Key Functions in Human Biology

Posted on Feb 5, 2026 in Medicine

Digestive and Metabolic Functions

1. Saliva and Stomach Acid

  • Saliva: Lubricates food for swallowing, moistens the mouth, and contains the enzyme Salivary Amylase (Ptyalin) which begins the digestion of carbohydrates (starches).
  • HCl (Hydrochloric Acid): Maintains an acidic pH (1.5–3.5) in the stomach to activate pepsinogen into pepsin (for protein digestion) and kills ingested bacteria/microbes.

2. Basal Metabolic Rate (BMR)

  • BMR: The minimum amount of energy (calories) required by the body to maintain vital life functions (breathing, circulation, cell production) while at complete rest.
  • Determination: It is determined using calorimetry (Direct Calorimetry measures heat production; Indirect Calorimetry measures oxygen consumption).

14. Blood Glucose Regulation and Gastric Acid

  • Insulin: Lowers blood glucose levels by promoting glucose uptake into cells.
  • Glucagon: Raises blood glucose levels by breaking down glycogen in the liver.
  • Gastric Acid: Essential for protein digestion and immune defense.

11. Digestive Processes

  • Hyperchlorhydria: A condition characterized by excess production of hydrochloric acid (HCl) in the stomach (acidity).
  • Peristalsis: The wave-like muscle contractions of the digestive tract that move food along the esophagus and intestines.

Respiratory and Renal Systems

3. Lung Volumes

  • Tidal Volume (TV): The volume of air inhaled or exhaled during a normal breath (approx. 500ml).
  • Vital Capacity (VC): The maximum amount of air a person can expel from the lungs after a maximum inhalation (VC=TV+IRV+ERV).
  • Total Lung Capacity (TLC): The total volume of air in the lungs after a maximum effort of inspiration.

8. Glomerular Filtration Rate (GFR)

  • GFR (Glomerular Filtration Rate): The total amount of filtrate formed by the kidneys per minute (Normal: approx. 125 ml/min).
  • Net Filtration Pressure (NFP): The total pressure that promotes filtration. It is the Glomerular hydrostatic pressure minus the opposing forces (Colloid osmotic pressure + Capsular hydrostatic pressure).

9. Renin-Angiotensin-Aldosterone System (RAAS)

A hormone system that regulates blood pressure and fluid balance. When blood pressure drops, the kidneys release Renin, which converts Angiotensinogen to Angiotensin I. ACE converts I to Angiotensin II (a vasoconstrictor), which stimulates the release of Aldosterone (increases Na+ and water retention), raising blood pressure.

Hormonal Control and Reproduction

4. General Function of Hormones

Hormones are chemical messengers secreted by endocrine glands directly into the blood. They regulate physiology, including growth and development, metabolism, sexual function, reproduction, and mood.

12. Pituitary Hormones

Anterior Pituitary

  • GH: Growth hormone (Growth).
  • TSH: Thyroid Stimulating Hormone (Thyroid function).
  • ACTH: Adrenocorticotropic Hormone (Adrenal function).
  • FSH/LH: Gonadotropins (Reproductive function).
  • Prolactin: Milk production.

Posterior Pituitary

  • Oxytocin: Uterine contractions/Milk ejection.
  • ADH (Vasopressin): Water retention.

13. Menstruation and Menopause

  • Menopause: The permanent cessation of menstruation (usually occurs between ages 45–55).
  • Phases of Menstrual Cycle: Menstrual Phase (Bleeding), Follicular /Proliferative Phase, Ovulation, Luteal/Secretory Phase.

5. Gametogenesis

The process of formation of gametes (sex cells).

  • Spermatogenesis: The formation of sperm (male gametes) in the testes.
  • Oogenesis: The formation of ova/eggs (female gametes) in the ovaries.

Nervous System Fundamentals

6. Nerve Impulse Transmission

  • Action Potential: A rapid rise and fall in voltage across a cellular membrane (nerve impulse) caused by the movement of ions (Na+ and K+), allowing a signal to travel down a neuron.
  • Synapse: The junction between two nerve cells (neurons) or between a neuron and a muscle, where impulses are transmitted by neurotransmitters.

7. Classification of Neurotransmitters

They can be classified chemically:

  • Amino Acids: e.g., GABA, Glutamate, Glycine.
  • Amines (Monoamines): e.g., Dopamine, Serotonin, Epinephrine, Histamine.
  • Peptides: e.g., Endorphins, Substance P.
  • Acetylcholine: (Distinct class).

15. Key Brain Structures

  • Hypothalamus: Maintains homeostasis (body temp, hunger, thirst) and controls the pituitary gland.
  • Cerebrum: Responsible for higher functions like thinking, memory, voluntary movement, and sensory perception.
  • Cerebellum: Coordinates voluntary movements, balance, and posture.

Organ Structure and Role

10. Organ Structure and Function

  • Gallbladder: A pear-shaped organ under the liver. Function: Stores and concentrates bile produced by the liver.
  • Lungs: Spongy, air-filled organs. Function: Gas exchange (O2 in, CO2 out).
  • Testis: Oval organs in the scrotum. Function: Produce sperm and the hormone testosterone.