医学专英课本chapter 2 Body structure.doc

Chapter 2 Body Structure Organization of the Body Before taking a look at the whole human body, we need to examine its parts. The human organism consists of cells, tissues, organs, and systems. These components are arranged in a hierarchical（分层的，分体系的）manner. That is, cells come together to form tissues, tissues come together to form organs, organs come together to form systems, and all the systems come together to form the whole organism. Cells An adult human body consists of about seventy-five trillion cells. These cells have much in common, yet those in different tissues vary in a number of ways. Cells vary considerably（相当地）in size. We measure cell sizes in units called micrometers. A micrometer equals one thousandth of a millimeter and is symbolized μm. A human egg cell is about 140μm in diameter and is just barely visible to an unaided eye. This is large when compared to a red blood cell, which is about 7.5μm in diameter, or the most common white blood cells, which vary from 10-12μm in diameter. On the other hand, smooth muscle cells can be between 20 and 500μm long. Cells also vary in shape, and typically their shapes are closely related to their functions. For instance（例如）, nerve cells often have long, threadlike extensions that transmit nerve impulse from one part of the body to another. Epithelial cells that line the inside of the mouth are thin, flattened, and tightly packed, somewhat like floor tiles. Muscle cells, which contract to pull structures closer together, are slender（细长的） and rod-like, with their ends attached to the parts they move. They are filled with contractile proteins. An adipose cell is little more than a blob of fat. A B lymphocyte is an antibody factory. [fig.2.1] Fig.2.1 Nerve cell, muscle cell，red blood cells and sperm cell —the nucleus and the cytoplasm. The nucleus is innermost and is enclosed by a thin membrane called the nuclear envelope. The cytoplasm is a mass of fluid that surrounds the nucleus and is itself encircled by