Upon measuring the blood pressure with a catheter inserted into the blood vessel, the observed value may include various types of measurment errors.
These errors are : (1) an error due to the resonance phenomenon relating to the fluid inertia within the catheter and the compliance of the pressure gage membrane, and (2) an error due to the reflection of the blood pressure in front of the catheter, the change of the blood pressure wave form and change of the blood flow form, and the attenuation of the pressure wave.
In this paper, the second subject is investigated theoretically and experimentally. At first, the relation between the blood flow and the blood pressure wave are obtained from the Navier-Stokes equation, the equation of motion for the elastic tube wall, and the equation of the continuity.
With the use of the pressure-voltage (or flow-current) analog, the change of the blood pressure at the exit of the heart, at the front and the end of the catheter, and the change of the reflection in front of the catheter and the change of the attenuation are calculated and observed.
Hence the heart is considered as a constant current source, and the large blood vessel a constant voltage source.
From these results, the value of the errors is thought to be mainly due to the reflection in front of the catheter and to be fairly large in size.
They show different values depending on the radii of the blood vessel and of the catheter, on the length of the inserted catheter, on the distance from the heart or the large blood vessel to the inserted catheter and on the frequency.
The blood pressure becomes sometimes larger than two times of the original blood pressure for these effects.
For the attenuation, we will have to take care of it, when the catheter is inserted into the small blood vessel, but will not do as the large blood vessel.

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The transit time distribution through the whole circulatory system is determined from the whole dilution curve which is the record of dye concentration of blood lead from the left ventricle after injection of dye into blood at the aortic root.
When a bolus of a substance is injected in the circulatory system, it is gradually mixed with blood as a consequence of the transit time difference and ventricular mixing. The temporary process of circulatory mixing is quantitatively expressed as the standard deviation of concentration for all the circulatory blood to the mean concentration and is computed with the use of the transit time distribution and the whole dilution curve.
The changes in transit time distributions and processes of circulatory mixing due to heart rates were examined in the experiments with anesthetised dogs during vagal nerve stimulation and extracorporal hypothermia.

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In the neurophysiological experiments, we often need to analyze the nerve impulse trains in the form of the time histograms such as the time interval histogram and post-stimulus time histogram. For these analyses, an on-line real-time computer, for example LINC-8, is very useful, and these analyses may be accomplished by the software of LINC-8 itself. Attaching a specially designed time-counter to LINC-8, we are, however, allowed to simplify the program and increase the facility of the system.
The time-counter described here has the following functions : (1) to clear the counter, (2) to start counting, (3) to stop counting, (4) to transfer the contents of the counter to the computer, (5) to transfer the contents into the counter from the computer, (6) to interrupt request to the computer, when a pulse is given from the external circuit such as an observed neuronal spike, or when the contents of the preset counter is decreased to empty.
This auxiliary time counter makes it easy to measure the elapsed time between successive impulses, and to display simultaneously the histogram on the scope. The time counter may be used to control the experiments, for example, to generate a programmed time series of stimuli for the automatically controlled experiments.

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