As Tears Go By

A look at animal and human tears

Children cry easily and even after a minor bump or hurt will shed tears. Adults may feel pain, cry and scream when hurt, but unlike children will not shed tears. An adult’s tears are associated with emotions or may be caused by some disorder, an eye infection or irritation, but not pain. And animals? They, too, have lacrimal (= tear) glands and can have watery eyes as the result of an infection or as part of a physiological control to remove excess salt from the body, but apparently not in connection with an injury. Sea turtles and a few other reptiles remove excess salt not only via their kidneys but with the help of their orbital eye glands as “white tears of saline” that drip out of their eyes.

Although human tears are not white, but watery, transparent and very slightly sticky because of mucins in them, they too contain salt  – as do, in fact, the tears of all land vertebrates that may not ‘shed tears’ but use the lacrimal fluid to lubricate their eyes and keep the cornea moist. Chemically tears are mostly water (ca. 98%); and apart from salts the lacrimal fluid contains a cocktail of amino acids and proteins, antibacterial enzymes and minute quantities of stress hormones. A tear’s chemical composition depends on the cause of its shedding and varies on whether the tear’s function is to wash out dust from the eye, to fight off irritants such as fumes (smoke or onions come to mind), to lubricate the eye’s surface, and as a response to physical pain and emotional upheaval. The autonomic nervous system through its parasympathetic branch governs the production and release of tears from the lacrimal glands, which are located in the upper region of the eye’s orbit. The tears are stored in the lacrimal sac near the nasal corner of the eye; from there the fluid via lacrimal canaliculi is released into the eye upon a signal from the parasympathetic nerve’s acetylcholine transmitter. In healthy individuals, there is a constant release of minute quantities that are distributed with each eye blink across the cornea, but of greater amounts if required. Excessive fluid is drained through the nasolacrimal duct and causes the ‘sniffle’ during weeping.

Basal tears are continually-produced via the 5th cranial nerve’s innervation to keep the eye’s cornea moist and to prevent bacterial infections. In humans, about 0.75-1.1 ml of the liquid is produced each day. Reflex tears are produced when the eye is irritated, and through their copious amount and high water content function to remove the irritation from the eye. Psychic, also known as ‘emotional’  tears, occur in response to strong feelings, which could be sadness, but also joy, stress and  physical pain. Because these tears contain such natural painkillers like leucine-enkephalin and prolactin, it may explain the role of the parasympathetic nervous system and that “a good cry can feel relieving”.  But it does not explain why men shed tears less often than women, a fact that is often explained with the traditional roles men and women are expected to play in life (the advice “boys don’t cry” is a case in point).

The fourth reason for tears is related to diseases and the release of tears accompanying other activities (e.g. yawning). Although elephants have been described as shedding emotional tears, crocodile tears are not an expression of emotional distress, but the result of compression of a nerve that controls the jaw muscles during feeding. In humans suffering from Bogorad syndrome “crocodile tears” also accompany swallowing. Reference to tears can generate resolve (Churchill’s famous “Blood, Sweat and Tears” comes to mind); tears evoke empathy: children know that (and actors train to shed tears at will) and tears appear in poems and songs (the record “Tears on my Pillow” is in my collection) and who wouldn’t remember Marianne Faithful’s beautiful song “As Tears Go By” or Eric Clapton’s touching “Tears in Heaven” (which I heard it for the first time in Chile in 1993). I actually heard of people who shed tears when listening to it.

© Dr V.B. Meyer-Rochow and, 2022. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.


I Hope It’s Never Happened Reading “Bioforthebiobuff”

At high school we had a history teacher by the name of Dr. L., who had spent 11 years in a Soviet prisoner-of-war camp before being released in 1955. He used to put the history book on the classroom’s desk, positioned himself comfortably on a chair near the side of the classroom and asked some of the best readers in class to take turns to read from the book. That’s how his lesson went. Although he can perhaps be forgiven for killing our interest in history by this behavior of his, his antics were also a cause of hilarity, especially when we noticed the regularity of his yawns and could predict when another big yawn of his would appear (silently counting: 14, 15, 16, 17, “yawn”!). But what made him yawn so much? Boredom, lack of sleep, or something else? And why is it so ‘contagious’? Mirror neurons perhaps?

The common view has always been that yawning was related to a lack of oxygen, a build-up of carbon dioxide and a room that was too warm and stuffy. Consequently, a call to open the window and let in ‘fresh air’, could often be heard in situations where people were seen to yawn frequently and appear sleepy. Yet, numerous studies have shown that lack of oxygen and carbon dioxide increases are by themselves not a cause of yawns. The situation is complex and although the amount of yawning appears to be correlated with boredom and sleepiness, it must leave us puzzled to notice that even after a good night’s sleep we wake up and then more often than not yawn upon awakening. Why yawn at that time? And cooling the brain in the morning or at other times by gaping wide: does it make sense? The idea that yawning is a component of thermoregulation has not yet achieved the acceptance it hoped to get.

If we examine objectively what happens during a yawn, we notice that it involves a wide open mouth and a long and deep inspiration of several seconds, sometimes accompanied by some soft vocalization during expiration. It is an involuntary behaviour that can be triggered by thinking and reading about yawning and/or seeing someone yawn. Yawning is communicative and is generally coupled with inactivity, lethargy and sluggishness (sometimes worry as well). To suppress the yawns can be difficult, especially when hindered to move as in boring meetings, lectures, and waiting rooms. And this actually gives us a clue: our bodies need us to stretch occasionally, to shake our arms and legs, to release tension.

The realization that yawning is a stretch response has been gaining attention ever since it was observed that when hemiplegic individuals that not normally can move their arms do move them when they pandiculate with an associated yawn. Yawning when pandiculating, i.e. stretching and thereby contracting and relaxing muscles, reduces muscular tension, is resetting and restoring the control over muscles, something that is critical for posture and movement and something that yoga instructors constantly emphasize. Obviously, the fact that the slow expiration following a yawn is associated with a sympathetic activation marked by an increase in blood pressure, suggests that at the start of the yawn it is associated with a sympathetic suppression that favours a parasympathetic dominance. This might also explain the observation of a paraplegic’s involuntary movement of its toes during a yawn.

Yawning must have ancient roots in the animal kingdom, for it can be observed in almost any animal group and is not even restricted to vertebrates alone as this delightful recording of a yawning leech shows here . Lizards, frogs, toads and even fish can be seen to yawn and all of them are ectothermic (often referred to as ‘cold-blooded’). As such, they would not be expected to use the yawning response to cool their brains as has been suggested for mammals, but could find yawning useful in connection with stretching and therefore the restoration of muscle control. Yawning:  a kind of physical exercise without having to get up? I think that that is a distinct possibility.

© Dr V.B. Meyer-Rochow and, 2021. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.

It Takes Energy to Stay Alive 

And to explain that to the students

I always felt teaching energetics to our undergraduate biology students was no easy task. And yet all life depends on energy. So, trying to avoid as much as possible entropy, enthalpy, the law of the conservation of energy and the laws of thermodynamics with its not exactly simple equations, I started my physiology energetics lecture with a thought-provoking example. Imagine, I said, you want to compare the amounts of the required energy to heat up and bring to the boil a cup of water and a bucket of water. You need less energy to heat up the smaller amount, but it also cools down much faster compared with the water in the bucket which had needed a much greater amount of energy to reach boiling point. If you decided to re-heat the water and bring it back to 100°C every 30 minutes, which would have required more input of energy over a 10 or a 100 hour period: the cup or the bucket? An important variable to be considered is of course the temperature down to which the boiled water would be allowed to cool before being re-heated. But all that is calculable and can be expressed in mathematical equations.

Students all know that the reverse reaction of photosynthesis is the one that provides us and other animals with energy by ‘burning fuel’, i.e. the food ingested, and that some of the energy is for growth and work, but some is converted to heat; in fact, ultimately all is dissipated as heat. Daily rates of minimum standard heat production in animals are related to body temperatures, so that small 42°C warm birds have the highest rate followed by mammals with body temperatures of 35-37°C and ectothermic animals like reptiles, amphibians and fishes. Body sizes and weights are further complicating factors, because the slopes of the relationship between rate of minimum energy expenditure and body weight are nearly the same for ectotherm (cold-blooded) and endotherm (warm-blooded) animals. The slopes are not 1.0, which would indicate a direct proportion: the slopes are approximately 0.75 and that indicates that a doubling in body weight does not double minimum energy expenditure. What is, however, interesting is that when the minimum metabolic rate is expressed per gram of body weight, one notes that energy expenditure rates shoot up exponentially as body weight decreases: small animals need more energy per gram body weight than larger ones. This explains why small warm-blooded animals, e.g. mice, shrews and humming birds need to ingest food more frequently than bigger species and, of course, fish, amphibians and reptiles with their lower resting metabolic rates.

Although resting metabolic rate and maximum longevity have been regarded as not always being ideal to explain ageing, there is nevertheless an obvious relationship between an animal’s body size and longevity (humans being long-lived but not terribly huge and heavy are a bit of an exception). In species of bigger animals the latter generally enjoy longer possible life spans than species that contain smaller individuals, cf. mice, dogs, horses, elephants and whales. Small animals have a greater surface area than bigger ones and tend to lose heat more readily to the environment than the latter, but this alone apparently does not explain the slope of 0.75, mentioned above and an even lower one of 0.67 if body surface area and body weight are correlated to each other. To explain this discrepancy some researchers suggested entering time as a 4th dimension into the equation. Since longevity in mammals is related to weight, total metabolic capacity has to be subject to the time that an organism spends being alive.

Humans appear to be a special case as some studies have shown that shorter rather than taller people have a greater life expectancy! However, it needs to be pointed out that socioeconomic status, relative weight, regular exercise, gender and health practice styles can influence the outcome. It has, for example been suggested on the basis of Japanese and Dutch studies, the latter involving 7800 men and women, that the taller Dutch (but not Japanese) women could expect to live longer than shorter individuals, but that did not apply to men. With such uncertainties abound, I think it’s comforting to be just average.

© Dr V.B. Meyer-Rochow and, 2021. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.