How Blood Knows Where to Unload Its Oxygen
See how partial pressure differences move oxygen and carbon dioxide across the alveoli and tissues, read the oxygen dissociation curve and the Bohr effect, follow the three ways carbon dioxide travels, and learn how breathing is regulated and what goes wrong in lung disorders.
How do gases move between air, blood and cells?
Every breath brings fresh oxygen to the alveoli, but that oxygen still has to cross into the blood, travel to every tissue and cross again into cells, while carbon dioxide makes the return journey.
This part covers diffusion and partial pressures, the oxygen dissociation curve, carbon dioxide transport, and the regulation and disorders of breathing.
This part covers diffusion and partial pressures, the oxygen dissociation curve, carbon dioxide transport, and the regulation and disorders of breathing.
How do partial pressure gradients drive the exchange of oxygen and carbon dioxide?
Each gas diffuses from where its partial pressure is higher to where it is lower, so oxygen moves from alveoli into blood and from blood into tissues, while carbon dioxide moves the opposite way across a very thin diffusion membrane.
Partial pressures in mm Hg:
- Oxygen — alveoli 104; deoxygenated blood 40; oxygenated blood 95; tissues 40
- Carbon dioxide — alveoli 40; deoxygenated blood 45; oxygenated blood 40; tissues 45
Worked example — the gradients at the alveoli.
**Why a small CO gradient is enough.** CO is 20 to 25 times more soluble than O, so far more of it diffuses per unit difference in partial pressure.
The diffusion membrane has three layers:
- Thin squamous epithelium of the alveoli
- Endothelium of the alveolar capillaries
- Basement substance between them
Together they are much less than a millimetre thick.
An everyday example. The smell of frying pakoras spreads from the kitchen to every room, moving from high concentration to low without any push.
The substance. Diffusion itself needs no energy — breathing and blood flow simply keep the gradients steep.
Partial pressures in mm Hg:
- Oxygen — alveoli 104; deoxygenated blood 40; oxygenated blood 95; tissues 40
- Carbon dioxide — alveoli 40; deoxygenated blood 45; oxygenated blood 40; tissues 45
Worked example — the gradients at the alveoli.
**Why a small CO gradient is enough.** CO is 20 to 25 times more soluble than O, so far more of it diffuses per unit difference in partial pressure.
The diffusion membrane has three layers:
- Thin squamous epithelium of the alveoli
- Endothelium of the alveolar capillaries
- Basement substance between them
Together they are much less than a millimetre thick.
An everyday example. The smell of frying pakoras spreads from the kitchen to every room, moving from high concentration to low without any push.
The substance. Diffusion itself needs no energy — breathing and blood flow simply keep the gradients steep.
What does the oxygen dissociation curve show, and how do pCO2, pH and temperature shift it?
**The oxygen dissociation curve plots the percentage saturation of haemoglobin against pO as an S-shaped curve; high pCO, high H concentration and high temperature shift it to the right, so haemoglobin releases oxygen more readily — the Bohr effect.
Oxygen transport:
- About 97 per cent** of O travels bound to haemoglobin in RBCs as oxyhaemoglobin
- About 3 per cent is dissolved in plasma
The sigmoid curve. Binding depends mainly on pO, and also on pCO, H concentration and temperature. Its steep middle portion means a modest fall in pO at the tissues releases a large amount of oxygen.
Conditions that decide loading:
- Alveoli — high pO, low pCO, lower H, lower temperature: oxyhaemoglobin forms
- Tissues — low pO, high pCO, high H, higher temperature: oxygen dissociates
An everyday example. During a fast sprint between the wickets, leg muscles grow warmer and more acidic, so blood reaching them unloads more oxygen.
The substance. A right shift is not a fault — it is exactly what gets extra oxygen to the busiest tissues.
Oxygen transport:
- About 97 per cent** of O travels bound to haemoglobin in RBCs as oxyhaemoglobin
- About 3 per cent is dissolved in plasma
The sigmoid curve. Binding depends mainly on pO, and also on pCO, H concentration and temperature. Its steep middle portion means a modest fall in pO at the tissues releases a large amount of oxygen.
Conditions that decide loading:
- Alveoli — high pO, low pCO, lower H, lower temperature: oxyhaemoglobin forms
- Tissues — low pO, high pCO, high H, higher temperature: oxygen dissociates
An everyday example. During a fast sprint between the wickets, leg muscles grow warmer and more acidic, so blood reaching them unloads more oxygen.
The substance. A right shift is not a fault — it is exactly what gets extra oxygen to the busiest tissues.
What are the three ways carbon dioxide is carried in blood, and what does carbonic anhydrase do in the chloride shift?
Carbon dioxide travels about 7 per cent dissolved in plasma, 20 to 25 per cent bound to haemoglobin as carbamino-haemoglobin, and about 70 per cent as bicarbonate, formed rapidly in RBCs by carbonic anhydrase; chloride ions enter RBCs to balance the bicarbonate leaving them.
Three modes:
- Dissolved in plasma — about 7 per cent
- Carbamino-haemoglobin — about 20 to 25 per cent; binding is favoured by high pCO and low pO in tissues
- Bicarbonate — about 70 per cent
Carbonic anhydrase. This enzyme, abundant in RBCs, speeds up the reaction in both directions:
- At tissues — high pCO drives it forward, forming HCO
- At alveoli — low pCO drives it back, releasing CO to be breathed out
Chloride shift. Bicarbonate made in RBCs diffuses out into plasma, and chloride ions move into the RBCs to keep the charges balanced. At the lungs, the exchange reverses.
An everyday example. Soda water fizzes when the bottle is opened — dissolved carbonic acid breaks back into carbon dioxide gas as the pressure falls.
The substance. **Most CO does not travel as CO at all** — it rides in the blood disguised as bicarbonate.
Three modes:
- Dissolved in plasma — about 7 per cent
- Carbamino-haemoglobin — about 20 to 25 per cent; binding is favoured by high pCO and low pO in tissues
- Bicarbonate — about 70 per cent
Carbonic anhydrase. This enzyme, abundant in RBCs, speeds up the reaction in both directions:
- At tissues — high pCO drives it forward, forming HCO
- At alveoli — low pCO drives it back, releasing CO to be breathed out
Chloride shift. Bicarbonate made in RBCs diffuses out into plasma, and chloride ions move into the RBCs to keep the charges balanced. At the lungs, the exchange reverses.
An everyday example. Soda water fizzes when the bottle is opened — dissolved carbonic acid breaks back into carbon dioxide gas as the pressure falls.
The substance. **Most CO does not travel as CO at all** — it rides in the blood disguised as bicarbonate.
How do the brain's respiratory centres regulate breathing, and what happens in asthma, emphysema and occupational lung disorders?
A respiratory rhythm centre in the medulla sets the breathing rhythm, a pneumotaxic centre in the pons fine-tunes it, and a chemosensitive area beside the rhythm centre speeds breathing when carbon dioxide and hydrogen ions rise; asthma, emphysema and dust-related disorders reduce the lungs' ability to move air or exchange gases.
Regulation:
- Respiratory rhythm centre — in the medulla; mainly responsible for regulation
- Pneumotaxic centre — in the pons; can reduce the duration of inspiration, changing the rate
- Chemosensitive area — next to the rhythm centre; highly sensitive to **CO and H, and adjusts breathing to remove them
- Receptors in the aortic arch and carotid artery** also detect CO and H changes
Disorders:
- Asthma — difficulty breathing with wheezing, due to inflammation of bronchi and bronchioles
- Emphysema — a chronic disorder in which alveolar walls are damaged, shrinking the respiratory surface; cigarette smoking is a major cause
- Occupational disorders — long exposure to dust in work such as stone grinding or breaking causes inflammation and fibrosis
An everyday example. Holding your breath until you must gasp shows the chemosensitive area at work — rising CO, not falling O, forces the next breath.
The substance. Emphysema cannot be fixed by clearing the airways, because the damaged alveolar walls themselves are lost.
Regulation:
- Respiratory rhythm centre — in the medulla; mainly responsible for regulation
- Pneumotaxic centre — in the pons; can reduce the duration of inspiration, changing the rate
- Chemosensitive area — next to the rhythm centre; highly sensitive to **CO and H, and adjusts breathing to remove them
- Receptors in the aortic arch and carotid artery** also detect CO and H changes
Disorders:
- Asthma — difficulty breathing with wheezing, due to inflammation of bronchi and bronchioles
- Emphysema — a chronic disorder in which alveolar walls are damaged, shrinking the respiratory surface; cigarette smoking is a major cause
- Occupational disorders — long exposure to dust in work such as stone grinding or breaking causes inflammation and fibrosis
An everyday example. Holding your breath until you must gasp shows the chemosensitive area at work — rising CO, not falling O, forces the next breath.
The substance. Emphysema cannot be fixed by clearing the airways, because the damaged alveolar walls themselves are lost.
Exam tip
What earns full marks on gas transport and regulation?
Learn the partial pressure values as a neat list, and draw the dissociation curve with labelled axes beside a right-shifted curve.
- **pO (mm Hg): alveoli 104, deoxygenated blood 40, oxygenated blood 95, tissues 40
- pCO (mm Hg): alveoli 40, deoxygenated blood 45, oxygenated blood 40, tissues 45
- O transport: 97 per cent as oxyhaemoglobin, 3 per cent in plasma
- CO transport: 70 per cent bicarbonate, 20 to 25 per cent carbamino-haemoglobin, 7 per cent dissolved
- Right shift**: high pCO, high H, high temperature
The trap. Saying that low oxygen is the main trigger for breathing. **Rising CO and H are the main signals.**
- **pO (mm Hg): alveoli 104, deoxygenated blood 40, oxygenated blood 95, tissues 40
- pCO (mm Hg): alveoli 40, deoxygenated blood 45, oxygenated blood 40, tissues 45
- O transport: 97 per cent as oxyhaemoglobin, 3 per cent in plasma
- CO transport: 70 per cent bicarbonate, 20 to 25 per cent carbamino-haemoglobin, 7 per cent dissolved
- Right shift**: high pCO, high H, high temperature
The trap. Saying that low oxygen is the main trigger for breathing. **Rising CO and H are the main signals.**
Did you know
Why does visiting a high mountain town leave you breathless?
High in the mountains, air still contains the same proportion of oxygen, but the air pressure is lower. So the partial pressure of oxygen in the alveoli falls, and the gradient pushing oxygen into the blood becomes smaller.
Visitors arriving in places such as Leh often feel breathless and tired for the first few days.
The body gradually adjusts: breathing becomes deeper, and the kidneys release a hormone that makes the bone marrow produce more red blood cells, raising the blood's oxygen-carrying capacity.
Visitors arriving in places such as Leh often feel breathless and tired for the first few days.
The body gradually adjusts: breathing becomes deeper, and the kidneys release a hormone that makes the bone marrow produce more red blood cells, raising the blood's oxygen-carrying capacity.
Exam relevance
How are gas transport and breathing regulation tested in NEET?
Gas exchange, transport and regulation complete Breathing and Exchange of Gases in NEET Biology, and the values and the curve are asked precisely.
What gets asked. Partial pressure values at alveoli, blood and tissues, conditions favouring formation and dissociation of oxyhaemoglobin, the shape and shifts of the dissociation curve, the share of CO carried in each form, locations of the respiratory centres, and features of asthma, emphysema and occupational disorders.
Question types. Statement-based questions, match-the-column lists, graph-based questions and assertion-reason questions.
The trap that costs marks. **Swapping the pO of deoxygenated and oxygenated blood** — 40 and 95 mm Hg.
What gets asked. Partial pressure values at alveoli, blood and tissues, conditions favouring formation and dissociation of oxyhaemoglobin, the shape and shifts of the dissociation curve, the share of CO carried in each form, locations of the respiratory centres, and features of asthma, emphysema and occupational disorders.
Question types. Statement-based questions, match-the-column lists, graph-based questions and assertion-reason questions.
The trap that costs marks. **Swapping the pO of deoxygenated and oxygenated blood** — 40 and 95 mm Hg.
Key takeaways
What must you be able to do from this part?
- Diffusion: gases move down partial pressure gradients across a three-layer membrane; CO is 20 to 25 times more soluble than O
- Oxygen transport: 97 per cent as oxyhaemoglobin; sigmoid dissociation curve; high pCO, H and temperature shift it right
- **CO transport: about 70 per cent bicarbonate via carbonic anhydrase and the chloride shift; 20 to 25 per cent carbamino-haemoglobin; 7 per cent dissolved
- Regulation and disorders**: rhythm centre in medulla, pneumotaxic centre in pons, chemosensitive area; asthma, emphysema, dust-induced fibrosis
Blood reaching an exercising muscle is warmer and more acidic than usual. Predict how the dissociation curve shifts and what that does to oxygen delivery.
- Oxygen transport: 97 per cent as oxyhaemoglobin; sigmoid dissociation curve; high pCO, H and temperature shift it right
- **CO transport: about 70 per cent bicarbonate via carbonic anhydrase and the chloride shift; 20 to 25 per cent carbamino-haemoglobin; 7 per cent dissolved
- Regulation and disorders**: rhythm centre in medulla, pneumotaxic centre in pons, chemosensitive area; asthma, emphysema, dust-induced fibrosis
Blood reaching an exercising muscle is warmer and more acidic than usual. Predict how the dissociation curve shifts and what that does to oxygen delivery.