Questions may ask for a basic understanding of the systems of the body, including their structure, function, and how they interact with each other over the course of a human life. These are the systems and terms with which you should be familiar and a brief description of what they involve:
The integumentary system consists of the skin, hair, and nails. Not only is the skin considered an organ but it is also the largest organ in/on the body. The integumentary system serves a variety of purposes but its primary role is to protect the body from injury and disease. It is also responsible for thermoregulation and conveying touch sensations such as pain and pressure.
The skin is made up of two layers:
Epidermis ― The outer epidermis is only four or five cellular layers depending on whether the skin is thick, such as on the soles of the feet, or thin, such as on the face; it is much thinner than the dermis. The epidermis does not contain any blood supply or accessory structures.
Dermis ― Blood vessels along with lymphatic vessels, hair follicles, sweat glands, sebaceous glands, and nerve endings are all located in the dermis. The dermis consists of two layers of connective tissue that give skin its elasticity.
Beneath the dermis is the hypodermis or subcutaneous layer, consisting of adipose tissue, but this layer is not considered part of the integumentary system.
During skin assessments, it is important to be aware of the appropriate terminology to describe the appearance of common abnormalities observed such as warts, freckles, pimples, moles, cysts, scars, rashes, burns, ulcers, and lacerations. Some of these findings may be benign based on the individual; others may indicate an acute injury or chronic disease process. The skin is often one of the first indicators of an underlying complication such as oxygen deprivation (cyanosis: blue), liver or biliary disease (jaundice: yellow), tissue bleeding (ecchymosis: purple), or fever/infection (erythema: red). Changes in skin color are easier to notice on patches of thin skin or on mucous membranes in non-white patients.
“Normal” skin should be warm, dry, pink at the nail beds and around the lips, and elastic when pinched. These findings indicate adequate hydration (turgor) and peripheral circulation.
The term musculoskeletal technically refers to two distinct organ systems (the muscular system and the skeletal system), which operate in such close conjunction with one another that they are often described in a combined fashion. Together, these organ systems physically support the body and make movement possible. Bones provide the underlying structure and the sites to which muscles attach. Muscles control and augment motion.
The skeletal system is made up of 206 independent bones. All bones are made up of living cells, minerals (primarily calcium), and protein in the form of collagen. Bones vary in structure: Compact or cortical bone is a calcium-dense, solid shell that makes up the diaphysis (shaft) of long bones; it is filled with yellow marrow, where fat cells are stored, and also serves as a pathway for nerves and blood vessels.
Spongy or cancellous bone is a lighter, hollow mesh network filled with red marrow where red blood cells, white blood cells, and platelets are produced. Spongy bone makes up the epiphysis or end of long bones as well as portions of small, irregular bones.
All bones are covered with a protective periosteum sheet.
Each bone is further categorized by its size and shape:
Long bones are longer than they are wide, such as the humerus or femur.
Short bones are as wide as they are long, such as the carpals or tarsals.
Flat* bones are thin and used for muscle attachment, such as the sternum and scapula.
**Sesamoid bones are found inside of muscle tissue, such as the patella.
Irregular bones do not fit any other category, such as the vertebrae.
Bones of the axial skeleton (including the skull, spine, shoulders, and hips) provide posture and protect precious internal organs. The appendicular skeleton, or the arms and legs, are attached to the axial skeleton by connective tissue ligaments, which can be found at all articulating joints. Cartilage is another flexible, connective tissue prevalent in children that matures (or ossifies) into bone with age (however, some soft body parts such as the nose and ears remain cartilaginous for life).
Bones are connected to muscles by tendons. There are at least 650 muscles in the human body depending on whether complex muscles are counted individually. Muscles make movement possible by metabolizing (as well as storing) glucose and oxygen to make energy, or ATP (adenosine triphosphate). ATP allows muscle fibers to contract on the cellular level. The two important protein fibers that make muscle contraction possible are thin, light actin and wide, dark myosin. This organ system also generates heat to maintain body temperature.
Muscle types are distinguished by whether they are voluntarily controlled and whether they are striated (meaning they have striped bands of actin and myosin):
Cardiac muscle is involuntary and striated. As its name indicates, it is found in the heart.
Smooth muscle is involuntary and non-striated. It is found around blood vessels, the urinary system, and the sensory organs.
Skeletal muscle is striated and is the only voluntary type of muscle.
Skeletal muscles are named by a combination of their shape, location, operation, and often their point of origin. All muscles are surrounded by a connective layer of tissue known as the fascia.
There are a number of degenerative diseases that impact the musculoskeletal system, but the most common injuries are bone fractures, sprains, and strains (sprains are stretched or torn ligaments, while strains are stretched or torn tendons or muscles―remember that both strain and tendon have the letter “t”).
The nervous system can be broken down into the central and peripheral nervous systems:
Central nervous system (CNS) ― This system consists of the brain and spinal cord. The brain controls the rest of the body in response to the information it receives from the sensory organs. It is responsible for voluntary motor movement, involuntary functions necessary for life (such as heart rate, blood pressure, and respiration), and higher-order thinking such as judgment and reasoning. It also processes and commands language, emotion, memory, and sleep cycles.
The brain is made up of a right and left hemisphere joined by the corpus callosum. Because of the pathway of nerve fibers, the right hemisphere typically controls and responds to input from the left side of the body while the left hemisphere controls and responds to input from the right side of the body. The corpus callosum allows each half to communicate with the other. Each side is also understood to be responsible for a more specialized set of functions.
In addition to two hemispheres, the brain consists of four parts:
Brainstem ― The brainstem includes structures of the midbrain and hindbrain (the pons and medulla oblongata) and controls basic life functions. As such, injury to this area is often fatal.
Diencephalon ― This part of the forebrain is responsible for maintaining body temperature (through the hypothalamus), the sleep/wake cycle (via the reticular formation), memory storage (in the hippocampus), and primal emotional responses (through the limbic system).
Cerebellum ― The cerebellum helps to coordinate movements initiated by the cerebrum by controlling balance and precision.
Cerebrum ― The cerebrum makes up the mass of tissue most people visualize when they imagine a brain. This is where voluntary movements are initiated and where specialized thinking occurs depending on the specific lobe: frontal (reasoning, planning, and emotions), parietal (logic, physical sensation, and written word), temporal (hearing and verbal language), and occipital (perception and vision).
The CNS also includes the spinal cord, which is a collection of neurons that start at the medulla oblongata and run through the vertebral column (to the lumbar region). This circuit is the pathway by which the brain communicates with the rest of the body and vice versa.
Peripheral nervous system (PNS) ― This system consists of nerve fibers and sensory organs. Nervous tissue in the PNS has two primary compositions:
Neurons communicate by exchanging electrical and end chemical signals known as neurotransmitters via an “action potential.” These cells never touch, but must transmit information across a “synaptic gap.” Neurons may be afferent (sending information to the CNS), efferent (sending information to the muscles), or interneurons (communicating with one another). Neuroglia are diverse cells that wrap and protect the neurons to facilitate rapid transmission.
The PNS is often discussed in terms of the autonomic or involuntary system (which regulates unconscious organ functions) and the somatic or voluntary system (which innervates skeletal muscle).
The ANS is further differentiated into a sympathetic nervous system, responsible for our “fight-or-flight” response, and a parasympathetic nervous system, which allows us to “rest-and-digest.”
Diseases of, or damage to, the CNS is incredibly dangerous, but this region of the nervous system is also protected by a network known as the blood brain barrier (intended to restrict cellular access to the brain) as well as the skeletal system. PNS conditions are often degenerative, meaning that symptoms progressively worsen over time (often irreversibly since nerve cells rarely regenerate). Nervous system impairments can disrupt any other organ system but commonly disrupt movement, bowel functions, and breathing.
The heart is the center of the cardiovascular, or circulatory, system which also includes blood vessels and blood itself. The most important role of this organ system is to carry nutrients and oxygen throughout the body.
The blood vessels responsible for this transport are:
Arteries ― These vessels are thick, elastic and move blood under high pressure away from the heart (remember that artery and away both begin with the letter “a”).
Veins ― Veins are much thinner and more fragile than arteries because they carry blood under low pressure toward the heart; in order to compensate for the low pressure, veins have valves inhibit backflow against gravity.
Capillaries ― These are the smallest blood vessels (one cell thick) that communicate directly with muscle cells and lung cells to exchange oxygen, carbon dioxide, and nutrients. Capillaries are nested into “beds” of tissue perfusion.
The heart is composed of four chambers: the right atrium, right ventricle, left atrium, and left ventricle. Note that “right” and “left” refer to the patient’s right and left, not the viewer’s. The upper atria receive blood while the muscular, lower ventricles contract and return it to circulation. (The left ventricle is more muscular than the right ventricle because it must force blood out to the entire body, resulting in measurable blood pressure.) The atria and ventricles are separated by valves that prevent backflow while the left and right side are separated by a thick septum. It is important to remember the pathway that blood travels through the body:
Deoxygenated blood from body > venae cavae > right atrium > tricuspid valve > right ventricle > pulmonary valve > pulmonary artery > lungs (where blood is oxygenated) > pulmonary veins > left atrium > mitral valve > left ventricle > aortic valve > aorta > aortic branches > the body
The initial half of this sequence from the body to the lungs is known as the pulmonary circuit. The second half where blood is returned to the body is the systemic circuit.
The mechanical pump of the heart is stimulated by involuntary electrical impulses. The dominant pacemaker is the sinoatrial (SA) node within the right atrium. From here, the impulse is slowed by the atrioventricular (AV) node between the right atrium and ventricle and transmitted to the Bundle of His, which branches into the Purkinje fibers to innervate the ventricular myocardium. This pathway can be traced using a noninvasive ECG.
Cardiovascular diseases are often quite serious and complications may be life-threatening. There are numerous blood and imaging tests that may give insight into the heart’s functions beyond simple vital signs.
There are about five liters of blood in the average human body and the entirety of this volume is circulated every 60 seconds. The heart achieves this by pumping 60 to 100 times per minute, though the specific rate varies by individual and is subjective to stress, illness, exercise, and emotion. Blood is made up of a number of important components:
Plasma accounts for 55% of blood volume; it is the liquid in which proteins, hormones, vitamins, minerals, and glucose are suspended.
Red blood cells or erythrocytes are the most common formed element in blood and have a unique disc shape caused by their lack of nucleus, which allows them to carry oxygen bound to iron molecules. Red blood cells also contain the Rh protein by which blood is “typed.”
White blood cells or leukocytes have an important role in infection control; depending on their specific classification, they respond to any agent deemed “foreign” to the body.
Platelets or thrombocytes allow for clotting (hemostasis) when a blood vessel has been damaged (they are summoned in response to a complex series of chemical signals).
Blood is also a component of the hematopoietic system, which is not a distinct organ system but rather refers to the the formation of blood cells. The organs involved in this process overlap with other body systems and include the bone marrow, spleen, liver, thymus, and lymph nodes.
The lymphatic system is distinct from the cardiovascular system, but the two work quite closely with one another because they both rely on blood vessels and lymph fluid is chemically similar to plasma. However, this system is not filled with blood, but rather with interstitial fluid collected from around tissues. The lymphatic system is sometimes referred to as the “immune system” because it works to fight infection and protect the body from disease by filtering this fluid to trap bacteria and other pathogens.
This organ system consists predominantly of a diffuse network of lymph nodes (where immune cells are stored) and lymph vessels (which connect nodes). Primary concentrations of lymph nodes are in the axillary (armpit) and inguinal (groin) areas. There are also a number of “accessory lymphoid tissues”; some of the important ones include:
Thymus ― Located behind the sternum, this gland is particularly active in childhood by creating mature T-cells. The thymus shrinks as a person ages.
Spleen ― The spleen sits in the left hypochondriac region of the abdomen where it traps and destroys bacteria and dying red blood cells. This organ also stores the materials for making new red blood cells in case of an emergency and produces antibodies in response to infection.
Tonsils ― There are three sets of tonsils near the pharynx which protect against diseases entering the oral cavity.
Because of their frequent, direct exposure to pathogens, these glands and organs often become swollen and infected themselves. If this occurs, they can be removed to prevent further complications, though this significantly impairs a person’s immune defense moving forward.
The respiratory system is in charge of receiving oxygen and the removing of carbon dioxide. It consists of the trachea, the bronchi, the diaphragm, and the lungs through the nasal and oral cavities. The pharynx and larynx are also involved.
Air can be taken up through either the nose or mouth and is directed by the pharynx into the larynx (or voice box). These structures make up the upper respiratory tract.
The lower respiratory tract begins at the trachea (or windpipe), which bifurcates into two main stem bronchi that lead to the right and left lung. The right lung is made up of three lobes and is bigger than the left lung, which only has two lobes. This is because the left lung shares space with the heart, which sits slightly to the left. Within the lungs, the bronchi further split into bronchioles that terminate in alveoli, which are bunches of hollow sacs that share membrane walls with capillary beds.
It is important to bear in mind that, the act of breathing is properly called ventilation, meaning mechanical gas exchange. Respiration technically refers to the cellular process of exchanging oxygen and carbon dioxide. This takes place microscopically within the lungs at the alveolar membrane. Diffusion across this membrane is what oxygenates the blood in exchange for carbon dioxide. This blood-air barrier is the junction between the respiratory and cardiovascular systems.
Ventilation has two phases:
During inspiration or inhalation, negative pressure is created when the lungs expand, allowing air to be passively drawn into the body. This occurs because the diaphragm, a skeletal muscle connected to the base of the lungs, contracts to increase the volume of the chest cavity.
During expiration or exhalation, the diaphragm relaxes, returning the lungs to their normal size and actively forcing air back out.
Although ventilation, as described above, is an involuntary process, it can also be hastened or slowed voluntarily due to the presence of multiple nerve pathways. Receptors near the heart and in the brain are normally responsible for detecting levels of carbon dioxide, which determines the rate of breathing. However, the intercostal muscles between the ribs and other accessory muscles can be consciously activated to enhance breathing, often in times of exertion or distress when the resting respiratory rate (12 to 20 breaths per minute) or depth is insufficient to meet the body’s demands for oxygen.
Respiratory ailments are fairly common and vary greatly in their severity from mild allergies to life-threatening infections. Every respiratory condition (as well as diseases of many other organs) is worsened by smoking.
As its name implies, the digestive system is responsible for breaking down food and absorbing nutrients. It is often referred to as the gastrointestinal system but includes organs beyond the stomach and intestines. One way to remember the organs involved is to follow the path of food as it passes through the alimentary canal:
Along the way, food is digested by a number of chemical enzymes:
Amylase digests carbohydrates and is found in saliva as well as secreted by the pancreas into the small intestine.
Lipase digests fats; a gastric version begins the process in the stomach but a more powerful form is released by the pancreas into the small intestine.
Pepsin digests proteins and is produced by the stomach.
Gastrin stimulates the release of hydrochloric acid by the stomach cells.
Bile is produced by the liver and stored by the gallbladder; it is released into the small intestine to help break apart lipids so they can be digested by lipase.
Food enters the body through the upper gastrointestinal tract:
It is masticated (or mechanically broken apart) in the mouth by the teeth and tongue. When swallowed, food enters the pharynx and must pass the epiglottis, which covers the larynx and trachea to direct the food bolus down the posterior esophagus instead.
Food is carried down the esophagus by peristalsis, a series of involuntary, muscular waves, to enter the stomach (first it must pass the cardiac sphincter, which is normally closed to prevent vomiting; however, this sphincter commonly becomes incompetent, allowing reflux of stomach contents or the stomach itself). The mucosa of the stomach is coated with mucus to prevent gastric acid from eroding the organ itself. Peristalsis also continues in the stomach to mechanically “churn” food into smaller particles.
The lowest section of the stomach is known as the pylorus where food can be stored temporarily (the stomach is able to swell to accommodate about one liter of food). After a few hours, the stomach slowly releases pieces of digested food known as chyme past the pyloric sphincter to enter the first part of the small intestine, known as the duodenum.
The duodenum is the beginning of the lower gastrointestinal tract:
The duodenum meets the jejunum, followed by the ileum. These portions of the small intestine are where the vast majority of nutrients (including amino acids, sugars, and some fats) are absorbed, leaving a liquid slurry. This is where several organs part of this system begin to come into play even though food does not pass through them:
The liver, an organ that also has extensive roles in detoxification, glucose synthesis, cholesterol production, nutrient storage, and protein formation, then releases bile into the small intestine so that pancreatic lipase can digest the remainder of it.
Peristalsis carries the remaining product into the large intestine whose primary responsibility is to resorb water and compact the waste product into stool. The five sections of the large intestine, in order, are the cecum, the ascending colon, the transverse colon, the descending colon, and the sigmoid colon. When the descending colon becomes filled with solid waste, this typically prompts the urge to defecate. During defecation, stool (or feces) passes through the rectum and then the exterior anus.
Common gastrointestinal discomforts are related to the buildup of gaseous byproducts, which are released as flatulence through the anus or eructation through the mouth. This organ system also has a unique and extensive supply of blood vessels and nerves. But because its role is less essential compared to other body systems, it is often one of the first to suffer damage and one of the last to recover following a significant disease or trauma.
During patient assessments, it is important to inquire about diet and toileting habits to see if any of these may be contributing to unpleasant symptoms. Nutrition is an important element of the digestive system that often goes undiscussed and many patients may avoid talking about their bowel movements out of embarrassment or discomfort.
The urinary system, also known as the renal system, performs numerous essential regulatory functions beyond the elimination of metabolic waste including maintenance of hydration, blood pressure, the blood’s electrolyte and acid-base balance, and the production of red blood cells.
Despite this, the urinary system is only made up of four organs: the kidneys (paired), the ureters (paired), the bladder (single), and the urethra (single). The kidneys interact closely with other organ systems in order to perform their various roles. They produce the hormone erythropoietin (which stimulates red blood cell production in the bone marrow), secrete the enzyme renin (which communicates a drop in blood pressure to the brain), and metabolize calcitriol (a form of vitamin D that promotes calcium absorption in the intestines).
The kidneys filter blood within the renal corpuscles and convert the harmful or unnecessary metabolic byproducts (primarily urea and uric acid) into urine, a process that takes place at the cellular level via millions of nephrons. After leaving the renal corpuscle (or glomerulus), urine flows through a series of microscopic tubules where ions and water molecule are exchanged to meet the body’s specific hydration needs. This is mediated by the reception of hormones such as ADH (antidiuretic hormone), which causes more water to be retained in the presence of dehydration.
After leaving the kidney, urine travels down the ureters, tubes of smooth muscle, to the bladder where it can be stored. The average bladder can swell to accommodate over 400 mL of urine, but many people feel the urge to urinate well before this. Voiding (or micturition) takes place through the urethra, located in the genitalia. Healthy kidneys also produce 1 to 1.5 liters of urine per day making urination a meaningful indicator of kidney function.
Urinary tract infections are extremely common in women because their urethra is less than half the length of a man’s (providing a shorter distance for bacteria to travel) and because of the proximity of the female urethra to the anus. Urinary symptoms, however, are often vague or may even go unnoticed despite being reflective of more complicated disease processes. Just as in the digestive system, it is important to establish a baseline for urinary habits and remember to look for additional signs of adequate hydration (such as mental status, mucous membranes, and skin turgor).
The reproductive system is a collection of sexual organs involved during sexual reproduction.
In men, these organs are predominantly external and include the penis, scrotum, testes, and urethra. However, internal glands and ducts are also involved in the production of sperm cells and sex hormones.
Testosterone is the hormone that stimulates sperm development and the visual secondary sex characteristics of increased pubic and body hair, muscle and bone growth, and thicker skin and vocal cords. This hormone is present from birth but becomes most active during puberty.
Sperm cells are produced within the testes and stored in a sac known as the epididymis. When puberty begins, the average man will produce millions of sperm cells per day (regardless of sexual activity) and will continue to produce sperm for the remainder of his life. Puberty in men typically occurs between the ages of 12 and 16.
During ejaculation, sperm travel from the epididymis, into the vas deferens, which joins with the seminal vesicle to form the ejaculatory duct. The ejaculatory duct joins with the urethra, the same tract used during urination.
Sperm travel in a fluid known as semen, which is a combination of sugar, protein, and lipids to nourish and support sperm cells. Semen also lubricates the reproductive tract during sexual intercourse. The chemical components of semen come from the seminal vesicles, followed by the prostate, followed by the bulbourethral glands.
Testicular self-exams are important to male sexual health, especially during adolescence. On the other end of the spectrum, prostate cancer is common among elderly men and after age 50, it is recommended that all men receive regular prostate exams.
Penises may be circumcised or uncircumcised which will influence how much of the glans is visible. Circumcision is believed to reduce the risk of certain sexually transmitted diseases but it sometimes causes complications. There are a variety of opinions on male circumcision, often with religious or ethnic overtones, so it is important to be culturally sensitive.
In women, the reproductive system is entirely internal. The external organ is referred to as the vulva and includes the labia, the clitoris, and the urethra (which, unlike in men, is not a part of this system). Internal organs include the ovaries, fallopian tubes, uterus, cervix, and vagina.
Women are born with millions of ova (or egg cells) and do not produce any new ones after birth; after puberty, they begin to release one (or more) ova every 28-days during their menstrual cycle. Although, estrogen is the hormone that initiates puberty and is associated with the development of secondary female sex characteristics (such as breast development and widening of the hips), follicle-stimulating hormone (FSH) and **luteinizing hormone (LH) play an important role in the regulation of the menstrual cycle.
When FSH is present, an ovum is released into the fallopian tube. If fertilization by sperm occurs, it often takes place in the fallopian tube. From there, the ovum travels to the uterus whose endometrium has been built up in preparation for pregnancy. If the ovum was fertilized, it develops into a zygote and implants into the uterine wall and continues to grow into an embryo and then a fetus. Human pregnancy lasts an average of 280 days from the last menstrual period or nine months. During this time, the cervix, uterus, and vagina undergo extensive changes in shape, size, and elasticity to prepare for childbirth.
If the ovum is not fertilized, the uterine lining is shed, which results in menstruation, a normal type of vaginal bleeding (which can be difficult to distinguish from abnormal bleeding). A woman’s first menstrual cycle is known as her menarche and may occur as early as age 10. When a woman’s menstrual cycle ends later in life, this is referred to as menopause.
The endocrine system is actually a collection of glands, which are tissues that secrete hormones (chemical messengers) into the bloodstream. Each hormone produced by each gland has a specific function and has a target receptor site located on an organ in another part of the body. This organ system thus allows all other organ systems to communicate with one another.
Endocrine glands also differ from exocrine glands (such as sweat, salivary, sebaceous, mammary, and lacrimal glands) which secrete substance directly onto a surface through a duct as opposed to into the bloodstream.
Compared to the nervous system, whose chemical and electrical impulses occur seemingly instantaneously, the endocrine system takes longer to produce enough hormones to stimulate a desired response. When hormones do initiate a response, the effect is often prolonged and lasts until the existing hormones are metabolized by the body or until a hormone with a competing function takes over.
There are dozens of endocrine glands, which each secreting an abundance of different hormones. Here are some of them:
Located in the brain:
Pineal gland ― secretes only melatonin, which causes drowsiness.
Hypothalamus ― primarily secretes hormones that tell the pituitary gland to secrete other hormones: thyrotropin-releasing hormone (TRH), gonadotropin-releasing hormone (GnRH), corticotropin-releasing hormone (CRH); also inhibits other hormones via dopamine and somatostatin; secretes vasopressin (antidiuretic hormone) in response to low blood volume.
Pituitary gland ― consists of an anterior and posterior lobe; many anterior lobe hormones are self-explanatory: growth hormone (GH), thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH). The pituitary gland also plays an important role in menstruation and pregnancy via luteinizing hormone (to stimulate ovulation) and prolactin (to make breast milk). The posterior lobe secretes oxytocin, which is active during childbirth and nursing.
Located in the thoracic cavity:
Thyroid gland― secretes triiodothyronine (T3) and thyroxin (T4), which increase the basal metabolic rate, as well as calcitonin, which reduces blood calcium levels.
Parathyroid gland ― secretes parathyroid hormone, which increases blood calcium levels (these glands are attached to the back of the thyroid gland).
Thymus ― secretes thymosin which allows for T-cell maturation; this function is active in childhood but decreases with age.
Located in the abdominal cavity:
Adrenal gland ― one atop each kidney; it consists of an outer cortex and an inner medulla; the cortex produces steroids including glucocorticoids (cortisol), mineralocorticoids (aldosterone), and androgens (testosterone); together these stimulate fat breakdown and increased blood pressure. The medulla secretes adrenaline and noradrenaline (both responsible for the fight-or-flight response) and dopamine which raises the heart rate and blood pressure.
Pancreas ― secretes insulin from beta cells to lower blood sugar and glucagon from alpha cells to raise blood sugar; also secretes somatostatin to inhibit the release of both hormones.
Located in the pelvic cavity:
Testes (male) ― secrete androgens (testosterone), which are responsible for male secondary sex characteristics including anabolic growth and sexual maturation.
Ovaries (female) ― secrete estrogens (estradiol), which are responsible for female secondary sex characteristics, and progesterone, which supports ovulation, pregnancy, and has anti-inflammatory properties.
In addition to these, organs from other organ systems (including the stomach, small intestine, liver, kidneys, heart, and bones) are able to produce and secrete hormones, but these are considered secondary to their dominant function.
Endocrine disorders are often complex because the problem may occur at any point in the information exchange: the gland may be damaged causing it to secrete too much or too little hormone, the circulation of the hormone may be impaired, the response to the hormone may be inhibited because receptor sites are blocked, or the receiving organ may not be functioning properly and therefore unable to interpret the signal. The specific interruption will determine the symptoms manifested and the necessary treatment.
The sensory system is technically a part of the nervous system that controls the four special senses: vision, hearing, smell, and taste.
The sensory system conducts and receives impulses along the twelve cranial nerves, often listed via roman numerals:
I) Olfactory ― processes sense of smell
II) Optic ― receives visual information
III) Oculomotor ― coordinates eye movement with nerves IV and VI
IV) Trochlear ― coordinates eye movement with nerves IV and VI
V) Trigeminal ― carries sensation from the face, controls chewing muscles
VI) Abducens ― coordinates eye movement with nerves IV and VI
VII) Facial ― allows facial expression
VIII) Vestibulocochlear ― receives auditory information, controls balance
IX) Glossopharyngeal ― controls swallowing and gag reflex
X) Vagus ― coordinates swallowing and voice box (longest nerve, runs to abdomen)
XI) Accessory ― ccntrols sternocleidomastoid and trapezius muscles
XII) Hypoglossal ― controls tongue movement
The special senses are the four senses that have “specialized” organs: vision (eyes), hearing (ears), smell (nose), and taste (tongue). Each of these organs has devoted nerve fibers that connect it to the CNS.
The eye is an orb filled with vitreous humour, effaced at the front with a lens and a central pupil. The lens is protected by a layer of aqueous humour kept in place by the cornea, which is the window through which light passes to reach the pupil. From there, light travels to the back of the eye where it stimulates rod receptors (which allow us to see shades of gray, peripheral images, and images in the dark) and cone receptors (which allow us to see daylight and color). These receptors are directly connected to cranial nerve I where the information is brought to the brain. The iris is the exterior colored portion of the eye that varies by individual.
PERRLA is the common acronym used to describe “normal” eyes: it stands for “pupils equal, round, reactive to light, and accommodating.” Pupils should be the same circular size and when light is shined in either eye, both pupils should contract (this is the meaning of accommodation).
Eye disorders are increasingly common with age. Many conditions of the eye that impair vision are due to irregularities or complications of the lens.
The ear is a paired organ that processes sound by the vibration of air. The portion of the air that is visible is actually only the outer ear (or pinna), which is responsible for catching sound waves. From there, sound enters the auditory canal and meets the eardrum (or tympanic membrane). Just like a drum, this membrane vibrates to stimulate three small bones known as the ossicles: the incus (shaped like an anvil), the malleus (shaped like a hammer), and the stapes (shaped like a stirrup). Beyond this is the inner ear, which includes the semicircular canals and the cochlea. The cochlea is connected to cranial nerve VIII, allowing the sound to be processed.
Hearing loss is also common with age. However, deafness can either be mechanical and caused by obstruction or interference with the auditory canal or it can be neurological where the brain is unable to interpret the sounds it receives.
The nose is a collection of cartilage with two passages that, aside from their role in respiration by humidifying inhaled air, also house receptors for smell particles that connect to *cranial nerve I. This nerve has the shortest distance to the brain and, because it connects to the cerebrum rather than the brain stem, often directly influences mood and memory.
Disorders of smell are uncommon, but smell is also closely linked to the sensation of taste (when the nasal passage is blocked, many taste sensations are diminished).
Taste buds, located on the tongue, come in five “varieties”: salty, sour, bitter, sweet, and umami (or meaty). These are what we process as “flavors” because they are actually pores for dissolved food. Cranial nerves IX and X which innervate parts of the tongue, then carry this information to the brain. Contrary to popular belief, certain tastes are not isolated to particular regions of the tongue but rather are located all over.
Taste buds can be damaged or lost, frequently by burning the tongue, but they are able to regenerate. However, their size and density tends to lessen with age, explaining why many elderly people may desire extra seasoning on their meals.
“Touch” is not considered a special sense, anatomically, because it is perceived all over the body, not just by the skin, but by the internal organs. Touch includes the sensations of pain, heat, pressure, vibration, and even relative position but these all rely on diffuse, generalized neural pathways.
Homeostasis is defined as the state of equilibrium between elements. In the case of the human body, this involves the coordination of physiological processes among organ systems to maintain the conditions necessary for life and health. Homeostasis occurs, and must be preserved, at every functional level of the body from cells to organ systems.
In the absence of homeostasis, such as when a particular cell or organ becomes diseased or damaged, the body begins to biologically and chemically change in an attempt to compensate for the impairment and return to its balanced state. Neighboring structures may temporarily take on new functions to compensate and the immune system is often activated to promote healing.
If the body is unable to regain homeostasis, permanent damage begins to occur. At the macro level, if a single organ fails to regulate its task, the entire organ system has been disrupted. From there, it is only a matter of time before other organ systems begin to fail. There is never a definite sequence of organ failure, but in the presence of overall infection or trauma, the respiratory system is commonly the first to fail followed by the digestive and urinary systems. Organ failure always requires medical intervention to provide artificial homeostasis.