Page 155 - 86 human physiology part-2

Chapter 19

19 DEVELOPMENT: BIRTH THROUGH DEATH

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Overview

O ur Birth to Death chapter is an all encompassing review of the physiological changes that occur

throughout a normal life span. In determining what a normal life consists of we included functions
that are likely to happen to a large percentage of the population. While any one person is not likely to
experience all of the events listed in this chapter they will undoubtedly go though some of the
processes.

Apoptosis

Apoptosis is the process of regulated cell death and removal. In some cases cell damage can trigger
apoptosis, but it is usually a normal function of the cell. Apoptosis results in controlled auto digestion
of the cells content. The cell membrane stays in place and the cells contents are not dispersed. When
this process is near completion "eat me" signals, like phosphatidylserine, appear on the surface of the
cell membrane. This in turn attracts phagocytic scavengers that complete the process of removing the
dead cell without eliciting an inflammatory response. Unlike necrosis, which is a form of cell death that
results from acute cellular injury, apoptosis is carried out in an ordered process that generally confers
advantages during an organism's life cycle.

Apoptosis Rates
The rate at which cells die varies widely between different cell types of the body. Some cells,
such as white blood cells, live for only a matter of hours where other cells can live an throughout
the entire lifetime of the host.

Homeostasis
Apoptosis is a regulated function that results in a relatively consistent number of cells in the
body. This balancing act is part of the Homeostasis (see chapter 1) required by living organisms
to maintain their internal states within certain limits. An example of this is that blood cells are
constantly being replaced and apoptosis takes place to eliminate a simular number of older cells.

Development
Apoptosis also plays a key role in growth and development. An example of how apoptosis
enables development is the differentiation of human fingers in a developing embryo. Apoptosis is
the function that enables the embryos fingers to separate.

Disorders
Too much apoptosis causes cell-loss disorders such as osteoporosis, whereas too little apoptosis
results in uncontrolled cell proliferation, namely cancer.

370 | Human Physiology