Transport of Oxygen — Revision Notes

⚡ 30-Second Revision

Primary Carrier: — Hemoglobin (Hb) in RBCs (97%). \n- Dissolved in Plasma: 3%. \n- Binding Site: $Fe^{2+}$ in heme group. \n- Oxyhemoglobin: $Hb + O_2 \rightleftharpoons HbO_2$. \n- Cooperative Binding: Binding of one $O_2$ increases affinity for subsequent $O_2$. \n- ODC Shape: Sigmoidal (S-shaped). \n- Right Shift (\( \downarrow \) Affinity, \( \uparrow \) Release): \( \uparrow pCO_2, \downarrow pH, \uparrow Temp, \uparrow 2,3-BPG \). \n- Left Shift (\( \uparrow \) Affinity, \( \downarrow \) Release): \( \downarrow pCO_2, \uparrow pH, \downarrow Temp, \downarrow 2,3-BPG \). \n- Bohr Effect: \( \uparrow pCO_2 \) and \( \downarrow pH \) cause right shift. \n- 2,3-BPG: Reduces Hb affinity for $O_2$, causes right shift. \n- Fetal Hb (HbF): Higher $O_2$ affinity than Adult Hb (HbA) due to less 2,3-BPG binding (left shift). \n- Oxygen Carrying Capacity: ~20 mL $O_2$ per 100 mL blood. \n- **Arterial $pO_2$ (lungs): ~100 mmHg, Hb saturation ~97%. \n- Venous $pO_2$ (tissues):** ~40 mmHg, Hb saturation ~75% (at rest).

2-Minute Revision

Oxygen transport is crucial for cellular energy. Most oxygen (97%) travels bound to hemoglobin within red blood cells, forming oxyhemoglobin. A small amount (3%) is dissolved in plasma. Hemoglobin's unique ability to bind four oxygen molecules cooperatively gives the oxygen-hemoglobin dissociation curve (ODC) its characteristic sigmoidal shape.

This shape ensures efficient oxygen loading in the lungs (high $pO_2$, plateau phase) and effective unloading in the tissues (low $pO_2$, steep phase). \n\nHemoglobin's affinity for oxygen is not constant but is modulated by several factors.

Conditions in active tissues – high carbon dioxide ($pCO_2$), increased acidity (low pH), and elevated temperature – all decrease hemoglobin's oxygen affinity, causing the ODC to shift to the right.

This is known as the Bohr effect for $pCO_2$ and pH. Similarly, an increase in 2,3-Bisphosphoglycerate (2,3-BPG), an RBC metabolite, also shifts the curve to the right, enhancing oxygen release.

Conversely, conditions like low $pCO_2$, high pH, low temperature, and low 2,3-BPG increase oxygen affinity, shifting the curve to the left. Fetal hemoglobin also exhibits a left-shifted curve, allowing it to efficiently extract oxygen from maternal blood.

Understanding these shifts and their physiological implications is key for NEET.

5-Minute Revision

Oxygen transport is a finely tuned process ensuring every cell receives its vital supply. The journey begins in the lungs, where oxygen, driven by a high partial pressure ($pO_2$ of ~104 mmHg), diffuses into pulmonary capillaries.

Here, it primarily binds to hemoglobin (Hb), a protein in red blood cells, forming **oxyhemoglobin ($HbO_2$)**. Each Hb molecule can bind up to four $O_2$ molecules. This binding is cooperative: the attachment of one $O_2$ enhances the affinity for subsequent $O_2$ molecules, leading to the sigmoidal oxygen-hemoglobin dissociation curve (ODC).

This S-shape is critical: the plateau at high $pO_2$ (lungs) ensures near-complete saturation (approx. 97% at 100 mmHg arterial $pO_2$), while the steep portion at lower $pO_2$ (tissues) allows for significant oxygen release with small $pO_2$ drops.

\n\nAs oxygenated blood reaches systemic capillaries, tissue $pO_2$ is lower (~40 mmHg), prompting $O_2$ dissociation. This unloading is further enhanced by local tissue conditions, which reduce hemoglobin's affinity for oxygen, causing a rightward shift of the ODC.

These factors include: \n1. **Increased $pCO_2$:** Active tissues produce more $CO_2$. \n2. **Decreased pH (Increased $H^+$):** $CO_2$ forms carbonic acid, increasing $H^+$. This is the Bohr effect.

\n3. Increased Temperature: Metabolic activity generates heat. \n4. Increased 2,3-Bisphosphoglycerate (2,3-BPG): Produced in RBCs, it stabilizes deoxyhemoglobin. \n\nConversely, a leftward shift (increased affinity, reduced release) occurs with opposite conditions (low $pCO_2$, high pH, low temperature, low 2,3-BPG).

Fetal hemoglobin (HbF) has a naturally left-shifted ODC compared to adult hemoglobin (HbA) due to weaker 2,3-BPG binding, enabling efficient oxygen uptake from the mother. Approximately 20 mL of $O_2$ is carried per 100 mL of blood, with only about 5 mL released to resting tissues, highlighting the body's reserve capacity.

Understanding these dynamic interactions is vital for NEET success.

Prelims Revision Notes

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Forms of Oxygen Transport: — \n * 97%: Bound to hemoglobin (Hb) in red blood cells, forming oxyhemoglobin ($HbO_2$). \n * 3%: Dissolved in blood plasma. \n2. Hemoglobin Structure: \n * Tetrameric protein (e.g., HbA: two alpha, two beta chains). \n * Each chain has a heme group with an $Fe^{2+}$ ion, which binds one $O_2$ molecule. Total 4 $O_2$ per Hb. \n3. Oxygen-Hemoglobin Dissociation Curve (ODC): \n * Shape: Sigmoidal (S-shaped) due to cooperative binding. \n * Cooperative Binding: Binding of first $O_2$ increases affinity for subsequent $O_2$. \n * **Plateau Region (high $pO_2$, 60-100 mmHg):** Ensures high saturation in lungs (e.g., ~97% at 100 mmHg). Safety margin. \n * **Steep Region (low $pO_2$, 0-40 mmHg):** Allows significant $O_2$ unloading in tissues (e.g., ~75% saturation at 40 mmHg in venous blood at rest). \n * **$P_{50}$:** $pO_2$ at which Hb is 50% saturated (normal ~25-27 mmHg). \n4. Factors Affecting ODC (Shifts): \n * **Right Shift (\( \downarrow \) Affinity, \( \uparrow \) $O_2$ Release):** Occurs in active tissues. \n * \( \uparrow pCO_2 \) (Bohr Effect) \n * \( \downarrow pH \) (\( \uparrow H^+ \), Bohr Effect) \n * \( \uparrow \) Temperature \n * \( \uparrow \) 2,3-Bisphosphoglycerate (2,3-BPG) \n * **Left Shift (\( \uparrow \) Affinity, \( \downarrow \) $O_2$ Release):** Occurs in lungs or under specific conditions. \n * \( \downarrow pCO_2 \) \n * \( \uparrow pH \) (\( \downarrow H^+ \)) \n * \( \downarrow \) Temperature \n * \( \downarrow \) 2,3-BPG \n * Fetal Hemoglobin (HbF) has higher affinity than Adult Hemoglobin (HbA) due to weaker 2,3-BPG binding. \n5. Partial Pressures (approximate values): \n * Alveolar $pO_2$: 104 mmHg \n * Arterial blood $pO_2$: 95-100 mmHg \n * Tissue $pO_2$: 40 mmHg (resting) \n * Venous blood $pO_2$: 40 mmHg \n6. Oxygen Carrying Capacity: \n * 1 gram of Hb carries ~1.34 mL of $O_2$. \n * 100 mL of blood (with ~15g Hb) carries ~20 mL of $O_2$. \n * At rest, ~5 mL of $O_2$ is delivered to tissues per 100 mL blood. \n7. Physiological Significance: Shifts ensure $O_2$ is loaded in lungs and unloaded in tissues according to metabolic demand (e.g., exercise, high altitude acclimatization).

Vyyuha Quick Recall

CADET, Face Right! \n\nThis mnemonic helps remember factors causing a Rightward Shift of the Oxygen-Hemoglobin Dissociation Curve (meaning Creased oxygen affinity and Decreased oxygen release to tissues): \n\n* C - Carbon dioxide (Increased $pCO_2$) \n* A - Acidosis (Decreased pH / Increased $H^+$) \n* D - DPG (Increased 2,3-BPG) \n* E - Exercise (Increased metabolic activity leading to above factors) \n* T - Temperature (Increased temperature) \n\nRemember: If these factors increase, the curve shifts Right, and hemoglobin 'lets go' of oxygen more easily, which is beneficial for active tissues.

The opposite conditions would cause a Left shift.