Grade 7 Chapter 12: The Muscular System and Digestive System in Human Beings

The Muscular System

Our body performs various movements like walking, running, jumping, and lifting objects. These movements are possible due to the contraction and relaxation of muscles. Muscles are bundles of special types of cells that can contract and relax, causing movement in different parts of the body. There are over 600 muscles in the human body. Muscles are always attached to bones by tough, inelastic tissues called tendons.

The study of muscles is called Myology.

Types of Muscles

Muscles are classified into three main types based on their structure and control:

1. Voluntary Muscles (Striated or Skeletal Muscles)

• These muscles are under our conscious control. We can move them according to our will.
• They are attached to bones and are responsible for all body movements like walking, speaking, eating, and lifting.
• They appear striated (striped) under a microscope, hence called striated muscles.
• Examples: Muscles of limbs (arms and legs), tongue, and face.
• Function: To bring about movement of bones.

2. Involuntary Muscles (Smooth or Non-striated Muscles)

• These muscles are not under our conscious control. Their movements occur automatically.
• They are found in the walls of internal organs like the stomach, intestines, blood vessels, and urinary bladder.
• They do not show striations, hence called non-striated or smooth muscles.
• Function: Control involuntary processes such as digestion, blood circulation, and breathing.

3. Cardiac Muscles

• These are special involuntary muscles found only in the wall of the heart.
• They are striated like voluntary muscles but function involuntarily.
• They contract and relax rhythmically and continuously throughout a person's life without tiring, pumping blood throughout the body.
• Function: To pump blood continuously.

Functions of the Muscular System

• Movement: Facilitates walking, running, lifting, and all other bodily movements.
• Posture: Maintains the body's posture, whether sitting or standing.
• Heat Production: Muscle contraction generates heat, helping to maintain body temperature.
• Protection: Muscles protect internal organs.

The Digestive System

The food we eat is complex and cannot be directly used by the body. It needs to be broken down into simpler, soluble forms that can be absorbed and utilized by the body's cells. This process of breaking down complex food substances into simpler, soluble forms is called digestion. The organs that work together to perform this function form the digestive system.

Parts of the Digestive System

The human digestive system consists of the alimentary canal (a long tube extending from the mouth to the anus) and associated digestive glands.

A. Alimentary Canal

The alimentary canal is about 9 meters long in an adult human and includes the following organs:

1. Mouth (Buccal Cavity)

• Teeth: Help in mechanical digestion (chewing/mastication) of food, breaking it into smaller pieces. There are different types of teeth: incisors (biting), canines (tearing), premolars and molars (grinding).
• Tongue: Helps in mixing food with saliva and pushing it towards the pharynx for swallowing. It also contains taste buds.
• Salivary Glands: Three pairs of salivary glands (parotid, submandibular, sublingual) secrete saliva. Saliva contains an enzyme called salivary amylase (or ptyalin) which begins the digestion of carbohydrates (starch) into simpler sugars.
• Food is chewed, mixed with saliva, and formed into a soft ball called a bolus.

2. Pharynx

• A short, funnel-shaped passage connecting the oral cavity to the esophagus and nasal cavity to the larynx.
• It serves as a common passage for food and air. The epiglottis, a flap-like structure, covers the windpipe during swallowing to prevent food from entering the respiratory tract.

3. Esophagus (Food Pipe)

• A muscular tube about 25 cm long that connects the pharynx to the stomach.
• Food moves down the esophagus by rhythmic waves of muscular contractions called peristalsis. This is an involuntary action.
• No digestion occurs here.

4. Stomach

• A J-shaped muscular bag located on the upper left side of the abdomen.
• The stomach churns the food, mixing it with gastric juices secreted by its inner lining.
• Gastric juice contains:
  • Hydrochloric acid (HCl): Kills bacteria, provides an acidic medium for enzymes, and activates pepsin.
  • Pepsin: An enzyme that begins the digestion of proteins into simpler peptides.
  • Mucus: Protects the stomach lining from the acidic action of HCl.
• Food stays in the stomach for about 3-4 hours and is converted into a semi-liquid paste called chyme.

5. Small Intestine

• A highly coiled and long tube (about 6-7 meters in adults).
• It is the site of complete digestion and absorption of food.
• It receives digestive juices from the liver (bile) and pancreas (pancreatic juice), and its own intestinal juice.
• The inner lining of the small intestine has millions of finger-like projections called villi, which increase the surface area for efficient absorption of digested food.
• Digested food (nutrients like glucose, amino acids, fatty acids, glycerol) passes through the villi into the blood vessels and is transported to various parts of the body for assimilation.

6. Large Intestine

• A wider and shorter tube (about 1.5 meters long) than the small intestine.
• No digestion occurs here. Its main functions are:
  • Absorption of water and some salts from undigested food.
  • Formation of feces (undigested solid waste).

7. Rectum and Anus

• Rectum: The final section of the large intestine where undigested waste (feces) is stored temporarily.
• Anus: The opening through which feces are eliminated from the body (egestion/defecation).

B. Associated Digestive Glands

1. Salivary Glands

• Three pairs (parotid, submandibular, sublingual) located in the mouth.
• Secrete saliva containing salivary amylase (ptyalin) to start carbohydrate digestion.

2. Liver

• The largest gland in the body, located in the upper right part of the abdomen.
• Produces bile juice, which is stored in the gallbladder. Bile is not an enzyme but helps in the emulsification of fats (breaking down large fat globules into smaller ones), making them easier for lipase enzymes to act upon.
• Also performs many other functions like detoxification, storage of glycogen, etc.

3. Pancreas

• A leaf-shaped gland located just below the stomach.
• Secretes pancreatic juice, which contains enzymes for the digestion of carbohydrates, proteins, and fats.
  • Pancreatic amylase: Digests carbohydrates.
  • Trypsin: Digests proteins.
  • Lipase: Digests fats.

Digestion Process Summary

• Mouth: Chewing (mechanical), Salivary Amylase starts starch digestion (chemical). Bolus formed.
• Esophagus: Peristalsis pushes bolus to stomach. No digestion.
• Stomach: Churning (mechanical), HCl (acidic medium, kills germs), Pepsin starts protein digestion (chemical). Chyme formed.
• Small Intestine:
  • Receives Bile from Liver (fat emulsification).
  • Receives Pancreatic juice from Pancreas (Amylase, Trypsin, Lipase for carbs, proteins, fats).
  • Secretes Intestinal juice (complete digestion of carbs, proteins, fats into glucose, amino acids, fatty acids/glycerol).
  • Absorption of digested nutrients into blood via villi.
• Large Intestine: Absorption of water and formation of feces.
• Rectum & Anus: Storage and elimination of feces.

Importance of a Healthy Digestive System

• Ensures proper absorption of nutrients for energy, growth, and repair.
• Prevents common digestive issues like indigestion, constipation, and diarrhea.
• Supports overall health and immunity.

Tips for Maintaining Digestive Health

• Eat a balanced diet rich in fiber.
• Chew food properly and slowly.
• Drink plenty of water.
• Avoid overeating and oily/spicy foods.
• Exercise regularly.
• Maintain proper hygiene.

Summary

• Muscles: Bundles of contractile cells responsible for movement.
• Types of Muscles:
  • Voluntary (Skeletal/Striated): Under conscious control, attached to bones (e.g., limb muscles).
  • Involuntary (Smooth/Non-striated): Not under conscious control, found in internal organs (e.g., stomach, intestines).
  • Cardiac: Involuntary, striated, found only in the heart.
• Digestive System: Breaks down complex food into absorbable forms. Consists of alimentary canal and digestive glands.
• Alimentary Canal (Mouth to Anus):
  • Mouth: Chewing, salivary amylase (starch digestion).
  • Esophagus: Peristalsis.
  • Stomach: HCl, pepsin (protein digestion).
  • Small Intestine: Complete digestion and absorption (aided by bile, pancreatic and intestinal juices; villi increase surface area).
  • Large Intestine: Water absorption, feces formation.
  • Rectum & Anus: Feces storage and elimination.
• Digestive Glands:
  • Salivary Glands: Secrete saliva.
  • Liver: Produces bile (fat emulsification).
  • Pancreas: Produces pancreatic juice (digests carbs, proteins, fats).
• Digestion: Mechanical (chewing, churning) and Chemical (enzymes).
• Absorption: Digested food passes into the bloodstream, mainly in the small intestine.