Veins in human legs only have one-way valves in them and they work when the skeletal muscles around them contract and release. The flexion/relaxation while walking pumps blood back to your torso. When standing still, you don’t get that pumping action naturally, which is why people can walk miles without any discomfort, but when standing in a place for even a few minutes makes them uncomfortable. Legs are sometimes considered a “second heart.”
Seasoned military personnel can stand at ease for long periods of time because they are actually swaying back and forth very slowly in micro-movements to contract their muscles and relieve the tingling and numb sensation you get when you keep standing for long periods of time. You too can do that if you ever feel lightheaded.
Your brain ignores a lot of sensory inputs. We blank out while driving but still reach our destinations with no incidents, we tune out smells pretty quickly (no matter how bad), we tune out physical sensations (you ignore how your tongue is resting in your mouth, you ignore it but you always see your nose), we ignore the the second ‘the’, we hit autopilot at work, etc.
It's probably an evolutionary trait that evolved from a need to be vigilant against unique sensory input, like sudden movements and noticing things that aren't supposed to be there. We’re good at recognizing patterns, and tune things out to reduce overall taxation from sensory inputs so we can instead focus on novel inputs.
Humans are rare in their inability to synthesize vitamin C. Almost every mammal can do it, with few exceptions like primates and guinea pigs. In fact, humans actually have the genetic coding to make vitamin C, but it’s turned off. A lot of species typically evolve the ability to make vitamins that they can't get easily in their diet. The human body can synthesize its own vitamin D because there aren't many food sources for it, but we can’t make vitamin C because we can find it in a lot of the food we eat. There are lots of other species that make their own vitamin C. It's a tradeoff between needing to find a variety of food and not needing the cellular machines to make more stuff.
Cells in the human body can only proliferate (divide) a certain number of times in a lifetime. This is called the Hayflick Limit. Every time your cells divide they lose a bit of DNA. Eventually, as you age you start losing useful bits of DNA. Human cells can divide about 50ish times before they hit the Hayflick Limit.
Being hard on your body basically boils down to killing off your cells and forcing them to divide to replace themselves, accelerating their aging. For example, if you are a boxer and take a lot of punches to your kidneys, you’ll force your kidneys to repair the damage via cellular division, and those are a finite resource. When people start breaking down of old age what basically happens is the number of cells that can continue to go through healthy divisions starts becoming scarce, so organs have a hard time maintaining themselves.
There’s a reason why you feel hungry in just 6-8 hours even after having a very heavy meal (2000+ calories). Hunger is not directly tied to calorie intake. Your body decides whether you should be eating depending on nutrient levels and how full your stomach is. That system is tuned to our evolutionary past when our diets were much less calorie-dense. When we were eating berries and lean meat, it was not possible to consume 2,000 calories in a single meal, so we had multiple meals per day. Someone who eats all that in a single burger will have enough energy to make it through the day, but their body will eventually start screaming at them to eat something anyway.
If you continue to eat one meal a day regularly though, you’ll eventually condition your body to stop feeling hungry throughout the day.
There’s also a reason for you to not feel hungry after not eating for a long time. If you haven't eaten in a long time, your body doesn't relax, it instead increases stress hormones, i.e., adrenaline and cortisol, which induce the metabolism of energy stores (fat tissues, glycogen) and helps mobilize you so that you can find food or fight for it. Once you eat, stress hormones decrease, blood flow to the intestines and visceral organs is restored, and you feel hungry again. This is why appetizers induce appetite.
There’s a reason we don’t have voluntary control over our sleep. Sleeping literally changes our very physiology. When we fall asleep our core body temperature drops which then allows certain proteins to work differently. It’s not something we'd want easy control over. The process of getting sleepy is highly regulated by not only our circadian rhythm but also by other hormone systems. We need to burn energy to feel fatigued (when we use ATP, we make adenosine as a byproduct, which signals fatigue in humans). We need a lack of blue-wavelength light to initiate the process of releasing melatonin at night, which makes us sleepy and helps initiate the sleeping-end of our circadian processes.
In short, we don't have voluntary control over our sleep because it's chemically and hormonally regulated.
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The human nose as an organ performs many functions aside from just being a conduit to the respiratory and olfactory systems. Within a split second, it modifies the moisture of inhaled air to a constant relative humidity of 75%, and the temperature to a constant of 98.6°F, irrespective of the temperature (winter or summer) outside. It’s the first line of defense for inhaled foreign particles - large particles are captured by hairs and smaller ones get caught in mucus.
The shape of the nose also changes vocal resonance and affects your voice. The nose also helps to keep water out of the body. We can't close our nose, but its shape, position, size of the nostrils, and nostril hairs themselves (which are hydrophobic), all serve to block water from entering the nasal passages.
9Human Mating Strategy
Like all other organisms, the human mating strategy is part and parcel of our overall survival strategy. In our case, we are extreme “K-specialists,” i.e., we devote a huge amount of investment and resources in our offspring, compared to, say, willows who just scatter their seed to the wind by the millions.
Human females have also developed a strategy of concealed ovulation. The current scientific consensus is that by concealing her ovulation and maintaining a perpetual state of potential sexual readiness, the human female makes it difficult for males to know whether her offspring are theirs. The male counter-strategy is to be at hand as often as possible to prevent cuckoldry. Together, this strategy and counter-strategy promote pair-bonding, monogamy, and dual parental investment, thus maximizing parental investment in offspring.
A pimple is an infection. Anaerobic bacteria sometimes colonize a hair follicle and consume the sebum (oily substance) produced by sebaceous glands. They then excrete byproducts that irritate the surrounding area. The resulting inflammatory reaction calls in neutrophils (a type of white blood cell), which just come in and dumps bleach on the bacteria. As neutrophils die, they accumulate and form what we call pus or the “white stuff”. It only has one immediate way out; through the hair follicle to the skin surface. That’s why it exists the way it does.
If left untouched, after a few days the pimple will resolve following absorption back into the body.