Human Physiology

Human physiology is the branch of biology that studies the functioning of the human body: circulation, respiration, thermoregulation, muscle contraction, nerve transmission. Many of these mechanisms translate into measurable physical quantities, accessible to a smartphone’s sensors.

Discover FizziQ

How to measure it in class

FizziQ makes it possible to measure several physiological quantities without medical equipment: the camera gives access to the pulse, the accelerometer to body movements and heartbeats, the stopwatch to reaction time.

Steps:

• Place a fingertip on the lens of the rear camera, flash on, and start the photoplethysmography measurement: the reflected light signal oscillates at the rhythm of the pulse.
• Count the number of oscillations over a known duration, for example 30 s, to obtain the heart rate in beats per minute. Stay perfectly still during the acquisition, because the slightest movement of the finger masks the signal.
• Lay the smartphone flat on the rib cage, lying down, and record the acceleration: the slow oscillations give the breathing rate, the fast peaks the heartbeats.
• Repeat the heart rate measurement after thirty squats, then every minute for five minutes, and plot the recovery curve.
• Measure reaction time by starting the stopwatch at a sound signal and stopping it as fast as possible, then repeat about ten times to obtain a mean and a spread.

Scientific activities on this topic

Smartphones make it possible to conduct fascinating experiments to better understand the functioning of the human body, in particular thanks to the camera or the accelerometer.

• Heart rate by accelerometer
• Photoplethysmography of a finger
• Cellular respiration of muscle
• Centripetal acceleration and the body’s tolerance: the astronaut and the spinner

Learn more

Photoplethysmography: reading the pulse with light

This is the principle of all optical heart rate sensors, from the connected wristband to the smartphone. The skin is illuminated and the light coming back is measured. At each systole, the heart sends a pressure wave that dilates the arterioles of the finger: the volume of blood in the illuminated tissue increases for a moment. Now hemoglobin absorbs light; the more blood there is under the skin, the less light returns to the sensor. The received light intensity therefore oscillates at the rhythm of the pulse. The useful signal is very weak, of the order of one percent of the total intensity: most of the light is absorbed by the tissues, the bones and the venous blood, which do not vary. It is this variable component, and it alone, that carries the cardiac information.

Why green light

The optical sensors of watches use green LEDs because hemoglobin absorbs strongly in the green, around 520 to 570 nm, which maximizes the contrast between systole and diastole. With a smartphone, one rather uses the white flash and exploits the red channel of the image, because red light passes through the tissues of the finger better: it comes out the other side with enough intensity to saturate the red channel of the sensor, whose variations are then tracked. The two approaches measure the same thing, but one works in reflection on the skin, the other in transmission through the finger.

Heart rate and adaptation to exercise

At rest, the heart rate of a teenager lies between 60 and 90 beats per minute. During exercise, it first increases because the sympathetic nervous system speeds up the heart, which makes it possible to increase blood flow to the muscles. The maximum rate decreases with age, according to the approximate formula 220 minus the age, which remains a rough and individual estimate. The pedagogical interest lies mainly in recovery: the speed at which the pulse comes back down after exercise is a good indicator of training, more reliable than the maximum value reached.

A frequent error: confusing heart rate and blood pressure

A smartphone measures a rate, not a pressure. The number of beats per minute says nothing about blood pressure, which requires a cuff and a measurement of the occlusion pressure of the artery. Likewise, no application measures oxygen saturation reliably with the camera alone: a real oximeter compares absorption at two distinct wavelengths, red and infrared, which a smartphone’s sensor does not allow. These classroom measurements are valid physical measurements, but they are in no way medical devices.

Orders of magnitude

Resting heart rate: 60 to 90 bpm for a teenager, 40 to 50 bpm for a trained endurance athlete. Resting breathing rate: 12 to 20 cycles per minute. Stroke volume: about 70 mL, that is a cardiac output of 5 L/min at rest and up to 25 L/min at maximal effort. Total blood volume: about 5 L. Reaction time to a visual stimulus: 200 to 250 ms; to a sound stimulus: 150 to 200 ms. Core body temperature: 37 °C, regulated to within a few tenths of a degree. Thermal power dissipated at rest: 70 to 90 W.

Formula

The heart rate is deduced from the period between two successive beats:

f = 60 / T

where:

• f: heart rate (beats per minute)
• T: duration between two consecutive beats (s)

It can also be computed by counting the beats over an observation period:

f = 60 × N / Δt

where:

• N: number of beats counted
• Δt: duration of the observation (s)

Cardiac output relates the rate to the volume sent at each beat:

Q = f × SV

where:

• Q: cardiac output (L/min)
• SV: stroke volume, volume of blood ejected at each beat (L)

Application examples

• A student counts 38 beats in 30 s: their heart rate is 60 × 38 / 30 = 76 bpm

• With a rate of 70 bpm and a stroke volume of 70 mL, the cardiac output is 70 × 0.070 = 4.9 L/min, that is the entire blood volume of the body circulating every minute

• Tracking heart rate after thirty squats makes it possible to plot a recovery curve and compare students with different levels of training

• Measuring reaction time with the stopwatch is a way to introduce the notions of mean and uncertainty over a series of measurements

• A hospital pulse oximeter applies the same optical principle as photoplethysmography, but with two wavelengths to distinguish oxygenated hemoglobin from reduced hemoglobin

• Activity wristbands combine photoplethysmography and an accelerometer to estimate both heart rate and energy expenditure

FAQ

Q: Why must you stay still during the pulse measurement? A: Because the useful signal represents only about 1% of the received light. The slightest slip of the finger changes the amount of transmitted light far more strongly than the passage of the pulse wave, and completely drowns the cardiac signal.

Q: How can an accelerometer detect heartbeats? A: At each contraction, the ejection of blood causes a very slight recoil of the body, by conservation of momentum. This movement is tiny, but sufficient for the accelerometer of a smartphone placed on the chest of a person lying down and motionless. This technique is called ballistocardiography.

Q: Can the smartphone replace a medical device? A: No. The measurements obtained are physically valid and pedagogically interesting, but no smartphone sensor is calibrated or certified as a medical device. These experiments serve to understand a principle, not to make a diagnosis.

Q: Why does the heart rate increase even before the effort begins? A: Because the nervous system anticipates. The mere fact of preparing to run triggers a sympathetic response that speeds up the heart, before the muscles have consumed any additional oxygen. It is a good example of anticipatory regulation.

Q: Why is the pulse measured at the finger and not elsewhere? A: Because the tip of the fingers is richly vascularized, thin and weakly pigmented under the nail, which lets enough light through. The earlobe and the wrist also work, but with a weaker signal or one more sensitive to movement.

Related concepts

Biomechanics - Accelerometer - Colorimeter - Absorbance - Pedometer