20.4 Digestive, Urinary, Reproductive & Endocrine Systems
Key Takeaways
- The alimentary canal runs from mouth to pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine, rectum, and anus, with the liver, gallbladder, and pancreas as accessory organs.
- The nephron is the functional unit of the kidney; filtration occurs at the glomerulus, followed by reabsorption and secretion along the tubule to produce urine.
- Urine travels from the kidney through the ureter to the bladder and out the urethra; women have a much shorter urethra, which is why urinary tract infection is far more common in women.
- The anterior pituitary secretes growth hormone, TSH, ACTH, FSH, LH, and prolactin, while the posterior pituitary stores and releases antidiuretic hormone and oxytocin produced by the hypothalamus.
- Type 1 diabetes results from autoimmune destruction of pancreatic beta cells with absolute insulin deficiency, while type 2 results from insulin resistance with relative deficiency.
Digestive System
Normal structure and function
The alimentary canal is a continuous muscular tube: mouth → pharynx → esophagus → stomach → small intestine → large intestine → rectum → anus. Food is moved by peristalsis, rhythmic smooth muscle contraction.
| Structure | Function |
|---|---|
| Mouth | Mastication; salivary amylase begins carbohydrate digestion |
| Esophagus | Transport; the lower esophageal sphincter prevents reflux |
| Stomach | Churns food into chyme; hydrochloric acid and pepsin begin protein digestion; intrinsic factor for B12 absorption |
| Small intestine — duodenum, jejunum, ileum | The primary site of digestion and absorption; villi and microvilli create enormous surface area; lacteals absorb fat |
| Large intestine — cecum, ascending, transverse, descending, sigmoid colon | Water and electrolyte reabsorption; bacterial flora synthesize vitamin K; forms and stores feces |
Accessory organs lie outside the canal but are essential:
- Liver — produces bile, metabolizes drugs and toxins, stores glycogen and fat-soluble vitamins, synthesizes plasma proteins and clotting factors
- Gallbladder — stores and concentrates bile, releasing it to emulsify fat
- Pancreas — exocrine function secretes amylase, lipase, and protease into the duodenum; endocrine function secretes insulin and glucagon into the blood
The pancreas performing both exocrine and endocrine roles is a recurring exam point.
Abnormal structure and function
| Condition | Description |
|---|---|
| GERD | Reflux of gastric contents through an incompetent lower esophageal sphincter; heartburn, regurgitation |
| Peptic ulcer disease | Mucosal ulceration, commonly from Helicobacter pylori or NSAIDs |
| Crohn disease | Inflammatory bowel disease affecting any part of the tract with skip lesions and full-thickness involvement |
| Ulcerative colitis | Inflammatory bowel disease limited to the colon and rectum, continuous and mucosal |
| Irritable bowel syndrome | Functional disorder with pain and altered bowel habit, without structural damage |
| Diverticulosis / diverticulitis | Outpouchings of colonic wall; inflammation or infection of them |
| Cholelithiasis | Gallstones; right upper quadrant pain after fatty meals |
| Hepatitis A / B / C | Viral liver inflammation; A is fecal-oral, B and C bloodborne |
| Cirrhosis | Irreversible hepatic fibrosis; jaundice, ascites, coagulopathy |
| Appendicitis | Appendiceal inflammation; right lower quadrant pain at McBurney point, rebound tenderness |
| Colorectal cancer | Screened from age 45; occult bleeding, change in bowel habit |
Urinary System
Normal structure and function
The system comprises two kidneys, two ureters, the bladder, and the urethra.
The nephron — about one million per kidney — is the functional unit. Urine formation has three steps:
- Filtration at the glomerulus, a capillary tuft within Bowman capsule, driven by blood pressure; water, electrolytes, glucose, and waste pass into the filtrate while cells and large proteins are retained.
- Reabsorption along the proximal tubule, loop of Henle, and distal tubule, returning water, glucose, and needed electrolytes to the blood.
- Secretion of additional waste, drugs, and hydrogen ions from blood into the filtrate.
What remains is urine, which drains through collecting ducts to the renal pelvis, down the ureter by peristalsis, into the bladder for storage, and out through the urethra.
Kidney functions extend beyond waste removal: regulation of fluid and electrolyte balance, acid-base balance, and blood pressure through renin; production of erythropoietin, which stimulates red cell production; and activation of vitamin D. This is why chronic kidney disease produces anemia and bone disease alongside uremia.
Anatomic sex difference. The female urethra is roughly 1.5 inches long and the male about 8 inches. The much shorter distance from the perineum to the bladder is why urinary tract infection is far more common in women — and why front-to-back perineal cleansing and the midstream clean-catch technique matter so much for specimen validity.
Abnormal structure and function
| Condition | Description |
|---|---|
| Urinary tract infection / cystitis | Dysuria, frequency, urgency, suprapubic pain; usually E. coli |
| Pyelonephritis | Kidney infection; fever, flank pain, costovertebral angle tenderness |
| Nephrolithiasis | Kidney stones; severe colicky flank pain radiating to the groin, hematuria |
| Chronic kidney disease | Progressive loss of function; eGFR below 60 for 3 months |
| Acute kidney injury | Abrupt decline in function; often reversible |
| Glomerulonephritis | Glomerular inflammation; hematuria, proteinuria, edema |
| Benign prostatic hyperplasia | Prostatic enlargement obstructing outflow; hesitancy, weak stream, nocturia |
| Incontinence | Stress, urge, overflow, or functional |
Reproductive System
Male: testes (within the scrotum) produce sperm and testosterone; sperm mature in the epididymis, travel the vas deferens, and mix with secretions from the seminal vesicles, prostate, and bulbourethral glands to form semen, exiting through the urethra within the penis.
Female: ovaries produce ova and estrogen and progesterone; the fallopian (uterine) tubes transport the ovum and are the usual site of fertilization; the uterus — fundus, body, and cervix, with the endometrial lining — houses the developing fetus; the vagina is the birth canal and copulatory organ. The breasts contain mammary glands that produce milk under prolactin stimulation.
The menstrual cycle averages 28 days: the menstrual phase (days 1–5, endometrial shedding), the follicular/proliferative phase (days 6–13, FSH-driven follicular maturation and endometrial rebuilding), ovulation (about day 14, triggered by an LH surge), and the luteal/secretory phase (days 15–28, the corpus luteum secreting progesterone to maintain the endometrium; without implantation it degenerates and menstruation begins).
Disorders include endometriosis (endometrial tissue outside the uterus, causing pain and infertility), uterine fibroids, polycystic ovary syndrome, pelvic inflammatory disease, benign prostatic hyperplasia and prostate cancer, erectile dysfunction, infertility, ectopic pregnancy, and the sexually transmitted infections.
A patient with chronic kidney disease has developed anemia. Which kidney function explains this complication?
Endocrine System
Normal structure and function
Endocrine glands are ductless and secrete hormones directly into the bloodstream, acting on distant target tissues. This contrasts with exocrine glands, which secrete through ducts onto a surface. Most endocrine activity is regulated by negative feedback: rising hormone levels suppress the signal that produced them.
| Gland | Principal hormones | Function |
|---|---|---|
| Hypothalamus | Releasing and inhibiting hormones; produces ADH and oxytocin | Links the nervous and endocrine systems; controls the pituitary |
| Anterior pituitary | Growth hormone, TSH, ACTH, FSH, LH, prolactin | The "master gland" — directs other endocrine glands |
| Posterior pituitary | Stores and releases ADH and oxytocin made by the hypothalamus | Water retention; uterine contraction and milk letdown |
| Thyroid | T3, T4, calcitonin | Metabolic rate; calcitonin lowers blood calcium |
| Parathyroid | Parathyroid hormone | Raises blood calcium — the opposite of calcitonin |
| Adrenal cortex | Cortisol, aldosterone, androgens | Stress response and glucose regulation; sodium and water retention |
| Adrenal medulla | Epinephrine, norepinephrine | Sympathetic "fight or flight" response |
| Pancreatic islets | Insulin (beta cells), glucagon (alpha cells) | Insulin lowers blood glucose; glucagon raises it |
| Ovaries / testes | Estrogen and progesterone / testosterone | Reproductive function and secondary sex characteristics |
| Pineal | Melatonin | Circadian rhythm |
| Thymus | Thymosin | T lymphocyte maturation |
Two pairings account for most endocrine exam questions. Insulin lowers glucose and glucagon raises it. Calcitonin lowers serum calcium (think "calcitonin tones calcium down") and parathyroid hormone raises it. The posterior pituitary is also a common trap: it stores and releases ADH and oxytocin but does not manufacture them — the hypothalamus does.
Abnormal structure and function
| Condition | Description |
|---|---|
| Type 1 diabetes mellitus | Autoimmune destruction of beta cells with absolute insulin deficiency; usually younger onset; requires insulin; risk of diabetic ketoacidosis |
| Type 2 diabetes mellitus | Insulin resistance with relative deficiency; associated with obesity and inactivity; managed with lifestyle change, oral agents, and sometimes insulin |
| Gestational diabetes | Glucose intolerance first recognized in pregnancy; screened at 24–28 weeks |
| Hypothyroidism | Deficient thyroid hormone; fatigue, weight gain, cold intolerance, constipation, bradycardia; TSH high, free T4 low |
| Hyperthyroidism / Graves disease | Excess hormone; weight loss, heat intolerance, tremor, tachycardia, exophthalmos; TSH low, free T4 high |
| Goiter | Thyroid enlargement, classically from iodine deficiency |
| Cushing syndrome | Cortisol excess; central obesity, moon face, buffalo hump, striae, hyperglycemia |
| Addison disease | Adrenal insufficiency; fatigue, weight loss, hypotension, hyperpigmentation |
| Acromegaly / gigantism | Growth hormone excess after / before epiphyseal closure |
| Diabetes insipidus | ADH deficiency; massive dilute urine output — unrelated to diabetes mellitus despite the shared name |
| Hyperparathyroidism | Excess PTH; hypercalcemia, bone demineralization, kidney stones |
The type 1 versus type 2 distinction is the single most tested endocrine item: autoimmune absolute deficiency versus resistance with relative deficiency. It explains why a patient with type 1 diabetes cannot be managed on oral agents alone and why diabetic ketoacidosis is characteristic of type 1 while hyperosmolar hyperglycemic state is more typical of type 2.
Equally confusable are diabetes mellitus and diabetes insipidus — the first is a disorder of glucose regulation, the second a disorder of water regulation from insufficient antidiuretic hormone. They share only a name derived from the increased urine output both produce.
Which statement correctly describes the relationship between calcitonin and parathyroid hormone?
A 19-year-old patient is newly diagnosed with type 1 diabetes mellitus. The patient asks why she cannot simply take pills like her grandfather does. What is the correct physiologic explanation?
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