Radiation damages cell DNA through direct interaction or indirect interaction by creating what are called ‘free radicals’. When cell DNA is damaged it impacts that cells ability to function properly. With high acute doses the primary concern is cell death. The DNA becomes so damaged cells start killing themselves to prevent spreading damaged DNA. Or the damage may be so severe that cell membranes rupture and the cell basically just falls apart.
Cells primarily start dying when they encounter certain checkpoints in their life cycle. During these checkpoints the cell does a sort of self-check. If all is not well the cell kills itself. A significant checkpoint occurs right before a cell divides. So the faster a cell multiplies the more likely it is to kill itself due to damage. If a cell rarely multiplies it could go on functioning relatively normally for a long time despite the damaged DNA.
The first serious effect of acute radiation sickness is called hematopoietic syndrome. This is when all your bone marrow cells (fast reproducers that make red blood cells) die and your hair falls out. Occurs at between 100 and 1000 rads whole body exposure. Takes about 3 weeks to fully present, with death occurring at a month or two usually due to hemmorage or infection since it kills or severely weakens your immune system. Survivable at lower exposures with medical intervention.
The second serious effect is called gastrointestinal syndrome. This is when the lining of your intestinal tract gets destroyed. Your intestines have lots of juices, acids and such that damage the intestinal wall so the cells must be regularly replaced to repair the walls. These cells get killed off and can no longer be replaced. Result is all those digestive juices eat at the cell walls until it’s gone. You basically digest yourself from the inside out. Though the primary concern is your body can no longer absorb water. So you usually die of dehydration. Requires exposures of 1000 rads or higher. Takes a few days to fully present and you die in a couple weeks. Few survive even with medical intervention.
The third most serious effect is Cardiovascuslar or Central Nervous System syndrome. Requires whole body exposure of greater than 5000 rads. Damage is so severe cells start to liquefy. The walls of your blood vessels start to break down and you start bleeding out of everything. Your brain starts to swell up and you die from bleeding out or to much pressure on the brain due to fluid build up. Symptoms present in a few hours. Dead within a couple days.
It’s important to understand that these are for whole body exposure. If only a limb or specific organ is exposed then it’s possible for very high doses to be received without detrimental consequences. External radiation exposure only affects the area exposed to it. It does not spread through the body unless radioactive material is injected, inhaled or consumed.
As a comparison a standard dental xray is only about .0005 rad, equal to exposure received on a 2 hr flight.