1. According to Canadian Blood Services, a unit of blood is approximately 450 ml. The amount of blood in one person is five litres or 10.5 pints. On average, 4.6 units of blood are required per patient.

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2. Blood sugar concentration, or glucose level, is tightly regulated in the human body. Normally, the blood glucose level is maintained between about 4 and 6 mmol/L (70 to 100 mg/dL). The total measurement of glucose in the circulating blood is therefore about 3.3 to 7g (assuming an ordinary adult blood volume of 5 liters). Glucose levels rise after meals and are usually lowest in the morning, before the first meal of the day.

http://en.wikipedia.org/wiki/Blood_sugar

How is it used?
The blood glucose test is ordered to measure the amount of glucose in the blood right at the time of sample collection. It is used to detect both hyperglycemia and hypoglycemia, to help diagnose diabetes, and to monitor glucose levels in persons with diabetes. Blood glucose may be measured on a fasting basis (collected after an 8 to 10 hour fast), randomly (anytime), post prandial (after a meal), and/or as part of an oral glucose tolerance test (OGTT / GTT). An OGTT is a series of blood glucose tests. A fasting glucose is collected; then the patient drinks a standard amount of a glucose solution to "challenge" their system. This is followed by one or more additional glucose tests performed at specific intervals to track glucose levels over time. The OGTT may be ordered to help diagnose diabetes and as a follow-up test to an elevated blood glucose.

The American Diabetes Association recommends either the fasting glucose or the OGTT to diagnose diabetes but says that testing should be done twice, at different times, in order to confirm a diagnosis of diabetes.

Most pregnant women are screened for gestational diabetes, a temporary form of hyperglycemia, between their 24th and 28th week of pregnancy using a version of the OGTT, a 1-hour glucose challenge. If either fasting glucose or a random glucose is above the values used to diagnose diabetes in those who are not pregnant, the woman is considered to have gestational diabetes and neither the screening nor the glucose tolerance test is needed. If the 1-hour level is higher than the defined value, a longer OGTT is performed to clarify the patient’s status.

Diabetics must monitor their own blood glucose levels, often several times a day, to determine how far above or below normal their glucose is and to determine what oral medications or insulin(s) they may need. This is usually done by placing a drop of blood from a skin prick onto a glucose strip and then inserting the strip into a glucose meter, a small machine that provides a digital readout of the blood glucose level.

In those with suspected hypoglycemia, glucose levels are used as part of the "Whipple triad" to confirm a diagnosis. (See “Is there anything else I should know?” section).

The urine glucose is seldom ordered by itself. At one time, it was used to monitor diabetics, but it has been largely replaced by the more sensitive and “real time” blood glucose. The urine glucose is, however, one of the substances measured when a urinalysis is performed. A urinalysis may be done routinely as part of a physical or prenatal checkup, when a doctor suspects that a patient may have a urinary tract infection, or for a variety of other reasons. The doctor may follow an elevated urine glucose test with blood glucose testing.

http://www.labtestsonline.org/understandin g/analytes/glucose/test.html