[MUSIC] High altitude environments are challenging to live in. Yet plants and animals, including humans, thrive here as a result of different physiological responses and genetic adaptations. In this lesson, we'll explore the effects of high-altitude environments on humans. We will then identify physiological responses that allow humans from lowland environments to survive in these challenging landscapes. And discuss genetic adaptations that allow humans to inhabit high-altitude mountainous regions. To start, let's hear from Phillip Ainslie, a physiologist at the University of British Columbia, Okanagan. Doctor Ainslie will give us a brief overview of what happens to our bodies at high elevations, including an illness called Acute Mountain Sickness, which we will discuss in more detail later on. >> Certainly at high altitude the body responds in a myriad of ways, depending on how high you are and how quickly you get there. Upon initial arrival, probably the best change is increase in breathing, so you ventilate more per breath. That's by far the most beneficial process you do retain. And that happens immediately within minutes of arrival, and carries on for years. The second benefit you get is an increase in haemoglobin and red blood cell mass, which helps the transport of oxygen around your body. There's really those two factors which are the main beneficial things which occur, ventilation first, then changes in blood parameters second. There's a number of negative consequences of high altitude. Blood pressure becomes higher, the pressure in your lungs becomes higher. And initially you can be very prone to all aspects of altitude illness. Acute Mountain Sickness is very, very common, it'll effect 50 to 60% of trekkers, or climbers who go at high altitude. If you've not had it before, it's very similar to a really bad hangover. And pretty bad headaches, nausea, vomiting, dizziness, and milder symptoms, but headache normally comes first. >> Let's examine more closely some of the effects, responses, and adaptations to high-altitude environments. First we need to review some fundamental principles. Why do humans need oxygen, and how is it transported through our bodies? At a very basic level, humans need oxygen to provide fuel for a series of reactions in our bodies that convert glucose, or sugar, to useable energy. Without this usable form of energy, we wouldn't have the ability to carry out essential processes, such as thinking, walking, talking, or digesting food. You probably know that oxygen travels through our body in our blood. However, it's the details of this movement that we need to review. If we were to rely only on the liquid component of our blood, called plasma, to deliver oxygen to our tissues, we would need to circulate over 180 liters of blood per minute. To do this, your heart would need to be several times larger than it is. Instead we only circulate about 5 liters of blood per minute, and still get enough oxygen to where it's needed. How is this possible? The answer is haemoglobin, a protein found in red blood cells that facilitates the transport of oxygen. In fact, haemoglobin transports about 97% of oxygen in our blood, while only 3% is transported in plasma. Haemoglobin has a high affinity for oxygen. So when oxygen dissolves in blood, it quickly moves out of the plasma to bind to haemoglobin. Haemoglobin consists of four protein molecules, and each of these protein molecules can carry an oxygen molecule. When we talk about oxygen saturation in the blood, we are referring to the percentage of haemoglobin protein molecules that have oxygen attached to them. The affinity between oxygen and haemoglobin is so high that at sea level, even during strenuous activity, the amount of oxygen saturation is 100%. This means that we're able to move enough oxygen from the lungs to our blood, so that 100% of the haemoglobin in our red blood cells is bound to oxygen. However, the challenges associated with high altitude changes this relationship. We'll come back to this later. For now, let's spend some time thinking about atmospheric pressure. Why does breathing become more difficult at higher altitudes? [SOUND] Atmospheric pressure is the pressure exerted by the weight of air in the atmosphere of the earth. Areas of low pressure have less atmospheric mass above their location. Whereas high pressure areas have greater atmospheric mass above their location. However, as elevation increases, there's less overlying atmospheric mass, so that atmospheric pressure generally decreases with increasing elevation. Atmospheric pressure was first measured in 1643, when Evangelista Torricelli invented the mercury barometer. A mercury barometer consists of a mercury filled glass tube, closed at the top and open at the bottom. This glass tube sits in a container that's also filled with mercury and open to the atmosphere. When the mercury level in the tube falls, it creates a vacuum in which gas pressure is less than atmospheric pressure. The barometer works by equalizing the pressure exerted by the mercury in the tube, and the atmospheric pressure. As atmospheric pressure increases, the greater downward pressure on the mercury in the container causes it to push up in the mercury in the tube, raising its height. In contrast, when atmospheric pressure decreases, reduced pressure on the mercury in the container causes it to exert less upward force on the mercury in the tube, making it fall. As a result of the use of barometers as instruments to measure atmospheric pressure, atmospheric pressure is sometimes also referred to as barometric pressure. In 1648, five years after Torricelli invented the mercury barometer, Blaise Pascal created an experiment to test his hypothesis that atmospheric pressure decreased with altitude. One person monitored atmospheric pressure at the base of the Puy-Do-Dome Mountain in France. And others trekked 1,000 meters up the mountain to measure atmospheric pressure there. They found that as they climbed the mountain, the mercury in the barometer fell. This provided the first conclusive evidence that atmospheric pressure decreases with altitude. The air we breathe at sea level is the same air we breathe on a mountain peak. This air is comprised of approximately 78% nitrogen and 21% oxygen. So how does decreased atmospheric pressure reduce our ability to intake oxygen? To answer this question, we need to first understand how oxygen is transported from the air into our tissues, a process called respiration. In humans, the respiratory system consist of our lungs and a series of connected tubes that transfer air to and from the lungs. Breathing moves air into the lungs where there are millions of tiny air sacs, called alveoli. Here oxygen moves through the thin walls of the alveoli and enters into the bloodstream, where it's circulated to oxygenate other tissues. At the same time, blood that is already circulated through the body returns to the lungs. This blood is relatively depleted of oxygen, and has a higher concentration of carbon dioxide. This carbon dioxide moves from the blood into the alveoli where it will be exhaled. The movement of oxygen and other gases between the alveoli and the bloodstream occurs through a process called diffusion, where gasses move from areas of high concentration to low concentration. The difference in the concentration of a substance, such as oxygen, between two areas is called a concentration gradient. This is similar to the pressure gradients that we discussed in the previous lesson on climate. The rate at which oxygen diffuses into blood depends on two properties, the surface area of the tissue across which diffusion occurs, and the concentration gradient. A larger surface area, or steeper concentration gradient, will both increase the rate of diffusion. The process of diffusion in our lungs is highly efficient because we have millions of alveoli, which together have a surface area equal to that of a tennis court. Because the surface area of our alveoli is consistently high, the rate of oxygen diffusion into our blood depends mostly on its concentration gradient. The concentration gradient of a gas, such as oxygen, is determined by the difference in its partial pressures between locations. Partial pressure is the pressure a gas exerts independent of others in a mixture. For example, oxygen makes up about 21% of the gases in the atmosphere. The partial pressure of oxygen is then equal to 21% of the total pressure of the atmosphere. In this sense, the partial pressure of oxygen in air is proportional to the total atmospheric pressure. As we've discussed in previous lessons, the total atmospheric pressure is reduced at high altitudes. Therefore, the partial pressure of oxygen at high altitudes is less than at sea level. At the same time, the partial pressure of oxygen in the venous blood returning to or entering the lungs is relatively similar, both at sea level and at higher altitudes. This means that the concentration gradient of oxygen between the lungs and the blood is reduced at higher altitudes, because there's a smaller difference in the partial pressure of oxygen. Decreased atmospheric pressure reduces our ability to uptake oxygen. As a result, the rate of diffusion of oxygen into blood is slower, and it's difficult to fully saturate haemoglobin on our red blood cells. To summarize, the decreased concentration gradient of oxygen leads to decreased oxygen diffusion from the lungs into the blood. Less oxygen in the blood means there is less haemoglobin saturation. This reduction in haemoglobin saturation, and really the reduction in oxygen, can lead to serious health problems. However, there are some remarkable adaptations that humans have, which operate to reduce these effects.
