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Living organisms carry out a wide range of functions each utilizing a unique set of anatomical structures. While two different organisms may share anatomical or physiological similarities, all organisms have at least one unique feature and function. However, in order to be considered living, every organism must meet a strict set of basic criteria:
Living organisms must be composed of one or more (1+) cells. Cells are the smallest unit considered living by science. Because of this, they are also known as the basic unit of all living things—all living things must be fundamentally composed of cells. Single-celled organisms such as bacteria only have one cell and are very small (i.e., yeast, E. coli). Multicellular organisms have more than one cell and can range from very small (i.e., tardigrade) to very large (i.e., elephant). Both categories satisfy the requirement of being composed of cells.
Living things must contain more complexity in their organization than an inanimate (non-living) object.
The adult human body is composed of approximately 37 trillion cells organized in specific locations, groups, and times. Each cell contains multiple internal compartments and structures which serve their own individual and shared cellular functions. These structures are composed of multiple chemicals which have their own unique properties. Some cells work alone while others work with one another to perform collective tasks. These tasks can change based on location in the body, internal environmental conditions, or time of day.
In comparison to living organisms, inanimate, or non-living, objects have significantly less complexity in their organization.
Living organisms must grow and develop over time.
Growth is the increase in size. Humans, like all multicellular organisms, grow by increasing the number of existing cells, increasing the amount of non-cellular material around cells (such as mineral deposits in bone), and, within very narrow limits, increasing the size of existing cells.
Development is all of the changes the body goes through in life. Development includes the process of differentiation, in which unspecialized cells become specialized in structure and function to perform certain tasks in the body. Development also includes the processes of growth and repair, both of which involve cell differentiation. You will learn more about cellular differentiation later in this course.
Living organisms must take in and use energy.
Cells require energy to perform certain functions such as growth and development. In order to obtain that energy, organisms must consume molecules (i.e., food) and convert them into energy. Humans consume molecules in food and beverages, some of which are converted into energy. That energy will be utilized for movement or building and maintaining body structures.
Living organisms must maintain a relatively stable internal environment.
All organisms survive within a range of various conditions including but not limited to temperature and concentration. If the conditions go outside of the normal range, various diseases or disorders can develop. If these changes become exaggerated or prolonged, it can result in death.
The body has anatomical features and physiological processes that function within specific ranges to keep the internal environment stable in order to maintain life.
For example, on a hot day, the temperature of the human body begins to rise. In order to maintain a stable internal temperature, the body can sweat to cool itself. In cold weather, the body can shiver to generate extra heat. These processes work to maintain the internal body temperature.
Living organisms must be able to respond (adjust) to changes in the internal or external environment, also known as responsiveness.
An example of responsiveness to external stimuli could include moving toward sources of food and water and away from perceived dangers. Changes in an organism’s internal environment can include increased or decreased light. Under low light, the black pupils in the eye expand to allow more light in. Under bright light, the pupils will constrict to allow less light in. This is the body’s response to changes in the environment.
Living organisms must be able to reproduce using DNA.
Reproduction is the formation of a new organism from parent organisms. In humans, reproduction is carried out by the male and female reproductive systems. Because death will come to all complex organisms, without reproduction, the line of organisms would end.
Almost every cell contains DNA, the genetic material that contains the instructions for life. When an organism reproduces, it provides the new organism with its own set of DNA. This process allows for the inheritance of genetic traits, evolution, and more.
Humans have been acclimating to life on Earth for at least the past 200,000 years. Earth and its atmosphere have provided us with air to breathe, water to drink, and food to eat, but these are not the only requirements for survival. Although you may rarely think about it, you also cannot live outside of a certain range of temperature and pressure that the surface of our planet and its atmosphere provides. The next sections explore these four requirements of life.
Humans require oxygen to make energy.
Atmospheric air is only about 20 percent oxygen, but oxygen is a key component of the chemical reactions that keep the body alive, including the reactions that produce energy. Brain cells are especially sensitive to a lack of oxygen because of their requirement for a high-and-steady production of energy. Five minutes without oxygen is likely to result in brain damage, and death is likely within ten minutes. This can occur if breathing is restricted or the delivery of oxygen is restricted in the case of a broken blood vessel during a stroke.
Humans require nutrients to produce energy as well as build and maintain body structures.
A nutrient is a substance in foods and beverages that is essential to human survival. The basic classes of nutrients are macronutrients and micronutrients (vitamins and minerals). These groups are defined by the amount the body requires. Macronutrients are required in large amounts while micronutrients are required in small amounts.
The most critical nutrient is water, one of the macronutrients. Depending on the environmental temperature and our state of health, we may be able to survive for only a few days without water. The body’s functional chemicals are dissolved and transported in water, and the chemical reactions of life take place in water. Moreover, water is the largest component of cells, blood, and the fluid between cells, and water makes up about 70 percent of an adult’s body mass. Water also helps regulate our internal temperature and cushions, protects, and lubricates joints and many other body structures.
The remaining macronutrients are carbohydrates, lipids (fats), and proteins. These large molecules yield energy and building blocks for building and maintaining cellular and body structures. You ingest these nutrients in foods and beverages, and the digestive system breaks them down into molecules small enough to be absorbed. Although you might feel as if you are starving after missing a single meal, you can survive without consuming these macronutrients for at least several weeks.
Micronutrients are vitamins and minerals. These elements and compounds participate in many essential chemical reactions and processes, such as nerve impulses, and some, such as calcium, also contribute to the body’s structure. Your body can store some of the micronutrients in its tissues, and draw on those reserves if you fail to consume them in your diet for a few days or weeks. Some other micronutrients, such as vitamin C and most of the B vitamins, are water-soluble and cannot be stored, so you need to consume them every day or two.
The body can also respond effectively to short-term exposure to cold. One response to cold is shivering, which is a random muscle movement that generates heat. Another response is the increased breakdown of stored energy to generate heat. When that energy reserve is depleted, however, and the core temperature begins to drop significantly, the brain will be deprived of oxygen and therefore energy, causing confusion, lethargy, and eventually loss of consciousness and death.
IN CONTEXT
Everyday Connection - Controlled Hypothermia
As you have learned, the body continuously engages in coordinated physiological processes to maintain a stable temperature. In some cases, however, overriding this system can be useful, or even life-saving. Hypothermia is the clinical term for an abnormally low body temperature (hypo below or under). Controlled hypothermia is clinically induced hypothermia performed in order to reduce the metabolic rate of an organ or of a person’s entire body.
Controlled hypothermia often is used, for example, during open-heart surgery because it decreases the metabolic needs of the brain, heart, and other organs, reducing the risk of damage to them. When controlled hypothermia is used clinically, the patient is given medication to prevent shivering. The body is then cooled to 25–32°C (79–89°F). The heart is stopped and an external heart-lung pump maintains circulation to the patient’s body. The heart is cooled further and is maintained at a temperature below 15°C (60°F) for the duration of the surgery. This very cold temperature helps the heart muscle to tolerate its lack of blood supply during the surgery.
Some emergency department physicians use controlled hypothermia to reduce damage to the heart in patients who have suffered a cardiac arrest. In the emergency department, the physician induces coma and lowers the patient’s body temperature to approximately 91 degrees. This condition, which is maintained for 24 hours, slows the patient’s metabolic rate. Because the patient’s organs require less blood to function, the heart’s workload is reduced.
Pressure is a force exerted by a substance that is in contact with another substance. Atmospheric pressure is pressure exerted by the mixture of gasses (primarily nitrogen and oxygen) in the Earth’s atmosphere. Although you may not perceive it, atmospheric pressure is constantly pressing down on your body. This pressure keeps gasses within your body, such as the gaseous nitrogen in body fluids, dissolved. If you were suddenly ejected from a spaceship above Earth’s atmosphere, you would go from a situation of normal pressure to one of very low pressure. The pressure of the nitrogen gas in your blood would be much higher than the pressure of nitrogen in the space surrounding your body. As a result, the nitrogen gas in your blood would expand, forming bubbles that could block blood vessels and even cause cells to break apart.
Atmospheric pressure does more than just keep blood gasses dissolved. Your ability to breathe—that is, to take in oxygen and release carbon dioxide—also depends upon a precise atmospheric pressure. Altitude sickness occurs in part because the atmosphere at high altitudes exerts less pressure, reducing the exchange of these gasses, and causing shortness of breath, confusion, headache, lethargy, and nausea. Mountain climbers carry oxygen to reduce the effects of both low oxygen levels and low barometric pressure at higher altitudes. This is also reflected when people mention that the air is “thinner” when traveling to higher altitudes such as the “mile high city,” Denver, Colorado (1592 m / 5223 ft above sea level).
IN CONTEXT
Homeostatic Imbalances - Decomposition Sickness
Decompression sickness (DCS) is a condition in which gasses dissolved in the blood or in other body tissues are no longer dissolved following a reduction in pressure on the body. This condition affects underwater divers who surface from a deep dive too quickly, and it can affect pilots flying at high altitudes in planes with unpressurized cabins. Divers often call this condition “the bends,” a reference to joint pain that is a symptom of DCS.
In all cases, DCS is brought about by a reduction in barometric pressure. At high altitude, barometric pressure is much less than on Earth’s surface because pressure is produced by the weight of the column of air above the body pressing down on the body. The very great pressures on divers in deep water are likewise from the weight of a column of water pressing down on the body. For divers, DCS occurs at normal barometric pressure (at sea level), but it is brought on by the relatively rapid decrease of pressure as divers rise from the high pressure conditions of deep water to the now low, by comparison, pressure at sea level. Not surprisingly, diving in deep mountain lakes, where barometric pressure at the surface of the lake is less than that at sea level is more likely to result in DCS than diving in water at sea level.
In DCS, gasses dissolved in the blood (primarily nitrogen) come rapidly out of solution, forming bubbles in the blood and in other body tissues. This occurs because when pressure of a gas over a liquid is decreased, the amount of gas that can remain dissolved in the liquid also is decreased. It is air pressure that keeps your normal blood gasses dissolved in the blood. When pressure is reduced, less gas remains dissolved. You have seen this in effect when you open a carbonated drink. Removing the seal of the bottle reduces the pressure of the gas over the liquid. This in turn causes bubbles as dissolved gasses (in this case, carbon dioxide) come out of solution in the liquid.
The most common symptoms of DCS are pain in the joints, with headache and disturbances of vision occurring in 10 percent to 15 percent of cases. Left untreated, very severe DCS can result in death. Immediate treatment is with pure oxygen. The affected person is then moved into a hyperbaric chamber. A hyperbaric chamber is a reinforced, closed chamber that is pressurized to greater than atmospheric pressure. It treats DCS by repressurizing the body so that pressure can then be removed much more gradually. Because the hyperbaric chamber introduces oxygen to the body at high pressure, it increases the concentration of oxygen in the blood. This has the effect of replacing some of the nitrogen in the blood with oxygen, which is easier to tolerate out of solution.
The dynamic pressure of body fluids is also important to human survival. For example, blood pressure, which is the pressure exerted by blood as it flows within blood vessels, must be great enough to enable blood to reach all body tissues, and yet low enough to ensure that the delicate blood vessels can withstand the friction and force of the pulsating flow of pressurized blood.
Source: THIS CONTENT HAS BEEN ADAPTED OPENSTAX "ANATOMY AND PHYSIOLOGY 2E" AT openstax.org/details/books/anatomy-and-physiology-2e
