Human Physiology Preview

The human body is a remarkable machine made up of interconnected systems. In Grade 11, you will study human physiology in much greater depth. This lesson previews four key areas: digestion, blood, the heart, and neurons.

The Digestive System (Detailed)

Digestion is the process of breaking down complex food into simple, absorbable molecules.

The journey of food:

1. Mouth: Mechanical digestion (chewing) and chemical digestion begin. Salivary amylase breaks starch into maltose. The tongue forms food into a bolus.

2. Oesophagus: Peristalsis (wave-like muscle contractions) pushes food to the stomach.

3. Stomach: HCl (hydrochloric acid) kills bacteria and activates pepsinogen to pepsin. Pepsin breaks proteins into peptides. The stomach churns food into chyme.

4. Small intestine: The main site of digestion and absorption.

   • Duodenum: Receives bile (from liver, stored in gall bladder) to emulsify fats, and pancreatic juice (amylase, trypsin, lipase)
   • Jejunum and Ileum: Absorption through villi (finger-like projections that increase surface area)

5. Large intestine: Absorbs water and electrolytes. Houses beneficial bacteria. Forms faeces.

6. Rectum and Anus: Storage and elimination of waste.

Blood Composition

Blood is a fluid connective tissue (about 5-6 litres in an adult).

• Plasma (55%): Liquid portion containing water, proteins (albumin, antibodies, fibrinogen), nutrients, hormones, and waste products
• Red Blood Cells (RBCs/Erythrocytes, 45%): Contain haemoglobin for oxygen transport. No nucleus. Produced in bone marrow. Lifespan: about 120 days.
• White Blood Cells (WBCs/Leucocytes): Defence against infection. Types: neutrophils, lymphocytes, monocytes, etc.
• Platelets (Thrombocytes): Cell fragments involved in blood clotting.

Haemoglobin: Hb + O₂ --> HbO₂ (oxyhaemoglobin) in lungs; releases O₂ in tissues.

Heart Structure

The human heart is a four-chambered muscular organ.

Chambers:

• Right atrium: Receives deoxygenated blood from the body (via vena cava)
• Right ventricle: Pumps deoxygenated blood to lungs (via pulmonary artery)
• Left atrium: Receives oxygenated blood from lungs (via pulmonary vein)
• Left ventricle: Pumps oxygenated blood to the body (via aorta) -- thickest wall

Valves prevent backflow: tricuspid (right side), bicuspid/mitral (left side), semilunar valves.

Double circulation: Blood passes through the heart twice in one complete circuit -- pulmonary circulation (heart to lungs) and systemic circulation (heart to body).

The Neuron

Neurons are the functional units of the nervous system. A typical neuron has:

• Dendrites: Receive impulses from other neurons or receptors
• Cell body: Contains the nucleus and most organelles
• Axon: Transmits impulses away from the cell body (can be very long -- up to 1 metre!)
• Synapse: The gap between two neurons where chemical neurotransmitters transfer the signal

Reflex arc: Receptor --> Sensory neuron --> Relay neuron (in spinal cord) --> Motor neuron --> Effector (muscle/gland). This is why you pull your hand away from a hot pan before you even feel the pain.

Nepal Connection

Nepal's community health programs educate people about nutrition and blood donation. The Nepal Red Cross regularly organizes blood donation camps. Understanding human physiology helps health workers in remote areas provide better care.

Key Takeaways

• Digestion breaks food down chemically and mechanically; most absorption occurs in the small intestine
• Blood has plasma, RBCs, WBCs, and platelets
• The heart has four chambers and maintains double circulation
• Neurons transmit signals; reflex arcs provide rapid unconscious responses