To determine the percentage of red blood cells in a whole blood sample, the cells must be separated from the liquid plasma. This separation can be rapidly accomplished by placing a tube of blood in a centrifuge. The force created by the spinning motion of the centrifuge causes the cells to be packed into the lower end of the tube. Then the quantities of cells and plasma can be measured, and the percentage of cells (hematocrit or packed cell volume) can be calculated.
1. To determine the percentage of red blood cells in a blood sample, follow these steps:
a. Lance the end of a finger to obtain a drop of blood. See the demonstration in Laboratory Exercise 38 for directions.
b. Touch the drop of blood with the colored end of a heparinized capillary tube. Hold the tube tilted slightly downward so that the blood will easily move into it by capillary action (fig. 39.1). To prevent an air bubble, keep the tip in the blood until filled.
c. Allow the blood to fill about two-thirds of the length of the tube.
d. Plug the blood end of the tube by pushing it with a rotating motion into sealing clay or by adding a plastic Critocap. By holding a finger over the tip of the dry end, blood will not drain out while sealing the blood end.
e. Place the sealed tube into one of the numbered grooves of a microhematocrit centrifuge. The tube's sealed end should point outward from the center and should touch the rubber lining on the rim of the centrifuge (fig. 39.1).
f. The centrifuge should be balanced by placing specimen tubes on opposite sides of the moving head, the inside cover should be tightened with the lock wrench, and the outside cover should be securely fastened.
g. Run the centrifuge for 3-5 minutes.
h. After the centrifuge has stopped, remove the specimen tube and note that the red blood cells have been packed into the bottom of the tube. The clear liquid on top of the cells is plasma.
i. Use a microhematocrit reader to determine the percentage of red blood cells in the tube. If a microhematocrit reader is not available, measure the total length of the blood column in millimeters (red cells plus plasma) and the length of the red blood cell column alone in millimeters. Divide the red blood cell length by the total blood column length and multiply the answer by 100 to calculate the percentage of red blood cells.
j. Record the test result in Part A of Laboratory Report 39.
2. Complete Part B of the laboratory report.
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.