The Peculiar Human Body- Part 1

Tushar Kaushik

Our body is a cluster of trillions of cells, millions of tissues, organs, and organ systems. A myriad of various physiological, molecular, and chemical processes are happening inside us every day. Many of these processes have been studied and are still being studied. In this series, you will read about the various peculiar features of our body that have always intrigued you and have drawn your curiosity from time to time.

How does an oxygen molecule travel in our body?

We breathe in about 18,000 times per day. A sublime and habitual process anyone rarely thinks about. But behind the curtains of our keratinous skin and organs, a bewildering drama of coordinated sequences of our cells plays out. All of our imperative organs and organ systems, such as the nervous system, the circulatory system, the digestive system, the respiratory system and the excretory system, work in sync to sustain our life by supplying and exchanging oxygen to tissues and cells throughout our body. Cells need oxygen because it’s one of the key ingredients of aerobic respiration, a multi-step process of producing cellular energy, a molecule ATP, which our cells use as a fuel to power their functions. But for an oxygen molecule to travel through our relatively giant bodies is a cumbersome task. Usually, gas enters cells by diffusing, a process where they travel from high concentration to low concentration, but that is only efficient over membranes and capillaries. So, for oxygen to reach every cell in our body, it must need an elaborate network for transportation. This is provided by the 20 trillion red blood cells in our body. A single RBC has approximately 270 million molecules of hemoglobin protein. Each hemoglobin molecule contains 4 heme groups, which possess hemic iron ions that provide the cell its distinctive scarlet red color. This protein can carry up to 4 molecules of oxygen. To produce an RBC, our body assimilates raw materials from our food, such as Vitamins B6, B9, B12, A, and Iron.  In our digestive system, food gets broken down into its smallest elements, one such element of importance is iron, the building block of hemoglobin. So, we can, in fact, say that the real journey of oxygen transportation initiates from our stomach itself. Iron travels in blood through cardiovascular system to the body’s ‘hematopoietic tissue’, the birthplace of red blood cells, and is enclosed within our bone marrow cavities. The renal system regulates our levels of red blood cells through the release of erythropoietin, a hormone which dictates the marrow to increase production.

Our bodies produce 2.5 million red blood cells per second so that oxygen we inhale will have ample transportation. But before oxygen can even reach our lungs and the respiration system comes into play, the brain needs to get involved. The pons in our brainstem initiates breathing by sending a message through our nervous system to muscles of the diaphragm and ribs. This causes them to contract, which increases the space inside the rib cage and allows the lungs to expand. This expansion drops the internal air pressure inside our lungs, and the air rushes in. We can picture our lungs as two big balloons, but they’re actually way more complicated. Why? The RBCs in the vessels can only pick up the oxygen that is in close contact with them. If our lungs were in the shape of mere balloons, the air that was not in the direct contact of the inner surface wouldn’t diffuse through. Fortunately, our lungs’ framework ensures that only minuscule amount of oxygen is wasted. The interior is divided into hundreds of millions of miniature sac-like projections called alveoli. These alveoli dramatically increase the surface area of lungs to somewhere around 140 square meters. The alveolar walls are made of squamous cells, or flat cells, and are surrounded by capillaries. Together, the alveolar wall and capillaries make a two-cell thick membrane that brings blood and oxygen close enough for diffusion. These oxygen-bonded RBCs then get transported from the lungs through the cardiovascular network, an extensive network of blood vessels that encloses every cell in the body. If we were to lay this system out end to end in a straight line, the line would stretch over 100,000 miles. For RBCs to travel through such an elaborate and extensive network must certainly require an external powerhouse pump; that’s where our heart comes in. The human heart pumps an average of about 100,000 times per day. It is the powerhouse that ultimately propels the RBC to each and every cell in our body, completing the body’s team effort. Just think: this entire complex system is built around the delivery of tiny molecules of oxygen. If just one part malfunctioned, so would we.

The story of Hiccup

Sometimes, out of the blue, when you are eating something, maybe watching TV, or even doing something as vague as running, you start to have these intense rounds of hiccups that end as abruptly as they started. Well, except in the case of Mr. Charles Osborne, who began to hiccup in 1922 after a hog fell on top of him. He wasn’t cured until his death in 1991.  In fact, he is now listed by Guinness as the World record holder for ‘hiccup longevity’. Beat that?

Doctors have pointed out a few causes of hiccups. They are often initiated by stimuli that stretch the stomach beyond what is ordinary, like swallowing air or gulping foods or drinks. Some associate hiccups with emotions such as anxiety and excitement, or a response such as laughing or crying.

But what happens inside us when we hiccup?

Hiccups are initiated by a series of anatomical and physiological changes, starting from the diaphragm (a dome-shaped muscle that gives support to our lungs) with an involuntary spasm or sudden contraction. This initiates an immediate closure of the vocal chords and the glottis, a slit-like opening between the vocal chords. This sudden change in the conditions of the diaphragm also initiates a sudden intake of air, but the closure of the vocal chords stops it from entering further into the trachea and reaching the lungs, which creates the sound “hic.”

But why have a hiccup at all? There has been no known beneficial function of hiccups to date. They don’t provide any medical or physiological advantage to humans at all. Why even begin to inhale air and then later suddenly stop it from entering into lungs? Scientists believe that the answer to hiccups lies in evolutionary biology. There have been many anatomical structures and physiological mechanisms in our body that don’t seem to serve any function but are remnants of our evolutionary past. They are just relics that are no longer used, and now only pose a challenge to the evolutionary biologists in understanding the functions that they used to perform.

One of the ideas proposed by biologists is that hiccups surfaced millions of years ago in our evolutionary cycle. The origin of hiccups is somewhat connected to the origin of lungs.The lung is thought to have evolved as an organ when the fishes living in warm, stagnant water with a limited supply of oxygen needed to take advantage of the abundant oxygen above the surface. Later, when descendants of these early aquatic ecosystems moved to terrestrial ecosystem, they shifted gradually over the course from Gill-based respiratory mechanism to respiration through lungs. It is almost similar to transition phase by amphibians, such as frogs, as they transition from tadpoles, which respire through gills, to adult frogs, who which respire through lungs. This hypothesis suggests that hiccuping is a relic of ancient transition from gills to lungs.The sudden closure of the glottis is somewhat similar to the process in which air is inhaled as water passes over the gills. This is also supported by evidence that the neural mechanism involved in the generation of a hiccup is almost identical to respiration is amphibians.

One of the hypotheses regarding the possibility that the hiccup is now used as a ‘glorified burp’ in mammals was best explained by Daniel Howes in this article: Hiccups: A new explanation for the mysterious reflex.

Their explanation for this involves the uniquely mammalian activity of nursing. The ancient hiccup reflex may have been adopted by mammals to help remove air from the stomach, like a burp. The sudden expansion of the diaphragm would raise air from the stomach, while a closure of the glottis would prevent milk from entering the lungs. Further backing this is that hiccups appear in human babies long before birth and are far more common in infants than adults.

Edited by: Kaylynn Crawford and Shreya Singireddy