Respiratory System - Notes, Biology

Table of Contents
1. What is Respiration?
2. Types of Respiration
3. Respiratory Organs in the Human Body
4. Transport of Gases in Blood
5. Control of Respiration
6. Respiratory Disorders in the Human Body
7. Respiratory Pigments
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What is Respiration?

Respiration comprises all physical and chemical processes by which organisms obtain oxygen (O₂), oxidise organic nutrients and liberate energy, producing carbon dioxide (CO₂) and water as by-products. At the cellular level, the overall aerobic oxidation of glucose can be summarized as:

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy (ATP)

Respiratory System in Humans 

Types of Respiration

(I) Anaerobic Respiration

• Occurs when nutrients are oxidized without the use of molecular oxygen; also called fermentation.
• In yeast, glucose is converted to ethyl alcohol and CO₂: glucose → ethanol + CO₂.
• In animal muscle (during intense activity when O₂ supply is limited), glucose is converted to lactic acid: glucose → lactic acid.
• Some parasitic or anaerobic organisms (for example certain stages of Ascaris, Fasciola, Taenia) obtain energy anaerobically.
• Anaerobic respiration yields relatively little energy (ATP) per glucose molecule compared with aerobic respiration.

(II) Aerobic Respiration

• Cells use molecular oxygen to oxidize nutrients completely to CO₂ and H₂O, releasing much more energy than anaerobic pathways.
• Major stages include glycolysis (cytoplasm), the link reaction, the Krebs (citric acid) cycle and the electron transport chain (mitochondrial inner membrane).
• Oxygen for aerobic respiration may be taken from atmospheric air (terrestrial animals and plants) or dissolved oxygen in water (aquatic animals, fish).
• Aerobic respiration is found broadly in most plants and animals and is the principal source of ATP for sustained activity.

Levels of Respiratory Processes

• External respiration: Exchange of O₂ and CO₂ between the external medium (air or water) and the blood (or haemolymph).
• Transport of gases: Movement of O₂ and CO₂ in the blood from respiratory surfaces to tissues and back.
• Internal (tissue) respiration: Exchange of gases between blood and body tissues across capillary walls.
• Cellular respiration: Oxidation of substrates inside cells to produce ATP.

Respiratory Organs in the Human Body

(I) Nasal Cavity

• Opens to the exterior through the nostrils (nares).
• Internal hair (vibrissae) and mucus trap dust and suspended particles, filtering the inhaled air.
• Extensive blood supply warms the air; mucous membranes moisten it before it reaches lower respiratory passages.
• Olfactory receptors in the upper nasal cavity are involved in the sense of smell.

(II) Pharynx

• Common chamber for food and air; lies behind the nasal cavity and mouth.
• Air from the nasal cavity passes through the pharynx to the larynx and trachea.
• The opening to the trachea (windpipe) is guarded by the glottis, and the epiglottis acts as a flap to prevent food entering the airway during swallowing.

(III) Larynx (Voice Box)

• Located at the top of the trachea; in males the prominent thyroid cartilage forms the Adam's apple.
• Contains the vocal cords (vocal folds); air forced through them causes vibration and sound production.
• Pitch of sound depends on the tension of the vocal cords: greater tension → higher pitch.

(IV) Trachea, Bronchi and Bronchioles

• The trachea is a tube about 10-12 cm long in adults, supported by incomplete C-shaped rings of cartilage that prevent collapse and allow flexibility.
• The trachea branches into two primary bronchi, one entering each lung; bronchi further divide into secondary and tertiary bronchi and then into many bronchioles.
• Bronchi and bronchioles are lined with ciliated epithelium and mucus-producing cells; cilia and mucus move trapped particles toward the pharynx for removal (mucociliary escalator).

(V) Lungs, Alveoli and Pleura

• The paired lungs occupy the thoracic cavity and are separated from the abdominal cavity by the diaphragm.
• Each lung is enclosed by a double-layered serous membrane called the pleura; the space between the layers is the pleural cavity, containing a thin film of fluid that reduces friction and helps maintain lung expansion.
• The right lung has three lobes; the left lung has two lobes and a cardiac notch to accommodate the heart.
• Bronchioles end in clusters of alveolar ducts and thousands of alveoli (air sacs). Alveoli provide a very large surface area and a thin respiratory surface for efficient gas exchange.
• Alveolar walls are lined by type I and type II pneumocytes; type II cells secrete surfactant, which reduces surface tension and prevents alveolar collapse.

(VI) Diaphragm 

• The diaphragm is the chief muscle of respiration; it is a dome-shaped sheet of skeletal muscle separating thorax and abdomen.
• Inspiration (inhalation): Diaphragm contracts and flattens; external intercostal muscles raise the ribs; thoracic cavity volume increases; intrapulmonary pressure falls below atmospheric pressure; air flows into lungs.
• Expiration (exhalation): Diaphragm relaxes and becomes dome-shaped; ribs descend; thoracic volume decreases; intrapulmonary pressure rises above atmospheric pressure; air flows out. Quiet expiration is usually passive; forced expiration uses abdominal and internal intercostal muscles.
• Normal resting respiratory rate and volumes can be measured by spirometry.

Transport of Gases in Blood

• Oxygen transport: Most oxygen in arterial blood is carried bound to haemoglobin (in red blood cells) as oxyhaemoglobin; a small portion is dissolved in plasma. The affinity of haemoglobin for O₂ is described by the oxygen-haemoglobin dissociation curve.
• Carbon dioxide transport: CO₂ is carried in blood as dissolved CO₂, chemically bound to haemoglobin (carbamino-haemoglobin) and principally as bicarbonate ion (HCO₃^-) formed in red blood cells via the enzyme carbonic anhydrase.
• Changes in partial pressures of O₂ and CO₂, PCO₂-dependent shifts in pH (Bohr effect) and temperature affect oxygen unloading at tissues.

Control of Respiration

• Breathing is primarily regulated by respiratory centres in the medulla oblongata and the pons of the brainstem.
• Central chemoreceptors (medulla) respond to changes in cerebrospinal fluid pH reflecting CO₂ levels; peripheral chemoreceptors (carotid and aortic bodies) sense low arterial O₂, high CO₂ and low pH and modify ventilation rate accordingly.
• Voluntary control of breathing is possible via cortical pathways, but chemical control predominates for long-term regulation.

Respiratory Disorders in the Human Body

• Asphyxia: Condition resulting from inadequate oxygen and/or excessive CO₂ concentration in tissues; may be due to airway obstruction, drowning or suffocation.
• Hypoxia: Shortage of oxygen supply to tissues; may be caused by respiratory, circulatory or environmental factors.
• Hypopnoea: Abnormally slow or shallow breathing.
• Hyperpnoea: Increased depth and rate of breathing (can be physiological during exercise).
• Apnoea: Temporary cessation of breathing.
• Dyspnoea: Difficult or laboured breathing; a subjective feeling of breathlessness.
• Orthopnoea: Difficulty breathing when lying flat; relieved by sitting or standing up (often associated with heart failure).
• Tachypnoea: Rapid shallow breathing.
• COPD (Chronic Obstructive Pulmonary Disease): A group of progressive lung diseases that include emphysema and chronic bronchitis; often associated with long-term smoking and air pollution.
• Emphysema: Destruction of alveolar walls leading to reduced surface area for gas exchange and difficulty in exhalation; associated with smoking.
• Tuberculosis (TB): Bacterial disease caused by Mycobacterium tuberculosis, affecting the lungs and sometimes other organs; characterised by cough, haemoptysis and weight loss.
• Pneumonia: Infection/inflammation of lung tissue (alveoli) often caused by bacteria (e.g., Streptococcus pneumoniae) or viruses; leads to fluid-filled alveoli and impaired gas exchange.
• Whooping cough (Pertussis): Infectious respiratory disease caused by Bordetella pertussis, characterised by severe coughing bouts.
• Asthma: Recurrent episodes of airway narrowing due to bronchial muscle spasm, inflammation and increased mucus production, producing wheeze and breathlessness.
• Sleep Apnoea Syndrome (SAS): Repeated episodes of airway obstruction during sleep (often associated with snoring), causing apnoea and sleep fragmentation.
• Hypercapnia: Elevated CO₂ levels in the blood.
• Cyanosis: Bluish discoloration of skin and mucous membranes due to increased deoxygenated haemoglobin.
• Pleurisy: Inflammation of the pleural membranes causing sharp chest pain, especially on breathing.

Respiratory Pigments

Practical Notes and Applications

• Understanding respiratory mechanics and gas transport is essential in clinical contexts (ventilation management, oxygen therapy, treatment of respiratory failure).
• Spirometry is used diagnostically to measure lung volumes and capacities (tidal volume, vital capacity, residual volume, forced expiratory volume) and to detect obstructive or restrictive lung disease.
• Public-health measures (vaccination against pertussis, anti-tuberculosis programmes, reduction of air pollution and smoking cessation) reduce the burden of respiratory diseases.
• Organisational and engineering considerations (ventilation, air quality control) are relevant for civil and electrical engineering applications where indoor air and occupational health matter; computer science applications include modelling respiratory gas exchange and analysing clinical data.