Breathing and Exchange of Gases — Revision Notes

⚡ 30-Second Revision

Breathing: — Mechanical movement of air. Inspiration (active: diaphragm & external intercostals contract, thoracic volume $\uparrow$, intra-pulmonary pressure $\downarrow$). Expiration (passive: muscles relax, thoracic volume $\downarrow$, intra-pulmonary pressure $\uparrow$).
Gas Exchange: — Simple diffusion down partial pressure gradients.

- Alveoli to Blood: $P_{O_2}$ (alveoli) $104\,\text{mmHg} > P_{O_2}$ (blood) $40\,\text{mmHg}$. $P_{CO_2}$ (blood) $45\,\text{mmHg} > P_{CO_2}$ (alveoli) $40\,\text{mmHg}$. - Blood to Tissues: $P_{O_2}$ (blood) $95\,\text{mmHg} > P_{O_2}$ (tissues) $40\,\text{mmHg}$. $P_{CO_2}$ (tissues) $45\,\text{mmHg} > P_{CO_2}$ (blood) $40\,\text{mmHg}$.

Oxygen Transport: — 97% by Hemoglobin ($HbO_2$), 3% dissolved in plasma.
Carbon Dioxide Transport: — 70% as $HCO_3^-$ (bicarbonate), 20-25% as Carbaminohemoglobin, 7-10% dissolved in plasma.
Enzyme: — Carbonic anhydrase in RBCs for $CO_2 + H_2O \rightleftharpoons H_2CO_3$.
Bohr Effect: — $\uparrow P_{CO_2}$, $\uparrow H^+ (\downarrow pH)$, $\uparrow \text{Temp} \implies$ Right shift of $O_2$-Hb curve, $\downarrow O_2$ affinity (more $O_2$ release).
Haldane Effect: — $\downarrow P_{O_2}$ (deoxygenated Hb) $\implies \uparrow CO_2$ binding affinity to Hb (more $CO_2$ transport).
Regulation: — Medullary Rhythm Centre (primary), Pneumotaxic Centre (pons, inhibits inspiration), Chemosensitive Area (medulla, sensitive to $CO_2$, $H^+$), Peripheral Chemoreceptors (aortic/carotid, sensitive to $\downarrow P_{O_2}$, also $CO_2$, $H^+$).

2-Minute Revision

Breathing is the mechanical process of moving air, while gas exchange is the diffusion of $O_2$ and $CO_2$ across membranes. Inspiration is active, involving diaphragm and external intercostal contraction, increasing thoracic volume and decreasing intra-pulmonary pressure to draw air in.

Expiration is usually passive, with muscle relaxation decreasing volume and increasing pressure to expel air. Gas exchange occurs at the alveoli (lungs) and tissues, driven by partial pressure gradients.

Oxygen moves from high $P_{O_2}$ (alveoli) to low $P_{O_2}$ (blood/tissues) and is mostly transported by hemoglobin (97%). Carbon dioxide moves from high $P_{CO_2}$ (tissues/blood) to low $P_{CO_2}$ (blood/alveoli) and is primarily transported as bicarbonate ions (70%), with carbonic anhydrase playing a key role.

The oxygen-hemoglobin dissociation curve illustrates $O_2$ binding, and its rightward shift (Bohr effect) due to increased $CO_2$, $H^+$, or temperature promotes $O_2$ release in tissues. Breathing is regulated by the medullary rhythm center, influenced by chemoreceptors sensitive to $CO_2$ and $H^+$ levels, with $O_2$ playing a lesser role under normal conditions.

5-Minute Revision

Let's consolidate the core concepts of 'Breathing and Exchange of Gases'. The entire process begins with ventilation, the physical act of breathing. During inspiration, the diaphragm contracts and flattens, while external intercostal muscles contract, lifting the rib cage.

This increases the thoracic cavity volume, which in turn decreases the intra-pulmonary pressure below atmospheric pressure, causing air to rush into the lungs. Expiration is typically passive, as these muscles relax, reducing thoracic volume and increasing intra-pulmonary pressure, forcing air out.

Forced breathing involves accessory muscles.

Once air is in the alveoli, gas exchange occurs via simple diffusion. In the lungs (external respiration), oxygen moves from the alveolar air ($P_{O_2} = 104\,\text{mmHg}$) into the deoxygenated blood ($P_{O_2} = 40\,\text{mmHg}$).

Simultaneously, carbon dioxide moves from the blood ($P_{CO_2} = 45\,\text{mmHg}$) into the alveoli ($P_{CO_2} = 40\,\text{mmHg}$). This is facilitated by the extremely thin respiratory membrane and vast alveolar surface area.

In the tissues (internal respiration), the oxygenated blood ($P_{O_2} = 95\,\text{mmHg}$) releases oxygen to the cells ($P_{O_2} = 40\,\text{mmHg}$), while the cells' waste carbon dioxide ($P_{CO_2} = 45\,\text{mmHg}$) diffuses into the blood ($P_{CO_2} = 40\,\text{mmHg}$).

Gas transport is crucial. Oxygen is primarily transported by hemoglobin (97%) as oxyhemoglobin, with a small amount dissolved in plasma (3%). Carbon dioxide is transported in three main ways: as bicarbonate ions ($HCO_3^-$) (70%), formed rapidly in RBCs by carbonic anhydrase; as carbaminohemoglobin (20-25%); and dissolved in plasma (7-10%).

The oxygen-hemoglobin dissociation curve shows how $O_2$ saturation of Hb varies with $P_{O_2}$. Factors like increased $P_{CO_2}$, increased $H^+$ (decreased pH), and increased temperature shift this curve to the right (Bohr effect), promoting $O_2$ release in active tissues.

Conversely, the Haldane effect states that deoxygenated hemoglobin has a higher affinity for $CO_2$ and $H^+$, facilitating $CO_2$ transport.

Regulation of respiration is primarily neural, centered in the medullary rhythm center, modulated by the pneumotaxic and apneustic centers in the pons. Chemical regulation is paramount: central chemoreceptors in the medulla are highly sensitive to $CO_2$ and $H^+$ levels, while peripheral chemoreceptors in the carotid and aortic bodies respond to significant drops in $O_2$ as well as $CO_2$ and $H^+$.

An increase in $CO_2$ or $H^+$ is the strongest stimulus for increasing breathing rate and depth. Remember common disorders like asthma (bronchial inflammation), emphysema (alveolar damage), and occupational lung diseases (fibrosis from dust exposure).

Prelims Revision Notes

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Respiratory Organs: — Humans have a pair of lungs, protected by the rib cage, vertebral column, sternum, and diaphragm. Air pathway: Nostrils $\rightarrow$ Pharynx $\rightarrow$ Larynx $\rightarrow$ Trachea $\rightarrow$ Bronchi $\rightarrow$ Bronchioles $\rightarrow$ Alveoli.
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Mechanics of Breathing:

* Inspiration: Active process. Diaphragm contracts (flattens), external intercostals contract (ribs up/out). Thoracic volume $\uparrow$, Intra-pulmonary pressure $\downarrow$ (below atmospheric).

Air enters. * Expiration: Passive process (normal). Diaphragm relaxes (domes), external intercostals relax (ribs down/in). Thoracic volume $\downarrow$, Intra-pulmonary pressure $\uparrow$ (above atmospheric).

Air exits. * Forced Breathing: Involves accessory muscles (e.g., internal intercostals, abdominal muscles).

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Gas Exchange (Diffusion): — Occurs across the respiratory membrane (alveolar wall + capillary endothelium + basement membrane). Rate depends on partial pressure gradient, solubility of gases, membrane thickness, and surface area.

* Partial Pressures (approx. mmHg): * Atmospheric: $P_{O_2} = 159$, $P_{CO_2} = 0.3$ * Alveolar: $P_{O_2} = 104$, $P_{CO_2} = 40$ * Deoxygenated Blood: $P_{O_2} = 40$, $P_{CO_2} = 45$ * Oxygenated Blood: $P_{O_2} = 95$, $P_{CO_2} = 40$ * Tissues: $P_{O_2} = 40$, $P_{CO_2} = 45$ * $CO_2$ is 20-25 times more soluble than $O_2$.

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Transport of Oxygen:

* 97% by Hemoglobin (Hb): Forms oxyhemoglobin ($HbO_2$). Reversible binding. Each Hb binds 4 $O_2$. * 3% dissolved in plasma. * Oxygen-Hemoglobin Dissociation Curve: Sigmoid shape. Factors shifting curve right (decreased $O_2$ affinity, increased $O_2$ release to tissues): $\uparrow P_{CO_2}$, $\uparrow H^+ (\downarrow pH)$, $\uparrow \text{Temperature}$, $\uparrow$ 2,3-BPG (Bohr effect).

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Transport of Carbon Dioxide:

* **70% as Bicarbonate Ions ($HCO_3^-$):** $CO_2 + H_2O \xrightarrow{\text{Carbonic Anhydrase}} H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$. Occurs in RBCs. $HCO_3^-$ moves to plasma, $Cl^-$ moves into RBCs (Chloride Shift). * 20-25% as Carbaminohemoglobin: Binds to amino groups of Hb. Favored by low $P_{O_2}$ (Haldane effect). * 7-10% dissolved in plasma.

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Regulation of Respiration:

* Neural: Respiratory Rhythm Centre (medulla, primary), Pneumotaxic Centre (pons, inhibits inspiration), Apneustic Centre (pons, prolongs inspiration). * Chemical: Chemosensitive area (medulla, sensitive to $\uparrow CO_2$, $\uparrow H^+$). Peripheral chemoreceptors (carotid/aortic bodies, sensitive to $\downarrow P_{O_2}$, also $\uparrow CO_2$, $\uparrow H^+$). $CO_2$ and $H^+$ are the most potent stimuli.

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Disorders: — Asthma (bronchial inflammation/constriction), Emphysema (alveolar wall damage, $\downarrow$ surface area), Occupational Respiratory Disorders (fibrosis from dust exposure).

Vyyuha Quick Recall

For the factors that shift the Oxygen-Hemoglobin Dissociation Curve to the RIGHT (meaning more oxygen released to tissues), remember: CADET, face RIGHT!

C — $CO_2$ (Increased $P_{CO_2}$)
A — Acid (Increased $H^+$ or decreased pH)
D — 2,3-DPG (or BPG) (Increased 2,3-Bisphosphoglycerate)
E — Exercise (leads to all the above)
T — Temperature (Increased Temperature)

All these conditions are found in metabolically active tissues, where oxygen is needed most, hence the curve shifts to the RIGHT, promoting oxygen unloading.