It’s one of those biological hierarchies we all learned in high school science class: Complex molecules aggregate to form cells; cells cooperate to form tissues; tissues organize to form organs; a group of organs comprises an organ system; and an assemblage of organ systems constitutes an organism.
So it is with humans, who are, of course, organisms. The conglomeration and interplay of various organ systems is what makes Homo sapiens the fascinating beings we are…and the tendency for these organ systems to malfunction is what justifies the existence of physicians, medicine men, shamans, and others of their ilk.
In fact, there are those within the healers’ realm whose lifelong attentions are focused on the function – and the malfunction – of a specific organ system, or even a single organ or tissue within an organ system.
Anatomy and physiology – two fields of knowledge that partially encompass the nature of organ systems – are complex and fluid disciplines. Practitioners of the healing arts spend their entire lives plumbing the depths of these sciences; therefore, while the uninformed or casual reader can gain a basic understanding of organ systems relatively quickly, extensive understanding only comes from dedicated study.
With that in mind, what follows is a primer on human organ systems and their basic functions.
The Integumentary System (Skin and Associated Structures)
The skin is the largest organ in the body. It separates us from a hostile environment, helps to provide recognizable form (important for attracting mates and expediting other social interactions), aids in excretion of certain toxins and wastes, and helps to stabilize internal temperatures.
Normal skin is composed of two layers: the outer layer, or epidermis, which has four or five layers depending on the skin’s location; and the dermis, a two-tiered layer of connective tissue, elastic fibers, blood vessels, and cells.
The epidermis is a layer of skin cell maturation and renewal; it is also responsible for the various shades of pigmentation that are typical of human skin. The dermis contains the hair follicles and the sebaceous and sweat glands, along with their ducts.
Skin can be thick or thin and hairy or hairless. Skin appendages (sweat and oil glands, nails, and hairs) and sensory glands (for detecting pressure, temperature, pain, etc.) vary in their concentrations throughout the integument, depending on where the skin is located on the body.
The Nervous System
The human nervous system is an incredibly complex organ system. Anatomically, it is composed of the brain, the spinal cord, the special sensory organs (eyes, nose, taste buds, skin receptors, and ears) and a plethora of nerves and supporting cells and tissues.
The nerves in the human body resemble a highly interconnected network of electrical cables and wires. Messages that are conveyed along this network are ultimately controlled by the brain – the “master computer” – which resides within the skull.
The spinal cord, a tubular cable of nerves and neuronal interconnections, resides inside a canal (the spinal canal) which runs the entire length of the backbone; the spinal cord passes through a hole in the bottom of the skull to connect to the base of the brain.
At each level of the backbone, a pair of nerves divides from the spinal cord – much like an exit ramp on a freeway – and exits the spinal canal to travel to specific areas of the body. These nerves carry messages from various regions of the body to the spinal cord and thence to the brain; they also relay messages that convey the brain’s response to incoming data. In this way, the brain regulates much of the body’s reactivity to its surroundings.
Likewise, the nervous system – through two counterbalancing divisions (sympathetic and parasympathetic) – regulates the body’s internal milieu: Blood pressure, hormone secretion, respiratory rate, gastric acid secretion, and a myriad of other physiologic activities are thus either directly or indirectly controlled by the nervous system.
The Cardiovascular System
The heart, arteries, veins, arterioles, venules, and capillaries – along with the fluids they contain – compose the cardiovascular, or circulatory, system. Together, these structures carry oxygen-rich blood to all of the tissues and cells of the body, and they act as conduits to transfer various metabolic wastes from distant tissues to sites where these toxins can be eliminated or neutralized.
Lymphatic vessels, which are sometimes included within the cardiovascular system, run roughly parallel to blood vessels and contain a filtrate of blood plasma, along with a high concentration of immune cells (see “Immune System” below).
The cardiovascular system is a closed-loop system: The heart acts as a four-chambered, two-sided pump that pressurizes blood and pushes it into either the aorta – the main artery that carries blood to the head and body – or the pulmonary artery, which carries blood to the lungs for re-oxygenation. Once the blood has made its way through the body (or through the lungs) it returns to the heart to repeat the circuit.
Using the lungs as a starting point, the cardiovascular circuit can be described as follows: Oxygenated blood leaves the lungs, courses through the pulmonary veins (the only veins in the body that carry oxygen-rich blood), and returns to the left atrium of the heart. From there, it travels sequentially to the left ventricle, through the aorta, into progressively smaller arteries and arterioles, into the capillaries where it releases its oxygen and accumulates cellular wastes, into the venules, thence to increasingly larger veins, and finally back to the right atrium of the heart. This deoxygenated blood then flows into the right ventricle, which pumps it into the pulmonary artery (the only artery in the body that carries deoxygenated blood) which leads back to the lungs.
The heart beats approximately once every second – three billion times in an average lifespan – and pushes the typical red blood cell through the cardiovascular circuit 250,000 times before the cell is returned to the bone marrow to be recycled.
The Respiratory System
Oxygen is critical to every cell in the body. Thus, the mechanism for providing this vital element must be capable of delivering it to even the most deeply hidden or remote tissue.
As we inhale a breath, oxygen-rich air is conveyed along the nasal and oral passages, into the trachea (windpipe), through the bronchi and bronchioles of the lungs, and finally to the alveoli, which are grape-like clusters of hollow sacs whose walls are only one cell thick.
The alveoli are enveloped within a meshwork of capillaries that are carrying deoxygenated blood from the right side of the heart – blood that is also rich in carbon dioxide released from distant tissues in the body. As this blood passes the oxygen-filled alveoli, a marvelously engineered molecular transfer occurs: Red blood cells relinquish their burden of carbon dioxide (which is eliminated with the next outward breath) greedily take on a supply of oxygen, and race back to the left side of the heart, which then pumps this oxygenated blood to the tissues of the body.
The Musculoskeletal System
Although the bones and muscles are often considered as separate systems, their interrelationship is so intimate that these structures can readily be considered as one organ system.
Simply put, the bones that make up the human skeleton offer structural support and protection for the surrounding organs and tissues, and the attachment of muscles to the bones affords us our ability to move through space and to perform some of the delicate movements that distinguish us from many other species.
The skeleton can be divided into an axial portion (the skull, vertebrae, and ribs) and an appendicular portion (the bones of the limbs and pelvis). The human skeleton contains 206 bones; individual bones articulate with others via joints, where they are held in position by ligaments, cartilaginous capsules, and/or tendons.
Certain parts of the skeleton serve as repositories of marrow, where blood and immune cells are born and recycled. The skeleton also plays an integral role in the storage and homeostasis of important minerals like calcium and phosphorus.
Muscles attach to bones via tendons; when the muscles contract, they create a levering action between adjacent bones, allowing for flexion, extension, and even circumduction around a specific joint.
Together, the bones and muscles provide a living scaffold that confers protection, support, mobility, and raw material for critical metabolic processes.
The Endocrine System
Endocrine tissues are those that produce hormones; hormones are chemical messengers that are released into the bloodstream or lymph and circulated to other parts of the body. These messengers influence a plethora of physiologic functions, including growth and maturation, immunity, reproductive behavior, digestion, metabolic rate, fluid and electrolyte balance, and even personality.
The endocrine system is composed of discrete organs – called glands – that are located throughout the body; the influences of these structures’ secretions can be exerted on the entire body or on specific organs or tissues. Included among the endocrine glands are the hypothalamus, the pituitary, the thyroid, the parathyroids, the thymus, the adrenals, the ovaries, the testes, and specialized cells within the pancreas. Additionally, certain tissues within other organ systems – such as the lining of the gastrointestinal tract and the placenta – perform endocrine functions.
The Urogenital System
An organ system that deals with filtering and eliminating metabolic waste may be functionally different from one that provides for reproduction, but the origin of these two systems from common tissues during fetal development – not to mention their anatomical proximity, particularly in males – is sufficient reason to consider them as one organ system.
The urinary system consists of the kidneys, the ureters, the bladder, and the urethra. Blood flowing through the kidneys is filtered, toxins and metabolic byproducts are removed (while some minerals, proteins, and other precious molecules are either retained or reabsorbed), and the waste fluid – known as urine – passes through the ureters to a hollow organ (the bladder) where it is temporarily stored until it can be voluntarily released through the urethra.
In addition to controlling fluid balance and the mineral and protein content of blood, the kidneys – through the secretion of hormone-like substances – participate in blood pressure control, calcium/phosphorus balance, and a variety of other important functions.
The reproductive system consists of the gonads, the sex organs, and accessory structures. Males possess testes (where sperm are produced); seminiferous tubules for transport of sperm; a prostate gland (a source of nutrients for sperm); and a penis. Females are endowed with ovaries (from which ova, or eggs, are released each month during a woman’s reproductive years); Fallopian tubes, or oviducts, which transport ova to the uterus; a uterus, which provides a hospitable environment for a developing fetus; and a vagina, which serves as both a conduit for sexual intercourse and a birth canal for passage of the newborn.
Sexual development – and much of one’s reproductive behavior – is largely controlled by hormones produced by the gonads and other endocrine organs.
The Digestive System
The digestive system, or alimentary canal, provides us with a means for procuring nutrition, breaking it down into usable particles, emulsifying and dissolving it for absorption, and eliminating the portions that are unusable or indigestible.
Digestion begins in the mouth, where food is chewed (masticated) and mixed with saliva. Once swallowed, foods and liquids descend through the esophagus to the stomach, where they are mixed with enzymes and hydrochloric acid, which further prepare the ingested material for introduction into the small intestine.
The small intestine provides its own enzymes, many of which are produced in the pancreas. Both enzymes and bile are introduced into the small intestine via ducts that lead from the pancreas and liver. As undulating waves of peristalsis move along the intestine, the food is mixed, churned, and reduced to a pulverized pudding whose essential elements are eventually transported across the lining of the gut and into the bloodstream.
Once absorbed from the digestive tract, the nutrients we eat are carried via blood vessels and lymphatics to the liver, where they are detoxified and chemically prepared for use by the various cells and tissues of the body.
The undigested material that remains in the gut after all nutrients are absorbed is passed along to the colon, where much of the moisture is reabsorbed. Finally, the residue is excreted as feces through the anus.
While the digestive tract can simply be viewed as a continuous, food-conducting tube from mouth to anus, this organ system is highly metabolically active and is intricately tied to other organ systems, including the circulatory system, the endocrine system, the immune system, and the nervous system.
The Immune System
Immunity is what allows humans to exist in an environment that is rich in microbes. Without an intact immune system, the microorganisms that surround us would quickly reduce us to our basic molecular components, and we would soon be assimilated into the soil.
The immune system is less well-defined than most other organ systems. While it possesses its discrete organs – the thymus, the lymph nodes and lymphatic vessels, and the spleen, for example – much of the immune system’s activity occurs at the cellular level in every organ system in the body.
Immune cells (lymphocytes, neutrophils, basophils, Langerhan’s cells, etc.) develop very specific abilities to deal with each and every foreign molecule in our environment. This immune cell “education” takes place in the thymus, the bone marrow, the lymph nodes, and in other specialized organs, but once the cells are mature they migrate into the bloodstream and find their way into every tissue in the body. Threats from viruses, bacteria, fungi, parasites, toxins, and allergens are a constant and omnipresent threat, and an efficient, rapid response must be mobilized to maintain optimal health and viability.
Through the production of antibodies, cytokines, and other important immune molecules, and through the packaging and elimination of dangerous foreign agents, our immune systems allow us to pass relatively unscathed through a microbe-rich environment.
Although the study of discrete organ systems allows the student of human anatomy and physiology to more easily comprehend how these systems work, the functional nuances and relationships between different organ systems must also be understood before the full complexity of the human organism can be appreciated.
