Q.What is the amount of O2 supplied to tissues through every 100 ml. of oxygenated blood under normal physiological conditions?
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Transport Of Oxygen
The Delivery Problem
Every one of the body's trillions of cells constantly burns fuel and needs a steady oxygen supply, yet oxygen is a gas and the cells are buried deep inside tissues. The body solves this with a dedicated carrier: a protein inside red blood cells called haemoglobin. Oxygen is not simply dissolved in blood — dissolved oxygen alone could barely supply the body — instead it binds chemically to haemoglobin, forming oxyhaemoglobin, which raises blood's oxygen-carrying capacity roughly seventy-fold.
How the Binding Works
When you inhale, oxygen diffuses from the alveoli into the surrounding capillaries and meets red blood cells packed with haemoglobin. Each haemoglobin molecule carries four iron-containing haem groups, and each haem group can bind one oxygen molecule. The reaction is reversible: haemoglobin picks up oxygen (Hb + 4 O₂ ⇌ Hb(O₂)₄) where oxygen is plentiful, in the lungs, and releases it where oxygen is scarce, in the tissues.
Oxyhaemoglobin, the oxygen-loaded form, is bright red; deoxyhaemoglobin, the oxygen-free form, is darker — this is why arterial blood looks bright red and venous blood looks darker.
Cooperative Binding
When the first oxygen molecule binds to haemoglobin, it triggers a shape change that makes it easier for the next three to bind — this is cooperative binding, and it produces an S-shaped (sigmoid) oxygen-haemoglobin dissociation curve rather than a straight line. At the oxygen level found in the lungs, haemoglobin is nearly fully saturated — almost every binding site is filled. At the lower oxygen level found in resting tissues, saturation drops to around 75%, meaning about a quarter of the carried oxygen is unloaded to the cells.
A healthy adult's blood carries about 20 mL of oxygen per 100 mL of blood; only a small fraction of that travels dissolved in plasma, the rest is bound to haemoglobin.
What Shifts the Curve
Haemoglobin's affinity for oxygen is not fixed — it changes with conditions in the tissues, letting the body fine-tune delivery:
| Factor | Change | Effect on oxygen release | Why it helps |
|---|---|---|---|
| Carbon dioxide level | High | Increases release | Active tissues produce more carbon dioxide |
| Acidity (pH) | Lower (more acidic) | Increases release | Active tissues produce lactic acid |
| Temperature | Higher | Increases release | Working muscles run hotter |
Under normal physiological conditions, every 100 millilitres of oxygenated blood delivers about 5 millilitres of oxygen to the tissues. …
Every 100 mL of oxygenated blood delivers roughly 5 mL of oxygen to the tissues under normal conditions.
Oxygen is carried mainly bound to haemoglobin as oxyhaemoglobin, loaded at the lungs where oxygen pressure is high and released at the tissues where oxygen pressure is low, carbon-dioxide pressure and hydrogen-ion concentration are high, and temperature is higher — all conditions that favour the dissociation of oxygen from haemoglobin. …
Method: Recalling a standard physiological transport value from its mechanism
This method applies to questions asking for a standard quantitative value (like "how much O2 is delivered per 100 mL blood") that is grounded in a described physiological process, not a formula you calculate from scratch.
Steps
Step 1: Identify the underlying mechanism that makes the number possible
Before recalling the number, be clear on why it exists: oxygen travels bound to haemoglobin as oxyhaemoglobin, loading in the lungs where oxygen pressure is high, and unloading in the tissues where oxygen pressure is low, carbon-dioxide pressure and hydrogen-ion concentration are high, and temperature is higher — conditions that all favour the oxygen–haemoglobin bond breaking.
Step 2: Recognise that not all bound oxygen unloads at once …
- AP EAPCET 2024Set ap-2024-05-17-AN1 markMCQQ.These factors are favourable for the formation of oxyhaemoglobin. I. High pCO2. II. High pO2. III. Lesser H+ concentration and lower temperature. IV. Higher H+ concentration and higher temperature. (A) II and III (B) I and IV (C) III and IV (D) I and III
›Reveal solutionSolution
This tests the Bohr effect; oxyhaemoglobin forms readily under high pO2, low H+ and low temperature — i.e., conditions II and III.
Concept and Intuition
Haemoglobin's affinity for oxygen is not fixed — it shifts with local conditions (the Bohr effect). In the lungs (alveoli), pO2 is high, pCO2 is low, H+ concentration is low, and temperature is comparatively lower — these conditions push the equilibrium toward Hb + O2 → HbO2 (oxyhaemoglobin formation/loading). In actively respiring tissues, the reverse conditions (high pCO2, high H+, high temperature) favour unloading of O2 from oxyhaemoglobin.
Step-by-Step Solution
- High pCO2 (I) shifts the equilibrium toward the deoxy form (favours O2 release, not uptake) — so I is NOT a favourable factor for formation.
- High pO2 (II) directly drives more Hb molecules to bind O2 — favourable for formation. …
- AP EAPCET 2023Set ap-2023-05-22-AN1 markMCQQ.Identify the correct option Statement –I (S-I): Haemoglobin binds with oxygen in oxygen rich areas and leave CO2 Statement – II (S-II): Haemoglobin is an amphoteric compound (A) S-I and S-II are incorrect. (B) S-I is correct and S-II is incorrect. (C) S-I is incorrect and S-II is correct. (D) S-I and S-II are correct.
›Reveal solutionSolution
Both statements describe genuine, textbook-correct properties of haemoglobin — its oxygen/CO2 exchange behaviour at the lungs and its amphoteric (protein buffer) nature.
Concept and Intuition
Haemoglobin's affinity for O2 and CO2 is reciprocal and location-dependent: in the oxygen-rich alveolar capillaries it binds O2 strongly and releases CO2 (which diffuses into the alveoli to be exhaled); in the tissues (oxygen-poor, CO2-rich) the reverse happens. Separately, being a protein, haemoglobin carries both –COOH and –NH₂ groups, making it amphoteric — able to act as both an acid and a base, which helps it buffer blood pH.
Step-by-Step Solution
- Evaluate S-I: at the lungs (oxygen-rich), Hb binds O2 to form oxyhaemoglobin and simultaneously releases the CO2 it was carrying — true.
- Evaluate S-II: Hb is a globular protein with ionizable acidic and basic groups, so it is amphoteric — true. …
- AP EAPCET 2021Set ap-2021-09-03-AN1 markMCQQ.Identify the correct group of statements(i) Oxygen is carried by Haemoglobin(ii) Oxygen is carried by carbonic anhydrase(iii) Caron dioxide is carried by Haemoglobin(iv) Sulphur dioxide is carried by Haemoglobin (A)(i) &(iii) only (B)(i) &(ii) only (C)(i) &(iv) only (D)(ii) &(iii) only
›Reveal solutionSolution
This tests the mechanisms of respiratory gas transport in blood — what haemoglobin does and does not carry.
Concept and Intuition
Haemoglobin is the primary carrier of oxygen (as oxyhaemoglobin, HbO₂) and also carries a substantial fraction of carbon dioxide (as carbaminohaemoglobin, HbCO₂), while most CO₂ is actually transported as bicarbonate ions in plasma. Carbonic anhydrase is an enzyme found in RBCs that speeds up the reaction CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ — it facilitates CO₂ transport as bicarbonate, but it does not itself "carry" oxygen. Sulphur dioxide has no physiological transport role in the blood.
Step-by-Step Solution
- Statement (i): Oxygen is carried by Haemoglobin — true, ~97% of O₂ is transported as oxyhaemoglobin.
- Statement (ii): Oxygen is carried by carbonic anhydrase — false, carbonic anhydrase is an enzyme facilitating CO₂ interconversion, not an oxygen carrier. …
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