Essential Human Biological Systems and Processes

1. Anatomy and Physiology of the Nervous System

The Brain

The brain is the primary organ of the central nervous system (CNS), located within the skull. It regulates body activities, thoughts, memory, emotions, movements, and vital functions.

Parts of the Brain

• Cerebrum: The largest part of the brain, divided into right and left hemispheres. Its outer layer is the cerebral cortex. Functions: Controls voluntary movements, thinking, intelligence, memory, learning, speech, and sensations (pain, touch, temperature).
• Cerebellum: Located below the cerebrum at the back of the brain. Functions: Maintains posture and balance, coordinates muscle movements, and controls fine motor skills.
• Brain Stem: Consists of the midbrain, pons, and medulla oblongata. Functions: Controls breathing, heart rate, blood pressure, and reflexes like swallowing and coughing.
• Spinal Cord: A long cylindrical structure extending from the medulla oblongata, protected by the vertebral column. Functions: Carries nerve impulses between the brain and body; controls reflex actions.

Reflex Arc

The pathway followed by nerve impulses during a reflex action. Components: Receptor (receives stimulus), sensory neuron (carries impulse to CNS), interneuron (processes information), motor neuron (carries response), and effector (produces action).

2. CSF, Meninges, and Neurons

Cerebrospinal Fluid (CSF)

A clear fluid surrounding the brain and spinal cord. Composition: Water, glucose, proteins, ions (sodium, potassium, chloride), and lymphocytes. Formation: Produced by the choroid plexus in the ventricles (approx. 500 ml daily). Circulation: Lateral ventricles → Third ventricle → Fourth ventricle → Subarachnoid space → Venous blood. Functions: Protects the CNS, provides nutrients, and removes waste.

Meninges and Ventricles

• Meninges: Three protective layers: Dura mater (outer), Arachnoid mater (middle), and Pia mater (inner).
• Ventricles: Four cavities filled with CSF: two lateral, one third, and one fourth ventricle.

Neurons

The structural and functional unit of the nervous system. Parts: Cell body, dendrites, and axon. Function: Receives and transmits nerve impulses.

3. Electrophysiology and Transmission

Action Potential of the Heart

Electrical changes in cardiac muscle cells. Phases: Depolarization (Na⁺ entry), Plateau phase (Ca²⁺ entry), and Repolarization (K⁺ exit). Importance: Maintains heartbeat rhythm and muscle contraction.

Neurotransmission

Communication between neurons via chemical messengers (neurotransmitters). Steps: Impulse reaches synapse → Neurotransmitter release → Binding to receptors → Response. Examples: Acetylcholine, Dopamine, Noradrenaline.

Neuromuscular Transmission

Impulse transmission from motor neuron to muscle fiber. Steps: Acetylcholine release → Muscle membrane stimulation → Muscle contraction.

4. Digestive System

Responsible for ingestion, digestion, absorption, and waste removal. Parts: Mouth (teeth/saliva), Pharynx, Oesophagus (peristalsis), Stomach (HCl/enzymes), Small Intestine (duodenum, jejunum, ileum), and Large Intestine (water/salt absorption). Accessory Organs: Liver (bile), Gall bladder (stores bile), and Pancreas (enzymes).

5. Metabolism and Acid Regulation

Stomach Acid Production

Parietal cells secrete H⁺ via the proton pump (H⁺/K⁺ ATPase), which combines with chloride to form HCl. Regulation: Gastrin, histamine, and acetylcholine increase secretion.

ATP and BMR

ATP is the cellular energy molecule formed via glycolysis, the Krebs cycle, and the electron transport chain. Significance: Powers muscle contraction, active transport, and metabolism. Basal Metabolic Rate (BMR) is the minimum energy required at rest to maintain vital functions like breathing and heart activity.

6. Respiratory System

Facilitates gas exchange. Parts: Nose (filters/warms), Pharynx, Larynx (voice), Trachea, Bronchi, and Lungs (alveoli). Gaseous Exchange: Oxygen moves into the blood; CO₂ moves into the alveoli. Functions: Supplies oxygen, removes CO₂, and maintains blood pH.

7. Urinary System

Maintains water, electrolyte, and acid-base balance. Parts: Kidneys, ureters, urinary bladder, and urethra. Nephron: The functional unit (Renal corpuscle, Glomerulus, Bowman’s capsule, Renal tubule). Urine Formation: Glomerular filtration, tubular reabsorption, and tubular secretion.

8. RAAS and Acid-Base Balance

RAAS: Renin-Angiotensin-Aldosterone System regulates blood pressure and fluid balance. Renin triggers angiotensin II, which constricts vessels and releases aldosterone to increase blood volume. Acid-Base Balance: The kidney removes excess H⁺ and reabsorbs bicarbonate to maintain blood pH (7.35–7.45).

9. Endocrine System

Glands producing hormones directly into the bloodstream. Key Glands: Pituitary, Thyroid, Parathyroid, Adrenal, Pancreas, Pineal, Testes, and Ovaries. Functions: Regulates growth, metabolism, reproduction, and homeostasis.

Pituitary and Thyroid Glands

• Pituitary: The "master gland" at the base of the brain. Hormones: GH, TSH, ACTH, FSH, LH, ADH, and Oxytocin.
• Thyroid: Butterfly-shaped gland in the neck. Produces T3 and T4 to control metabolism and temperature. Synthesis: Iodine and tyrosine combine to form hormones, stored as thyroglobulin.
• Other Glands: Pineal (melatonin/sleep cycle) and Thymus (thymosin/immunity).