LC

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.