Chapter 20 - The Endocrine System and Reproduction

Section 1 - The Endocrine System


  The endocrine system produces chemicals that control many of the body's daily activities. The endocrine system also regulates long-term changes, such as growth and development. The endocrine system is made up of glands. Endocrine glands produce and release their chemical products directly into the bloodstream. The blood then carries those chemicals throughout the entire body.

  The chemical product of an endocrine gland is called a hormone. Hormones turn on, turn off, speed up, or slow down the activities of different organs and tissues. Because hormones are carried by blood, they can regulate activities in tissues and organs far from the glands that produced them. Hormones are released when nerve impulses travel to the brain and are interpreted. The brain then sends a nerve impulse to a specific endocrine gland. The gland then releases the hormone into the bloodstream.

  A hormone interacts only with certain target cells, cells that recognize the hormone's chemical structure. A hormone and its target cell fit together the way a key fits into a lock. The endocrine glands include the hypothalamus, pituitary, thyroid, parathyroid, adrenal, thymus, and pancreas. They also include the ovaries in females and testes in males.

  The nervous system and the endocrine system work together. The hypothalamus, a tiny part of the brain near the middle of your head, is the link between the two systems. Nerve messages controlling sleep, hunger, and other conditions come from the hypothalamus. The hypothalamus plays a major role in maintaining homeostasis.

  The pituitary gland is located just below the hypothalamus. The pituitary gland communicates with the hypothalamus to control many body activities. Nerve impulses or hormone signals from the hypothalamus cause the pituitary gland to release hormones. Some hormones from the pituitary gland turn on other endocrine glands. Other pituitary hormones control body activities directly. Pituitary hormones regulate growth from infancy to adulthood and the amount of water in the blood.

  The type of signal used in a heating system is called negative feedback because the system is turned off by the condition it produces. For example, suppose you set a thermostat at 20°C. If the temperature falls below 20°C, the thermostat signals the furnace to turn on. Once the furnace heats the room to the proper temperature, the thermostat sends a negative signal to the furnace that means "no more heat.” Many hormones are controlled by a negative feedback system. Through negative feedback, when the amount of a particular hormone in the blood reaches a certain level, the endocrine system sends signals that stop the release of that hormone. Negative feedback is an important way that the body maintains homeostasis.




Section 2 - The Male and Female Reproductive Systems


  An egg is the female sex cell. A sperm is the male sex cell. The joining of a sperm and an egg is called fertilization. Fertilization is an important part of sexual reproduction, the process by which living things produce new individuals of the same type. Sexual reproduction involves the production of eggs by the female and sperm by the male. The egg and sperm join together during fertilization. When fertilization occurs, a fertilized egg, called a zygote, is produced. Chromosomes are structures in the cells that carry the information that controls inherited characteristics.

  The male reproductive system is specialized to produce sperm and the hormone testosterone. Its structures include the testes, scrotum, and penis. The testes are the organs in which sperm are produced. The testes also produce the hormone testosterone, which controls the development of male physical characteristics. The testes are located in an external pouch called the scrotum.

  Sperm mix with fluids. This mix of sperm cells and fluids is called semen. Semen leaves the body through an organ called the penis. The male urethra runs through the penis. Semen and urine leave the body through the urethra.

  The role of the female reproductive system is to produce eggs and, if an egg is fertilized, to nourish a developing baby until birth. The organs of the female reproductive system include the ovaries, fallopian tubes, uterus, and vagina. The ovaries are located slightly below the waist, one on each side of the body. The ovaries produce both egg cells and hormones. One hormone, estrogen, triggers the development of some adult female characteristics. Each ovary is located near a fallopian tube. The two fallopian tubes are passageways for eggs. Fertilization usually occurs in the fallopian tubes. Each month, one of the ovaries releases an egg. The egg moves through a fallopian tube, which leads to the uterus. The uterus, or womb, is a hollow muscular organ. If the egg has been fertilized, it remains in the uterus and begins to develop. The vagina is a muscular passageway leading to the outside of the body.

  The menstrual cycle is a monthly cycle of changes that occurs in the female reproductive system. During the menstrual cycle, an egg develops in an ovary. At the same time, the uterus prepares for the arrival of an embryo. At the start of the menstrual cycle, an egg starts to mature in one of the ovaries, and the lining of the uterus begins to thicken. About halfway through the cycle, the mature egg is released from the ovary into a fallopian tube. This process is called ovulation. If it is not fertilized, the egg and the lining of the uterus break down. The extra blood and tissue of the thickened lining pass out of the body through the vagina. This process is called menstruation.




Section 3 - The Human Life Cycle


  A fertilized egg is called a zygote. After fertilization, the zygote begins to divide. The zygote develops first into an embryo and then into a fetus.The developing human is called an embryo from the two-cell stage through the eighth week of development. After the ninth week of development until birth, the embryo is called a fetus.

  Soon after the embryo attaches to the uterus, membranes form. The membranes and other structures that form during development protect and nourish the developing embryo and then the fetus. One membrane surrounds the embryo and develops into a fluid-filled sac called the amniotic sac. The fluid cushions and protects the developing baby. Another membrane that forms is the placenta. The placenta links the developing embryo and the mother. A ropelike structure called the umbilical cord connects the embryo and the placenta.

  After about nine months, the baby is born. The birth of a baby takes place in three stages-labor, delivery, and afterbirth. During the first stage of birth, called labor, strong muscular contractions of the uterus cause the cervix to enlarge, eventually allowing the baby to fit through the opening. During delivery, the second stage, the baby is pushed completely out of the uterus, through the vagina, and out of the mother's body. In the third stage, called afterbirth, contractions push the placenta and other membranes out of the uterus through the vagina.

  The delivery of more than one baby from a single pregnancy is called a multiple birth. There are two types of twins. Identical twins develop from a single fertilized egg. Fraternal twins develop when two eggs are released from the ovary and are fertilized by two different sperm.

  The changes that take place between infancy and adulthood include physical changes, such as an increase in size and coordination. They also include mental changes, such as the ability to communicate and solve complex problems. Infancy is the first two years of life. During infancy, a baby's shape and size change greatly. Its nervous and muscular systems become better coordinated, and the baby develops new physical skills. Childhood is the life stage from about two to 12 years of age. Children gradually become more active and independent, and they experience many physical and mental changes. Starting at about the age of 12, children gradually begin to change from a child to an adult, which is called adolescence. Sometime between the ages of about 9 and 15 years, a child enters puberty, which is the period of sexual development in which the body becomes able to reproduce. After about the age of 30, a process known as aging begins.




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