Definition
One type of syringe comprises two parts: (1) the plunger and (2) the barrel. Having these essential parts, we call this syringe the 2-part syringe.
Basically, a syringe is a medical device used to inject or remove fluids from the body. That fluid could be in the form of blood or medication.
Parts
Plunger
This part is responsible for pushing or pulling any form of liquid in or out of the barrel. It is basically made up of a straight plastic rod which is in turn made up of polypropylene. So it is actually a piston-type rod that could go down and up the barrel.
Barrel
The barrel is the hollow and tube-like part of the syringe that holds the fluid to be injected or aspirated.
If the plunger is pushed down the barrel, the liquid will be forced out of the syringe. If it is pulled out, the fluid will be drawn inside the barrel. The barrel has an opening at the bottom end. This opening is provided with a tapering tip where the needle is eventually attached.
To know the volume of fluid currently inside the syringe, we can find the graduation lines marked on the side of the barrel. These graduation lines are expressed in terms of milliliters or fractions of a milliliter, depending on the size of the syringe. The larger the size is, the more prominent will be the interval between the graduation lines.
Types
The syringe tips could be in the form of Luer-slip.
Luer-slip
In the Luer-slip syringe tip, the needle is held in place by the force of friction. The needle is reasonably secure, but there is a possibility that it may come off if not firmly or snugly attached or if considerable pressure is applied on it.
Under this type of 2-part syringe, there are two sub-types: (1) the centric and (2) the eccentric.
In the centric style, the syringe tip is at the center of the bottom end.
In the eccentric type, the syringe tip is off-center. This is used when the needle needs to be parallel with the injection plane, such as in the intradermal route of administration or puncturing superficial blood vessels.
Sizes of syringes
Syringes are available in different sizes ranging from 1 to 60 ml. The basic rule is that when the size of the syringe to be used is being determined, the capacity of the syringe, which is next in a size larger than the volume to be measured, should be selected. For instance, if the fluid volume to be injected is 2.3 ml, then a 3 ml syringe should be used. If 3.8 ml of fluid needs to be injected, then a 5 ml syringe should be selected.
Through this method, the graduation marks on the syringe to be used will be in the smallest possible increments for the volume to be measured, leading to a more accurate measurement of the needed volume. In addition, as a precaution, a syringe should not be filled to its maximum capacity when aspirating a fluid because the plunger may be dislodged from the barrel.
Hypodermic needles
A 2-part syringe cannot discharge its essential functions if a needle is not attached to its tip. Thus, there is a need to discuss the proper selection of a needle.
Parts of a needle
There are three essential parts of a needle: (1) the hub, (2) the shaft, and (3) the bevel.
The hub is located at one end of the needle, and it is the structure that attaches to the syringe tip. The hub is made of PP.
The shaft is the long and slender stem of the needle that is beveled at one end to form a point. It is made of stainless steel.
The bore of the needle shaft, which is hollow and cylindrical, is known as the lumen.
Needle size
The size of a needle is based on the length and gauge. The length is measured in inches from the meeting point of the hub and the shaft up to the tip of the point. The lengths of needles range from 5/16 to 3 ½ inches. For some particular uses, some needles are available in longer lengths. On the other hand, the gauge of a needle that actually measures the lumen’s size ranges from 31 (the finest) up to 13 (the largest).
Selection of the needle size
In the selection of the needle size to use, two bases will be considered.
(1) the viscosity or rate of flowing of the solution and
(2) the nature of the rubber closure on the vial(s) of the medications.
A fluid is said to be very viscous if its rate of flowing is very slow. Thus, an oily medication is considered a viscous substance because it flows very slowly. For drugs considered viscous, needles with larger lumens (lower gauges) should be used.
If the rubber closure is easier to puncture, then needles with smaller lumens (higher gauges) should be used. Suppose needles with larger lumens are used for easily pierced rubber closure. In that case, tearing of the rubber could take place, and it will contaminate the medication.
Needle gauge recommendations
The following table shows the most appropriate gauges of the needle for a particular situation.
| Gauge | Appropriate Use | Comments |
| 16-18 | IV infusion: -In adults and adolescents -Of viscous fluids and large volumes -At rapid infusion rate | Large vessel required Insertion may be painful |
| 19-20 | IV infusion: -In adults, adolescents, and older children -Of blood products and other viscous fluids IM injection | Large vessel required Insertion may be painful |
| 21 | IV injection or infusion -In most ages IM injection | |
| 22-23 | IV infusion -In all ages including infants and elderly -Of non-viscous fluids -At slow to moderate infusion rates IM injection | Suitable for small or fragile veins Infusion control devices may be required Insertion through tough skin may be difficult |
| 24-27 | IV infusion -In all ages including infants, toddlers, and elderly -Of non-viscous fluids -At slow to moderate infusion rates Subcutaneous (SC) injection Intradermal (ID) injection (25-26 gauge) | Especially useful for very small veins Infusion control devices may be required Insertion through tough skin may be difficult |
