1. Thirst as a Warning Signal
You must have experienced the feeling of thirst: dry mouth, burning sensation throughout your body, and the moment you see water, you rush towards it, scooping it up and gulping it down.
What does thirst represent?
Thirst indicates that your body is lacking water, and not just a little. Medical researchers have tested that when a person feels thirsty, they have already lost at least 2% of their body water. This means that for a person weighing 50 kilograms, they have lost at least 50 kg × 0.02 = 1 kg of water. Thus, thirst serves as a signal, warning us, "You need to drink water!" Without thirst, one might forget to drink water for a lifetime, leading to severe dehydration.
Looking back at medical history, we find that thirst is a sensation, a response to insufficient saliva in the mouth. It is also a natural instinct, similar to hunger prompting us to eat, nothing particularly special.
This explanation raises a series of questions for modern scientists: Is thirst really that simple? Who regulates the sensation of thirst in the body? How is thirst generated? With these questions in mind, researchers began their investigations.
Let’s look at an interesting experiment.
In this experiment, a major vein in a rat's abdomen (the inferior vena cava) was tightly ligated. As a result, all blood returning from the hind legs and abdomen could not reach the heart, causing the heart to lose 40% of its blood, leading the rat into a state of low blood volume. When the rat woke up from anesthesia, its first action was to drink water excessively. This indicates that low blood volume is a cause of thirst. Now the question arises: why does low blood volume cause thirst? Researchers conducted numerous experiments, and one particularly surprising result was observed: rats that had their kidneys removed did not exhibit excessive thirst even when the inferior vena cava was ligated. Why is that?
Next, scientists crushed the removed kidneys, filtered out the debris, extracted the juice, and injected it into the blood vessels of those rats. Guess what? The rats began to drink water vigorously. Researchers discovered a chemical substance in the kidney juice called angiotensin, which triggered thirst in the blood-deficient rats. Thus, the issue seemed clearer. On hot days, when we run and sweat profusely, our blood volume decreases. When blood volume drops, the kidneys produce angiotensin, leading to thirst.
At this point, one might think the experiment should conclude. However, scientists continued to question why angiotensin causes thirst. Is there anything else that can induce thirst?
Many scientists speculated that angiotensin must enter the brain through the bloodstream after being produced by the kidneys, and there are likely sensitive areas in the brain for this chemical (which researchers named angiotensin receptors). Scientists searched for these receptors in animal brains. However, the search results sparked debate. Many believed these receptors were located in a part of the brain called the organum vasculosum of the lamina terminalis. Yet, several scientists destroyed this part of the rat's brain and injected angiotensin, expecting the rats to stop feeling thirsty, but after a week or two, the rats still exhibited intense thirst when injected with angiotensin. Other researchers argued that the organum vasculosum is not the only area that can trigger thirst; there must be other parts of the brain involved, which have yet to be discovered. Additionally, some scientists severed the vagus nerve near the diaphragm in rats. After the rats returned to normal, they attempted to induce thirst using various methods (including the previously mentioned low blood volume method), but the rats showed no thirst response at all. Thus, these scientists concluded that thirst is not solely determined by the brain; other systems must also be involved in generating the sensation of thirst.
While it is universally accepted that low blood volume can cause thirst, the exact mechanism behind it remains a topic of debate, and no satisfactory answer has been reached to date.
2. The Secret of Pain
Few people have not suffered from pain. Toothaches, headaches, stomachaches, and many others are part of life’s trials. According to medical experts, there are over a thousand types of pain. Some pains, like toothaches or post-surgical pain, can be terrifying at the moment but fade away once the illness is resolved. However, some pains persist, tormenting individuals for years. In the United States alone, there are approximately 30 million arthritis patients, 70 million with chronic back pain, 20 million with migraines, and millions more suffering from various long-term pains. The most frightening are cancer patients, who constantly live in pain.
Pain is essentially a warning system for the body, alerting us to potential health issues. If a person experiences no pain when ill, it would be disastrous. However, both in China and abroad, cases of children born without the sensation of pain have been documented. These children may not realize they are injured, making them prone to danger.
What causes pain? Why does it strike suddenly without warning? Why does it sometimes persist? These questions remain unanswered.
According to a brain specialist, some types of pain can leave a lasting impression on the nervous system. Even if the source of pain disappears, the sensation can remain, making it as difficult to eliminate as erasing a memory. In ancient Egypt, such pain was attributed to divine or ghostly influences.
Modern understanding of pain has evolved over the past two decades. Contemporary science explains that pain signals are released by certain chemical substances stored near nerve endings. Among these substances are neurochemical agents that can transmit pain information to the brain.
Research indicates that pain is a complex experience, both psychological and physical, often difficult to distinguish. Here’s a story:
A new soldier from the United States was deployed to a Pacific island for the first time. When a shell exploded nearby, he felt intense pain. He was carried to a medical station, where doctors found no injuries, only that his water bottle had been hit. He was sent back to the front lines. As the shelling intensified, he suddenly felt a sharp pain in his head, and blood began to flow. Upon examination, doctors found only minor injuries from shrapnel embedded in his face. After treatment, he returned to the front lines, where almost all his comrades were killed. The third time, his foot was blown off, but he felt no pain at all.
After sharing his experience with an expert, he remarked that the most painful moment was when his water bottle was hit, followed by the injury to his face. The expert explained, "The perception of pain varies from person to person." Fear, anxiety, tension, and the anticipation of disaster can sometimes amplify pain. However, strong motivation can also suppress the sensation of pain. In the movie "Little Soldier Zhang Ga", when Zhang Ga was chasing the enemy and was shot, he didn’t even realize he was injured; this is a representative example.
Additionally, we can adopt various methods to alleviate pain, such as using analgesics, anesthetics, or even listening to music.
3. The Secret of Breathing
Our lungs are incredibly busy. They continuously inhale fresh air, extracting oxygen while expelling carbon dioxide, the waste produced by the body. Even while sleeping, they never rest.
The human body cannot survive without oxygen, nor can it tolerate the accumulation of waste gases. Otherwise, all chemical processes in the body would be thrown into chaos!
However, did you know that before birth, a fetus’s lungs are collapsed, containing no air at all? Instead, they are filled with fluid (medically referred to as "lung liquid"). Thus, upon birth, complications arise.
First, all the fluid in the lungs must be expelled; otherwise, how can air enter?
Second, the collapsed lungs need to expand, which requires the newborn to take a breath. But how does a newborn know to breathe upon birth? How does this first breath occur?
4. The Mysterious Disappearance of "Water"
The fluid in a fetus's lungs can range from 60 to 200 milliliters, which is quite significant.
Yet, when a baby is born, as soon as they take their first breath, most of this fluid seems to vanish. Where does it go? Medical researchers have been trying to find out.
Some medical experts suggest that as the fetus exits the mother's "home" (the uterus), it must pass through a narrow and constricted passage (the birth canal), which helps expel the lung fluid through the mouth and nose. Measurements indicate that about two-thirds of the expelled fluid comes from this process. So, what happens to the remaining fluid?
Further studies have shown that when a newborn takes their first breath, they exert considerable effort, drawing in a large volume of air, which then pushes the remaining fluid from the lung's alveoli. The lymphatic vessels in the lungs quickly absorb this fluid. After a few breaths, the lungs are cleared of any remaining fluid!
However, this explanation faced opposition from other medical experts, who noted that cesarean-born infants do not pass through the birth canal, yet their lung fluid is still expelled quickly. Additionally, relying on lymphatic vessels to absorb fluid may not be effective if the volume is too great.
To this day, researchers still do not know how the fluid in a newborn's lungs is removed!
5. How Do Collapsed Lungs Expand?
The second intriguing question is: how do a newborn's lungs begin to breathe upon birth?
Most medical experts believe that when a newborn leaves the warmth of the mother’s womb for the cool outside world, the cold stimulates the lung nerves responsible for breathing, thus initiating respiration. They conducted experiments with sheep fetuses, placing them in water at 10°C. Initially, the fetuses did not breathe, but as the water temperature increased, they continued to breathe until it reached 40°C, at which point breathing ceased. This suggests that cold is a trigger for initiating breathing in fetuses.
However, some experts disagree, arguing that some infants do not breathe at birth, regardless of how much cold water is applied. Furthermore, in tropical regions, temperatures can exceed maternal body temperature, yet infants still initiate breathing regardless of external temperature.
Thus, another hypothesis emerged. It is suggested that humans have an innate tendency to gasp when startled or faced with unexpected events. The birth experience is a sudden change that may startle the infant into taking a gasp, thus beginning respiration. However, serious researchers have measured the force of a newborn's first breath, which is three times stronger than a typical breath, while the gasp from being startled is not nearly as forceful. Therefore, this explanation seems somewhat forced.
Other medical experts propose that after birth, the oxygen supplied by the mother ceases, but the newborn's lungs have not yet received the signal to start breathing. Consequently, the body experiences a lack of oxygen and an accumulation of waste gases, prompting the brain to react to the dual stimuli of low oxygen and waste buildup. This urgency may awaken the part of the nervous system that controls breathing, sending commands to the lungs to start breathing. This explanation sounds reasonable, but opposing experts raise a question: during the fetal stage, the oxygen supply is already minimal, so this little oxygen is sufficient for them. Thus, the fetal brain has adapted to low oxygen levels, and even if the newborn does not breathe for two to three minutes, would it truly panic?
There are many other theories surrounding this topic. The multitude of explanations indicates that the correct answer has yet to be found.
