Human Anatomy – Organ Systems, Cavities & Levels

Organ Systems & Their Key Organs

• Endocrine System

  • Major glands: \text{pancreas}, \text{testes}, \text{ovaries}, thyroid, parathyroids, adrenals, pineal, pituitary, hypothalamus.
  • Core functions:
    • Secretes chemical messengers (hormones) directly into the bloodstream.
    • Coordinates long-term growth, metabolism, reproduction, and stress responses.
  • Significance: Allows body-wide communication slower than nerves but longer-lasting; integrates with the nervous system to form the neuro-endocrine axis.

• Lymphatic / Immune System

  • Structures: lymph nodes, spleen, thymus, tonsils, red bone marrow, lymphatic vessels.
  • Functions:
    • Produces/distributes immune cells (T-cells, B-cells, natural-killer cells).
    • Returns interstitial fluid to the bloodstream; absorbs lipids (lacteals) from the intestine.
  • Example: Swollen “glands” during infection represent hyperactive lymph nodes producing armies of lymphocytes.

• Cardiovascular System

  • Organs: heart, arteries, veins, capillaries, blood.
  • Key roles:
    • Transports oxygen from lungs and nutrients from the digestive tract to tissues.
    • Removes (\text{CO}_2) and metabolic wastes.
    • Liquid connective tissue (blood) also carries hormones, heat, immune cells.
  • Clinical tie-in: Atherosclerosis narrows arteries, starving tissues of \text{O}_2 and glucose.

• Digestive System

  • Occupies most of the abdominal cavity.
  • Route: oral cavity → pharynx → esophagus → stomach → small intestine (duodenum, jejunum, ileum) → large intestine → rectum → anus.
  • Accessory organs: liver, gallbladder, pancreas (exocrine portion).
  • Central portion (umbilical region) dominated by the small intestine—especially the jejunum.

• Nervous System

  • Brain + spinal cord fill most of the dorsal body cavity (cranial & vertebral cavities).
  • Rapid, electro-chemical communication network; basis for sensation, movement, cognition.

• Respiratory System

  • Primary acid-base balancer via regulation of blood P{\text{CO}2}.
  • Organs: nasal cavities, pharynx, larynx, trachea, bronchi, lungs (each lung lies in its own pleural cavity).
  • Gas exchange occurs across alveolar–capillary membrane.

• Urinary (Renal) System

  • Cooperates with respiratory system to maintain pH by excreting H^+ and reabsorbing HCO_3^-.
  • Major organs in abdominal–pelvic region: kidneys, ureters, bladder (in true pelvis), urethra.

• Reproductive System

  • Primarily located in the pelvic cavity.
    • Male: testes (in scrotum), epididymides, vas deferens, seminal vesicles, prostate, penis.
    • Female: ovaries, uterine tubes, uterus, vagina.
  • Works closely with endocrine system (gonads = mixed endocrine/exocrine glands).

Levels of Structural Organization

• Chemical level → Cell → Tissue → Organ → Organ-system → Organism.
• The level between the cell and the organ is the tissue level (aggregates of similar cells working together, e.g.
  epithelial, connective, muscle, nervous tissues).

Body Cavities & Landmark Organs

• Dorsal Body Cavity

  • Cranial cavity: brain.
  • Vertebral (spinal) cavity: spinal cord.
  • Dominant organ system: nervous.

• Ventral Body Cavity

  • Thoracic cavity:
    • Two pleural cavities – each houses a lung.
    • Mediastinum (central) – contains heart (in pericardial cavity), esophagus, trachea, thymus, major vessels.
    • “Large organ located in the mediastinum” = heart.
  • Abdominopelvic cavity (separated by diaphragm):
    • Abdominal portion – digestive organs; small intestine sits mainly in the umbilical region.
    • Pelvic portion – urinary bladder, internal reproductive organs, distal colon.

Acid–Base Homeostasis (Respiratory + Urinary)

• Blood pH tightly controlled around 7.35–7.45.
• Key equation (bicarbonate buffer):
  pH \; \approx \; 6.1 + \log\left(\dfrac{[HCO3^-]}{0.03\times P{CO_2}}\right)
• Respiratory compensation: (\uparrow) ventilation → (\downarrow P{CO2}) → more basic; opposite for acidosis.
• Renal compensation: kidneys excrete H^+, generate/reabsorb HCO_3^-.
• Failure of either system leads to respiratory or metabolic acidosis/alkalosis.

Integrative & Clinical Connections

• Endocrine and nervous systems form the body’s dual-control hierarchy (fast vs. slow).
• Lymphatic & cardiovascular systems are anatomically intertwined—lymph eventually returns to venous blood.
• Digestive, cardiovascular, and urinary systems cooperate for nutrient uptake, distribution, and waste removal.
• Pelvic inflammatory disease (PID) illustrates anatomical proximity of reproductive and urinary tracts.
• Trauma to the mediastinum can impair heart and great vessels, rapidly compromising circulation.

Common Exam Triggers & Mnemonics

• “T.O.P” glands in endocrine (Testes, Ovaries, Pancreas) frequently tested.
• Body cavity landmark: “H&L” (Heart in Mediastinum, Lungs in Pleura).
• Acid–base pair: “R and U keep pH true” (Respiratory + Urinary).
• Levels of organization: “C.T.O” (Cell → Tissue → Organ) fills the gap question.