Unit 4: Locomotion and Excretion

Part 1: Locomotion

1. Muscle Types

Muscles are tissues that contract to produce movement.

2. Actin and Myosin (Sliding Filament Theory)

Skeletal muscle contracts via the Sliding Filament Theory.

• Muscle fibers are made of myofibrils.
• Myofibrils are composed of repeating units called sarcomeres.
• Sarcomeres contain two types of protein filaments:
  • Actin (Thin filaments): Anchored to the Z-disc.
  • Myosin (Thick filaments): Have "heads" that can bind to actin.

[Image of a sarcomere showing actin and myosin filaments]

Mechanism of Contraction:

1. A nerve signal causes myosin heads to bind to actin, forming "cross-bridges."
2. The myosin heads then pull the actin filaments *past* them, towards the center of the sarcomere.
3. This "sliding" action shortens the sarcomere, and thus the entire muscle, causing it to contract.
4. ATP is required to detach the myosin head and "re-cock" it for the next pull.

3. Bone and Cartilage

Both are types of specialized connective tissue that provide structural support.

• Bone:
  • Structure: A rigid, living tissue. Its matrix is hardened with calcium phosphate crystals.
  • Function: Provides the main structural framework, protects organs, allows movement (via muscles), stores calcium, and produces blood cells (in the bone marrow).
• Cartilage:
  • Structure: A firm, flexible, rubbery connective tissue. It is *avascular* (lacks a blood supply).
  • Function: Provides cushioning and shock absorption at joints (articular cartilage), forms flexible structures (e.g., nose, ears), and connects ribs to the sternum.

4. Human Skeleton

The 206 bones of the human skeleton are divided into two parts.

[Image of the human skeleton, color-coded for axial and appendicular]

a) Axial Skeleton (80 bones)

Forms the central axis of the body.

• Skull: Protects the brain.
• Vertebral Column (Spine): Protects the spinal cord and supports the body.
• Rib Cage (Ribs and Sternum): Protects the heart and lungs.

b) Appendicular Skeleton (126 bones)

Consists of the limbs and the girdles that attach them to the axial skeleton.

• Pectoral Girdle: Clavicle (collarbone) and Scapula (shoulder blade).
• Upper Limbs: Humerus, Radius, Ulna, Carpals, Metacarpals, Phalanges.
• Pelvic Girdle: Hip bones.
• Lower Limbs: Femur, Tibia, Fibula, Tarsals, Metatarsals, Phalanges.

5. Disorders of Skeletal System

• Osteoporosis: A condition where bone density decreases, making bones weak, brittle, and prone to fractures. Common in older adults, especially post-menopausal women.
• Arthritis: Inflammation of one or more joints, causing pain, stiffness, and swelling. (e.g., Osteoarthritis, Rheumatoid Arthritis).
• Fractures: A break in the bone, caused by trauma or disease.

Part 2: Excretion

6. Definition of Excretion

Excretion is the physiological process of removing metabolic wastes (waste products from chemical reactions in the body) and other non-useful, toxic, or excess substances from an organism.

The primary metabolic waste product in humans is urea, which is produced from the breakdown of amino acids (proteins).

7. Structure of Kidney

The kidneys are two bean-shaped organs that are the primary excretory organs in humans. They filter waste from the blood and regulate water/salt balance.

[Image of a cross-section of the human kidney]

Key Regions:

• Renal Cortex: The outer layer of the kidney.
• Renal Medulla: The inner part, arranged in pyramid-shaped structures.
• Renal Pelvis: The central collecting region that funnels urine into the ureter (the tube leading to the bladder).
• Blood Supply: The Renal Artery brings "dirty" blood to the kidney, and the Renal Vein carries "clean" blood away.

8. Structure of Nephron

The nephron is the microscopic functional unit of the kidney, responsible for filtering blood and forming urine. Each kidney contains over a million nephrons.

[Image of the structure of a nephron and its associated blood vessels]

Key Parts:

1. Renal Corpuscle:
   • Glomerulus: A tangled ball of capillaries where blood is filtered.
   • Bowman's Capsule: A cup-shaped structure that surrounds the glomerulus and collects the filtrate (water, salts, urea, glucose, etc. forced out of the blood).
2. Renal Tubule: A long, coiled tube where the filtrate is processed.
   • Proximal Convoluted Tubule (PCT): Reabsorbs most of the good substances (glucose, amino acids, water) back into the blood.
   • Loop of Henle: Dips into the medulla. Establishes a salt gradient to reabsorb water.
   • Distal Convoluted Tubule (DCT): Fine-tunes salt and water balance.
3. Collecting Duct: Receives processed filtrate (now urine) from many nephrons and carries it to the renal pelvis.

9. Urea Cycle

The Urea Cycle is a metabolic pathway that converts highly toxic ammonia (NH₃) into less toxic urea, which can then be safely transported in the blood and excreted by the kidneys.

• Source of Ammonia: The breakdown of amino acids (proteins) in the liver releases ammonia.
• Location: Primarily in the liver.
• Outcome: 2 Ammonia (NH₃) + 1 Carbon Dioxide (CO₂) → 1 Urea + 1 Water.

[Image of the Urea Cycle (simplified)]

10. Composition of Human Urine, Sweat, and Sebum

a) Human Urine

The final filtrate produced by the kidneys.

• ~95% Water
• Metabolic Wastes: Urea (main component), creatinine, uric acid.
• Excess Ions: Sodium (Na⁺), Potassium (K⁺), Chloride (Cl⁻), phosphates.
• Note: Normal urine should *not* contain glucose, proteins, blood cells, or bacteria. Their presence indicates disease.

b) Sweat

A fluid secreted by sweat glands in the skin for thermoregulation (cooling).

• ~99% Water
• Salts (Electrolytes): Mainly Sodium Chloride (NaCl), which is why it tastes salty.
• Trace Wastes: Small amounts of urea and lactic acid.

c) Sebum

An oily, waxy substance secreted by sebaceous glands in the skin.

• Composition: A complex mixture of lipids (fats), waxes, and cholesterol.
• Function: It is *not* primarily excretory. Its function is to lubricate and waterproof the skin and hair.