The body produces various physiological signals. The accessibility to these signals is important because
(1) they can be internal (blood pressure)
(2) they may emanate from the body (infrared radiation)
(3) they may be derived from a tissue sample (blood or tissue biopsy)
All physiological signals can be grouped into the following categories –
(4)dimensions( for example : imaging)
(5)displacement (such as velocity, force, andacceleration)
(8) chemical concentration and composition.
Sources of Physiological Signals
Physiological signals are generated by the body during the functioning of various physiological systems. Hence physiological signals hold information which can be extracted from these signals to find out the state of the functioning of these physiological systems. The process of extracting information can be very simple as feeling the pulse to find the state of heart beats and it can be complex which may require analysis of the structure of tissue by a sophisticated machine. Depending on type of energy, the physiological signals can be:
(a) Bioelectrical signals: These signals are generated by nerve cells and muscle cells. The source of these signals are cells which undergo change of state from resting potential to action potential under certain conditions. The change of potential in many cells generate an electric field which fluctuates and in this process it is to emit bioelectric signal. ECG and EEG are obtained from the biosignals from heart and brain respectively.
(b) Biomechanical Signals: These signals are generated by some mechanical function of a physiological system. These signals are related to motion, displacement, pressure and flow of the physiological system. The respiratory physiological system functions with the movement of chest which can be analysed.
(c) Bioacoustic Signals: These are created by the physiological systems which are dealing with the flow of blood and air. The flow of the blood in the heart, the opening and closing of chest in respiratory system generate unique accoustic signals.
(d) Biomagnetic Signals: Weak magnetic fields are generated by various organs like heart, brain and lungs while functioning. Magneto encephalograph is obtained from the biomagnetic signals from the brain.
(e) Biochemical signals: The information is obtained by chemical measurements from the living tissues or analysis of the samples obtained from the body. The concentrations of various constituents in the blood and the measurement of partial pressure of oxygen and carbon dioxide in respiration are found out by this method.
(f) Bioimpedence Signals: The impedence of the skin depends upon the composition of skin, blood distribution and blood volume through the skin. The measurement of impedance helps in finding the state of skin and functioning of various physiological systems. The voltage drop by the tissue impedence is nothing but a bioimpedence signal.
(g) Bio optical Signals: These signals are produced by the optical variations by the functioning of the physiological system. The blood oxygenation can be measured by measuring transmitted and reflected light from the blood vessel.
Transducer is a device which converts one form of variable or energy into another form of variable or energy. Generally, transducer is required to convert physiological variables into electrical signals which are easier to be processed. The relationship between input and output variable can be linear, logarithmic or square. The transducer can be active or passive depending upon conversion of non electrical variable into electrical signal. The active transducer directly converts input variable into electrical signals while passive transducer modifies either excitation
voltages or modulates the carrier signals. The passive transducers are externally powered while active transducers are self generating
and require no external power.
Modern digital computers make the application of these transducers absolutely very essential. Type of transducer, principle of operation and typical applications are tabled as under :–
- Short Note on Biotelemetry System (biomedikal.in)
- Biomedical Summer Training 2011 @ Cardea Labs, Delhi (biomedikal.in)
- What are the various Bio-Accoustic Signals present in human Body? (biomedikal.in)