CPR manually forces blood through your body when the heart can no longer pump on its own. At best, chest compressions deliver about 30% to 40% of the brain’s normal blood supply, which is enough to keep brain cells alive for precious extra minutes until advanced medical care arrives. The process is physically violent by necessity, and understanding what it does to the body helps explain both why it works and why it leaves marks.
How Compressions Move Blood
When the heart stops, blood pressure drops to zero within seconds. Chest compressions create an artificial version of the heartbeat by squeezing the chest between the breastbone and the spine at a depth of about 5 centimeters (roughly 2 inches) and a rate of 100 to 119 compressions per minute. That rhythm and force are specific: studies of more than 13,700 patients found improved survival to hospital discharge at that compression rate compared to slower or faster speeds, and survival improved when depth reached at least 5 centimeters compared to shallower compressions.
There are two competing explanations for how this actually moves blood. The cardiac pump theory says compressions squeeze the heart directly between the breastbone and spine, pushing blood out of its chambers. The thoracic pump theory says compressions raise pressure inside the entire chest cavity, and because veins have one-way valves, blood can only move forward. In reality, both mechanisms likely contribute. The important result is the same: each compression pushes oxygen-carrying blood toward the brain and heart, and each release lets the chest recoil so blood can refill.
What Happens to the Brain
Brain cells begin dying within four to six minutes without oxygen. Even high-quality CPR only restores 30% to 40% of normal blood flow to the brain, but that fraction is enough to slow the damage significantly. Think of it as keeping the brain on life support at a reduced level rather than letting it go dark entirely. Every minute without any circulation accelerates irreversible injury, so even imperfect blood flow buys critical time.
This is the core reason bystander CPR matters so much. Data from the National Institutes of Health shows that people who received bystander CPR had a 28% greater chance of surviving compared to those who received no CPR before paramedics arrived. That gap exists almost entirely because of what those compressions do for the brain in the first few minutes.
How the Heart Benefits
The heart muscle itself needs blood flow to have any chance of restarting. During cardiac arrest, a measurement called coronary perfusion pressure tracks how much blood is reaching the heart’s own arteries. Research published by the American Heart Association found that when this pressure reaches at least 15 mmHg, the odds of the heart restarting rise substantially. Compressions build this pressure gradually, which is why interruptions are so damaging. Every time compressions stop, that pressure drops back toward zero and has to be rebuilt from scratch.
When paramedics arrive, they often administer epinephrine (adrenaline), which works alongside compressions by tightening blood vessels throughout the body. This redirection effect funnels more of the limited blood flow toward the heart and brain, the two organs that matter most for survival. Compressions provide the force, and the medication helps aim it where it’s needed.
What CPR Does to Breathing
Chest compressions move some air in and out of the lungs, but not much. Measurements show each compression cycle moves roughly 70 to 100 milliliters of air, which is less than the volume of the airways themselves (the “dead space” that never reaches the parts of the lung where oxygen actually transfers into the blood). In practical terms, compressions alone generate almost no meaningful gas exchange.
Animal studies confirm that ventilation from compressions alone deteriorates significantly after about 10 minutes and fails to sustain adequate oxygen and carbon dioxide exchange. This is why rescue breaths or a bag-valve mask remain part of the protocol. For bystanders uncomfortable with mouth-to-mouth, compression-only CPR still works in the first several minutes because the blood already contains residual oxygen from the person’s last normal breaths. But the clock on that reserve is short.
Physical Damage From Compressions
Effective CPR requires enough force to compress an adult’s chest by about two inches, over and over, for what can stretch to 30 minutes or longer. That kind of repeated impact breaks things. A study published in the Journal of the American Heart Association performed full-body CT scans on 104 patients who survived out-of-hospital cardiac arrest. The results were striking: 74% had rib fractures and 18% had fractures of the sternum (breastbone).
These fractures are not a sign that CPR was done wrong. They are a predictable consequence of compressions performed at the correct depth on a real human chest, especially in older adults whose bones are less flexible. Ribs typically fracture along the sides where they curve, and the sternum can crack at its midpoint. Survivors often report significant chest soreness during recovery, and healing generally takes several weeks.
Injuries Beyond Broken Bones
Internal organ damage is less common but does occur. Solid organ injuries, particularly to the liver and spleen, happen in roughly 0.6% to 3% of cases depending on the study. The liver is especially vulnerable because it sits partially behind the lower end of the sternum. If hand placement drifts too low or excessive downward force is applied, the pointed lower edge of the breastbone can lacerate the liver’s left lobe. Splenic injuries follow a similar mechanism. These complications can occur even with correct hand placement and appropriate pressure, though improper technique raises the risk.
Lung bruising (pulmonary contusion) and small air leaks around the lungs are also documented. None of these injuries change the calculus of whether to perform CPR. A person in cardiac arrest will die without intervention. Broken ribs heal; brain death does not.
Why Depth and Speed Matter So Much
The specific numbers in CPR guidelines exist because of a narrow performance window. Compressions shallower than 4 centimeters don’t generate enough pressure to move blood effectively. Compressions deeper than 6 centimeters are associated with reduced survival, likely because excessive depth increases internal injury without improving circulation. Similarly, compressing faster than 120 times per minute doesn’t give the chest enough time to fully recoil between compressions, which limits how much blood refills the heart.
Full chest recoil is just as important as the compression itself. When you push down, you force blood out. When you let the chest spring back completely, you create a brief negative pressure that pulls blood back into the heart from the veins. Leaning on the chest between compressions, even slightly, eliminates this refill phase and can cut the already limited blood flow by a significant margin. This is why fatigue is such a practical concern: after about two minutes of continuous compressions, most people’s technique degrades, and guidelines recommend switching rescuers every two minutes if possible.
What the Body Looks Like After CPR
If CPR is successful and the person’s heart restarts, the aftermath is physically significant. The chest wall is often bruised and tender, sometimes visibly deformed if multiple ribs fractured. Soreness when breathing, coughing, or moving the upper body is common and can persist for weeks. Some survivors develop fluid or air collections around the lungs that need monitoring or drainage.
Internally, the period after the heart restarts brings its own set of challenges. Blood that was stagnant or poorly oxygenated during the arrest now floods back into tissues, triggering inflammatory responses throughout the body. The brain, despite being kept partially supplied during CPR, may still show effects ranging from temporary confusion to more lasting cognitive difficulties depending on how long circulation was compromised. Recovery timelines vary enormously based on how quickly CPR started, how long the arrest lasted, and what caused it in the first place.