The Lab / Core medicine / Lesson 9
Your blood picks up oxygen in the lungs and hands it over in the tissues, using the same molecule for both jobs. See how one S-shaped curve makes that possible, then watch what slides it, and what robs it.
Educational, not for clinical use
Build the curve
Most cells need oxygen to produce energy. Blood picks it up in the lungs, where air meets blood, and drops it off in the tissues, the working body parts such as muscle and brain. Red blood cells do the carrying.
Build the curve
Each red blood cell is stuffed with hemoglobin, a protein that holds oxygen. One hemoglobin molecule has four oxygen seats: spots where one oxygen molecule can sit. Oxygen sits loosely, so it can get on and off again.
Build the curve
Oxygen pressure measures how much oxygen is pressing to move from one place to another. It is high in the lungs, where fresh air arrives, and low in working tissues, which use oxygen up. Oxygen always moves from high pressure to low.
Build the curve
The higher the oxygen pressure around hemoglobin, the more of its seats fill. The share of seats holding oxygen is called saturation: empty at no pressure, almost completely full at high pressure.
Build the curve
Binding means an oxygen sticking to a seat. The first oxygen is the hardest to bind. Once it sits, the molecule changes shape and its other seats bind more easily. This teamwork is cooperative binding. Letting go works the same way in reverse.
Build the curve
Plot saturation against oxygen pressure and you get the oxygen–hemoglobin dissociation curve, dissociation meaning letting go. Teamwork gives it an S shape: a slow start, a steep middle where small pressure changes move lots of oxygen, and a flat top.
Build the curve
Blood leaving the lungs sits on the plateau, the curve’s flat top: almost every seat is full. Because the top is flat, even if lung oxygen pressure dips a little, hemoglobin still loads nearly fully. That is a built-in safety margin.
Build the curve
Working tissues have a low oxygen pressure, which lands on the curve’s steep slope. There, a small drop in pressure empties many seats, so hemoglobin unloads, or hands over, plenty of oxygen exactly where it is needed.
Build the curve
The gap in height between the lungs point and the tissues point is the oxygen unloaded. At rest, most oxygen stays on board: a reserve for exercise, when the heart also pumps more blood (Lesson 3).
Build the curve
The half-saturation pressure is the oxygen pressure at which half the seats are full. It marks where the curve sits. A curve moved right, a right shift, needs more pressure to fill, so lets go easily; moved left, a left shift, it holds on tighter.
Build the curve
Saturation says what share of seats are full, not how many seats there are. Oxygen content is the total oxygen the blood carries: saturation combined with the amount of hemoglobin. Less hemoglobin lowers the ceiling, even if every seat is full.
Build the curve
Lungs load hemoglobin on the flat top; tissues unload it on the steep slope; the drop between is what they receive. Two key measures: the half-saturation pressure, which says where the curve sits, and the oxygen content, which says how much the blood carries.
What changes it
Busy tissue makes carbon dioxide, a waste gas, and acid, and it warms up. Red cells also make more 2,3-BPG, a chemical that loosens hemoglobin’s grip. All four shift the curve right: at the same tissue pressure, hemoglobin lets go of more oxygen.
What changes it
Fetal hemoglobin, the kind a baby makes before birth, grips oxygen tighter, so it can pull oxygen from the mother’s blood. Cold and alkalosis (blood less acidic than usual) also shift the curve left: hemoglobin loads easily but lets go less in the tissues.
What changes it
Carbon monoxide, a colourless gas from fires and faulty heaters, sticks to oxygen seats far more tightly than oxygen does. Fewer seats carry oxygen, and the rest grip tighter, a left shift. Blood oxygen pressure stays normal, yet content falls.
What changes it
Anemia means too little hemoglobin in the blood, for example after blood loss. Each remaining seat fills normally, so saturation looks normal, but the ceiling drops and content falls. Tissues pull more from each seat, so their oxygen pressure falls.
Start with Next, or use the arrow keys. Curves, heights and seat counts are illustrative, not measurements.
Loading reference values…
Normal teaching baseline.
The blue line on each bar is an illustrative teaching model, not a patient measurement. These textbooks give single approximate values rather than ranges, so each bar shows the textbook’s value as a black tick; the model is called close when it is within a tenth of that value. Arrows come from the cited source for the selected state.
An illustrative teaching model of blood passing from the lungs to resting tissue and back. Change the oxygen reaching the lungs, where the curve sits, how much hemoglobin there is, how much carbon monoxide is stuck to it, and how hard the tissues are working. The faint dashed lines are the normal state.
The curve follows the Hill equation, saturation = Pn ÷ (Pn + P50n), where P is oxygen pressure. Its steepness n and its half-saturation pressure P50 are fitted at load time through two textbook points from the Clinical values data file: arterial blood (pressure and saturation) and venous blood (pressure and saturation). That fit lands the model’s P50 a little above the separate textbook P50 shown in Clinical values; textbooks differ slightly, and the model keeps Guyton’s two points exact instead.
Blood leaves the lungs at the arterial pressure (scaled by the lung slider) and loads to the saturation the curve gives there. Oxygen content is saturation × the hemoglobin present × the oxygen each gram binds (from the data file). The tissues remove a fixed amount of oxygen per pass, set at rest from the textbook utilization coefficient and multiplied by the tissue-use slider; the venous saturation is what is left, and the venous oxygen pressure is read back off the curve. Curve position multiplies P50. Carbon monoxide takes a share of seats out of use and lowers P50 for the seats that remain (by 0.6 × that share, an illustrative choice). So with carbon monoxide the model keeps two saturations apart: functional saturation (oxygen-holding seats as a share of the seats still free for oxygen), on which the curve and P50 are defined, and fractional saturation (oxygen-holding seats as a share of all seats), which the Clinical values rows print. At P50, functional saturation is one half, while fractional saturation is lower. If the tissues would need more oxygen than the blood carries, the model stops at a near-empty floor and says the tissues are short.
The model leaves out oxygen dissolved directly in plasma, blood flow and the heart, carbon dioxide transport, and all timing. The sizes of the drug and disease changes are illustrative; only their directions are meant to teach.