Complete Biology notes on Human Body Systems: Digestive, Circulatory, Respiratory, and Nervous System for BPSC and Other Competitive Exams in 2025

Complete Biology notes on Human Body Systems: Digestive, Circulatory, Respiratory, and Nervous System for BPSC and Other Competitive Exams in 2025

General Science- 16 (Biology)

Human Body Systems: Digestive, Circulatory, Respiratory, Nervous System

The human body is a marvel of biological engineering, with various organ systems working in a coordinated manner to perform life-sustaining functions. Understanding these systems is crucial for general science knowledge, especially for competitive exams like BPSC CCE.

1. Digestive System

The digestive system is responsible for breaking down the food we eat into simpler substances that can be absorbed and utilized by the body’s cells, and for eliminating undigested waste.

Purpose:

• Digestion: Breakdown of complex food molecules into simpler, absorbable forms.
• Absorption: Uptake of digested nutrients into the bloodstream.
• Elimination (Egestion): Removal of undigested waste materials from the body.

Major Organs/Components:

• Alimentary Canal (Gastrointestinal Tract): A long, muscular tube extending from the mouth to the anus.
  • Mouth (Buccal Cavity): Mechanical digestion (chewing) and initiation of chemical digestion (salivary amylase for carbohydrates).
  • Pharynx: Passageway for both food and air.
  • Esophagus (Food Pipe): Tube connecting pharynx to stomach; food moves by peristalsis (rhythmic muscular contractions).
  • Stomach: Muscular bag; stores food, mixes it with gastric juices (HCl, pepsin), initiates protein digestion. Food becomes a semi-liquid paste called chyme.
  • Small Intestine: Longest part of the alimentary canal (approx. 6-7 meters). Primary site for nutrient absorption and completion of digestion. Divided into:
    • Duodenum: Receives bile from the liver/gallbladder and pancreatic juice from the pancreas.
    • Jejunum: Middle section.
    • Ileum: Final section, connects to the large intestine.
    • Inner lining has finger-like projections called villi and microvilli, which increase surface area for absorption.
  • Large Intestine: Wider and shorter than the small intestine. Absorbs water and electrolytes, forms and stores feces. Divided into Cecum, Colon, and Rectum.
  • Rectum: Stores feces temporarily.
  • Anus: External opening for defecation.
• Associated Digestive Glands: These produce enzymes and other substances vital for digestion but food does not pass through them.
  • Salivary Glands: Produce saliva (contains salivary amylase for starch digestion).
  • Liver: Largest gland in the body. Produces bile (emulsifies fats), detoxifies chemicals, metabolizes drugs, produces proteins.
  • Gallbladder: Stores and concentrates bile produced by the liver.
  • Pancreas: Produces pancreatic juice (contains enzymes like amylase, lipase, trypsin for carbohydrates, fats, and proteins respectively) and hormones (insulin, glucagon).

Process of Digestion:

1. Ingestion: Taking food into the mouth.
2. Mechanical Digestion: Chewing (mastication) in the mouth, churning in the stomach.
3. Chemical Digestion: Breakdown of food molecules by enzymes (e.g., salivary amylase in mouth, pepsin in stomach, pancreatic enzymes in small intestine).
4. Absorption: Digested nutrients (amino acids, simple sugars, fatty acids, glycerol) pass through the intestinal wall into the bloodstream or lymphatic system.
5. Elimination (Egestion): Undigested waste material is expelled from the body.

Key Concepts:

• Peristalsis: Wave-like muscular contractions that push food along the alimentary canal.
• Enzymes: Biological catalysts that speed up chemical reactions (e.g., amylase for starch, pepsin for protein, lipase for fat).
• Bile: Produced by liver, stored in gallbladder; emulsifies fats (breaks large fat globules into smaller ones).
• Villi: Finger-like projections in the small intestine that increase surface area for absorption.
• Chyme: Semi-liquid food paste in the stomach.
• Bolus: Chewed food mixed with saliva in the mouth.

2. Circulatory System (Cardiovascular System)

The circulatory system is a transport network that moves blood, oxygen, nutrients, hormones, and waste products throughout the body.

Purpose:

• Transport of Oxygen: From lungs to tissues.
• Transport of Nutrients: From the digestive system to cells.
• Transport of Hormones: From endocrine glands to target organs.
• Transport of Waste Products: Metabolic wastes (e.g., CO2​, urea) from cells to excretory organs.
• Regulation: Helps regulate body temperature and pH.
• Protection: Contains components of the immune system.

Major Components:

• Heart: A muscular pumping organ, about the size of a fist, located in the chest cavity between the lungs.
  • It has four chambers: two upper atria (receiving chambers) and two lower ventricles (pumping chambers).
  • Right Atrium: Receives deoxygenated blood from the body.
  • Right Ventricle: Pumps deoxygenated blood to the lungs.
  • Left Atrium: Receives oxygenated blood from the lungs.
  • Left Ventricle: Pumps oxygenated blood to the rest of the body (most muscular chamber).
  • Valves ensure one-way blood flow.
• Blood Vessels: A network of tubes through which blood circulates.
  • Arteries: Carry oxygenated blood away from the heart to various parts of the body (except the pulmonary artery, which carries deoxygenated blood to the lungs). They have thick, elastic, muscular walls to withstand high pressure.
  • Veins: Carry deoxygenated blood towards the heart from various parts of the body (except the pulmonary vein, which carries oxygenated blood from the lungs to the heart). They have thinner walls and contain valves to prevent backflow of blood, as pressure is lower.
  • Capillaries: Microscopic, thin-walled vessels that connect arteries and veins. They are the sites where exchange of gases, nutrients, and waste products occurs between blood and body cells.
• Blood: A specialized connective tissue that flows throughout the circulatory system.
  • Plasma: The liquid matrix of blood (about 55%). Contains water, proteins, salts, hormones, nutrients, and wastes.
  • Red Blood Cells (RBCs) / Erythrocytes: Contain hemoglobin, an iron-rich protein that binds to oxygen, giving blood its red color. Transport oxygen.
  • White Blood Cells (WBCs) / Leukocytes: Part of the immune system, fight infections and foreign invaders.
  • Platelets / Thrombocytes: Cell fragments involved in blood clotting to prevent excessive bleeding.

Process of Circulation (Double Circulation in Humans): Blood circulates through two main pathways:

1. Pulmonary Circulation:
   • Deoxygenated blood from the body enters the right atrium, then moves to the right ventricle.
   • The right ventricle pumps this blood to the lungs via the pulmonary artery.
   • In the lungs, blood releases CO2​ and picks up O2​ in the alveoli.
   • Oxygenated blood returns from the lungs to the left atrium via the pulmonary vein.
2. Systemic Circulation:
   • Oxygenated blood from the left atrium moves to the left ventricle.
   • The left ventricle pumps this oxygenated blood to the rest of the body via the aorta (the largest artery).
   • Blood delivers O2​ and nutrients to tissues and collects CO2​ and wastes.
   • Deoxygenated blood returns from the body to the right atrium via the vena cava (largest veins).

Key Concepts:

• Heartbeat: Rhythmic contraction and relaxation of heart chambers.
• Pulse: The throbbing sensation felt in arteries due to blood flow, corresponding to the heartbeat.
• Blood Pressure: The force exerted by blood against the walls of blood vessels. (Systolic/Diastolic).
• Hemoglobin: Protein in RBCs responsible for oxygen transport.
• Blood Groups: Classified based on antigens on RBC surface (A, B, AB, O, Rh factor). Crucial for blood transfusions.
• Lymphatic System (Brief mention): A parallel system that collects excess tissue fluid (lymph) and returns it to the blood, also plays a role in immunity.

3. Respiratory System

The respiratory system is responsible for the exchange of gases (oxygen and carbon dioxide) between the body and the external environment.

Purpose:

• Gas Exchange: Taking in oxygen (O2​) and expelling carbon dioxide (CO2​).
• Vocalization: Produces sounds as air passes over vocal cords.
• Smell: Olfactory receptors in the nasal cavity detect odors.
• Thermoregulation: Helps regulate body temperature through evaporative cooling.

Major Organs/Components:

• Nose / Nasal Cavity: Filters (hair), warms, and moistens inhaled air. Contains olfactory receptors for smell.
• Pharynx (Throat): Common passageway for air and food.
• Larynx (Voice Box): Contains vocal cords, responsible for sound production. The epiglottis (a flap of cartilage) covers the glottis during swallowing to prevent food from entering the trachea.
• Trachea (Windpipe): A tube reinforced with C-shaped cartilaginous rings (to prevent collapse) that extends from the larynx to the bronchi.
• Bronchi: The trachea divides into two primary bronchi, one leading to each lung. These further divide into smaller secondary and tertiary bronchi.
• Bronchioles: Even smaller branches of bronchi that lack cartilage and lead to alveolar sacs.
• Alveoli (Air Sacs): Tiny, balloon-like structures at the end of the bronchioles. They are the primary sites of gas exchange with the blood capillaries. The walls are extremely thin to facilitate diffusion.
• Lungs: Spongy, elastic organs located in the chest cavity, housing the bronchioles and alveoli. The right lung has three lobes, the left lung has two.
• Diaphragm: A dome-shaped muscular partition separating the thoracic (chest) cavity from the abdominal cavity. It plays a crucial role in breathing.

Process of Respiration:

1. Breathing (Ventilation): The mechanical process of moving air in and out of the lungs.
   • Inhalation (Inspiration): Diaphragm contracts and moves downward, intercostal muscles contract (rib cage moves up and out). This increases chest volume, decreases lung pressure, and air rushes in.
   • Exhalation (Expiration): Diaphragm relaxes and moves upward, intercostal muscles relax (rib cage moves down and in). This decreases chest volume, increases lung pressure, and air is forced out.
2. External Respiration: Exchange of gases (O2​ and CO2​) between the lungs (alveoli) and the blood in the capillaries. Oxygen diffuses from alveoli into blood, carbon dioxide diffuses from blood into alveoli.
3. Internal Respiration (Cellular Respiration): Exchange of gases (O2​ and CO2​) between blood in capillaries and body cells. Oxygen diffuses from blood into cells, carbon dioxide diffuses from cells into blood. (Note: Cellular respiration also refers to the biochemical process in cells that produces energy).

Key Concepts:

• Respiration vs. Breathing: Breathing is the physical act of inhaling and exhaling; respiration includes both breathing and gas exchange at the cellular level.
• Vital Capacity: The maximum amount of air a person can exhale after a maximum inhalation.
• Cilia: Tiny hair-like structures lining the respiratory tract that trap dust and pathogens and sweep them upwards.
• Epiglottis: Flap that prevents food from entering the windpipe.

4. Nervous System

The nervous system is the body’s control and communication network. It collects information from sensory organs, processes it, and sends signals to muscles and glands, enabling the body to respond to stimuli and maintain homeostasis.

Purpose:

• Control and Coordination: Integrates all body functions.
• Sensory Input: Receives information from the internal and external environment.
• Information Processing: Interprets sensory input and determines appropriate responses.
• Motor Output: Sends signals to muscles and glands to execute responses.
• Learning and Memory: Higher cognitive functions.

Divisions of the Nervous System:

A. Central Nervous System (CNS):

• Brain: The command center, located in the skull.
  • Cerebrum: Largest part, responsible for voluntary actions, thought, memory, learning, language, sensory interpretation (e.g., sight, hearing). Divided into two hemispheres.
  • Cerebellum: Located at the back of the brain, coordinates voluntary movements (balance, posture, coordination, speech), important for motor learning.
  • Brainstem: Connects the cerebrum and cerebellum to the spinal cord. Controls vital involuntary functions like breathing, heart rate, sleep, consciousness.
• Spinal Cord: A long, cylindrical bundle of nerves extending from the brainstem down the back, protected by the vertebral column.
  • Function: Transmits signals between the brain and the rest of the body. Also mediates reflex actions (involuntary, rapid responses).

B. Peripheral Nervous System (PNS):

• Consists of all the nerves extending from the brain and spinal cord to the rest of the body (muscles, organs, skin).
• Main Subdivisions:
  1. Somatic Nervous System (Voluntary): Controls voluntary movements of skeletal muscles and transmits sensory information from skin, muscles, and joints to the CNS.
  2. Autonomic Nervous System (Involuntary): Controls involuntary body functions (e.g., heart rate, digestion, breathing, glandular secretions) that are not under conscious control.
     • Sympathetic Nervous System: “Fight-or-flight” response. Prepares the body for stress (increases heart rate, dilates pupils, inhibits digestion).
     • Parasympathetic Nervous System: “Rest-and-digest” response. Calms the body down and conserves energy (slows heart rate, constricts pupils, promotes digestion).

Basic Unit of the Nervous System: Neuron (Nerve Cell)

• Neurons are specialized cells that transmit electrical and chemical signals (nerve impulses).
• Structure of a Neuron:
  • Cell Body (Soma): Contains the nucleus and other organelles.
  • Dendrites: Short, branched extensions that receive signals from other neurons.
  • Axon: A long, single extension that transmits signals away from the cell body to other neurons, muscles, or glands. It can be covered by a myelin sheath (insulating layer that speeds up impulse transmission).
  • Axon Terminals: Ends of the axon that release neurotransmitters into the synapse.
  • Synapse: The small gap between the axon terminal of one neuron and the dendrite or cell body of another neuron, where signal transmission occurs.

Process of Nerve Impulse Transmission:

• A stimulus generates an electrical impulse (action potential) in the neuron.
• This impulse travels along the axon to the axon terminal.
• At the synapse, neurotransmitters are released, which cross the gap and bind to receptors on the next neuron, transmitting the signal.

Reflex Arc:

• The neural pathway that mediates a reflex action (an involuntary, rapid, and automatic response to a stimulus).
• Components:
  1. Receptor: Detects the stimulus (e.g., pain receptors in skin).
  2. Sensory Neuron (Afferent Neuron): Transmits signal from receptor to CNS.
  3. Interneuron (Relay Neuron): Processes the signal in the spinal cord (or brainstem).
  4. Motor Neuron (Efferent Neuron): Transmits signal from CNS to effector.
  5. Effector: Muscle or gland that carries out the response (e.g., muscle contracting to withdraw hand).
• Example: Withdrawal of hand immediately upon touching a hot object.

Key Concepts:

• Neurotransmitters: Chemical messengers that transmit signals across synapses (e.g., acetylcholine, dopamine, serotonin).
• Myelin Sheath: Fatty insulation around axons that increases the speed of nerve impulse conduction.
• Sensory Neurons: Carry signals to the CNS.
• Motor Neurons: Carry signals from the CNS to muscles/glands.
• Interneurons: Connect sensory and motor neurons within the CNS.

Practice Questions

1. Which of the following organs is the primary site for the absorption of digested nutrients in the human body
   • A) Stomach
   • B) Small Intestine
   • C) Large Intestine
   • D) Esophagus
   • Rationale: The small intestine’s villi significantly increase its surface area, making it ideal for nutrient absorption.
2. The rhythmic contraction and relaxation of muscles in the esophagus that push food down to the stomach is called
   • A) Digestion
   • B) Absorption
   • C) Peristalsis
   • D) Mastication
   • Rationale: Peristalsis is the involuntary wave-like movement in the alimentary canal.
3. Bile, which helps in the digestion of fats, is produced by the
   • A) Gallbladder
   • B) Pancreas
   • C) Liver
   • D) Stomach
   • Rationale: The liver produces bile, which is then stored and concentrated in the gallbladder.
4. Which enzyme present in saliva begins the digestion of carbohydrates
   • A) Pepsin
   • B) Lipase
   • C) Trypsin
   • D) Salivary Amylase
   • Rationale: Salivary amylase (ptyalin) starts the breakdown of starch in the mouth.

5. Which chamber of the human heart pumps oxygenated blood to the rest of the body
   • A) Right Atrium
   • B) Right Ventricle
   • C) Left Atrium
   • D) Left Ventricle
   • Rationale: The left ventricle is the most muscular chamber and is responsible for pumping oxygenated blood to the systemic circulation.
6. The blood vessels that carry deoxygenated blood from the body towards the heart are called
   • A) Arteries
   • B) Veins
   • C) Capillaries
   • D) Arterioles
   • Rationale: Veins typically carry deoxygenated blood back to the heart (except pulmonary vein).
7. What is the main function of hemoglobin in red blood cells
   • A) Fighting infections
   • B) Clotting blood
   • C) Transporting oxygen
   • D) Transporting nutrients
   • Rationale: Hemoglobin binds to oxygen in the lungs and releases it in tissues.
8. The exchange of gases, nutrients, and waste products between blood and body cells occurs primarily in the
   • A) Arteries
   • B) Veins
   • C) Capillaries
   • D) Aorta
   • Rationale: Capillaries have thin walls that facilitate efficient exchange of substances.

9. The primary site for the exchange of oxygen and carbon dioxide in the lungs are the
   • A) Bronchi
   • B) Trachea
   • C) Larynx
   • D) Alveoli
   • Rationale: Alveoli are specialized for efficient gas exchange with surrounding capillaries.
10. Which structure prevents food from entering the windpipe during swallowing
    • A) Pharynx
    • B) Larynx
    • C) Trachea
    • D) Epiglottis
    • Rationale: The epiglottis covers the opening of the trachea during swallowing.
11. During inhalation, the diaphragm
    • A) Relaxes and moves upward
    • B) Relaxes and moves downward
    • C) Contracts and moves downward
    • D) Contracts and moves upward
    • Rationale: Contraction of the diaphragm and its downward movement increases chest volume for inhalation.
12. The C-shaped cartilaginous rings are present in which part of the respiratory system to prevent its collapse
    • A) Bronchioles
    • B) Alveoli
    • C) Trachea
    • D) Larynx
    • Rationale: The trachea (windpipe) is reinforced with C-shaped cartilage rings to keep it open.

13. Which part of the brain is primarily responsible for coordinating voluntary movements, balance, and posture
    • A) Cerebrum
    • B) Cerebellum
    • C) Brainstem
    • D) Spinal Cord
    • Rationale: The cerebellum is crucial for motor control and coordination.
14. The basic structural and functional unit of the nervous system is the
    • A) Nephron
    • B) Alveolus
    • C) Neuron
    • D) Myelin
    • Rationale: The neuron is the specialized cell responsible for transmitting nerve impulses.
15. Which component of the autonomic nervous system is responsible for the “fight-or-flight” response
    • A) Somatic nervous system
    • B) Sympathetic nervous system
    • C) Parasympathetic nervous system
    • D) Central nervous system
    • Rationale: The sympathetic nervous system prepares the body for stressful situations.
16. In a reflex arc, which type of neuron transmits signals from the central nervous system to the effector (e.g., a muscle)
    • A) Sensory neuron
    • B) Interneuron
    • C) Motor neuron
    • D) Association neuron
    • Rationale: Motor neurons carry efferent signals from the CNS to muscles or glands.