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BarrelEditThe barrel holds the medication that will be injected. The markings on it indicate the amount of an injected dose. Insulin syringes, whether they are U 100 or U 40 syringes, have markings that are in International Units. Some insulin syringes have markings for half-units, some for whole units and still others for every 2 units; this depends on the size and type of syringe for insulin (and sometimes on the company who produces them). We can see from the barrel markings that this is a U 100 1/2 cc insulin syringe that holds up to 50 International Units of insulin, but we can only see the numeric unit markings in increments of 5 IU's. Both 3/10 cc (up to 30 IU's of U 100 insulin) and 1/2 cc (up to 50 IU's of U 100 insulin) syringes have numbers in increments of 5; 1 cc (up to 100 IU's of insulin) syringes are numbered in increments of 10. 
Other types of syringes use percentages of cc's (cubic centimeters) for their barrel markings. If you compare a 1 cc insulin syringe and a 1 cc tuberculin syringe, each of them holds a maximum of 1 cubic centimenter of fluid. The tuberculin syringe has barrel markings indicating fractions of 1 cubic centimeter--the insulin syringe measures out the cubic centimeter in terms of International Units (of insulin).
The plunger and its black rubber stopper fits inside the hollow barrel. Moving the plunger partly out of the barrel allows you to draw up a dose of insulin; pushing it back in lets you inject it. Using the back section of the rubber plunger as a guide will result in underdosing; the rubber plunger is solid-the drawn insulin is only in front of it. Measure from the front section of the plunger--the one closest to the needle area.
Needle and needle hubEdit
Some syringes for other uses have detachable needle hubs, but insulin syringes have theirs permanently attached. When they were reusable and made of glass, there was sometimes a problem with "dead space"-the area where the needle and its hub fit onto the syringe. It was common for this area to hold insulin that wasn't being injected when the shot was given. 
When you use only one insulin for injection with a syringe, the dead space can leave you slightly underdosed as this amount of insulin was still in the syringe when it was discarded, and also a waste of insulin.
But when someone was using two insulins in the same syringe, the situation could be just the opposite. . The dead space insulin from the fast-acting insulin that's drawn first would then be returned to you when the slower-acting insulin was drawn, so you'd be getting a slight overdose of your faster-acting insulin. It was capable of causing hypoglycemia for some people 
The dead space made it difficult to accurately measure the real number of insulin units, as the amount of it would vary from syringe to syringe when used for injections. 
The advent of plastic disposable insulin syringes that allowed the needle to be directly attached to the syringe barrel put an end to the dilemma. If you look carefully at syringe specifications, you'll see they all claim either a low or no dead space.  This means less dosage variability-more accuracy and continuity-from injection to injection whether you use one insulin or two in a syringe.
Other sub partsEditThe cap is to keep the needle sterile and prevent it being bent before use, as well as to prevent unplanned "sticks". The finger grip is for an easier hold of the syringe.
- ↑ Diabetes Mellitus. Washington State University School of Veterinary Medicine.
- ↑ Terumo 1 cc syringes.
- ↑ FAQs-Dead Space. Safeguard Medical.
- ↑ Definition of Dead Space in Syringes. Biotop.
- ↑ Easy Touch/MVP Vet Insulin Syringes-No Dead Space information.
- ↑ Corcoran JS, Yudkin JS. (1985). How inaccurate is insulin mixing? Patient variability and syringe dead space effect. Diabetic Medicine.
- ↑ Danger of Dead Space in U 100 Insulin Syringes. BMJ (1984).
- ↑ Arnott RD, Cameron MA, Stepanas TV, Cohen M. (1982). Insulin syringes: dangers of dead space. Australian Medical Journal.
- ↑ Evril Insulin Syringes-No Dead Space.
- ↑ Terumo Insulin Syringes-Low Dead Space.
- ↑ Monoject Insulin Syringes-Low Dead Space.
- ↑ BD Syringes-Low Dead Space.
- ↑ Easy Touch-No Dead Space.
- ↑ MVP Vet U 40 Insulin Syringes made by Easy Touch-No Dead Space.