Editing Anatomy and Physiology

'''Anatomy and Physiology''' is the study of the structures of the human body and the functions those structures perform. In nursing, this subject is the reference point for every assessment: you cannot recognize an abnormal finding until you know what normal looks like, and you cannot anticipate a complication until you understand the mechanism that produces it. This article organizes the material by body system, with nursing application notes throughout.

== How to Study This Subject for the NCLEX ==

Anatomy and physiology is almost never tested as isolated recall on the licensure exam. Instead, normal structure and function is embedded in questions about assessment, complications, and prioritization. When you study, keep asking three questions about every structure:

* '''What does it do?''' The function predicts the symptom when the structure fails.
* '''What does it need?''' Most organs need perfusion, oxygen, and a narrow chemical environment. Loss of any of these produces predictable dysfunction.
* '''What would I see at the bedside?''' Translate every mechanism into an observable sign, a vital sign change, or a laboratory value.

A useful habit is the "backward flow" rule for any pump or tube: when forward flow is blocked, fluid backs up behind the blockage. This single idea explains left- versus right-sided heart failure, hydronephrosis, bowel obstruction, and increased intracranial pressure.

== Levels of Organization and the Cell ==

The body is organized from chemicals to cells to tissues to organs to organ systems to the whole organism. The '''cell''' is the smallest living unit.

Key cellular structures and their nursing relevance:

* '''Cell membrane''': a phospholipid bilayer that is selectively permeable. It controls what enters and leaves the cell and maintains the electrical gradient that makes nerve and muscle function possible.
* '''Mitochondria''': the site of aerobic ATP production. When oxygen delivery fails, cells shift to anaerobic metabolism, produce lactic acid, and the patient develops metabolic acidosis. This is the chemistry behind shock.
* '''Nucleus''': contains DNA. Cells that divide rapidly (bone marrow, gastrointestinal mucosa, hair follicles) are the cells most damaged by chemotherapy and radiation, which explains myelosuppression, mucositis, and alopecia.
* '''Lysosomes''': digestive enzyme sacs involved in cellular cleanup and in the inflammatory response.
* '''Ribosomes and endoplasmic reticulum''': protein synthesis. The liver's abundant endoplasmic reticulum underlies its role in making albumin and clotting factors.

=== Transport Across Membranes ===

* '''Diffusion''': solutes move from higher to lower concentration. Gas exchange in the alveoli is diffusion.
* '''Osmosis''': water moves across a semipermeable membrane toward the higher solute concentration. This is the mechanism behind every IV fluid decision.
* '''Facilitated diffusion''': carrier-assisted movement down a gradient, such as glucose entry into most cells with insulin's help.
* '''Active transport''': movement against a gradient using ATP. The sodium-potassium pump moves three sodium ions out for every two potassium ions in, maintaining the resting membrane potential.
* '''Filtration''': movement driven by hydrostatic pressure, as in the glomerulus and at the arterial end of capillaries.

'''Nursing application:''' IV fluid tonicity follows directly from osmosis.
* '''Isotonic''' (0.9% sodium chloride, lactated Ringer's): stays in the extracellular space, expands vascular volume, used for dehydration and hypovolemia. Risk is fluid overload.
* '''Hypotonic''' (0.45% sodium chloride): water shifts into cells; used for cellular dehydration. Risk is cerebral edema and worsening hypotension, so it is avoided in head injury and in hypovolemic shock.
* '''Hypertonic''' (3% sodium chloride, dextrose 5% in 0.9% sodium chloride): pulls water out of cells into the vascular space. Used cautiously for severe hyponatremia and cerebral edema. Risk is fluid overload and phlebitis, so infuse slowly and monitor closely.

== Homeostasis and Feedback ==

'''Homeostasis''' is the maintenance of a stable internal environment despite external change. It is maintained mostly by '''negative feedback''', in which a change triggers a response that reverses the change. Rising blood glucose triggers insulin release, glucose falls, and insulin secretion decreases. Falling blood pressure triggers baroreceptor-mediated sympathetic activation, blood pressure rises, and the stimulus is removed.

'''Positive feedback''' amplifies a change until an endpoint is reached. There are few physiologic examples: oxytocin release during labor intensifies contractions until delivery, and the clotting cascade amplifies until a clot forms.

'''Nursing application:''' Compensation is a negative-feedback response, and it hides deterioration. A patient in early compensated shock may have a normal blood pressure because tachycardia and vasoconstriction are holding it up. Rising heart rate, narrowing pulse pressure, cool skin, restlessness, and falling urine output appear before hypotension. Waiting for a low blood pressure to act is waiting too long. This is also why children compensate impressively and then decompensate abruptly.

== Integumentary System ==

The skin has three layers: the '''epidermis''' (avascular, nourished by diffusion, contains melanocytes and keratinocytes), the '''dermis''' (blood vessels, nerve endings, hair follicles, sweat and sebaceous glands, collagen), and the '''subcutaneous (hypodermis)''' layer of fat and connective tissue.

Functions: protection and barrier defense, temperature regulation through vasodilation, vasoconstriction, and sweating, sensation, vitamin D synthesis, and fluid and electrolyte conservation.

'''Nursing application:'''
* Pressure injury staging follows anatomy. Stage 1 is intact skin with nonblanchable erythema, stage 2 is partial thickness loss into the dermis, stage 3 is full thickness with visible subcutaneous fat, and stage 4 exposes muscle, tendon, or bone. Unstageable injuries are obscured by slough or eschar; deep tissue injury appears as persistent nonblanchable deep red, maroon, or purple discoloration.
* Because the epidermis is avascular and the dermis holds the vessels, capillary refill and blanching tell you about dermal perfusion.
* Burns destroy the barrier function, so the immediate threats are massive fluid shift and hypovolemia first, and infection later.
* Older adults have thinner dermis, less subcutaneous fat, and reduced elasticity, which increases skin tear and pressure injury risk and makes skin turgor an unreliable hydration indicator. Check turgor over the sternum or forehead in older adults instead of the back of the hand.

== Skeletal System ==

The adult skeleton has 206 bones divided into the '''axial skeleton''' (skull, vertebral column, ribs, sternum) and the '''appendicular skeleton''' (limbs, shoulder girdle, pelvic girdle).

Functions: support, protection of organs, movement through leverage with muscles, mineral storage (about 99% of body calcium is in bone), and '''hematopoiesis''' in red bone marrow.

Bone is dynamic tissue. '''Osteoblasts''' build bone, '''osteoclasts''' resorb it, and '''osteocytes''' maintain it. Remodeling is driven by mechanical stress, parathyroid hormone, calcitonin, vitamin D, and sex hormones.

Joint types include synovial (freely movable, such as the knee and shoulder), cartilaginous (slightly movable, such as intervertebral discs), and fibrous (immovable, such as skull sutures).

'''Nursing application:'''
* Immobility causes bone resorption to outpace formation, releasing calcium into the blood. Expect disuse osteoporosis, hypercalcemia, and renal calculi in immobilized patients. Weight bearing and mobilization are therapeutic, not merely comfort measures.
* Estrogen restrains osteoclast activity, so postmenopausal women lose bone rapidly. This drives fall precautions and calcium and vitamin D teaching.
* Long bones and the pelvis contain marrow, so fractures there cause substantial blood loss and carry a risk of fat embolism, which classically presents with respiratory distress, confusion, and petechiae.
* Compartment syndrome is a pressure problem inside a fascial compartment. The earliest and most reliable sign is pain out of proportion to the injury and pain on passive stretch. Pulselessness is a very late finding. Report suspected compartment syndrome immediately and do not elevate the limb above heart level or apply ice.

== Muscular System ==

Three muscle types:
* '''Skeletal''': striated, voluntary, attached to bone by tendons.
* '''Cardiac''': striated, involuntary, connected by intercalated discs with gap junctions that allow the heart to contract as a functional unit.
* '''Smooth''': nonstriated, involuntary, found in vessel walls, the gastrointestinal tract, the bladder, the uterus, and the airways.

Skeletal muscle contracts by the '''sliding filament mechanism'''. A nerve impulse releases acetylcholine at the neuromuscular junction, the muscle fiber depolarizes, calcium is released from the sarcoplasmic reticulum, calcium binds troponin and exposes binding sites on actin, and myosin heads pull the actin filaments inward using ATP. Relaxation requires ATP as well, which is why rigor mortis occurs when ATP production stops.

'''Nursing application:'''
* Calcium is required for contraction and potassium is required for repolarization, so electrolyte disturbances present as muscular and cardiac symptoms. Hypocalcemia causes tetany, Chvostek and Trousseau signs, and laryngospasm. Hyperkalemia causes weakness and life-threatening dysrhythmias.
* Smooth muscle relaxation explains many drug effects: bronchodilators relax airway smooth muscle, and vasodilators relax vessel walls and can cause orthostatic hypotension.
* Isometric exercises maintain muscle mass in patients who cannot bear weight.
* Disuse atrophy and contractures begin within days of immobility, which is the rationale for range-of-motion exercises and correct positioning.

== Nervous System ==

=== Organization ===

* '''Central nervous system (CNS)''': brain and spinal cord.
* '''Peripheral nervous system (PNS)''': cranial and spinal nerves, divided into the somatic (voluntary) and autonomic divisions.
* '''Autonomic nervous system''': the '''sympathetic''' division produces the fight-or-flight response (increased heart rate and contractility, bronchodilation, pupil dilation, decreased gastrointestinal motility, glycogenolysis), and the '''parasympathetic''' division produces rest-and-digest effects (decreased heart rate, bronchoconstriction, pupil constriction, increased gastrointestinal motility). The vagus nerve carries most parasympathetic output.

=== The Neuron and Impulse Transmission ===

A neuron has dendrites (receive), a cell body, and an axon (transmit). Myelin, produced by Schwann cells in the periphery and oligodendrocytes in the CNS, speeds conduction by saltatory transmission. Demyelination, as in multiple sclerosis and Guillain-Barre syndrome, slows or blocks conduction.

Impulse transmission depends on sodium influx (depolarization) and potassium efflux (repolarization), with the sodium-potassium pump restoring the gradient. Neurotransmitters cross the synapse: acetylcholine, dopamine, serotonin, norepinephrine, gamma-aminobutyric acid, and glutamate.

=== Brain Regions ===

* '''Frontal lobe''': voluntary movement, judgment, personality, expressive speech (Broca area).
* '''Parietal lobe''': sensory interpretation, spatial orientation.
* '''Temporal lobe''': hearing, memory, receptive speech (Wernicke area).
* '''Occipital lobe''': vision.
* '''Cerebellum''': balance, coordination, fine motor control. Cerebellar injury produces ataxia and a positive Romberg sign.
* '''Brainstem''' (midbrain, pons, medulla): the medulla contains the cardiac, vasomotor, and respiratory centers. Brainstem compression is lethal.
* '''Hypothalamus''': temperature regulation, thirst, hunger, and control of the pituitary.
* '''Thalamus''': sensory relay station.

=== Cranial Nerves ===

There are 12 pairs. Those most often assessed in nursing practice include II (optic, vision), III (oculomotor, pupil constriction and most eye movement), V (trigeminal, facial sensation and chewing), VII (facial, facial expression and taste on the anterior tongue), VIII (vestibulocochlear, hearing and balance), IX and X (glossopharyngeal and vagus, gag and swallow), and XII (hypoglossal, tongue movement).

'''Nursing application:''' Cranial nerves IX, X, and XII govern swallowing. Assessing gag, cough, and voice quality before giving anything by mouth after a stroke is an aspiration-prevention measure grounded directly in this anatomy.

=== Intracranial Pressure ===

The '''Monro-Kellie doctrine''' states that the skull is a fixed container holding brain tissue, blood, and cerebrospinal fluid. An increase in any one component must be offset by a decrease in another or intracranial pressure rises. Normal intracranial pressure in an adult is approximately 5 to 15 mmHg.

Early signs of increasing intracranial pressure: a change in level of consciousness (the earliest and most sensitive sign), restlessness, headache, and pupillary changes. Late signs: '''Cushing triad''' of widening pulse pressure with rising systolic pressure, bradycardia, and irregular respirations, along with posturing and fixed dilated pupils.

'''Nursing application:''' Elevate the head of the bed about 30 degrees with the head midline to promote venous drainage, avoid hip flexion, avoid neck flexion or rotation, cluster care to limit stimulation, and prevent Valsalva maneuvers such as straining, coughing, and vomiting. Suction only briefly and as needed. Hypotonic fluids are avoided because they worsen cerebral edema.

=== Spinal Cord ===

The spinal cord transmits motor commands down and sensory information up, and mediates reflexes. Injury above T6 risks '''autonomic dysreflexia''', an exaggerated sympathetic response to a stimulus below the level of injury (most often a distended bladder or impacted bowel) causing severe hypertension, pounding headache, bradycardia, flushing above the injury, and pallor below it. The nursing response is to sit the patient upright first, then find and remove the stimulus.

Injury at or above C3 to C5 threatens diaphragm innervation by the phrenic nerve, which is why the memory aid "C3, 4, 5 keeps the diaphragm alive" matters clinically.

== Endocrine System ==

The endocrine system uses hormones carried in the bloodstream for slow, widespread, sustained regulation, in contrast to the fast and targeted nervous system.

* '''Hypothalamus''': controls the pituitary and links the nervous and endocrine systems.
* '''Anterior pituitary''': growth hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, follicle-stimulating hormone, luteinizing hormone, and prolactin.
* '''Posterior pituitary''': stores and releases '''antidiuretic hormone (ADH)''' and oxytocin. ADH causes water reabsorption in the collecting ducts. Too little ADH causes diabetes insipidus with large volumes of dilute urine and hypernatremia; too much causes the syndrome of inappropriate antidiuretic hormone with water retention, concentrated urine, and dilutional hyponatremia.
* '''Thyroid''': triiodothyronine and thyroxine set metabolic rate; '''calcitonin''' lowers serum calcium by moving it into bone.
* '''Parathyroid''': '''parathyroid hormone''' raises serum calcium by pulling it from bone, increasing renal reabsorption, and activating vitamin D for gut absorption. Parathyroid hormone and calcium move together; calcium and phosphorus move in opposite directions.
* '''Adrenal cortex''': '''aldosterone''' (a mineralocorticoid) retains sodium and water and excretes potassium; '''cortisol''' (a glucocorticoid) raises glucose, suppresses inflammation and immunity, and follows a diurnal rhythm.
* '''Adrenal medulla''': epinephrine and norepinephrine, the hormonal arm of the sympathetic response.
* '''Pancreas''': alpha cells secrete '''glucagon''' (raises glucose), beta cells secrete '''insulin''' (moves glucose and potassium into cells), delta cells secrete somatostatin.

'''Nursing application:'''
* Insulin drives potassium into cells, which is why insulin with dextrose is used to treat hyperkalemia and why hypokalemia can develop during treatment of diabetic ketoacidosis. Potassium must be monitored closely.
* Aldosterone's sodium-water-retention and potassium-wasting profile explains the electrolyte pattern in Addison disease (low sodium, high potassium, hypotension, hypoglycemia) versus Cushing syndrome (high sodium, low potassium, hypertension, hyperglycemia).
* Abrupt withdrawal of long-term corticosteroids can precipitate adrenal crisis because exogenous steroid suppresses the hypothalamic-pituitary-adrenal axis. Steroids must be tapered.
* Thyroid surgery risks accidental parathyroid injury, so watch for hypocalcemia and tetany, and keep calcium gluconate available.

== Cardiovascular System ==

=== Structure and Blood Flow ===

The heart has four chambers. Deoxygenated blood returns through the superior and inferior vena cava to the '''right atrium''', passes the '''tricuspid valve''' into the '''right ventricle''', exits through the '''pulmonic valve''' to the pulmonary arteries and lungs. Oxygenated blood returns by the pulmonary veins to the '''left atrium''', passes the '''mitral valve''' into the '''left ventricle''', and is ejected through the '''aortic valve''' into the aorta.

The left ventricle has the thickest wall because it pumps against systemic vascular resistance.

The coronary arteries fill during '''diastole''', which is why tachycardia, by shortening diastole, worsens myocardial ischemia.

=== Conduction ===

The '''sinoatrial (SA) node''' is the normal pacemaker at 60 to 100 beats per minute. The impulse travels to the '''atrioventricular (AV) node''' (40 to 60 beats per minute intrinsic rate), where it is briefly delayed to allow atrial filling of the ventricles, then through the bundle of His, the bundle branches, and the Purkinje fibers (20 to 40 beats per minute).

On the electrocardiogram: the '''P wave''' is atrial depolarization, the '''QRS complex''' is ventricular depolarization, and the '''T wave''' is ventricular repolarization.

=== Cardiac Output ===

Cardiac output equals '''heart rate multiplied by stroke volume''', normally about 4 to 8 liters per minute in an adult. Stroke volume depends on:
* '''Preload''': the volume stretching the ventricle at end diastole. Increased by fluid overload, decreased by hemorrhage, dehydration, and venous dilators.
* '''Afterload''': the resistance the ventricle pumps against. Increased by hypertension, vasoconstriction, and aortic stenosis.
* '''Contractility''': the force of contraction, improved by positive inotropes and reduced by ischemia, acidosis, and hypoxia.

The '''Frank-Starling law''' states that greater stretch produces stronger contraction, up to a point beyond which the failing ventricle cannot keep up.

Blood pressure is a product of cardiac output and systemic vascular resistance. '''Mean arterial pressure''' of roughly 65 mmHg or more is generally needed to perfuse the vital organs.

=== Vessels ===

Arteries carry blood away from the heart and are thick and elastic. Arterioles are the main site of resistance. Capillaries are one cell thick and are the only site of exchange. Veins are thin walled, hold about two thirds of blood volume, and rely on valves and skeletal muscle pumping for return.

'''Starling forces''' at the capillary determine edema: hydrostatic pressure pushes fluid out and oncotic pressure from plasma proteins pulls it back. Low albumin (liver failure, malnutrition, nephrotic syndrome) lowers oncotic pressure and causes edema despite a normal or low blood volume. Normal serum albumin is about 3.5 to 5.0 g/dL.

=== Baroreceptors and the Renin-Angiotensin-Aldosterone System ===

Baroreceptors in the carotid sinus and aortic arch detect pressure changes within seconds and adjust sympathetic and parasympathetic output. Over hours to days, low renal perfusion triggers renin release, which produces angiotensin I, converted by angiotensin-converting enzyme to angiotensin II, a potent vasoconstrictor that also stimulates aldosterone release and sodium and water retention.

'''Nursing application:'''
* This system explains the mechanism of angiotensin-converting enzyme inhibitors and angiotensin receptor blockers, and explains why they can cause hyperkalemia.
* Baroreceptor sensitivity declines with age, which is why orthostatic hypotension and falls are common in older adults. Teach patients to change position slowly and dangle their legs before standing.
* '''Left-sided heart failure''' backs blood into the pulmonary circulation: dyspnea, orthopnea, paroxysmal nocturnal dyspnea, crackles, cough, and pink frothy sputum. '''Right-sided heart failure''' backs blood into systemic veins: jugular venous distention, peripheral edema, hepatomegaly, ascites, and weight gain. Daily weight is the most reliable indicator of fluid status; about 1 kilogram equals about 1 liter of fluid.

== Hematologic and Immune Systems ==

Blood is about 55% plasma and 45% formed elements.

* '''Erythrocytes (red blood cells)''' carry oxygen bound to hemoglobin. Production is stimulated by '''erythropoietin''' from the kidney in response to hypoxia. Life span is about 120 days. Typical adult reference ranges are hemoglobin about 14 to 18 g/dL in men and 12 to 16 g/dL in women, with hematocrit about 42% to 52% in men and 37% to 47% in women.
* '''Leukocytes (white blood cells)''' defend against infection. A typical adult range is about 5,000 to 10,000 per cubic millimeter. Neutrophils are the first responders in bacterial infection; lymphocytes mediate adaptive immunity; eosinophils rise in allergy and parasitic infection.
* '''Thrombocytes (platelets)''' initiate clotting. A typical range is about 150,000 to 400,000 per cubic millimeter.

Hemostasis proceeds through vasoconstriction, platelet plug formation, and the coagulation cascade producing a fibrin clot. The liver synthesizes most clotting factors and requires vitamin K for factors II, VII, IX, and X.

'''Immunity''' is innate (skin, mucous membranes, inflammation, phagocytes, complement) or adaptive. Adaptive immunity includes humoral immunity by B lymphocytes producing antibodies and cell-mediated immunity by T lymphocytes. Active immunity is produced by infection or vaccination and is long lasting; passive immunity comes from transferred antibodies (placenta, breast milk, immune globulin) and is temporary.

'''Nursing application:'''
* Chronic kidney disease reduces erythropoietin production, which explains the anemia of chronic kidney disease.
* Neutropenia, not a low total white count alone, drives infection risk; neutropenic patients may not mount a fever or purulent drainage because the inflammatory response requires white cells. Any temperature elevation in a neutropenic patient is an emergency.
* Liver disease and warfarin therapy both prolong the prothrombin time and international normalized ratio because both interfere with vitamin K dependent factor production.
* Splenectomy increases lifelong risk of infection with encapsulated organisms, so vaccination status matters.

== Respiratory System ==

=== Structure ===

The upper airway (nose, pharynx, larynx) warms, humidifies, and filters air. The lower airway (trachea, bronchi, bronchioles, alveoli) conducts air and performs gas exchange. There are roughly 300 million alveoli, each wrapped in capillaries. '''Surfactant''', produced by type II alveolar cells, lowers surface tension and prevents alveolar collapse. Insufficient surfactant causes respiratory distress syndrome in the preterm newborn.

The right mainstem bronchus is shorter, wider, and more vertical than the left, so aspirated material and a malpositioned endotracheal tube most often go right.

=== Mechanics of Breathing ===

Inspiration is active: the diaphragm contracts and flattens, the external intercostals lift the ribs, thoracic volume increases, intrapleural pressure falls, and air flows in. Expiration at rest is passive elastic recoil. Negative intrapleural pressure keeps the lung expanded against the chest wall; when air enters the pleural space, the lung collapses, which is a pneumothorax.

=== Gas Exchange and Control ===

Oxygen and carbon dioxide move by simple diffusion down partial pressure gradients across the alveolar-capillary membrane. Oxygen is carried mostly bound to hemoglobin; carbon dioxide is carried mostly as bicarbonate.

Breathing is controlled by the medulla and pons. '''Central chemoreceptors''' respond primarily to rising carbon dioxide (sensed as falling cerebrospinal fluid pH), which is the main normal stimulus to breathe. '''Peripheral chemoreceptors''' in the carotid and aortic bodies respond to low oxygen.

Typical adult arterial blood gas reference values: pH 7.35 to 7.45, PaCO2 35 to 45 mmHg, PaO2 80 to 100 mmHg, bicarbonate 22 to 26 mEq/L, and arterial oxygen saturation 95% to 100%.

'''Nursing application:'''
* Ventilation is reflected by PaCO2 and oxygenation by PaO2 and saturation. A patient can have a normal saturation while retaining carbon dioxide, so a normal pulse oximetry reading does not rule out respiratory failure.
* '''Ventilation-perfusion mismatch''' underlies most hypoxemia. Pulmonary embolism creates ventilation without perfusion; pneumonia and atelectasis create perfusion without ventilation.
* Pulse oximetry is unreliable in poor perfusion, hypothermia, motion, and carbon monoxide poisoning, where the reading may be falsely normal.
* Positioning matters: the "good lung down" principle places the healthier lung dependent to improve perfusion where ventilation is best, except in lung abscess or hemorrhage where the affected side is kept down to protect the healthy lung.
* Early hypoxia presents as restlessness, anxiety, tachycardia, and tachypnea. Cyanosis and bradycardia are late. Confusion in an older adult may be the first sign of hypoxia.

== Gastrointestinal System ==

The tract runs from mouth to anus: mouth, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine, rectum, anus. Accessory organs are the salivary glands, liver, gallbladder, and pancreas.

* '''Stomach''': secretes hydrochloric acid, pepsinogen, and intrinsic factor. Intrinsic factor is required for vitamin B12 absorption in the ileum, which is why gastric bypass, gastrectomy, and pernicious anemia cause B12 deficiency.
* '''Small intestine''': the primary site of nutrient absorption, with villi and microvilli providing enormous surface area. The duodenum receives bile and pancreatic enzymes.
* '''Large intestine''': water and electrolyte reabsorption and formation of stool. This is why diarrhea causes dehydration and metabolic acidosis from bicarbonate loss.
* '''Liver''': protein synthesis (albumin, clotting factors), drug and hormone metabolism, bile production, glycogen storage and gluconeogenesis, and conversion of ammonia to urea. Failure of ammonia conversion produces hepatic encephalopathy.
* '''Gallbladder''': stores and concentrates bile, released in response to cholecystokinin when fat enters the duodenum. Bile emulsifies fat and enables absorption of fat-soluble vitamins A, D, E, and K.
* '''Pancreas''': exocrine secretion of amylase, lipase, protease, and bicarbonate; endocrine secretion of insulin and glucagon.

Peristalsis moves contents forward and is stimulated by the parasympathetic nervous system. Opioids, anticholinergics, and immobility slow it; hence constipation risk.

'''Nursing application:'''
* Prolonged vomiting or nasogastric suction removes hydrochloric acid and produces '''metabolic alkalosis''' with hypokalemia; prolonged diarrhea removes bicarbonate and produces '''metabolic acidosis'''.
* Liver failure explains bleeding (clotting factor loss), edema and ascites (low albumin), jaundice (bilirubin), and confusion (ammonia) as one connected picture.
* Bowel sounds return in a predictable order after abdominal surgery: small intestine first, then stomach, then colon. Absent bowel sounds with distention and vomiting suggest ileus or obstruction.
* Fat-soluble vitamin deficiency follows any condition that blocks bile flow, so watch for bleeding from vitamin K deficiency in biliary obstruction.

== Renal and Urinary System ==

The '''nephron''' is the functional unit; each kidney has about one million. Blood enters the glomerulus through the afferent arteriole and leaves through the narrower efferent arteriole, creating the hydrostatic pressure that drives filtration. Filtrate passes to the proximal convoluted tubule (where most reabsorption occurs), the loop of Henle (which concentrates urine), the distal convoluted tubule (site of aldosterone action on sodium and potassium), and the collecting duct (site of ADH action on water).

Kidney functions: excretion of nitrogenous wastes, regulation of fluid volume and electrolytes, acid-base regulation by excreting hydrogen ions and reabsorbing bicarbonate, blood pressure regulation through renin, erythropoietin production, and activation of vitamin D.

Typical adult reference values: blood urea nitrogen about 10 to 20 mg/dL, serum creatinine about 0.6 to 1.2 mg/dL. Creatinine is the more specific indicator of kidney function because blood urea nitrogen also rises with dehydration, high protein intake, and gastrointestinal bleeding.

Adequate urine output in an adult is at least 30 mL per hour.

'''Nursing application:'''
* The kidney is the long-term regulator of both blood pressure and acid-base balance, so kidney failure produces hypertension, metabolic acidosis, hyperkalemia, fluid overload, anemia, and hypocalcemia with hyperphosphatemia. Each of these follows from a function listed above.
* Urine output below 30 mL per hour is reported promptly; it signals inadequate renal perfusion.
* Nephrotoxic exposures include aminoglycosides, nonsteroidal anti-inflammatory drugs, and iodinated contrast media. Hydration and monitoring of creatinine are protective.
* The bladder holds roughly 300 to 500 mL before a strong urge to void; urinary retention with overflow incontinence is a common postoperative and post-anesthesia problem.

== Fluid, Electrolyte, and Acid-Base Balance ==

Total body water is roughly 60% of adult body weight, distributed intracellularly (about two thirds) and extracellularly (about one third, split into interstitial and intravascular compartments). Infants have a higher proportion of body water and older adults a lower proportion, so both dehydrate faster.

Common adult reference ranges:
* Sodium 135 to 145 mEq/L
* Potassium 3.5 to 5.0 mEq/L
* Total calcium 9.0 to 10.5 mg/dL
* Magnesium 1.3 to 2.1 mEq/L
* Chloride 98 to 106 mEq/L
* Bicarbonate 22 to 26 mEq/L

'''Sodium''' is the main extracellular cation and determines water distribution, so sodium disorders present neurologically. Hyponatremia causes cerebral swelling with confusion, headache, and seizures; hypernatremia causes cellular dehydration with thirst, agitation, and neurologic changes.

'''Potassium''' is the main intracellular cation and governs cardiac and neuromuscular excitability. Both hypokalemia and hyperkalemia cause dysrhythmias. Potassium is '''never''' given by IV push and is always diluted and infused on a pump.

'''Calcium''' stabilizes nerve membranes. Low calcium means increased excitability (tetany, Chvostek and Trousseau signs, laryngospasm); high calcium means decreased excitability (lethargy, weakness, constipation, kidney stones).

'''Magnesium''' behaves much like calcium: low magnesium causes hyperexcitability and torsades de pointes; high magnesium causes depressed reflexes and respiratory depression. Deep tendon reflexes are the bedside monitor during magnesium sulfate therapy.

=== Acid-Base ===

The lungs correct acid-base disturbance in minutes by changing carbon dioxide elimination; the kidneys correct over hours to days by handling hydrogen ion and bicarbonate.

A practical interpretation approach:
# Look at pH: below 7.35 is acidosis, above 7.45 is alkalosis.
# Look at PaCO2 (respiratory) and bicarbonate (metabolic).
# Whichever one moves in the same direction as the pH abnormality is the cause; if PaCO2 moves opposite to pH, the problem is respiratory, and if bicarbonate moves in the same direction as pH, the problem is metabolic.
# Check whether the other value has shifted to compensate.

Common causes: respiratory acidosis from hypoventilation (opioid oversedation, chronic obstructive pulmonary disease, atelectasis); respiratory alkalosis from hyperventilation (anxiety, pain, early sepsis); metabolic acidosis from diabetic ketoacidosis, shock and lactic acid, diarrhea, and kidney failure; metabolic alkalosis from vomiting, nasogastric suction, and excessive antacid intake.

== Reproductive System and Selected Life-Span Changes ==

Female structures include the ovaries (oocytes, estrogen, progesterone), fallopian tubes (usual site of fertilization), uterus, cervix, and vagina. Male structures include the testes (sperm and testosterone), epididymis, vas deferens, seminal vesicles, prostate, and penis. The menstrual cycle is regulated by follicle-stimulating hormone and luteinizing hormone from the anterior pituitary, with ovulation at roughly day 14 of a 28-day cycle.

Selected physiologic changes in '''pregnancy''': plasma volume rises more than red cell mass, producing physiologic anemia of pregnancy; cardiac output and heart rate increase; the enlarged uterus can compress the inferior vena cava in the supine position, so left lateral positioning improves venous return and placental perfusion; progesterone relaxes smooth muscle, contributing to reflux, constipation, and urinary stasis.

Selected changes with '''aging''': decreased skin elasticity and subcutaneous fat, reduced bone density, decreased lung elasticity and cough effectiveness, stiffer arteries with higher systolic pressure, reduced glomerular filtration rate and drug clearance, decreased gastric acid, and blunted thirst sensation. These changes explain higher risk of falls, dehydration, pneumonia, adverse drug reactions, and atypical presentation of illness in which confusion may be the only sign of infection.

== Putting It Together: A Prioritization Framework ==

When a question asks who to see first or what to do first, anatomy and physiology supplies the ranking:
# '''Airway''' problems threaten oxygenation in minutes.
# '''Breathing''' problems come next.
# '''Circulation''' problems follow.
# '''Neurologic''' deterioration, especially a change in level of consciousness, outranks stable chronic findings.
# '''Expected''' findings for a given condition are lower priority than '''unexpected''' findings, even when the unexpected finding sounds milder.

Applying this consistently turns anatomy and physiology from memorization into a decision tool.

[[Category:Anatomy and Physiology]]
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