called a “universal donor;” his blood may be safe for patients having blood belonging to any of the three other groups. Similarly, a patient having Group AB blood may be called a “universal recipient”; it may be safe to give him blood belonging to any of the three other groups because his serum contains no alpha or beta to react against transfused red cells.