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The assumption, the faster death, the more important.

1) Breathing: allows oxygen to enter the lungs, where it diffuses into the bloodstream. The heart then pumps oxygenated blood through the arteries to tissues throughout the body. Oxygen is essential for mitochondrial oxidative phosphorylation, the primary process by which cells generate ATP (adenosine triphosphate), the energy currency of the cell. If this machinery is interrupted for more than a few minutes, particularly in the brain, ATP production declines rapidly. When energy levels fall below a critical threshold, ion gradients cannot be maintained, cellular homeostasis fails, and cell death pathways are triggered (via necrosis or apoptosis, depending on severity and duration). Sudden loss of effective cerebral perfusion means that blood flow to the brain becomes insufficient to sustain metabolism. When this occurs, several critical processes stop simultaneously:

• Oxygen delivery ceases
• Glucose delivery ceases
• Carbon dioxide and metabolic acid removal stop
• Blood pressure falls below the level required to maintain neuronal electrical activity

Because neurons have minimal energy reserves and high metabolic demand, loss of perfusion leads to rapid ATP depletion. As ATP levels collapse:

• Ion pumps (e.g., Na⁺/K⁺-ATPase) fail
• Membrane potentials dissipate
• Calcium influx increases
• Cellular damage cascades begin

If perfusion is not restored promptly, irreversible neuronal injury follows.

The system...

• Lungs: bring oxygen into the bloodstream
• Circulation: vascularization, oxygen diffuses from capillaries → cells → mitochondria
• Heart: pumps the blood around the body
• Blood: oxygen binds to hemoglobin in red blood cells
• Mitochondria

Airway → Lung mechanics → Gas exchange → Blood transport → Heart pumping → Vascular delivery → Cellular uptake → Mitochondrial ATP production → Neural function

Weakness at any point reduces overall efficiency.

2) Water: severe dehydration ultimately kills by impairing circulation unlike oxygen or ATP, the body keeps reserves. ~60% of body mass is water, blood plasma is mostly water, cells are mostly water, all biochemical reactions occur in aqueous solution. Without water, kidneys fail and circulation becomes inadequate.