Essentials for Life

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Chapter: Anatomy and Physiology for Health Professionals: Levels of Organization : Introduction to Human Anatomy and Physiology

1. What factors are necessary to sustain life in humans? 2. What elements are needed by the body for survival? 3. Describe metabolism and the effect of atmospheric pressure on the body.

Essentials for Life

Humans and other animals share many similar traits. All body cells are interdependent as we are multicellu-lar organisms. Vital body functions occur over various organ systems, which contribute to overall body health.


The body’s boundaries are maintained to keep the internal environment distinct from the external environment. All body cells are surrounded by selectively permeable membranes. The skin encloses and protects the body as a whole from factors such as dryness, bacteria, heat, sunlight, and chemicals.


Movement of the body is achieved via the muscular and skeletal systems. Inside the body, the cardiovascular, digestive, and urinary systems too use movement to transport blood, food materials, and urine. Even cells move, such as when muscle cells move by shortening, which is known as contractility.


The ability to sense and respond to environmental stimuli (changes) is known as responsiveness, which is also referred to as excitability. An example is the way we quickly withdraw our hands from a hot saucepan. Nerve cells are highly excitable. They com-municate with rapid electrical impulses, and, there-fore, the nervous system is the most responsive of all body systems. However, all body systems have some degree of excitability.


Humans require specific nutrients to remain healthy and to grow and develop normally. Energy is gained from the breakdown, digestion, absorption, and assimilation of food. Digestion breaks down food materials to simple, more easily absorbed molecules. Absorbed nutrients move throughout the body’s circulation. Nutrient rich blood is distributed, via the cardiovascular system, to the entire body. Respiration brings in oxygen that works with nutrients to grow and repair body parts. The unusable parts of these processes are then excreted as waste.


The body’s metabolism controls all these processes. It includes all chemical reactions inside body cells, the breaking down of substances into simpler forms (catabolism), creating more complex cellular componentsfrom simpler substances (anabolism), and the use of nutrients and oxygen to produce energy rich adenosine triphosphate (ATP) molecules (via cellular­ respiration).

In metabolism, nutrients and oxygen from the digestive and respiratory systems are circulated to all body cells. Hormones from the endocrine system glands have strong regulatory control over metabolism.


The process of removing wastes from the body is known as excretion. Nonessential substances that are produced during digestion and metabolism must be removed. The digestive system removes food compo-nents that cannot be digested via the feces. The urinary system removes urea and other metabolic wastes containing nitrogen via the urine. The blood carries carbon dioxide to the lungs for it to be exhaled.


Reproduction is a process that occurs at several levels. At the cellular level, reproduction means cell division. Cells divide to produce two identical daughter cells, which the body uses for growth and repair. At the organism level, the human reproduction system unites a sperm with an egg. A fertilized egg is formed, devel-oping into a baby inside the body of the mother. The function of the production of offspring is controlled by endocrine system hormones. Reproductive structures differ between the sexes, with the female structures providing a fertilization site for the male sperm cells. The female reproductive structures protect the developing fetus and nurture its growth until birth.


An increase in the size of an organism or its body parts is called growth. Most often, growth is achieved by an increase in the amount of cells. In fact, even when the cells do not divide, they can increase in size. True growth occurs when constructive activities occur more quickly than destructive activities. The various characteristics of life are listed in TABLE 1-1.


Human beings need several substances for survival: food (nutrients), water, oxygen, pressure, and heat in specific quantities and with specific qualities.


Food provides nutrients for energy, growth, and regulation of the chemical reactions in the body. Some of these chemicals are used as energy sources or sup-ply the raw materials needed for building new living matter; other chemicals help to regulate vital chemical­ reactions. Plant-based foods contain high levels of carbohydrates, vitamins, and minerals. Carbohydrates are the primary energy fuel for body cells. Certain vitamins and minerals are needed for chemical reac-tions inside cells and for oxygen transport in the blood. Calcium is a mineral that assists in making bones harder and is needed for blood clotting. Animal-based foods contain high levels of proteins and fats. Proteins are the most essential component required for build-ing cell structures. Fats assist in this process and are a great source of energy-providing fuel for the body.


Water is required for metabolic processes and makes up most of the body’s actual structure, transporting substances and regulating temperature. It accounts for 60% to 80% of body weight and is the most abundant chemical in the body. Water allows chemical reactions to occur and is also the fluid base for secretions and excretions. Water is mostly obtained from ingested liquids or foods, and is lost in the urine, by evapora-tion from the lungs and skin, and also in other body excretions.


Oxygen is a gas that drives metabolic processes by releasing energy from food that is consumed and by bringing nutrients to cells throughout the body. This energy release involves oxidative reactions, for which oxygen is required. Therefore, all nutrients require oxygen for them to be effectively used. Human cells only survive for a few minutes without oxygen. Oxygen makes up approximately 20% of the air that we breathe. It is made available to the blood and body cells by both the respiratory and cardiovascular sys-tems. Appropriate amounts of oxygen sustain life, but even oxygen may be toxic in excessive quantities.

Atmospheric Pressure

Appropriate pressure, specifically atmospheric pres-sure, is essential for breathing and gas exchange. Blood pressure is a form of hydrostatic pressure that forces the blood through the veins and arteries. Atmospheric pressure may be defined as the force that air exerts upon the body’s surface. Gas exchange, in higher alti-tudes, may be insufficient to support cellular metabo-lism because at these altitudes, atmospheric pressure is lower and the air is thinner. At sea level, the average atmospheric pressure is 760 mm of mercury (Hg).

Body Temperature

Heat energy is produced from metabolic reactions,­ influencing their speed. The muscular system generates the most body heat. Body heat is measured as tempera-ture. Normal body temperature must be maintained if chemical reactions are to sustain life continually. If the temperature is too high, chemical reactions occur very quickly, and proteins in the body change shape and cease functioning. If body temperature­ drops below 98.6°F (37°C), metabolic reactions slow down and eventually stop. Death may occur also because of either variation in temperature.

1. What factors are necessary to sustain life in humans?

2. What elements are needed by the body for survival?

3. Describe metabolism and the effect of atmospheric pressure on the body.

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