So, when we talk about ischemia, we’re usually talking about this lack of blood flow to a specific area of tissue. For example, with a heart attack, a coronary artery in the heart that supplies the left ventricle with blood gets blocked, so that localized area of heart tissue doesn’t get enough blood and oxygen; that damage is localized to that left ventricle. Shock is like ischemia, but on a global scale. In other words, it’s a circulatory failure of the whole body; blood flow to tissues is dangerously low, which leads to cellular injury, possibly damages multiple organs, and can even lead to multiple organ failure if not treated immediately.
Okay, so with shock, the body’s tissues aren’t getting enough oxygen via the blood, right? Normally, blood perfuses through tissue and delivers oxygen because there’s enough pressure in the circulatory system to push it through; so, blood pressure majorly affects the amount of blood perfusing through tissues.
Now, blood pressure is determined by two components: the resistance to blood flow in the blood vessels, which is affected by things like vessel length, blood viscosity, and vessel diameter; and the cardiac output, which is the volume of blood pumped by the heart through the body per minute. You can break that down into heart rate, or the number of beats per minute, multiplied by stroke volume, or the amount pumped out each beat. The stroke volume is found by taking the total volume of blood left over after contraction, which is called the end-systolic volume, and subtracting it from the total volume in the heart after filling, or the end-diastolic volume.