2 Vital Functions for Human Life

In this section, you will be introduced to the major organ systems of the body. We have grouped the organ systems according to the Vital Functions they perform. In the units that follow, with the exception of Levels of Organization, and Homeostasis, you will learn and explore each body system in-depth. To put these systems in context, we will first discuss vital functions of life. Think about your own body and its many functions. Which functions are essential? What functions does your body need to perform each day in order to survive?

Within any organism, there are a multitude of functions taking place at any given time. Humans, for example, can breathe, talk, digest food, process visual images, and move their bodies all at the same time. While all of these activities are important, some are essential to the survival of the human body itself. They are vital functions – processes or actions of the body on which life is directly dependent. An organ system is an integrated collection of organs in the body that work together to perform a vital function. This course will organize the organ systems of the body based on the vital functions defined below.

All multicellular organisms need to do the following in order to survive:

• Exchange material with the environment
• Transport fluids and materials within the body
• Facilitate structure, support and movement
• Regulate and control processes

Exchange with the Environment

An organism constantly interacts with its environment. Every cell needs to take in nutrients and get rid of waste products. This is accomplished via processes that exchange molecules across the cell membrane. For a single-celled organism, this is accomplished through direct exchange with the environment. It gets more complicated with larger organisms where cells are not in direct contact with the “outside” environment. In such organisms, including humans, there are specialized organ systems that serve as interfaces with the environment, moving things into, and out of, the body to keep it functioning. In order to survive, the human body must obtain food, water, and oxygen from the world around it. The human body must also rid itself of wastes before they build up to toxic levels. Two organ systems are primarily responsible for exchange of material with the environment.

The digestive system brings food and water into the body and eliminates solid wastes. During digestion and metabolism, food is broken down into simpler substances (including some waste products), and some of those simpler substances—for example, simple sugars—can be metabolized in a way that they release energy, especially when oxygen is present. The products of these processes include solid waste, liquid waste, and carbon dioxide. These waste materials must be eliminated from the body to prevent illness.

The digestive system is a continuous tube (the digestive tract or alimentary canal). Areas along this tube are specialized to perform different functions related to getting the nutrients from your food to the cells that need them. Accessory organs add secretions into different areas along the tube.

The respiratory system brings in oxygen and removes carbon dioxide. When we take a breath, air enters and exits via the respiratory system. This allows the body to obtain oxygen, which is needed for metabolic processes, and eliminate carbon dioxide, which is a metabolic waste product and can affect the body’s pH homeostasis. Like the digestive system, the respiratory system can be thought of as a tube, or rather, as a branching series of tubes that get smaller and smaller as they branch off. Unlike the digestive system, which moves solids and liquids in a single direction, the respiratory system moves gases in both directions, when we inhale and exhale.

The urinary system doesn’t take in anything from the environment, but does eliminate waste products of metabolism from the body fluid. One of the most important of the waste products removed from the blood is urea, the main end product of protein metabolism. Other waste products and some toxins are also removed from the blood by the kidneys. However, this is just one of several roles that the urinary system plays in maintaining homeostasis of body fluids, so it will be explored more extensively in the next vital function category involving body fluids.

Fluid Transport within the Body

Single-celled organisms can absorb nutrients and oxygen directly from the environment into the cells, where they are used to support basic cell functions. Waste products are excreted from these single cells in a similar fashion. In multi-celled organisms like humans, however, most cells are not exposed directly to the outside environment. Instead, body cells rely on organ systems to transport and regulate the composition of internal fluids throughout the body. Our body constantly transports molecules from one place to another to carry out the required metabolic functions for daily living. As we take in nutrients and oxygen, these molecules must be delivered to the cells, tissues, and organs that need them for metabolic processes. Furthermore, our body must have a system for disposing of wastes generated by metabolism and foreign particles, which may potentially disturb optimal body function. Three main body systems, the cardiovascular system, the lymphatic system, and the urinary system take care of this vital bodily function. The cardiovascular and lymphatic systems also participate in the function of immunity, to help defend the body’s cells from foreign organisms that may enter the body tissues or fluids. The lymphatic system returns interstitial fluid to blood and the urinary system filters blood to regulate body fluid homeostasis, including volume, pressure, and chemical composition.

The cardiovascular system consists of the blood, heart, and blood vessels. The heart supplies the force to circulate blood throughout the body via the blood vessels. This system also helps to maintain homeostasis of fluid volume, pH, and temperature. Blood is the main fluid used to deliver nutrients to the body’s tissues and to receive wastes that will ultimately be removed from the body by the urinary system. The respiratory and cardiovascular systems work together to take oxygen in from the external environment and deliver it, via blood, to cells. In a similar fashion, these systems also use the blood to remove carbon dioxide from cells and release it into the external environment. The red blood cells or erythrocytes are responsible for the transport of respiratory gasses.

The lymphatic system consists of lymphatic vessels which are near blood vessels and several organs such as lymph nodes, the thymus gland, and spleen. It returns excess tissue fluid to the cardiovascular system while initiating immune responses to ensure that the delivered blood is not compromised, responding to foreign invasion in multiple ways. The lymphatic system is an integrated part of the immune system to cleanse body fluids of dead cells, toxins, and pathogens. Together, the transport systems of the body coordinate the efforts of nutrient delivery and removal of wastes and other harmful substances. In addition to transporting the fluid, fluid/electrolyte balance must be maintained by integrated function of these organ systems: urinary, lymphatic and cardiovascular.

Our body is in constant exchange with the environment, through breathing, eating and other activities. Therefore, it is important to screen the body and its components regularly to identify foreign invaders that might enter during these activities (or in any other manner). Further, it is important to rapidly and effectively remove these invaders before they can cause significant harm. Our body has specialized transport systems to carry out these functions. The cardiovascular and lymphatic systems work together to transport excess fluids (blood and lymph fluid, respectively) away from body tissues. Once fluid enters the lymphatic system it is termed lymph. Lymph travels through lymph vessels and passes through many lymph nodes which filter and clean the lymph. One additional function of the lymphatic system is to transport absorbed fat from the digestive system to the body cells. The immune system also produces and matures immune cells, which protect the body from invasion by agents that cause disease.

The urinary system filters blood and adjusts the composition of blood/interstitial fluid by removing excess water, salt, acid, and metabolic waste from the body as urine. This allows the urinary system to control body fluid volume, blood pressure, pH, and electrolyte balance. It is a critical system for maintaining homeostasis. The kidneys, the main organs of the urinary system, serve as a filtration and reabsorption system, where soluble substances are filtered and then those that the body needs to keep are reabsorbed.

Structure, Support, Protection, and Movement

For the organs of the human body to function, they must be protected from potentially damaging substances in the environment. One level of defense is provided by the integumentary system, made up of the skin, hair, and nails. This system prevents many potentially harmful irritants from entering the body. Eyelashes, for example, help keep sand or other items out of the eyes, where they could potentially cause serious damage, and the skin prevents most pathogens (disease-causing microorganisms) from entering the body and destroying healthy body cells. Certain parts of the skeletal system, such as the skull and ribcage, also help to protect the internal organs, such as the brain, heart and lungs, from damage. The skeletal system and the muscular system also support the body and allow it to move away from danger, toward food sources, etc. The cardiovascular and lymphatic/immune systems also help defend the body’s cells from foreign organisms that may enter the body tissues or fluids through the process of immunity. The immune system produces immune cells, which protect the body from invasion by agents that cause disease.

The skeletal system, which includes the skeleton and articulations (joints), provides support and protection for soft tissues and organs, aids in movement, serves as a reservoir of calcium, and produces all blood cells. The numerous organs and structures of the skeletal system allow it to serve an important role in the support and protection of our body. Bones are very strong, yet flexible which makes them perfect for supporting our weight and allowing movement. The connective tissues such as cartilage, ligaments, and tendons aid in protecting our joints and providing stability. The red bone marrow inside the bone is vital for hematopoiesis or the production of all blood cells. Bones are also a reservoir for calcium. If your diet is deficient in calcium, a hormone will mobilize calcium from the bones to the blood, and your bones will be weaker.

The muscular (musculoskeletal) system generates force for movement of bones around articulations, facial expression, breathing, posture, and assists with temperature regulation. The muscular system contains muscle tissues and interconnects with both the nervous system and skeletal system. Nerves control the muscles and allow us to consciously direct movements. Some muscles, such as the muscles that control the pupil of your eye, cannot be controlled consciously but react to nerve stimuli. The skeletal system provides a stiff support for muscles to pull on. Muscles generate force to lift as well as to balance us. The energy produced by contracting muscles (such as when shivering) in the muscle system helps keep us warm.

The integumentary system is one of the most active parts of our body, even though we are not as aware of its activity as we are with the heart, lungs or stomach. The integumentary system encapsulates and protects the body. The skin is actually the largest organ in the body because of its large surface area. The skin integrates with muscles and allows for movements such as facial expression. It also contains many nerves that are related to the sense of touch. The skin can also be thought of as an immune system organ, since it protects the body from foreign organisms. The immune system also coordinates with other systems to respond to disease and infection. This response can provide two types of immunity:

• Nonspecific (innate) immunity, in which the body uses several general methods such as physical barriers (that is, the skin and mucous membranes), fever, inflammation, specific action by immune cells, and enzyme activity to protect itself against general harmful agents.
• Specific (adaptive) immunity, in which specialized cells (such as T and B cells) recognize specific foreign molecules called antigens within the body and respond to them.

Control and Regulation

To keep itself in a state of equilibrium, an organism must constantly gather information and react accordingly. In humans, the nervous system, made up of the brain, nerves, spinal cord and sensory organs, reacts to stimuli in the environment and signals other systems when actions are needed to bring the body back into balance. The endocrine system, which produces hormones and other regulatory substances, plays a key role in maintaining balance among chemical messengers within the body. Locally, most body cells can produce chemical messages that influence the metabolism of other cells. And there are some organs in other body systems that produce chemicals that can travel through the body to regulate metabolic processes in other organs. The nervous and endocrine systems work together to control many of the body’s other organ systems through electrical signals and chemical messengers.

The nervous system transmits signals to different parts of the body to coordinate function. The basic functional units of the nervous system that transmit messages are cells called neurons. Signals travel through a neuron as electrical impulses. Neurons release chemical substances, known as neurotransmitters, to transmit information to other neurons, to muscles, or to glands. These electrochemical signals are processed in the brain and sent down the spinal cord, which runs the length of the back. From the spinal cord, peripheral nerves send signals out to the extremities. Return signals come in through sensory nerves and either return to the spinal cord for processing or back to the brain. The spinal cord processes reflexes and repeated patterns.

The endocrine system is an equally important method of sending messages within the body for control and coordination of multiple body systems. The functional unit of the endocrine system is a gland, or a group of cells that secrete chemicals called hormones. Hormones circulate throughout the body within the bloodstream and act as long-term messengers. In comparison with neurotransmitters, hormones act over long distances for a longer time.

The systems integrate with each other, so that control systems within the nervous system regulate many activities of the endocrine system and hormones of the endocrine system can affect some of the functions of the nervous system. The chemical messengers produced by each individual system are responsible for many homeostatic functions, through feedback mechanisms. The activities of the two systems coordinate many of the body’s metabolic activities.

As you can see, several organ systems work together to accomplish these various vital functions throughout the body. Since the organ systems are distributed throughout large regions of the human body, it is necessary to define orientation within the body and communicate the proper terminology as you study these integrated structures and functions.