Sources of Biomedical Signals

Biomedical signals/physiological signals are those signals (phenomenon that conveys information) which are used primarily for extracting information on a biological system under investigation. Our body produces various physiological signals. The accessibility to these signals is important because these signals:

• Can be internal (Blood pressure)
• May emanate from the body (infrared radiation)
• Maybe derived from tissue sample (Blood or tissue biopsy)

All physiological signals can grouped as:

• Bio potential
• Pressure
• Flow
• Dimensions (imaging)
• Displacement (velocity, force, acceleration)
• Impendence
• Temperature
• Chemical concentration and composition

A transducer converts a physical signal into an electrical output. A transducer should only respond to the targeted form of energy existing in the physiological signal and it must exclude all other energies. It should also interface with the living system in such a way that it extracts minimum energy and it should not be invasive.

How are Physiological Signals generated?

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 either a simple as feeling the pulse to find the state of heart beats or it can be complex as analysing the structure of internal soft issues by an ultrasound. (link to ultrasound article)

Biomedical signals/Physiological signals are classified according to where they originate from within the body.

Biomedical Signals are classified as follows:

1. Bioelectric signals: These signals are generated by the nerve and muscle cells. Their basic source is the cell membrane which under certain conditions maybe excited to generate an action potential. The electric field generated by the action of many cells constitutes the bioelectric signal. The most common examples of bioelectric signals are the ECG (Electrocardiographic) and EEG (Electroencephalographic) signals. (Link to amazon products for ECG and EEG)
2. Biomechanical signals: These signals are generated due to some mechanical function of a physiological system. They include all types of motion and displacement signals, pressure, flow signals etc. in the physiological system. The respiratory physiological system performs its function by the chest movement. This movement can be measure and analysed.
3. Biocoustic signals: These signals are created by the physiological system in which either flow of blood or air takes place. The flow of the blood in the heart as well as inspiration and expiration of the lungs takes place accompanied with unique acoustic signals.
4. Bio-impedance signals: The impedance of the skin depends upon; the composition of the 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 due to the tissue impedance is a bio-impedance signal.
5. Biochemical signal: The signals which are obtained as a result of chemical measurements from the living tissue or from samples analysed in the laboratory. The examples of these include; measurement of partial pressure of carbon-dioxide (pCO₂), partial pressure of oxygen (pO₂) and concentration of various ions in the blood.
6. Bio-optical signals: These signals are produced by the optical variation by the functioning of the physiological system. The blood oxygenation can be determined by measuring transmitted and reflected light occurring from the blood vessel.
7. Biomagnetic signals: Extremely weak magnetic fields are produced by various organs such as the brain, heart and lungs.  The measurement of these signals provides information which is not available in other types of bio-signals such as bioelectric signals. A typical example is the Magnetoencephalography which is obtained by recording the biomagnetic signals from the brain (link to amazon).

You can also read: The Introduction to Biomedical Instrumentation System