Human Body Chapter Notes | Biology Class 6 ICSE PDF Download

Introduction

•  The human body is a complex structure made up of various organ systems that work together to perform essential functions. 
•  This chapter focuses on three vital systems: the digestive system, which breaks down food into nutrients; 
•  the respiratory system, which enables breathing and oxygen exchange; 
•  and the circulatory system, which transports blood and nutrients throughout the body. 
•  Understanding these systems helps us appreciate how our body functions and how to maintain its health through proper nutrition and exercise. 

The Digestive System

• Comprises the alimentary canal and digestive glands.
• Alimentary canal is a long tube from mouth to anus, including mouth, oesophagus, stomach, small intestine, large intestine, rectum, and anus.
• Digestive glands include salivary glands, liver, and pancreas, which assist in digestion.

Mouth

• Food enters through the mouth, opening into the buccal cavity.
• Contains tongue, teeth, and salivary glands.
• Teeth chew food into smaller pieces, mixing with saliva.
• Saliva, secreted by three pairs of salivary glands, moistens food for swallowing.

Tongue

• A muscular organ, attached at the back of the buccal cavity, free at the front.
• Moves in all directions to assist in swallowing and mixing food with saliva.
• Contains taste buds to detect sweet, bitter, salty, and sour tastes.
• Aids in speaking.

Teeth

• Located in upper and lower jaws, fixed to gums.
• Break food into smaller pieces for swallowing and grind it into pulp.
• Assist in speaking.
• Four types: incisors, canines, premolars, and molars.
• Incisors (8 total, 4 in each jaw) are flat, sharp-edged for cutting and biting.
• Canines (4 total, 2 in each jaw) are sharp, pointed for piercing and tearing.
• Premolars (8 total, 4 in each jaw) crush and grind food.
• Molars (12 total, 6 in each jaw) chew and grind food; the last molar is the wisdom tooth, appearing at 17-20 years.
• Babies have 20 milk teeth (8 incisors, 4 canines, 8 premolars) from 6-7 months, replaced by 32 permanent teeth (8 incisors, 4 canines, 8 premolars, 12 molars) between 6-8 years.

Oesophagus (Food Pipe)

• A tube from throat to stomach, moving food via peristalsis (wave-like muscle contractions).
• No digestion occurs here.

Stomach

• A muscular, hollow, thick-walled, flattened organ on the left side of the abdomen.
• Widest part of the alimentary canal, receives food from the oesophagus.
• Gastric glands in the inner lining secrete gastric juices for partial digestion.
• Transfers partially digested food to the small intestine.

Small Intestine

• A 7m long, 2.5cm wide coiled tube divided into duodenum, jejunum, and ileum.
• Inner wall has villi, finger-like projections that aid in absorbing digested food.

Large Intestine, Rectum, and Anus

• Large intestine is 1.5m long, 6.5cm wide, with three parts: caecum, colon, and rectum.
• Ileum opens into caecum, which leads to colon, surrounding the small intestine.
• Colon leads to rectum, which opens to the anus.
• Undigested food is expelled as faeces through the anus.

Salivary Glands

• Three pairs in the mouth secrete saliva containing salts, mucus, and enzymes.
• Saliva moistens food, dissolves some food for taste, converts starch to sugar, and cleans the mouth by destroying germs.

Liver

• Largest gland, reddish-brown, located in the upper right abdomen.
• Secretes bile juice, which aids in fat digestion and is associated with the small intestine.

Pancreas

• Cream-coloured gland associated with the small intestine.
• Secretes pancreatic juice with enzymes that break down proteins, starch, and fats into simpler forms.

The Process of Digestion

Food in the Mouth

• Food is chewed and mixed with saliva containing salivary amylase, which converts starch to maltose.
• Forms a soft ball called bolus, which moves to the oesophagus via the pharynx.

Food in the Stomach

• Food is churned by stomach muscles, mixed with gastric juice containing water, hydrochloric acid, pepsin, and renin.
• Hydrochloric acid kills bacteria and activates pepsin; mucus protects stomach lining.
• Pepsin breaks proteins into proteoses and peptones.
• Renin converts milk protein casein into insoluble curd (paracasein).
• Food becomes a pulp-like paste called chyme, which moves to the small intestine.

Food in the Small Intestine

• Duodenum receives bile from the liver and pancreatic juice from the pancreas.
• Bile emulsifies fats into tiny droplets and provides an alkaline medium.
• Pancreatic juice contains trypsin (breaks proteins, proteoses, peptones into polypeptides), amylase (converts starch to maltose), and lipase (breaks fats into fatty acids and glycerol).
• In the ileum, enzymes erepsin (converts peptones and peptides to amino acids), maltase (converts maltose to glucose), sucrase (converts sucrose to glucose and fructose), and lactase (converts lactose to glucose and galactose) complete digestion.

Absorption of Digested Food

• Small intestine absorbs digested food via villi, which have blood capillaries.
• Nutrients enter the bloodstream through capillaries for distribution.

Food in the Large Intestine

• Absorbs water from undigested food, forming semi-solid faeces.
• Faeces move to the rectum and are expelled through the anus (egestion).

Assimilation of Digested Food

• Nutrients are stored or converted for later use.
• Liver stores excess glucose as glycogen or converts it to fat for adipose tissue.
• Fatty acids provide energy or are stored under the skin.
• Amino acids form proteins; excess is converted to urea, removed by kidneys.

The Respiratory System

Nose and Nasal Cavity

• Air enters through nostrils into the nasal cavity.
• Hair traps dust; mucus traps dust and germs, preventing lung entry.
• Blood vessels warm inhaled air to body temperature.
• Breathing through the nose is recommended for cleaner, warmer air.

Pharynx, Larynx, and Trachea

• Pharynx is behind the nose and mouth, leading to the larynx (voice box).
• Larynx has vocal cords that vibrate to produce sound.
• Trachea (windpipe) has a glottis opening guarded by the epiglottis, which closes during swallowing to prevent food entry.
• Food in the trachea causes coughing if the epiglottis fails.

Bronchi, Bronchioles, and Alveoli

• Trachea divides into two bronchi, one for each lung.
• Bronchi divide into bronchioles, ending in alveolar sacs with millions of alveoli.
• Alveoli have blood capillaries for gas exchange.
• Ciliated epithelium in trachea, bronchi, and bronchioles pushes out unwanted particles.

Lungs

• Spongy, elastic organs in the chest cavity, protected by the ribcage.
• Left lung has two lobes; right lung has three.
• Covered by pleural membranes with pleural fluid for lubrication and gas diffusion.
• Diaphragm, a muscular sheet below lungs, aids in breathing.

Mechanism of Respiration

Breathing

• Physical process of inhaling oxygen and exhaling carbon dioxide.
• Inhalation: Ribs move up and out, diaphragm flattens, increasing chest space, lowering air pressure, and inflating lungs.
• Exhalation: Ribs move down and in, diaphragm moves up, decreasing chest space, increasing air pressure, and deflating lungs.
• One inhalation and exhalation form one breath.

Gaseous Transport

• Oxygen in alveoli diffuses into blood capillaries, binding with haemoglobin to form oxyhaemoglobin.
• Blood carries oxygen to body cells, releasing it and freeing haemoglobin.
• Carbon dioxide from cells binds with haemoglobin as carbaminohaemoglobin, transported to alveoli for exhalation.

Cellular Respiration
Biochemical process in cells, breaking down glucose with oxygen to produce carbon dioxide, water, and energy (stored as ATP).

Differences between Breathing and Cellular Respiration

• Breathing is physical, involves oxygen intake and carbon dioxide release, occurs in lungs, and releases no energy. 
• Cellular respiration is biochemical, oxidizes glucose to produce energy, occurs in cells, and releases energy as ATP. 

Respiratory Diseases

• Asthma: Allergic reaction causing breathing difficulty and wheezing, triggered by pollen, dust, or foods.
• Bronchitis: Viral infection of air passages, causing mucus cough, wheezing, and shortness of breath.
• Pneumonia: Bacterial lung inflammation with fluid in alveoli, causing cough, fever, chills, and shortness of breath.
• Tuberculosis: Bacterial disease affecting lungs, causing mucus or blood cough, weight loss, appetite loss, and lung pain.

The Circulatory System

• Consists of heart, blood vessels (arteries, veins, capillaries), and blood.
• Transports nutrients, oxygen, and waste products throughout the body.

The Heart

• The two upper chambers are called the atria (singular—atrium) or auricles.
• The two lower chambers are called the ventricles.
• The right auricle opens into the right ventricle.
• The left auricle opens into the left ventricle.
• There are valves between the auricle and the ventricle on each side.
• The valves allow blood to flow only in one direction, thus preventing the backflow of blood.
• There are no valves in between the two auricles or in between the two ventricles.
• The right side of the heart remains completely separated from the left side.
• There is no mixing of the deoxygenated blood (rich in carbon dioxide) with the oxygenated blood (rich in oxygen).

Function
The heart acts as a pump which brings about the circulation of blood throughout the body.

Blood Vessels

Inside the body, there is a system of pipes or channels through which the blood flows. These pipes or channels are called the blood vessels.
Three types of blood vessels, arteries, veins, and capillaries are present in the body. Arteries carry blood away from the heart, while veins bring blood back into the heart. A network of fine blood vessels forms the connection between the arteries and veins. These fine blood vessels are called capillaries.
Arteries

• Blood vessels that take blood away from the heart are called arteries. Arteries are deep seated except at some places (like the wrist, the temple and along the side of the neck) where these are close to the skin.
• They have thick elastic walls.
• They carry oxygenated (pure) blood (except the lung artery called the pulmonary artery which carries deoxygenated blood).
• Valves are absent in arteries.

Blood flows rapidly and at a high pressure through the arteries. The high pressure of blood strongly pushes the thick, elastic artery walls. This force exerted by the blood on the walls of the blood vessels is called blood pressure. The walls stretch and shrink as the blood passes by, giving rise to a pulse. The throbbing caused by the blood flowing through the arteries is called the pulse. It can be felt at the places where arteries lie close to the skin.
When you visit a doctor, he often holds your wrist and counts something. He is, in fact, counting your pulse rate by feeling the pressure of the movement of blood through the artery in your wrist.

Veins

• Veins are blood vessels that carry blood back to the heart from all parts of the body.
• They lie closer to the surface of the skin and can be seen as greenish-blue lines in our hands and legs.
• They have thinner walls than arteries.
• Valves are present in veins to stop the backward flow of blood.
• Blood, therefore, moves towards the heart.
• Blood in the veins does not move under high pressure and so does not push strongly on the walls of the veins.
• Veins carry deoxygenated (impure) blood rich in carbon dioxide from all parts of the body to the heart (except the lung vein called the pulmonary vein which carries oxygenated blood).

Capillaries

• When an artery reaches an organ, it splits into smaller and smaller vessels. The smallest blood vessels are called capillaries.
• Capillaries are extremely thin-walled (only one cell thick).
• They spread throughout the organ so that all cells have blood passing close to them.
• When blood leaves an organ, capillaries join together to form larger and larger vessels, and ultimately form the veins.

Blood

• Red fluid in blood vessels, transports nutrients, oxygen, and waste, fights infections, aids clotting, and regulates body temperature.
• Consists of plasma (liquid part) and cells: red blood corpuscles (RBCs), white blood cells (WBCs), and platelets.
• RBCs contain haemoglobin, carry oxygen, and give blood its red colour.
• WBCs fight infections by destroying germs.
• Platelets aid in blood clotting to prevent excessive bleeding.

Blood Groups

• Blood has antigens (A, B on RBCs) and antibodies (A, B in plasma).
• Four groups: A (antigen A, antibody B), B (antigen B, antibody A), AB (antigens A and B, no antibodies), O (no antigens, antibodies A and B).
• Group O is a universal donor; group AB is a universal recipient.

Circulation of Blood

• Pulmonary vein carries oxygenated blood from lungs to left atrium, then to left ventricle, which pumps it to body tissues via arteries.
• Arteries divide into capillaries, delivering nutrients and oxygen, collecting carbon dioxide and waste.
• Deoxygenated blood flows through capillaries to veins, then to right atrium and ventricle.
• Right ventricle pumps blood to lungs via pulmonary artery for oxygenation, returning to left atrium.

Heartbeat

• Heart muscles contract (systole) and relax (diastole) rhythmically, forming a cardiac cycle (0.8 seconds).
• Atria contract first, then ventricles, producing “lub” (atria) and “dub” (ventricles) sounds.
• Average heartbeat is 72 times per minute, heard via a stethoscope.

Keeping the Heart Healthy

Healthy Eating Habits

• dairy products (cheese, paneer and ghee)
• oily food
• meat

which contain high amounts of fat.
Also, when a fat-rich diet is consumed regularly, a person becomes obese. This puts extra strain on the heart, as it has to push the blood to a greatly enlarged body. This extra strain on the heart and the accumulation of fatty substances in the arteries can lead to heart attack. Obesity can be overcome by eating a balanced diet and increasing physical activity.

Exercise

• Physical exercise is necessary to maintain good health.
• When we exercise, the heart pumps more blood into arteries and the lungs expand fully to take in maximum oxygen.
• This means that blood circulation becomes faster and more oxygen goes to the cells for oxidation of food.
• In this process, more energy is produced by the food.
• The heart muscles and lungs also become stronger by doing more work.
• Exercise also helps in relieving tension, and a person feels and looks better.

Keywords

• Saliva: Watery substance in mouth from salivary glands.
• Wisdom tooth: Last molar in each jaw, appears at 17-20 years.
• Oesophagus: Tube from mouth to stomach for food passage.
• Villi: Finger-like projections in small intestine for food absorption.
• Bile juice: Liver secretion aiding fat digestion and enzyme activation.
• Digestion: Converting complex food to absorbable forms.
• Emulsification: Breaking fats into tiny droplets for enzyme action.
• Diaphragm: Muscular sheet below lungs aiding breathing.
• Breathing: Physical process of oxygen intake and carbon dioxide release.
• Arteries: Vessels carrying blood away from the heart.
• Veins: Vessels carrying blood to the heart.
• Systole: Heart contraction phase.
• Diastole: Heart relaxation phase.
• Heartbeat: Rhythmic heart contraction and relaxation.