THE CIRCULATORY SYSTEM

The Heart
The heart is a hollow organ, with four chambers, made up of muscles that continuously contract and relax. It is located beneath the breastbone in the chest, mostly on the left side, although a small portion extends over to the right side of the chest. The heart pumps the blood through the body. Without it, we could not live.

The Appendix
The appendix is a small extension of the large intestines, connected to the cecum and located in the lower right part of the abdomen. It is shout the length of your middle finger but only about as thick as a pencil. The appendix has no real function, but it kicks up quite a bit of trouble every once in a while by becoming inflamed or in¬fected. When that happens, it is usually removed.

The Adrenal Glands
The adrenal glands lie on top of the kidneys in the upper portion of the abdomen. They are shaped like triangles, and are about the size of ordinary cookies. Even though the adrenals are small, they are absolutely necessary to life. They supply important hormones, such as adrenalin and cortisone, and they control what happens to various essential substances, including salt and other chemicals in the blood.

The Kidneys
The kidneys are bean-shaped organs about the size of a fist. There are two of them, one on each side, under the ribs in the back part of the abdomen. The kidneys get rid of waste materials from the blood, which is then excreted in the form of urine. The kidneys also make sure that the blood retains those substances that the body needs to function normally.

The Ureters
The ureters are two long tubes connecting the kidneys with the bladder in the lower part of the abdomen. Their purpose is to carry to the bladder the urine that has been produced by the kidneys.

The Bladder
The urinary bladder is a hollow sac located in the lowermost part of the abdomen. The two ureters empty into it from above, and the urethra empties it from below. When the bladder is empty, it is much like a balloon without any air in it. When it is filled with urine, it is large and firm, like a blownup balloon. The purpose of the bladder is to store urine until we are ready to urinate.

The Urethra
The urethra is a tube whose function is to carry urine from the bladder to the outside. In the male, the urethra is located within the penis; in the female, it is located just above the opening of the vagina.

The Male Reproductive Organs
The Penis
The penis has two main purposes: to carry urine from the bladder to the outside, and to carry the sperm to the vagina during intercourse.

The Testicles
The two testicles, one on each side, are located in the scrotal sac just beneath the penis. The testicles manufacture the sperm that unite with the female egg to form a new human being. The testicles also manufacture the male sex hormone, which is supplied directly into the bloodstream. When a male reaches twelve to fourteen years of age, this hormone is responsible for his gradually turning from a boy into a man.

The Prostate Glend
This gland is located around the urethra at the bottom of the bladder. It supplies the fluid in which the sperm are carried.

The Seminal Vesicles
There are two seminal vesicles, one on each side, just above the prostate gland. They supply the fluid (semen) in which the sperm are carried.

The Female Reproductive Organs
The Uterus
The uterus is a pear-shaped organ made up of a cavity lined with membrane and surrounded by a thick muscle wall. It is located deep in the lower part of the abdomen in front of the rectum and just behind the urinary bladder. The uterus connects with the vagina below it and the two Fallopian tubes above it. The uterus is also called the womb. It is the organ within which the unborn child develops.

The Fallopian Tubes
The two Fallopian tubes extend for three to four inches from the top of the uterus, one on each side. Their purpose is to carry to the uterus the eggs that come from the ovaries. If pregnancy is going to take place, the sperm meet and fertilize the egg while it is in a Fallopian tube.

The Ovaries
There are two ovaries, one on each side of the uterus, next to the Fallopian tubes. Once a month, in a grown girl or woman, one of the ovaries produces a tiny egg-about the size of a pinpoint-that leaves the ovary and enters one of the Fallopian tubes. If the egg meets sperm in the Fallopian tube, it may become fertilized and form an embryo. If the egg is not fertilized, it will be expelled naturally. The ovaries also manufacture the female sex hormone, which is supplied directly into the bloodstream. When a female reaches eleven to thirteen years of age, this hormone is responsible for her gradually changing from a girl into a wuman.

The Vagina
The vagina is a membrane-lined canal, whose opening is located between the anus and the urethra. It extends from the outside up to the entrance to the uterus. The penis is placed within the vagina during intercourse, and the sperm that come from the penis are deposited in the vagina. During childbirth, the baby leaves the uterus and comes out through the vagina.

The Skin
The skin covers and protects the entire body. It also helps to control how much fluid we have in our bodies. On hot days our pores open, and we lose a lot of water through perspiration. On cold days our pores remain closed, and we hold more water within our bodies. Certain waste materials are also gotten rid of through the skin in perspiration.

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