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When the heart is at work to help transport blood throughout the body, it is doing one of two things: pumping or resting. Pumping, or contracting, is the act of forceful decrease in size, which forces blood out of the heart and into circulation. Relaxation, contrarily, refers to the time after contraction when the heart relaxes back to its initial size. End-diastolic volume is the amount of blood left in the heart, or specific ventricle, when the relaxation phase is completed.
When clinicians take a person's blood pressure, they gather two measurements, usually in the form of x/y. The first number, or numerator, is termed systolic blood pressure, while the denominator refers to diastolic blood pressure. Diastole refers to the relaxation of the heart, whereas systole is a term used to describe the contracting heart. There are pressure differences in the cardiovascular system dependent upon which phase is currently active, hence the disparity in measurements.
End-diastolic volume is generally a higher number than its partner. This can all be understood logically if a person pictures the heart contracting and relaxing. If an object is decreasing in size, fluid, in this case blood, is forced out or ejected. This is due to the inverse relationship between pressure and volume; while volume is decreasing during contraction, pressure increases to a level exceeding the external environment, causing a pressure gradient. This pressure gradient in a physical sense is what causes ejection of blood.
Diastole, the opposite of contraction, can be understood in a similar fashion. As the size of the heart's chambers increase, while it relaxes, the pressure builds outside of the heart related to internal pressure. This allows for a gradual increase in end-diastolic volume due to this gradient. Once pressures have equalized, end-diastolic volume is level and will not change until the next relaxation period.
End-diastolic volume is important physiologically for a number of reasons. The heart cannot pump out what it does not first have, therefore the greater the end-diastolic volume, the higher in volume an ejection of blood will be within the heart. If someone has a malfunctioning heart that cannot obtain a reasonably high end-diastolic volume, the blood flow and amount of nutrients delivered to tissues of the body may be compromised. Luckily, there are measures, such as regular exercise, or if need be even surgery, which can help improve end-diastolic volume.