[김홍표의 과학 한 귀퉁이] lungs are light

Humans have two lungs. the left one is a bit smaller It is because it makes room for the left-leaning heart. It is also understandable that the right hand extended to the left chest when saluting the flag is related to this anatomy. But where did the word lung come from? I do not know. However, lung, the English word for lung, is said to have the meaning of light.

These two light lungs hang from the bronchi and settle in the upper part of the digestive tract. From a long evolutionary perspective, lungs of this shape appeared long after fish came to land. Lung fish, which live in South America, can take in air by sticking their heads out of the water. They have peanut-shaped lungs, but unlike humans, lungfish do not have paired lungs. Only in the reptile salamander did two pairs of lungs finally appear.

In the human airway, which is about 12 cm long, the bronchi, which are largely divided into left and right branches, are connected to the alveoli by branching more than 20 times. The surface area inside them is about 130 square meters. It is the size of one and a half badminton courts. Tetrapods that left the water and successfully settled on land were equipped with a pair of lungs that greatly expanded their surface area.

A large breathing surface area is not necessarily desirable. This is because the capillary network must be built to match it. Only then can the oxygen gas in the air be drawn into the liquid blood. When a human breathes once, the amount of air entering the lungs is about 500 ml. Assuming an average of 15 breaths per minute, the amount is approximately 8 liters. During that time, 5 liters of blood returning to the heart must pass through the lungs. So the two meet. John West, who climbed Mount Everest in 1981 with an altitude sickness research team, emphasized that the basement membrane, where capillaries and alveoli meet, must be thin and highly elastic in order for the lungs to withstand the stress of breathing. It was also around that time that traces of bleeding were found in the respiratory tract of all the horses that had finished the race.

We feel everything going smoothly in our lungs, to the point where we are not even conscious of breathing. Is it really so? In 2020, scientists from Stanford, USA, reported in ‘Nature’ the observation that pulmonary capillaries are composed of two types of cells. The main characters were gCap cells and aerocytes, which are usually called capillaries. Gas cells are found only in the lungs and control the flow of air. On the other hand, gCap mainly maintains the vessel wall. This research result may mean the integrity of the lungs, but on the other hand, it also means that the pulmonary capillaries are under severe stress. The surprise didn’t stop there.

In 2017, scientists at the University of California reported to Nature that platelets are made in the lungs of mice. It has been known that platelets, which prevent bleeding by repairing capillary wounds, are produced in bone marrow giant cells. As if taking off a sujebi, giant cells divide their body into about 500 platelets. This was also observed in the lungs. As such, about 10 million platelets are formed per minute, and half are made in the bone marrow and half in the lungs. The scientists who conducted the experiment hurriedly conducted an experiment to see if giant cells in the lungs could be transplanted or moved to the bone marrow. Indeed, the giant cells in the lungs were able to travel to the bone marrow at some point. But I’m more interested in why platelets have a special place in the lungs.

Platelets are specialized tinker cells that mend blood vessels, but recently they have been reborn as beings with immune functions. When a foreign entity such as a bacterium is encountered or tissue is injured, the immune system immediately kicks in. The icing on the cake would be if the platelets, which rush to the surface when blood vessels are injured, act like immune cells. Oxygen respiration is essential for life forms such as mammals, which are serene and have a high basal metabolic rate. This is because it is almost the only way to extract maximum energy from food.

At the apex is the human. Humans can’t run as fast as lions, but they can walk far and advance to high and cold places. This is because the lungs have finished adapting. However, the lungs of athletes who challenge the extreme, those who have smoked cigarettes for a long time and have thinned the lung basement membrane, or those who have put too much strain on the heart due to repeated stress may find their lungs bleeding at some point. We live in this world with two lungs with a vast breathing surface area. It must be the struggle of life to send oxygen in the air to the depths of the cells. Every time we breathe, we stand at the interface between land and water.