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Chapter 23: Human Growth and Development
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Introduction (page 941)
Growth refers to an increase in size; development is the process of changing from one phase of life to another.
Prenatal Period (page 942)
1. Period of cleavage a. Fertilization occurs in a uterine tube and results in a zyg°te.
b. The zygote undergoes mitosis, and the newly formed cells divide mitotically too.
c. Each subsequent division produces smaller and smaller cells.
d. A solid ball of cells (morula) forms, and it becomes a hollow ball called a blastocyst.
e. The inner cell mass that gives rise to the embryo proper forms within the blastocyst.
f. The blastocyst implants in the uterine wall.
(1) Enzymes digest the endometrium around the blastocyst.
(2) Fingerlike processes from the blastocyst penetrate into the endometrium.
g. The period of cleavage lasts through the first week of development.
h. The trophoblast secretes hCG, which helps maintain the corpus luteum, helps protect the blastocyst against being rejected, and stimulates the developing placenta to secrete hormones.
2. Embryonic stage a. The embryonic stage extends from the second through the eighth weeks.
b. It is characterized by the development of the placenta and the main internal and external body structures.
c. The embryonic disk becomes cylindrical and is attached to the developing placenta by the connecting stalk.
d. Chorionic villi develop and are surrounded by spaces filled with maternal blood.
e. The cells of the inner cell mass fold inward, forming a gastrula that has two and then three primary germ layers.
(1) Ectoderm gives rise to the nervous system, portions of the skin, the lining of the mouth, and the lining of the anal canal.
(2) Mesoderm gives rise to muscles, bones, blood vessels, lymphatic vessels, reproductive organs, kidneys, and linings of body cavities.
(3) Endoderm gives rise to linings of the digestive tract, respiratory tract, urinary bladder, and urethra.
f. The embryo develops head, face, upper limbs, lower limbs, and mouth, and appears more humanlike.
g. The placental membrane consists of the epithelium of the villi and the epithelium of the capillaries inside the villi.
(1) Oxygen and nutrients diffuse from the maternal blood through the placental membrane and into the fetal blood.
(2) Carbon dioxide and other wastes diffuse from the fetal blood through the placental membrane and into the maternal blood.
h. The placenta develops in the disk-shaped area where the chorion contacts the uterine wall.
(1) The embryonic portion consists of the chorion and its villi.
(2) The maternal portion consists of the uterine wall and attached villi.
i. A fluid-filled amnion develops around the embryo.
j. The umbilical cord is formed as the amnion envelops the tissues attached to the underside of the embryo.
(1) The umbilical cord includes two arteries and a vein.
(2) It suspends the embryo in the amniotic cavity. k. The chorion and amnion fuse.
l. The yolk sac forms on the underside of the embryonic disk.
(1) It gives rise to blood cells and cells that later form sex cells.
(2) It helps form the digestive tube.
m. The allantois extends from the yolk sac into the connecting stalk.
(1) It forms blood cells.
(2) It gives rise to the umbilical vessels.
n. By the beginning of the eighth week, the embryo is recognizable as a human.
3. Fetal stage a. This stage extends from the end of the eighth week and continues until birth.
b. Existing structures grow and mature; only a few new parts appear.
c. The body enlarges, upper and lower limbs reach final relative proportions, the skin is covered with sebum and dead epidermal cells, the skeleton continues to ossify, muscles contract, and fat is deposited in subcutaneous tissue.
d. The fetus is full term at the end of the ninth month, which equals approximately 266 days.
(1) It is about 50 centimeters long and weighs 6-8 pounds.
(2) It is positioned with its head toward the cervix.
4. Fetal blood and circulation a. Umbilical vessels carry blood between the placenta and the fetus.
b. Fetal blood carries a greater concentration of oxygen than does maternal blood.
c. Blood enters the fetus through the umbilical vein and partially bypasses the liver by means of the ductus venosus.
d. Blood enters the right atrium and partially bypasses the lungs by means of the foramen ovale.
e. Blood entering the pulmonary trunk partially bypasses the lungs by means of the ductus arteriosus.
f. Blood enters the umbilical arteries from the internal iliac arteries.
Postnatal Period (page 961)
1. Neonatal period a. This period extends from birth to the end of the fourth week.
b. The newborn must begin to respire, obtain nutrients, excrete wastes, and regulate its body temperature.
c. The first breath must be powerful in order to expand the lungs.
(1) Surfactant reduces surface tension.
(2) A variety of factors stimulate the first breath.
d. The liver is immature and unable to supply sufficient glucose, so the newborn depends primarily on stored fat for energy.
e. Immature kidneys cannot concentrate urine very well.
(1) The newborn may become dehydrated.
(2) Water and electrolyte imbalances may develop.
f. Homeostatic mechanisms may function imperfectly, and body temperature may be unstable.
g. The circulatory system changes when placental circulation ceases.
(1) Umbilical vessels constrict.
(2) The ductus venosus constricts.
(3) The foramen ovale is closed by a valve as blood pressure in the right atrium falls and pressure in the left atrium rises.
(4) The ductus arteriosus constricts.
2. Infancy a. Infancy extends from the end of the fourth week to one year of age.
b. Infancy is a period of rapid growth.
(1) The muscular and nervous systems mature, and coordinated activities become possible.
(2) Communication begins.
c. Rapid growth depends on an adequate intake of proteins, vitamins, and minerals in addition to energy sources.
3. Childhood a. Childhood extends from the end of the first year to puberty.
b. It is characterized by rapid growth, development of muscular control, and establishment of bladder and bowel control.
4. Adolescence a. Adolescence extends from puberty to adulthood.
b. It is characterized by physiological and anatomical changes that result in a reproductively functional individual.
c. Females may be taller and stronger than males in early adolescence, but the situation reverses in late adolescence.
d. Adolescents develop high levels of motor skills, their intellectual abilities increase, and they continue to mature emotionally.
5. Adulthood a. Adulthood extends from adolescence to old age.
b. The adult remains relatively unchanged physiologically and anatomically for many years.
c. After age thirty, degenerative changes usually begin to occur.
(1) Skeletal muscles lose strength.
(2) The circulatory system becomes less efficient.
(3) The skin loses its elasticity.
(4) The capacity to produce sex cells declines.
6. Senescence a. Senescence is the process of growing old.
b. Degenerative changes continue, and the body becomes less able to cope with demands placed upon it.
c. Changes occur because of prolonged use, effects of disease, and cellular alterations.
d. An aging person usually experiences losses in intellectual functions, sensory functions, and physiological coordinating capacities.
e. Death usually results from mechanical disturbances in the cardiovascular system or from disease processes that affect vital organs.
Aging (page 970)
1. Passive aging a. Passive aging entails breakdown of structures and slowing or failure of functions.
b. Connective tissue breaks down.
c. DNA errors accumulate.
d. Lipid breakdown in aging membranes releases lipofuscin.
e. Free radical damage escalates.
2. Active aging a. In autoimmunity, the immune system attacks the body.
b. Apoptosis is a form of programmed cell death. It occurs throughout life, shaping organs.
3. The human life span a. The theoretical maximum life span is 120 years.
b. Life expectancy, based on real populations, is 76.5 years in the U.S., and may be quite lower in poorer nations and those ravaged by AIDS.
c. Medical technology makes life expectancy more closely approach life span.
Critical Thinking Questions
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.