Human Body Part 4

ENDOCRINE SYSTEM

Hormones are chemical messengers that influence virtually every body cell and function. Often working in concert with each other, as well as with the nervous system, hormones control growth, metabolism, digestion, blood pressure, reproduction and response to stress, among many other functions. Understandably, hormonal imbalances can have profound effects throughout the body.

Although scientists know that hormones are key to almost every body process, much remains to be learned about how they work. For example, we still do not understand how the thymus and pineal glands and their hormones work. And from time to time, yet another hormone is discovered.

In addition to being produced by various endocrine glands, hormones are secreted by other organs, including the lungs, intestines, heart and kidneys. Regardless of their origin, however, they all travel through the bloodstream in very small amounts, seeking out target organs or cells, which they then stimulate to perform a particular function. Some hormones, such as insulin are too large to actually enter a cell; instead, they attach themselves to a preprogrammed receptor that triggers the desired response. Other hormones, such as the steroids produced by the adrenal glands, are small enough to penetrate target cells and elicit the desired response from its genetic material.

REPRODUCTIVE SYSTEM

Reproduction remains one of life’s most profound wonders. Just the notion that two barely visible cells can merge and form a new human being in just nine months is nothing short of a miracle. Of course, many things can go awry along the way, but most babies are healthy at birth with all the organs needed to grow into a normal adult.

Sex hormones – principally testosterone in men and estrogen in women – directly control reproduction. But many factors influence both the male and female reproductive system including overall health, nutrition and stress. Genetics are also instrumental. Both the mother and father contribute half of the genes needed to make a new human being and it is this genetic material that determines many of the off-spring’s characteristics, such as eye and hair color, height, body build and blood type.

Sex is also determined at the moment of conception and depends upon which sex chromosome is donated by the father. Female cells have two X chromosomes; thus, when an egg divides, it must have an X chromosome. In contrast, males have an X and a Y chromosome and a sperm can carry either one. So if an egg is fertilized by an X sperm, the baby will be a girl with two female X chromosomes; if the father contributes a Y sperm, the offspring will be a boy with the characteristic male XY chromosomes.

THE IMMUNE SYSTEM

The human body is constantly bombarbed by millions of viruses, bacteria and other disease – causing microorganisms, or pathogens. Fortunately, most of these are thwarted by the body’s own protective physical and chemical barriers, such as the skin, saliva, tears, mucus and stomach acid. The millions of bacteria that live on the skin and the body’s mucus membranes also help protect against certain invaders. When a pathogen does manage to evade these defences and enter the body, it is attacked almost immediately by one or more components of the immune system.

The immune system uses extremely sensitive chemicals sensors to recognise a foreign organism or tissue, especially one that can cause disease. Sometimes it over reacts to a harmless substance, such as pollen or a certain food or medication; this can set the stage for allergic reaction. In other cases, the immune system mistakenly attacks normal body tissue as if it were foreign, resulting in an autoimmune disease such as lupus or rheumatoid arthritis. Most of the time, however, the immune system holds fast as our first line of defense against a host of potentially deadly diseases.

End of Human Body Part 4/4.