Physiology of Respiration

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Physiology of Respiration involves the exchange of oxygen and carbon dioxide in the body. It begins with inhalation, where the diaphragm contracts, expanding the chest cavity and drawing air into the lungs. Oxygen is then absorbed into the bloodstream while carbon dioxide is released.

1. Breathing

Inspiration (Inhalation):

  • The diaphragm contracts and flattens, and the external intercostal muscles contract, lifting the rib cage up and out.
  • This increases the volume of the thoracic cavity, reducing the pressure inside the lungs compared to atmospheric pressure.
  • Air flows into the lungs to equalize the pressure.

Expiration (Exhalation):

  • The diaphragm relaxes and returns to its dome shape, and the external intercostal muscles relax, allowing the rib cage to move down and in.
  • This decreases the volume of the thoracic cavity, increasing the pressure inside the lungs compared to atmospheric pressure.
  • Air is pushed out of the lungs to equalize the pressure.

2. External Respiration

  • Occurs in the alveoli of the lungs.
  • Oxygen from the air in the alveoli diffuses into the pulmonary capillaries because the partial pressure of oxygen is higher in the alveoli than in the blood.
  • Carbon dioxide diffuses from the blood into the alveoli because the partial pressure of carbon dioxide is higher in the blood than in the alveolar air.
  • This gas exchange is facilitated by the thin respiratory membrane and the large surface area of the alveoli.

3. Transport of Respiratory Gases

Oxygen Transport:

  • Oxygen binds to hemoglobin in red blood cells to form oxyhemoglobin (HbO2).
  • A small amount of oxygen is also dissolved in the plasma.
  • Hemoglobin releases oxygen in the tissues where the partial pressure of oxygen is low.

Carbon Dioxide Transport:

  • Carbon dioxide is transported in three forms: dissolved in plasma (7-10%), chemically bound to haemoglobin as carbaminohemoglobin (20-30%), and as bicarbonate ions (HCO3– ) in plasma (60-70%).
  • In the tissues, carbon dioxide diffuses into the blood and is converted to bicarbonate ions by the enzyme carbonic anhydrase in red blood cells.
  • In the lungs, bicarbonate is converted back to carbon dioxide, which diffuses into the alveoli to be exhaled.

4. Regulation of Respiration

  • Controlled by the respiratory centers in the medulla oblongata and pons of the brainstem.
  • The medulla contains the dorsal respiratory group (DRG) and the ventral respiratory group (VRG), which generate the basic rhythm of breathing.
  • The pons contains the pontine respiratory group, which modulates the rhythm of breathing for smooth transitions between inhalation and exhalation.
  • Chemoreceptors in the carotid bodies, aortic bodies, and central chemoreceptors in the medulla observe the levels of carbon dioxide, oxygen, and pH in the blood and cerebrospinal fluid.
  • Changes in these levels trigger adjustments in the rate and depth of breathing to maintain homeostasis.

Respiratory Capacity

Tidal Volume: The amount/volume of air breathed in and out during normal respiration.

Normal value: 500-600 ml.

Respiratory Minute Volume (RMV): RMV can be obtained by multiplying tidal volume by respiratory rate per minute.

Normal value: 6-6.5 L/minute

Inspiratory Reserve Volume(IRV): The extra amount of air that can be inspired forcefully after a normal inspiration.

Normal value: 2-3.3L

Expiratory Reserve Volume (ERV): The extra amount of air that can be breathed out by maximum expiratory effort after a normal expiration. Normal value: 1-1.5L

Residual Volume (RV): The amount of air which remains in the lungs after a maximal expiration.

Normal range:- 1-1.2L (approx).

Inspiratory Capacity (IC): Maximum amount of air that can be inspired by normal inspiration and forceful inspiration.

Normal range:-  3.-3.5 L

Expiratory Capacity (EC): Total amount of air that can be expired by normal expiration and forceful expiration. { EC= TV + ERV } 

Normal range: 2-2.5L

Functional Residual Capacity (FRC): Volume of air remaining in the lungs after normal expiration. (FRC=RV+ERV)

Normal range: 2-3 lit.

Total Lung Capacity (TLC): Volume of air that the lung can hold after a maximum possible respiration is called Total Lung Capacity.

[TLC = RV + ERV+TV+IRV ] 

Normal range: 5-7 lit.

Vital capacity (VC): Volume of air that can be breathed out by maximum expiratory effort after a maximum forceful inspiration. 

[VC = TV+ IRV + ERV]

Normal range:- 4-5 Lit

Dead Space (DS): Volume of air stays inactive on respiratory tracts like trachea, bronchus, nasopharynx etc.

Normal range:- 120-130 ml (approx)

Instrument that measures lung capacity – Spirometer

Transport of Oxygen in Blood

Binding to Haemoglobin:

  • Approximately 98% of oxygen in the blood is transported by binding to haemoglobin (Hb) in red blood cells.
  • Each haemoglobin molecule can carry up to four oxygen molecules (O2).

Oxygen Dissociation Curve:

  • The relationship between the partial pressure of oxygen (pO2) and the percentage saturation of hemoglobin is sigmoidal (S-shaped), known as the oxygen-hemoglobin dissociation curve.
  • This curve reflects haemoglobin’s affinity for oxygen and facilitates oxygen loading in the lungs and unloading in the tissues.

Influence of pH and CO2:

  • The Bohr effect describes how lower pH (more acidic conditions) and higher levels of CO2 reduce haemoglobin’s affinity for oxygen, promoting oxygen release in tissues where it is needed.

Oxygen Transport in Plasma:

  • Only about 1-2% of oxygen is transported dissolved in the plasma, as oxygen has low solubility in blood.

Transport of Carbon Dioxide in Blood

Dissolution in Plasma:

  • About 7-10% of CO2 is transported dissolved directly in the plasma.

Formation of Bicarbonate Ions:

  • Approximately 70% of CO2 is transported in the form of bicarbonate ions (HCO3– ) in the plasma.
  • CO2 diffuses into red blood cells and reacts with water to form carbonic acid (H2CO3), which quickly dissociates into bicarbonate (HCO3– and hydrogen ions (H+), a reaction catalyzed by the enzyme carbonic anhydrase.

Carbaminohemoglobin:

  • Around 20-23% of CO2 binds to haemoglobin to form carbaminohemoglobin (HbCO2).
  • CO2 binds to the amino groups on the haemoglobin molecule, which is different from the oxygen-binding sites.

Haldane Effect:

  • The Haldane effect describes how deoxygenated haemoglobin has a higher affinity for CO2 and H+, facilitating CO2 transport from the tissues to the lungs.
  • In the lungs, oxygenation of haemoglobin promotes the release of CO2 and H+, aiding in CO2 excretion.

Release and Exhalation:

  • In the lungs, CO2 is released from haemoglobin, bicarbonate is converted back to CO2, and dissolved CO2 diffuses into the alveoli to be exhaled.

Respiratory Pigments:

Respiratory pigments are proteins that transport oxygen in blood or hemolymph. They play a crucial role in respiratory physiology by binding oxygen molecules at respiratory surfaces and releasing them in tissues. There are several types:

Haemoglobin:

  • It is an iron-containing protein in red blood cells that binds oxygen.
  • Example: Found in vertebrates, including humans. 

Myoglobin:

  • Similar to haemoglobin but with a higher affinity for oxygen, aiding in oxygen storage within muscles.
  • Example: Found in muscle tissues of vertebrates.

 Hemocyanin:

  • Contains copper instead of iron, giving it a blue colour when oxygenated.
  • Example: Found in arthropods (like crabs and lobsters) and mollusks (like octopuses).

 Hemerythrin:

  • An iron-containing pigment that turns violet-pink when oxygenated.
  • Example: Found in some marine invertebrates, such as annelids and brachiopods.

 Chlorocruorin:

  • Similar to haemoglobin but with a green colour when oxygenated.
  • Example: Found in some annelids, such as marine worms.

Carbon Monoxide (CO):

Carbon monoxide (CO) poisoning occurs when CO gas, a colourless and odourless substance, is inhaled. It binds with haemoglobin in the blood, forming carboxyhemoglobin, which inhibits the blood’s ability to carry oxygen. 

Causes of Carbon Monoxide (CO) Release:

  • Smoke: Smoke from coal fires.
  • Wildfires: Natural events like forest fires can emit significant amounts of CO.
  • Vehicle Emissions: Internal combustion engines in cars, trucks, and other vehicles are major sources.
  • Residential Heating: Malfunctioning or improperly vented furnaces, wood stoves, and gas heaters can release CO.

Effects of Carbon Monoxide (CO) Poisoning:

  • Headaches.
  • Dizziness.
  • Nausea and vomiting.
  • Shortness of breath.
  • Loss of consciousness.
  • Long-term neurological damage.
  • Cardiac complications.
  • Death if exposure is prolonged and untreated.

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