This is a large and vital topic covering five key systems. Focus on understanding the pathways, key organs, and functions of each system. Diagrams are very important here.

1. The Process of Digestion

Short Notes & Key Concepts

• Digestion: The breakdown of large, insoluble food molecules into small, water-soluble molecules that can be absorbed into the bloodstream.

  • Mechanical Digestion: Physical breakdown of food (e.g., chewing).

  • Chemical Digestion: Breakdown of food using enzymes.

• The Digestive Tract Pathway: Mouth → Oesophagus → Stomach → Small Intestine → Large Intestine → Rectum → Anus

• Key Organs & Functions:

  • Mouth: Mechanical digestion by teeth. Chemical digestion of starch begins here with the enzyme salivary amylase (ptyalin).

  • Stomach: Secretes hydrochloric acid (HCl) to kill bacteria and provide the right pH for enzymes. Chemical digestion of protein begins with the enzyme pepsin.

  • Small Intestine: The main site for chemical digestion and absorption.

    • Receives bile from the liver (stored in the gall bladder). Bile emulsifies fats (breaks large drops into smaller ones).

    • Receives pancreatic juice containing enzymes: amylase (for starch), trypsin (for protein), and lipase (for fats/lipids).

    • The intestinal wall secretes its own enzymes to complete digestion.

  • Absorption: The inner wall of the small intestine is lined with villi to increase surface area for efficient absorption of digested food into the bloodstream.

  • Large Intestine: Main function is to absorb water from undigested food.

  • Liver: Produces bile, stores glucose as glycogen, and plays many other metabolic roles.

• End Products of Digestion:

2. The Process of Respiration

• Respiration: The process that releases energy from food (glucose). Do not confuse this with breathing!

• Breathing (External Respiration): The mechanical process of moving air into and out of the lungs (inspiration and expiration).

• Cellular Respiration: The chemical reaction inside cells that releases energy.

  • Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP)

• Respiratory System Pathway: Nasal Cavity → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli

• Alveoli: Tiny air sacs in the lungs. They are the respiratory surface where gas exchange occurs.

  • Adaptations for Gas Exchange:

    1. Large surface area.

    2. Thin walls (one-cell thick).

    3. Moist surface.

    4. Rich blood supply (surrounded by capillaries).

• Aerobic vs. Anaerobic Respiration:

  • Aerobic: Requires oxygen. Produces a large amount of energy (ATP).

  • Anaerobic: Does not require oxygen. Produces a small amount of energy. In humans, it produces lactic acid, which causes muscle cramps.

3. The Process of Excretion

• Excretion: The removal of metabolic waste products (waste made by chemical reactions in cells) from the body. Note: Faeces are undigested food, not metabolic waste, so defecation is not excretion.

• Main Excretory Organs:

  • Kidneys: Excrete urea, excess salts, and excess water in the form of urine.

  • Lungs: Excrete carbon dioxide and water vapour.

  • Skin: Excretes sweat (water, salts, and a small amount of urea).

• The Urinary System: Kidneys → Ureters → Bladder → Urethra

• The Nephron: The functional filtering unit of the kidney.

  • Ultrafiltration: Blood is filtered under high pressure in the glomerulus, forcing small molecules (water, glucose, urea, salts) into the Bowman's capsule. Blood cells and large proteins remain in the blood.

  • Selective Reabsorption: As the filtrate passes through the tubule, all glucose, most of the water, and some salts are reabsorbed back into the blood. Urea and excess water/salts are left behind to form urine.

4. The Process of Circulation

• Function: To transport oxygen, nutrients, hormones, and waste products around the body.

• Components of Blood:

  • Plasma: The liquid part; transports everything.

  • Red Blood Cells: Contain haemoglobin to transport oxygen. Have a biconcave shape and no nucleus to maximize oxygen-carrying capacity.

  • White Blood Cells: Fight disease by phagocytosis and producing antibodies.

  • Platelets: Cell fragments that help in blood clotting.

• Blood Vessels:

  • Arteries: Carry blood away from the heart. Thick, muscular walls to withstand high pressure.

  • Veins: Carry blood towards the heart. Thinner walls, lower pressure, and have valves to prevent backflow.

  • Capillaries: Tiny vessels that connect arteries and veins. Site of exchange with tissues.

• The Heart & Double Circulation:

  • Four Chambers: Right Atrium, Right Ventricle, Left Atrium, Left Ventricle.

  • Double Circulation: Blood passes through the heart twice for each complete circuit of the body.

    1. Pulmonary Circuit: Heart → Lungs → Heart (to pick up oxygen).

    2. Systemic Circuit: Heart → Body → Heart (to deliver oxygen).

  • Right side of the heart deals with deoxygenated blood.

  • Left side of the heart deals with oxygenated blood.

5. Coordination & Homeostasis

• Coordination: How the body's different parts work together, controlled by the nervous and endocrine systems.

• Homeostasis: The maintenance of a constant internal environment (e.g., temperature, blood glucose).

• Nervous System:

  • Central Nervous System (CNS): Brain and spinal cord.

  • Peripheral Nervous System (PNS): Nerves connecting the CNS to the rest of the body.

  • Neuron: The basic nerve cell.

  • Reflex Arc: The pathway a nerve impulse takes in a reflex action (e.g., pulling your hand from a hot object). Pathway: Receptor → Sensory Neuron → Interneuron (in CNS) → Motor Neuron → Effector (muscle/gland).

• Endocrine System: Uses chemical messengers called hormones transported by the blood.

  • Insulin & Glucagon (from Pancreas): Control blood glucose levels.

  • Adrenaline (from Adrenal Glands): Prepares the body for "fight or flight".

  • Thyroxin (from Thyroid): Controls the body's metabolic rate.

Exam Tips & Tricks

1. Diagrams are Key: Be able to draw and label simplified diagrams of the digestive system, respiratory system (especially alveoli), the nephron, the heart, and a reflex arc.

2. Pathways & Sequences: Memorize the flow of food, air, blood, and urine through their respective systems. Exam questions often ask you to list the organs in order.

3. Know Your Enzymes: For digestion, know the enzyme, what it acts on, and the end product (e.g., Amylase acts on Starch → Glucose).

4. Blood Vessel Comparison: A common question is to compare the structure of an artery, vein, and capillary.

Homeostasis Examples: Be ready to explain how the body controls temperature (sweating/shivering) and blood sugar (insulin/glucagon). This is a high-level application question.