Respiratory System in Humans 

The respiratory system is a complex network of organs and tissues that inhale oxygen and exhale carbon dioxide. It maintains the balance of gases in the blood and supports the body’s metabolic activities.

1.0Parts of the Human Respiratory System

1. Nose and Nasal Cavity

• We have a pair of external nostrils opening out above the upper lip. Since the external nostrils are a pair (two), this condition is called a dirhinous condition.
• It leads to a nasal chamber through the nasal passage.
• The nasal chamber opens into the pharynx, a portion of which is the common passage for food and air.
• The pharynx opens through the larynx region and opens into the trachea.

2. Pharynx

• A muscular passage that connects the nasal cavity to the larynx.
• Acts as a common pathway for both air and food.
• Divided into the nasopharynx, oropharynx, and laryngopharynx.

3. Larynx (Voice Box)

• It is a cartilaginous box that helps produce sound and is therefore called the sound box.
• During swallowing, the glottis can be covered by a thin elastic cartilaginous flap called the epiglottis to prevent the entry of food into the larynx.
• The larynx is the sound-producing organ in which two types of vocal cords are present.
• False vocal cords provide moisture to the true vocal cords.
• True vocal cords are helpful in phonation (Sound production).

4. Trachea (Windpipe)

• The trachea is a straight tube (diameter 2.5 cm) (length 12 cm) extending up to the mid-thoracic cavity, which divides at the level of the 5th thoracic vertebra into right and left primary bronchi.
• Each bronchus undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles, ending up in very thin terminal bronchioles.
• The tracheae, primary, secondary and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings.

• On the dorsal side of the rings, "trachealis muscles" are present. These are involuntary muscles that help with forcible breathing and dilate the trachea.
• Each terminal bronchiole gives rise to several fragile, irregular-walled and vascularised bag-like structures called alveoli.
• The branching network of bronchi, bronchioles and alveoli comprises the lungs.

5. Bronchi and Bronchioles

• The trachea divides into two primary bronchi—each entering one lung.
• Inside the lungs, bronchi branch into smaller bronchioles, which end in tiny air sacs called alveoli.
• This branching network ensures efficient distribution of air within the lungs.

6. Lungs

• Endodermal in origin.
• We have two lungs (the right lung is three-lobed, whereas the left lung is two-lobed), which are covered by a double-layered pleura, with pleural fluid between them.
• Pleural fluid reduces friction on the lung surface and acts as a shock absorber.
• The outer pleural membrane is in close contact with the thoracic lining, whereas the inner pleural membrane is in contact with the lung surface.
• The lungs are situated in the thoracic cavity, which is anatomically an air-tight chamber.
• The thoracic chamber is formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs and on the lower side by the dome-shaped diaphragm.
• The anatomical setup of the lungs in the thorax is such that any change in the volume of the thoracic cavity will be reflected in the lung (pulmonary) cavity. Such an arrangement is essential for breathing, as we cannot directly alter the pulmonary volume.
• Mammalian lungs are solid and spongy without muscles, so the power of self-contraction and self-relaxation is not present in mammalian lungs.

7. Alveoli

• Alveoli are structural and functional units of the lungs.
• Alveoli are lined with two types of cells called pneumocytes.
• Pneumocytes-I are larger cells and help in gaseous exchange, whereas pneumocytes-II are smaller cells that secrete a phospholipid LECITHIN. Lecithin acts as a surfactant, reduces the surface tension of alveoli, and keeps them always open for efficient gaseous exchange.
• On the outer side of alveoli, yellow fibrous connective tissue is present.
• The total number of alveoli in a man's lungs is 300 million.
• The exchange part is the site of actual diffusion of O2 and CO2 between blood and atmospheric air.

8. Diaphragm 

• It is the muscular structure that separates the thoracic cavity from the abdominal cavity.
• It is made up of muscles called radial muscles.
• Word related to the diaphragm is phrenic.
• The diaphragm is the principal muscle of breathing and aids in ventilation.
• The diaphragm is also involved in parturition (childbirth), micturition (passing urine) and defaecation (defecation).
• At the time of inspiration, the diaphragm contracts and becomes flattened, so the volume of the thoracic chamber increases in the anteroposterior axis.

Intercostal Muscles

• Spaces between two pairs of ribs are called intercostal spaces, whereas muscles in these spaces are called intercostal muscles.
• There are two types of intercostal muscles -
• (1) External intercostal muscles (EICM)
• (2) Internal intercostal muscles (IICM)
• Muscles that connect the dorsal face of the upper rib with ventral face of the lower rib are called external intercostal muscles or inspiratory muscles.
• Muscles that connect ventral face of the upper rib with the dorsal face of the lower rib are called the internal intercostal muscles or expiratory muscles.
• At the time of inspiration, EICM contracts so that the sternum moves outwardly, and ribs move upwardly; hence, the volume of the thoracic cavity is increased in the dorso-ventral axis.

2.0Mechanism of Breathing

• Breathing involves two stages: inspiration, during which atmospheric air is drawn in, and expiration, during which alveolar air is released.
• A pressure gradient between the lungs and the atmosphere drives the movement of air into and out of the lungs.
• Inspiration can occur when the pressure within the lungs (intrapulmonary pressure) is less than atmospheric pressure, i.e., there is a negative pressure in the lungs relative to atmospheric pressure.
• Similarly, expiration occurs when the intrapulmonary pressure exceeds atmospheric pressure.
• The diaphragm and a specialised set of muscles - external and internal intercostals between the ribs- help in the generation of such gradients.

Inspiration

• Inspiration is initiated by the contraction of the diaphragm, which increases the volume of the thoracic chamber in the anteroposterior axis.
• The contraction of external intercostal muscles lifts the ribs and the sternum, causing an increase in the volume of the thoracic chamber in the dorso-ventral axis.
• The overall increase in thoracic volume results in a similar increase in pulmonary volume.
• An increase in pulmonary volume decreases intrapulmonary pressure to below atmospheric pressure, forcing air from outside into the lungs (i.e., inspiration).

Expiration

• Relaxation of the diaphragm and the intercostal muscles returns the diaphragm and sternum to their normal positions and reduces the thoracic volume and thereby the pulmonary volume. This leads to an increase in intra-pulmonary pressure to slightly above the atmospheric pressure, causing the expulsion of air from the lungs, i.e., expiration.
• We can increase the strength of inspiration and expiration with the help of additional abdominal muscles.
• On average, a healthy human breathes 12-16 times/minute.
• The volume of air involved in breathing movements can be estimated by using a spirometer, which helps in the clinical assessment of pulmonary functions.
• In fever, the breathing rate increases.
• When the CO₂ concentration in the blood increases, the breathing rate increases.
• Breathing rate in infants is greater than adults.

3.0Gas Exchange in Humans

Gas exchange occurs primarily at the alveoli due to differences in partial pressures of oxygen and carbon dioxide.

• Oxygen Transport: Alveolar air has a higher partial pressure of
• than the blood in the capillaries. This causes O2​ to diffuse from the alveoli into the blood. Once in the blood, O₂​ binds to hemoglobin in red blood cells to form oxyhemoglobin and is transported to the tissues.
• Carbon Dioxide Transport: At the tissues, the partial pressure of $$CO_2 \, (p_C O2)$$ is high. CO_2​ diffuses from the tissues into the blood. In the blood, CO_2​ is transported in three forms: dissolved in plasma, as bicarbonate ions, and bound to hemoglobin. At the alveoli, the blood has a higher $$p_C O2$$ than the alveolar air, causing CO₂​ to diffuse from the blood into the alveoli to be exhaled.

4.0Transport of Gases

Oxygen Transport

• About 97% of oxygen is transported in the blood bound to hemoglobin in red blood cells, forming oxyhemoglobin.
• The remaining 3% is dissolved in plasma.

Carbon Dioxide Transport

• About 70% of CO₂ is carried as bicarbonate ions (HCO₃⁻) in plasma.
• 23% is bound to hemoglobin as carbaminohemoglobin.
• 7% is dissolved directly in plasma.

5.0Regulation of Breathing

• The medulla oblongata and pons in the brain regulate the rate and depth of breathing.
• Chemoreceptors in the brain and arteries detect changes in CO₂ and pH levels.
• An increase in CO₂ concentration triggers faster and deeper breathing to restore balance.

6.0Respiratory Volumes and Capacities

7.0Disorders of the Respiratory System

Several diseases affect the efficiency of the respiratory system:

• Asthma: Inflammation and constriction of bronchioles, causing difficulty in breathing.
• Emphysema: Damage to alveolar walls, reducing surface area for gas exchange.
• Bronchitis: Inflammation of the bronchi due to infection or smoking.
• Pneumonia: Infection causing fluid accumulation in alveoli.
• Tuberculosis (TB): Bacterial infection caused by Mycobacterium tuberculosis.

8.0Functions of the Human Respiratory System

• Supply of oxygen for cellular respiration
• Removal of carbon dioxide from the body
• Regulation of blood pH
• Production of sound through vocal cords
• Olfaction (smelling) via nasal receptors