HemoClimb: Oxygen Delivery

A nod to Chuckie Egg (A&F Software, 1983). Instead of collecting eggs, you’re a red blood cell collecting oxygen in the lungs at the top of the screen and carrying it down to a target cell at the bottom. Pathogens patrol the platforms, hypoxic cells try to steal your oxygen, Pneumococcus and carbon monoxide drift around the lungs, and, just as in the original, the lift will happily carry you up into the roof if you don’t step off in time.

The science

• You are a red blood cell (erythrocyte), a flexible disc with no nucleus, packed with about 270 million molecules of haemoglobin.
• Why four oxygen molecules? Each haemoglobin molecule has four haem groups, each holding an iron atom that binds one O₂. The game’s limit of four is one haemoglobin molecule’s worth.
• Picking up and dropping off. Oxygen enters the blood in the lungs’ alveoli, where there is plenty of it, and is released in tissues that need it. Haemoglobin lets go of oxygen more easily where oxygen is low and carbon dioxide and acid are high (the Bohr effect and the oxygen–haemoglobin dissociation curve). Cells short of oxygen are hypoxic.
• Dangers in the lungs. Pneumococcus (Streptococcus pneumoniae) is a common cause of pneumonia. Under the microscope it shows up in pairs (diplococci) inside a capsule, and it stains purple in the Gram stain. Carbon monoxide, from exhaust fumes and faulty heaters, binds haemoglobin more than 200 times more tightly than oxygen: each CO molecule takes a haem out of use, which is why carbon monoxide poisoning is so dangerous.
• Where the oxygen ends up. In the target cell’s mitochondria, oxygen is the last stop of the electron transport chain: complex IV (cytochrome c oxidase) turns it into water while pumping protons that power ATP synthesis. I worked on the structure of a caa3-type cytochrome c oxidase, a complex IV with its own cytochrome c built in: Joseph Lyons solved it as a PhD student, and I harvested the crystals and supervised the crystallographic work.
• A crystallography classic. Haemoglobin was one of the first protein structures solved by X-ray crystallography, by Max Perutz, who shared the 1962 Nobel Prize in Chemistry with John Kendrew.
• A new red blood cell. Now and then erythropoietin (EPO) floats by. The kidneys release it when oxygen runs low, and it tells the bone marrow to make more red blood cells (erythropoiesis).

Where the game cuts corners

• Red blood cells don’t climb, jump or ride lifts. The heart pumps them round the body, and a full lap takes about a minute. The lift is pure Chuckie Egg.
• One red blood cell carries far more than four oxygen molecules: with about 270 million haemoglobins, each holding four, that’s about a billion. The game’s four is one haemoglobin’s worth, to keep the counting manageable.
• Real red blood cells last about 120 days and then are recycled by the spleen and liver, which is a gentler ending than the roof.