Respiratory System

Main Parts and Functions

The respiratory system is a complex biological system responsible for facilitating gas exchange between the body and the external environment. Its primary function is to take in oxygen (O2), which is essential for cellular respiration, and to expel carbon dioxide (CO2), a waste product of metabolism. This system includes a series of organs and structures that work together to ensure efficient breathing and gas exchange.

1. Nose and Nasal Cavity:
   • Structure: The external nose leads into the nasal cavity, which is lined with mucous membranes and small hairs (cilia). It is divided by the nasal septum.
   • Function: The nose serves as the primary entry point for air into the respiratory system. Its functions include:
     • Filtering: Hairs and mucus trap dust, pollen, and other airborne particles, preventing them from reaching the lungs.
     • Warming: A rich blood supply beneath the mucous membranes warms the incoming air to body temperature.
     • Moistening: Water vapor from the moist mucous membranes humidifies the air, preventing the delicate lung tissues from drying out.
     • Olfaction: Contains olfactory receptors responsible for the sense of smell.
2. Pharynx (Throat):
   • Structure: A muscular tube extending from the nasal cavity down to the esophagus and larynx. It is divided into three regions: nasopharynx, oropharynx, and laryngopharynx.
   • Function: The pharynx serves as a common passageway for both air (to the larynx) and food (to the esophagus). During swallowing, a flap of cartilage called the epiglottis closes off the opening to the larynx, preventing food from entering the trachea.
3. Larynx (Voice Box):
   • Structure: A cartilaginous structure located at the top of the trachea, containing the vocal cords.
   • Function: The larynx is primarily responsible for sound production (phonation). As air passes over the vocal cords, they vibrate, producing sound. It also acts as a protective barrier, preventing food and foreign objects from entering the lower respiratory tract.
4. Trachea (Windpipe):
   • Structure: A tube approximately 10-12 cm long and 2.5 cm in diameter, extending from the larynx down into the chest cavity. It is supported by 16-20 C-shaped rings of hyaline cartilage, which prevent it from collapsing. The inner lining is ciliated pseudostratified columnar epithelium with goblet cells.
   • Function: The trachea provides a clear and unobstructed pathway for air to travel from the larynx to the bronchi. The cilia and mucus work together in the mucociliary escalator to trap and sweep inhaled particles upwards towards the pharynx, where they can be swallowed or expelled.
5. Bronchi and Bronchioles:
   • Structure: The trachea divides into two main (primary) bronchi, one leading to each lung. These primary bronchi further divide into smaller secondary and tertiary bronchi, and then into progressively smaller tubes called bronchioles. The walls of bronchioles contain smooth muscle, allowing them to constrict or dilate.
   • Function: The bronchial tree (bronchi and bronchioles) acts as a conducting zone, distributing air to the various regions of the lungs. The smooth muscle in the bronchioles regulates airflow to the alveoli.
6. Lungs:
   • Structure: A pair of spongy, cone-shaped organs located in the thoracic cavity, enclosed by the pleura (a double-layered membrane). The right lung has three lobes, and the left lung has two. Within the lungs, the bronchioles terminate in tiny air sacs called alveoli.
   • Function: The lungs are the primary organs of respiration. Their main function is to facilitate the exchange of oxygen and carbon dioxide between the air and the blood. This vital process occurs across the thin walls of the alveoli and the surrounding capillaries.
7. Alveoli:
   • Structure: Microscopic, thin-walled air sacs, numbering around 300-500 million in each lung, providing an enormous surface area for gas exchange. Each alveolus is surrounded by a dense network of capillaries.
   • Function: The alveoli are the functional units of the respiratory system where gas exchange takes place. Oxygen diffuses from the high concentration in the alveoli into the blood in the capillaries, while carbon dioxide diffuses from the high concentration in the blood into the alveoli to be exhaled.

Muscles of Respiration:

• Diaphragm: A dome-shaped muscle located at the base of the chest cavity, separating the thoracic and abdominal cavities. It is the primary muscle of quiet breathing.
• Intercostal Muscles: Muscles located between the ribs. External intercostals are involved in inhalation, and internal intercostals are involved in forced exhalation.

Difference between Respiration and Breathing

It is common to use the terms "respiration" and "breathing" interchangeably, but in biology, they refer to distinct processes. Understanding this difference is crucial for comprehending how organisms obtain and utilize energy.

Mechanism of Breathing (Pulmonary Ventilation)

Breathing, also known as pulmonary ventilation, is the physical process of moving air into and out of the lungs. It is a mechanical process driven by changes in pressure within the thoracic cavity, which are primarily brought about by the contraction and relaxation of respiratory muscles. It consists of two main phases: inhalation and exhalation.

• Inhalation (Inspiration - Breathing In): This is an active process that brings air into the lungs.

  • Diaphragm: The primary muscle of inspiration. It contracts and flattens, moving downwards towards the abdominal cavity. This action increases the vertical dimension of the thoracic cavity.
  • External Intercostal Muscles: These muscles, located between the ribs, contract, pulling the rib cage upwards and outwards. This action increases the anterior-posterior and lateral dimensions of the thoracic cavity.
  • Effect on Thoracic Volume and Pressure: The combined action of the diaphragm and external intercostals significantly increases the overall volume of the thoracic cavity. According to Boyle's Law (which states that for a fixed amount of gas at constant temperature, pressure and volume are inversely proportional), this increase in volume leads to a decrease in the intrapulmonary pressure (pressure within the lungs) below atmospheric pressure.
  • Air Movement: Because the pressure inside the lungs is now lower than the atmospheric pressure outside, air from the atmosphere (higher pressure) rushes into the lungs (lower pressure) until the pressures equalize.
  • Accessory Muscles (for forced inhalation): During strenuous activity or respiratory distress, additional muscles like the sternocleidomastoid and scalenes may contract to further increase thoracic volume.

• Exhalation (Expiration - Breathing Out): This is typically a passive process during quiet breathing, expelling air from the lungs.

  • Diaphragm: Relaxes and moves upwards, returning to its dome shape. This decreases the vertical dimension of the thoracic cavity.
  • External Intercostal Muscles: Relax, allowing the rib cage to move downwards and inwards due to gravity and elastic recoil of the chest wall and lungs.
  • Effect on Thoracic Volume and Pressure: The combined relaxation of these muscles decreases the volume of the thoracic cavity. This decrease in volume leads to an increase in the intrapulmonary pressure above atmospheric pressure.
  • Air Movement: Because the pressure inside the lungs is now higher than the atmospheric pressure, air is forced out of the lungs until the pressures equalize.
  • Forced Exhalation: During activities like coughing, sneezing, or speaking loudly, exhalation becomes an active process. The internal intercostal muscles contract, pulling the ribs further downwards and inwards, and the abdominal muscles contract, pushing the diaphragm upwards. These actions rapidly and significantly decrease thoracic volume, leading to a much greater increase in intrapulmonary pressure and a more forceful expulsion of air.

Common Respiratory Diseases

The respiratory system is susceptible to various diseases and conditions that can impair its function, leading to breathing difficulties and other symptoms. Here are some common respiratory diseases:

• Asthma:
  • Description: A chronic inflammatory disease of the airways (bronchi and bronchioles) characterized by recurrent episodes of wheezing, breathlessness, chest tightness, and coughing, particularly at night or in the early morning. These symptoms are often triggered by allergens (e.g., pollen, dust mites), irritants (e.g., smoke, pollution), exercise, cold air, or stress.
  • Mechanism: In asthma, the airways become inflamed, swollen, and produce excess mucus. The muscles around the airways also tighten (bronchoconstriction), further narrowing the air passages and making it difficult to breathe.
  • Management: Managed with bronchodilators (to relax airway muscles) and anti-inflammatory medications (e.g., corticosteroids) to reduce inflammation.
• Bronchitis:
  • Description: Inflammation of the lining of the bronchial tubes, which are the main air passages to the lungs. It can be acute (short-term, often due to viral infection) or chronic (long-term, often associated with smoking or exposure to irritants).
  • Symptoms: Persistent cough, often producing mucus (sputum), wheezing, shortness of breath, chest discomfort, and fatigue.
  • Mechanism: Inflammation causes the bronchial lining to swell and produce excessive mucus, leading to narrowing of the airways and obstruction of airflow.
• Pneumonia:
  • Description: An infection that inflames the air sacs (alveoli) in one or both lungs. The alveoli may fill with fluid or pus, leading to difficulty breathing, cough, fever, chills, and chest pain.
  • Causes: Most commonly caused by bacteria (e.g., Streptococcus pneumoniae) or viruses (e.g., influenza virus), but can also be caused by fungi or other microorganisms.
  • Impact: Impairs gas exchange as the alveoli are filled with exudate instead of air.
• Tuberculosis (T.B.):
  • Description: A serious infectious bacterial disease caused by Mycobacterium tuberculosis. It primarily affects the lungs but can also spread to other parts of the body, such as the kidneys, spine, and brain.
  • Transmission: Spread through the air when an infected person coughs, sneezes, or speaks.
  • Symptoms: Chronic cough (sometimes with blood), fever, night sweats, weight loss, and fatigue.
  • Treatment: Requires a long course of multiple antibiotics.
• Emphysema:
  • Description: A chronic, progressive lung disease that is a form of Chronic Obstructive Pulmonary Disease (COPD). It involves the gradual damage and destruction of the walls of the alveoli, leading to the formation of larger, less efficient air sacs.
  • Causes: Primarily caused by long-term exposure to airborne irritants, especially cigarette smoke, but also air pollution and chemical fumes.
  • Mechanism: The destruction of alveolar walls reduces the surface area available for gas exchange and causes the lungs to lose their elasticity, making exhalation difficult and trapping air in the lungs.
• Cystic Fibrosis (CF):
  • Description: A genetic disorder that affects the cells that produce mucus, sweat, and digestive juices. These secreted fluids normally are thin and slippery. However, in people with CF, a defective gene causes the secretions to become thick and sticky.
  • Impact on Respiratory System: The thick, sticky mucus clogs the airways, leading to persistent lung infections, inflammation, and progressive lung damage, including bronchiectasis (widening of airways) and respiratory failure.
• Lung Cancer:
  • Description: Uncontrolled growth of abnormal cells in the lungs. It is one of the leading causes of cancer-related deaths worldwide.
  • Causes: Strongest risk factor is smoking, but also exposure to radon gas, asbestos, and air pollution.
  • Symptoms: Persistent cough, chest pain, shortness of breath, weight loss, and fatigue.
• Sleep Apnea:
  • Description: A potentially serious sleep disorder in which breathing repeatedly stops and starts during sleep. The most common type is obstructive sleep apnea, where the throat muscles intermittently relax and block the airway.
  • Symptoms: Loud snoring, gasping for air during sleep, daytime sleepiness, and morning headaches.
  • Consequences: Can lead to serious health problems, including high blood pressure, heart problems, and diabetes.