from:油管
Cardiac output（心輸出量）is the amount of blood pumped by each ventricle in one minute.
It is the product of stroke volume（每搏輸出量） – the amount of blood pumped in one heartbeat, and heart rate（心率) – the number of beats in one minute.
An increase in either stroke volume(每搏輸出量)or heart rate results in increased cardiac output(心輸出量), and vice versa.
For example, during physical exercises(體育鍛鍊), the heart beats faster to put out more blood in response to higher demand of the body.
It is noteworthy that the ventricles do not eject all the blood they contain in one beat.
In a typical example(典型的例子), a ventricle is filled with about 100ml of blood at the end of its load, but only 60ml is ejected during contraction(收縮).This corresponds to an ejection fraction(射血分數) of 60%.
The 100ml is the end-DIASTOLIC volume, or EDV.(舒張末期容積）.The 40ml that remains in the ventricle after contraction is the end-SYSTOLIC volume, or ESV(收縮末期容積).
The stroke volume（每搏輸出量）equals EDV minus(減) ESV, and is dependent on 3 factors: contractility（收縮力）, preload（前負荷）, and afterload（後負荷）.
Contractility refers to the force of the contraction of the heart muscle.The more forceful the contraction, the more blood it ejects.
Preload is related to the end-diastolic volume.Preload, by definition, is the degree of stretch of cardiac myocytes（心肌細胞）at the end of ventricular filling（心室充盈末期）, but since this parameter is not readily measurable in patients, EDV is used instead.
This is because the stretch level of the wall of a ventricle increases as it’s filled with more and more blood; just like a balloon - the more air it contains, the more stretched it is.
According to the Frank-Starling mechanism, the greater the stretch, the greater the force of contraction（收縮）.In the balloon analogy（類似）, the more inflated（膨脹）the balloon, the more forceful it releases air when deflated.
Afterload, on the other hand, is the resistance that the ventricle must overcome to eject blood.Afterload includes 2 major components:
-Vascular pressure(血管壓力): The pressure in the left ventricle must be greater than the systemic pressure(系統壓力) for the aortic valve(主動脈瓣) to open.
Similarly, the pressure in the right ventricle(右心室)must exceed pulmonary pressure to open the pulmonary valve(肺動脈瓣).
In hypertension(高血壓) for example, higher vascular pressures make it more difficult for the valves to open, resulting in a reduced amount of ejected blood.
Damage to the valves, such as stenosis(狹窄), also presents higher resistance and leads to lower blood output.
該視頻為醫學英語札記整理髮布，因平時工作較忙，稍微有點倉促，如有不當之處，歡迎指正，共同進步