细胞构成组织，组织构成器官，器官又进一步构成系统。人 体的每个系统都有其特定功能，但是，请于其他系统而存在， 是整体系统共同作用保持了机体内部的稳定状态，即体内平衡。
Cells are organized into tissues, and tissues are arranged into organs, which in turn are grouped into systems. Each body system serves its specific functions. Bear in mind however that the body functions as a whole---no system is independent of the others. They work together to maintain the body’s state of internal stability, termed homeostasis.
practice
It is estimated that there are nearly 50 million hypertensives in the USA (systolic BP >= 140 mmHg and/or diastolic >= 90 mmHg, or taking antihypertensive medication). For unknown reasons, the prevalence of hypertension seems to be decreasing in the USA. Hypertension occurs more often in black adults (32%) than in white (23%) or Mexican American (23%) adults, and morbidity and mortality are greater in blacks.