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Effects of Age on Human Regulation Control

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Published: 8th Feb 2020 in Biology

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Age-Reaction Time Correlation Using a ruler dropping Experiment



The importance of regulation control in human biology can be explained by the theory of homeostasis, excretion process, endocrine and the nervous system. In practice, the nervous and hormonal system directly influence our reaction time. But does our age affect our reaction time as well?  To access this, an experiment was set up to test the reaction time of people with different ages. We used a ruler dropping method to calculate the time the participant takes to catch the dropping ruler; then we compared and analysed the data and found that, in general, as human age increases, the reaction time increases. Elderly people react more slowly than those in their prime. We concluded that this is because the regulation control varies between elderly and younger participants. But we also found the exception. the reaction time of middle-aged people is not faster than elderly people. Further research is needed to explain this phenomenon.



1. Homeostasis

In order to survive and be active the human body needs to keep a constant internal environment which requires the condition of tissue fluids (e.g. pH, blood temperature, salt concentration) to vary within a reasonable range. This is known as homeostasis. (Boyle & Senior, 2008)

1.1 Excretion system

The excretion system plays an important role in homeostasis control. It removes waste products out of the body during metabolism. As metabolism occurs all the time, nitrogenous waste products are produced within the body. These nitrogenous waste products contain urea, uric acid and nitrogen. If their amount builds up to a certain level, they can be toxic and change the condition of tissue fluid. Therefore, removing these nitrogenous waste products is essential to homeostasis. (William, 2015)

1.2 Kidney

The excretion system needs the cooperation of a series of organs.  The kidneys function as the main organ to filter waste products out of the blood and produce urine.

(Figure 1 The structure of a kidney) (Fielding, 2018)

(Figure 2 The structure of a nephron) (Fielding, 2018)

As shown the structure of a kidney in the figure 1, a kidney contains:

  • Renal capsule: The capsule covers the outer surface of a kidney to protect it from injury.
  •  Renal artery: It delivers the blood which contains oxygenated, nutrition and waste products to the kidney.
  • Cortex and medulla: They offer a large surface area for millions of nephrons and capillaries to spread over, which speed up the rate of blood filtration and substance reabsorption.
  • Nephrons: a nephron (figure 2) contains a Bowman’s capsule, a long tubule, a loop of Henle to work together with capillaries to filter the unwanted substance out of the blood and reabsorb the useful molecules (e.g. protein, glucose, water, sodium) back to the blood, and the collecting duct deliver the filtrate to the pelvis.
  • Pelvis: It collects the waste fluids from the nephrons and push them to the bladder along the ureter.
  •  Renal vein: The filtered blood passes the renal vein to leave the kidney to the heart.

(Geatrell et al., 2008)

1.3 Nephron

The structure of a nephron (figure 2) plays an important role in ultrafiltration and selective reabsorption.

  • Ultrafiltration

The ultrafiltration occurs in glomerulus and a Bowman’s capsule of a nephron. The blood flows through the afferent arteriole into the glomerulus. The high pressure in glomerulus forces the blood into the Bowman’s capsule. The blood filters through three layers (figure 3, the capillary wall, basement membrane and the epithelium of the Bowman’s capsule), and the large molecules like protein and blood cells can’t pass through, so they return to the capillary whilst the filtrate of small molecules enter the tubule. (Kent, 2000)

(Figure 3 the structure of three layers in ultrafiltration) (Fielding, 2018)

  • Selective reabsorption

In figure 2, the filtrate leaves the Bowman’s capsule and passes through proximal convoluted tubule (PCT), loop of Henle and the distal convolute tubule (DCT) where the selective reabsorption occurs. In the PCT, with the help of active transport and facilitated diffusion, useful substance like glucose, amino acids and sodium are reabsorbed; in the loop of Henle and DCT, the water molecules are absorbed by the difference of water potential. The leftover fluid is urine which leaves the nephron through the ureter. (Kent, 2000)

2. Negative feedback

Apart from the excretion system, the endocrine and nervous systems also contribute to homeostasis control. Negative feedback is the regulatory mechanism to keep things like body temperature, blood sugar and water content around a normal level. (Robert, 2015)

2.1 receptors and effectors

The principle of homeostasis is the way to maintain a constant internal environment to keep the cells and enzymes function well. But the internal environment will be changed by a stimulus. The stimulus will be processed in a certain pathway to trigger the negative feedback.





   (e.g. skin/ gland)


             Against                                                                      Against

Hormonal system

Neuronal system


                                             Negative feedback


(e.g. liver cells)


(e.g. muscle cells)

                                                   (Figure 4)

In figure 4, the receptor detects the stimulus and send this information through the neuronal or hormonal system to the effector to bring about a response. Negative feedback is the mechanism which reverses the changed conditions back to normal.

(Geatrell, et.al., 2008)

2.2 Temperature regulation

The structure of skin determines its role in regulating body temperature. As figure 5 has shown, the skin contains a part called dermis which is important in regulating body temperature.

(Figure 5 the structure of skin) (Kent, 2000)

  • Temperature receptors in the dermis can detect the change of the external temperature and send impulses to the brain via nerves.
  • Blood vessels such as capillary network and arterioles dilate (vasodilation) in a hot environment, resulting in more blood flows to the surface vessels to lose more heat to lower the temperature; or in a cold environment, the blood vessels constrict (vasoconstriction), causing less blood flows to the vessels to lose less heat.
  • Adipose cells act as thermal insulation to maintain body temperature.
  • Sweat glands produce sweat when the body feels hot. The sweat evaporates and takes the heat away from the skin to lower body temperature.
  • Erector pili muscle contracts to make the hairs stand up to form a insulation layer to reduce the heat loss when the body feels cold; erector pili muscle relax to make the hairs lie flat when it’s hot, without the insulation layer, the heat can be lost quickly. (Fielding, 2018)

2.3 Blood glucose level control

Regulation of blood glucose is another example of homeostasis.  To maintain blood glucose at a normal level, two hormones called insulin and glucagon have to work antagonistically by the mechanism of negative feedback. Figure 6 explains when the blood glucose concentration goes up above or goes down below the normal level, the insulin or glucagon is released to regulate the concentration of blood glucose back to normal by negative feedback.

(Figure 6 Regulate blood glucose level by negative feedback) (Fielding, 2018)

2.4 Water content control

The water content in blood needs to be maintained at a normal level, as it is important for homeostasis. The regulation of water content in blood involves receptors (hypothalamus), hormonal system (pituitary gland) and effectors (kidney) working together.

  • In a dehydrated condition,

(1)  The hypothalamus in the brain detects blood water content decreasing.

(2)  The pituitary gland secrets more ADH into the blood.

(3)  More ADH makes nephron tubules and collecting ducts in kidneys more permeable, so more water is reabsorbed into the blood.

(4)  The water content in blood rises to the normal level, and a small amount of concentrated urine is produced.

  • In a hydrated condition,

(1)  The hypothalamus in the brain detects that the water content in the blood is too high.

(2)  The pituitary gland produces less ADH into the blood.

(3)  Less ADH makes nephron tubules and collecting ducts less permeable, so less water is reabsorbed into the blood.

(4)  The water content in the blood drops to the normal level, and large amount of dilute urine is formed. (Kent, 2000)

2.5 Nerves vs hormones

A nerve is a strand of tissue which connects the CNS with the organs and tissues of the body; hormone is a substance which is secreted by an endocrine gland and circulates in the blood. The similarities between nerves and hormones are:

  • Both of them act as the communicator between cells, tissues, and organs to perform regulation control of the body.
  • Both nervous and hormonal system are manipulated by negative feedback.

The differences between nerves and hormones are:




Electrical impulse

Chemical messenger

Transmitted by









(Table 1)

(Rachna, 2017)

3. Nervous system

The nervous system contains two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS contains the brain and spinal cord and the PNS mainly contains the sensory neurone and the motor neurone. In response to an external stimulus, the receptor communicates with the effector through the nervous system.

3.1 Reflex arc

As the structures of sensory neurone and motor neurone shown in figure 7, both of them contain the axon, dendrites and myelin sheath which are essential in a reflex arc.

  • Axon: electrical impulses pass through the long axon
  • Dendrites: they connect individual neurons together to pass impulses.
  • Myelin sheath: it performs as an electrical insulator which help impulse to travel faster.  

(Figure 7 The structure of a sensory neurone and a motor neurone) (Fielding, 2018)

Apart from above similar components, the sensory neuron contains a receptor to detect the external stimuli and a motor neurone has an effector to take a response to the stimuli. The passage of an impulse in a reflex is known as reflex arc.

(Figure 8 The structure of a reflex arc) (Kent, 2000)

Figure 8 illustrates a reflex arc; the sensory neuron converts external stimuli into internal electrical impulses and sends these impulses to a relay neuron. This is because the reflex action is involuntary, so the conscious part of a brain doesn’t participate in a reflex arc. The sensory neurone links to a relay neurone in the unconscious part of a brain or in the spinal cord which connects to the specific motor neurone. The effector in motor neurone take a response to the stimuli to avoid harm.

3.2 Neurone mechanism

An impulse is a change in the potential difference across the plasma membrane of a neurone. Figure 9 shows the potential difference between the inside and outside of the axon. The following procedures characterized the transmission of an impulse along a nerve cell.

(Figure 9 The potential difference across the membrane of an axon) (Kent, 2000)

  • Resting potential

When not conveying an impulse, the potential difference is -70mV inside with respect to the outside. In figure 10, the sodium-potassium pumps in the plasma membrane pumps three Na+ out for every two K+ into the cell. Also, the membrane is much more permeable to K+ than Na+. Therefore, more K+ diffuse out of the cell down their concentration gradient, accounting for the resting potential.

(Figure 10 The ion channels and sodium-potassium pump in a resting potential)  

 (Kent, 2000)

  • Depolarization phase

Figure 11 shows that when a stimulus is applied, the axon becomes depolarized which means that the inside becomes less negative due to the Na+ channels being open and the K+ channels closed on the cell membrane.

  • Action potential peak

The action potential occurs when a stimulus is powerful enough to exceed the threshold level. The inside of the membrane becomes positively charged and the potential difference reaches a peak around +35mV.

  • Repolarization phase

After the peak of the potential difference, it drops down because on the cell membrane (see figure 11) the Na+ channels are closed and K+ channels are open, causing the inside to be more negative than the outside of the membrane. The potential difference undershoots the resting potential and then returns to it. (Kent, 2000)

(Figure 11 the change of ion channels and action potential) (Kent, 2000)

3.3 Reaction time experiment

The reaction time can be measured by setting up an experiment. A few factors might affect human ‘s reaction time, and age is one of them.

Question: how will human age affect their reaction time?

Hypothesis: If a group of adults (aged 18-75) are tested for reaction time, the older they are, the longer reaction time they need to response to an external stimulus.

Independent variable: age

Dependent variable: reaction time

Controlled variable: same ruler throughout the experiment.

Predicted relationship: as the age increases, the reaction time increases.



  • A ruler (30cm)

Experiment procedure:

(1)  Rest the arm which hold one end of the ruler on the edge of a table to prevent it moving or shaking.

(2)  Instruct the participant to place his/her thumb and index finger/middle finger at the zero of ruler. So, the ruler hangs between the two fingers of participant without touching it.

(3)  Drop the ruler without warning, the participant closes the two fingers to catch the ruler as the ruler falls.

(4)  The measurement where the participant’s two finger catch is the displacement of the ruler. 

(5)  Repeat the test 3 times on each participant and calculate the mean.

(6)  Record each participant’s age and result of each test. Calculate their reaction time.

(7)  Set all the data in a table and draw the graph.

(Fielding, 2018)

Method to calculate the reaction time:

Once the data of the ruler displacement are collected, the reaction time can be calculated as follows:

In physics, V=final speed, V0=initial speed, a=acceleration, s=distance

V2– V02= 2as

In this experiment, V0=0, a=g=9.8ms-2 , s=ruler displacement

So V= 2gs

(equation 1)

Also, in this experiment, V0=0, so V= ΔV (changing speed)

and a= ΔVt

, so g= ΔVt 

 ΔV=gt=V (equation 2)

combine equation 1 and 2: gt= 2gs

 (gt)2=2gs  t2= 2sg

so, t= 2s9.8

For example, if the mean displacement is 0.14m,

the reaction time will be t= 2*0.149.8




Data table



Ruler displacement (m)

Reaction Time


Trial 1

Trial 2

Trial 3



Amy S







Sophie G







Harper F







Callum B







Alex F







Liam G







Luke C







Casey H







Sarah S







Mary N







Steve M







Christin J







Heather J







Ashley B







Linlin L







Hui F







Adam J







Eric P







Carol P







Pauline J







(Table 2)

Reaction time – Age Graph

(Graph 1)


In the above experiment, the result of graph 1 shows that people aged from 18 to 31 years old have the lowest reaction time less than 0.1433s. The trend line goes up in the reaction time for the subsequent ages from 32-55, and 56-72 years old. However, compared to the people aged from 48-55 years old, the result of reaction time for people ages from 56 – 72 years old doesn’t increase rapidly.

The cause of this increase in reaction time is likely related to their ages. Generally, as the age increases, the reaction time seems to increase as well. The explanations for this phenomenon might be:

  • Hormone levels – adrenaline improves the action of muscles and prepare the body being alert. (Robert, 2015) The level of adrenaline may decrease as people gets older.
  • Reflex – a reflex arc mediates an involuntary response to an external stimulus. As people gets older, the reflex arc in the nervous system might not perform as well as young people.
  • Brain function—the cerebellum part in the brain controls people’s movement. As people gets older, the cerebellum may not work well as in young people.

Therefore, the reaction time increases as people gets older. As for the 56-72 years old group, their reaction time is not greater than the 48-55 years old. The reason for this result may be because they are not as mentally stressed or physically occupied as middle-aged people. A report from the University of Michigan indicates that the reaction times of elderly people can compare to the people in their 30s and 40s. (Traci, 2018) However, the data in the above experiment doesn’t support this claim. But the possibility is that an experiment with larger samples conducted in this field might show a sensible explanation of this phenomenon.

There are some errors in the above experiment. Firstly, the duration time before dropping the ruler affects the result. This is because the participants are highly alert in the first 30s, if the ruler fall during this period, he or she would react rapidly. If the person hold the ruler longer than 30s, the participants are less likely to focus on the ruler. So we realised we should control the duration time before dropping the ruler for each participant. Furthermore, different human attitudes affect the result as well. Some participants felt frustrated if they didn’t react quickly at first trial. The feeling of frustration would affect their second or third trial. Some participants wanted to try more times to get a better result. To avoid arguing with them, we often fulfilled their request. But their competitive attitude and our compromise affected the overall data. So we began to inform participants that only 3 trials would be allowed before they started and also refused their retry requests kindly. Final problem we noticed in this experiment was that the sample size is not big enough to make an accurate conclusion. The performance of 20 participants cannot represent all humans. So a larger sample size covers all ages is needed to have a more precise result.


Generally, our result data of the ruler dropping experiment are consistent with the hypothesis – as an adult’s age increases, the reaction time increases.  In graph 1, the reaction time of 18 – 37 years olds are within the range of 0.15 – 0.2s, while the reaction time of 38 – 72 years olds is between 0.2 – 0.25s. The trendline gradient is positive which proves our hypothesis is correct. We analysed the reason from nervous and hormone system which degrade as people get older. So, elderly people have longer reaction time than youngsters. Also, from some research in neuroscience on various websites, we self-generalized that the people between 18-38 years old have a shorter reaction time due to their increased brain and neurological activity.

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However, we also notice the people who take the longest reaction time are not the people aged 60+, but the ones aged 40-55. Therefore, as shown in our data, after 40 years old, as people get older, the reaction time may not take longer. This is not quite consistent with our hypothesis. We mentioned the reason might be the difference of mental stress and personal lifestyle between middle aged people and elderly people, which is currently not clearly explained. So, further experimentation and explanation in this direction are needed to allow us to understand this phenomenon scientifically.











  • Boyle, M and Senior, K. (2008) Human Biology (3rd edition) London; Collins 
  • Fielding, D. (ed) (2018) A-Level Biology OCR A Newcastle; CGP
  • Geatrell, B., Loweir, P. and Tilley, A. (2008) Human Biology Harlow; Heinemann 
  • Kent, M. (2000) Advanced Biology Oxford; Oxford University Press 
  • Willams, G. (2015) Advanced Biology for You (2nd edition) Cheltenham; Nelson Thornes 
  • Hocking, S., Sochacki, F. and Winterbottom, M. (2015) OCR A Level Biology A 2 (2nd edition) London; Pearson 
  • Robert, S. H. (ed) (2015) Dictionary of Biology (7th edition) Oxford; Oxford University Press 
  • Rachna, C. (2017) Difference Between Nervous System and Endocrine System. Bio Differences. Retrieved from https://biodifferences.com/difference-between-nervous-system-and-endocrine-system.html
  • Traci, P. (2018) As We Age, Loss if Brain Connections Slows Our Reaction Time. Psych Central. Retrieved from https://psychcentral.com/news/2010/09/13/as-we-age-loss-of-brain-connections-slows-our-reaction-time/18031.html


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