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HomeDiagnostic factsMercury Sphygmomanometer: Parts and Functions

Mercury Sphygmomanometer: Parts and Functions

Blood pressure measurement apparatus: types, parts, and functions

The sphygmomanometer is the instrument routinely used for recording arterial blood pressure in humans. 

They are different types of blood pressure instruments “sphygmomanometer” are as follows:

Mercury sphygmomanometer

Arenoid sphygmomanometer

Automatic digital sphygmomanometer 

Mercury sphygmomanometer is replaced in many places with arenoid sphygmomanometer due to mercury toxicity whereas Mercury sphygmomanometer is more accurate than arenoid sphygmomanometer.

Sphygmomanometer parts and functions:

Sphygmomanometer (the ‘‘BP apparatus’’). 1: Spring-loaded clip; 2: Lid of the apparatus; 3: Graduated glass tube; 4 One-way valve; 5: Mercury reservoir; 6: Stop cock; 7: Armlet; 8: Air pump (rubber bulb, with leak valve.)

Mercury manometer. The manometer is fitted to the lid of the instrument. One arm of the manometer is the reservoir for mercury—a broad and short well that contains enough mercury to be driven up in the other limb—the graduated glass tube.

Graduated tube. The manometer glass tube is graduated in mm from 0 to 300, each division representing 2 mm. 

A stopcock when closed will prevent the mercury from entering the glass tube.

The one-way valve fitted at the top of the mercury well prevents the spilling of mercury when the lid is closed while allowing pressure to be transmitted from the rubber bag to the mercury reservoir.

A spring-loaded clip at the top of the tube prevents mercury leakage from the device. 

The armlet (rubber bag; Riva Rocci cuff). Riva Rocci cuff consists of an inflatable rubber bag, 24 cm × 12 cm, which is fitted with 2 rubber tubes—one connecting it to the mercury reservoir and the other to a rubber bulb (air pump). The bag is enclosed in a long strip of inelastic cloth with a long tapering-free end. 

The rubber bag is 12 cm wide which is enough to form a pressure cone that reaches the underlying artery even in a thick arm. As a general rule, the length of the rubber bag should be 2/3 and the width should be 1/3 of the mid-arm circumference of the subject. 

The recommended width of Riva Rocci cuff of the bag in different age groups:

Infants: 2.5 cm 

Below 4 years: 5 cm 

Below 8 years: 8 cm 

Adults: 12 cm 

Air pump (rubber bulb). It is an oval-shaped rubber bulb. It has a one-way valve at its free end, and a leaking valve with a knurled screw, at the other where the rubber tube leading to the cuff is attached. The cuff can be inflated by turning the leak valve screw clockwise, and alternately compressing and releasing the bulb. Deflation of the bag is achieved by turning this screw anti-clockwise. 

Aneroid manometer. This manometer consists of metal bellows, mechanical links, and a calibrated dial that replaces the mercury manometer, it is common in hospital settings.

Pic source :  Medilog Bio Health

Dr. Sanjay Maharjan, MD
Dr. Sanjay Maharjan, MD
MD in Basic and Clinical Physiology


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