Info

Structure

Major changes that occur in the newborn's circulatory system.

Adjustment

In the Adult

Constricts Constricts

Becomes ligamentum teres that extends from the umbilicus to the liver Becomes ligamentum venosum that is superficially embedded in the wall of the liver Valve fuses along margin of foramen ovale and is marked by a depression called the fossa ovalis

Adjustment

In the Adult

Umbilical vein Ductus venosus Foramen ovale

Ductus arteriosus Umbilical arteries

Constricts Constricts

Closes by valvelike septum primum as blood pressure in right atrium decreases and pressure in left atrium increases Constricts

Distal portions constrict

Becomes ligamentum teres that extends from the umbilicus to the liver Becomes ligamentum venosum that is superficially embedded in the wall of the liver Valve fuses along margin of foramen ovale and is marked by a depression called the fossa ovalis

Becomes ligamentum arteriosum that extends from the pulmonary trunk to the aorta Distal portions become lateral umbilical ligaments;

proximal portions function as superior vesical arteries

On the other hand, females attain full growth at earlier ages, and in late adolescence, the average male is taller and stronger than the average female.

The periods of rapid growth in adolescence, which usually begin between the ages of eleven and thirteen in females and between thirteen and fifteen in males, cause increased demands for certain nutrients. It is not uncommon for a teenager to consume a huge plate of food, go back for more—and still remain thin. In addition to energy sources, foods must provide ample amounts of proteins, vitamins, and minerals to support growth of new tissues. Adolescence also brings increasing levels of motor skills, intellectual ability, and emotional maturity.

Adulthood

Adulthood (maturity) extends from adolescence to old age. As we age, we become gradually aware of certain declining functions—yet other abilities remain adequate throughout life. The "Life-Span Changes" sections in various previous chapters have described the effects of aging on particular organ systems. It is interesting to note the varying ages at which particular structures or functions peak.

By age eighteen, the human male is producing the highest level of the sex hormone testosterone that he will ever have in his lifetime, and sex drive is strong. In the twenties, muscle strength peaks in both sexes. Hair is its fullest, with each hair as thick as it will ever be. By the end of the third decade of life, obvious signs of aging may first appear as a loss in the elasticity of facial skin, producing small wrinkles around the mouth and eyes. Height is already starting to decrease, but not yet at a detectable level.

The age of thirty seems to be a developmental turning point. After this, hearing often becomes less acute. Heart muscle begins to thicken. The elasticity of the ligaments between the small bones in the back lessens, setting the stage for the slumping posture that becomes apparent in later years. Some researchers estimate that beginning roughly at age thirty, the human body becomes functionally less efficient by about 0.8% every year.

During their forties, many people weigh 10 to 20 pounds (4.5 to 9 kilograms) more than they did at the age of twenty, thanks to a slowing of metabolism and decrease in activity level. They may be 1/8 inch (0.3 centimeter) shorter, too. Hair may be graying as melanin-producing biochemical pathways lose efficiency, and some hair may fall out. Vision may become farsighted or nearsighted. The immune system is less efficient, making the body more prone to infection and cancer. Skeletal muscles tend to lose strength as more and more connective tissue appears within the muscles; the cardiovascular system is strained as the lumens of ar-terioles and arteries narrow due to accumulations of fatty deposits; skin loosens and wrinkles as elastic fibers in the dermis break down.

The early fifties bring further declines in the functioning of the human body. Nail growth slows, taste buds die, and the skin continues to lose its elasticity. For most people, the ability to see close objects becomes impaired, but for the nearsighted, vision improves. Women stop menstruating, although this does not necessarily mean an end to or loss of interest in sex. Delayed or reduced insulin release by the pancreas, in response to a glucose load, may lead to diabetes. By the decade's end, muscle mass and weight begin to decrease. A male produces less semen but is still sexually active. His voice may become higher as his vocal cords degenerate. A man has half the strength in his upper limb muscles and half the lung function as he did at age twenty-five. He is about 3/4 inch (2 centimeters) shorter.

The sixty-year-old may experience minor memory losses. A few million of the person's billions of brain cells have been lost over his or her lifetime, but for the most part, intellect remains quite sharp. By age seventy, height decreases a full inch (2.5 centimeters). Sagging skin and loss of connective tissue, combined with continued growth of cartilage, make the nose, ears, and eyes more prominent. Figure 23.25 outlines some of the anatomical and physiological changes that accompany aging.

Senescence

Senescence (se-nes'ens) is the process of growing old. It is a continuation of the degenerative changes that begin during adulthood. As a result, the body becomes less able to cope with the demands placed on it by the individual and by the environment.

Senescence is a result of the normal wear-and-tear of body parts over many years. For example, the cartilage covering the ends of bones at joints may wear away, leaving the joints stiff and painful. Other degenerative changes are caused by disease processes that interfere with vital functions, such as gas exchanges or blood circulation. Metabolic rate and distribution of body fluids may change. The rate of division of certain cell types declines, and immune responses weaken. As a result, the person becomes less able to repair damaged tissue and more susceptible to disease.

Decreasing efficiency of the central nervous system accompanies senescence. The person may lose some intellectual functions. Also, the physiological coordinating capacity of the nervous system may decrease, and homeo-static mechanisms may fail to operate effectively. Sensory functions decline with age also.

Death usually results, not from these degenerative changes, but from mechanical disturbances in the cardiovascular system, failure of the immune system, or disease processes that affect vital organs. However, the loss of function that often precedes death in the elderly is associated with inactivity, poor nutrition, and chronic disease. Table 23.4 summarizes the major phases of postnatal life and their characteristics, and table 23.5 lists some aging-related changes.

Decrease in height (about one inch); further decline in sense of taste; nose, ears, and eyes appear prominent as skin sags; facial fat decreases, but cartilage grows

Decrease in height (about 3/4 inch); decrease in lung capacity

Skin sags; decline in visual acuity; menopause; decline in sense of taste; decline in nail growth; increased risk of diabetes; declines in muscle mass, weight, metabolism, and memory

Back slumps; increase in weight; decrease in height; hair grays and thins; declining number of white blood cells; farsightedness

Peak hair thickness; skin less elastic; declines in hearing and height; peak of female sexuality; heart muscle thickens

Essentials of Human Physiology

Essentials of Human Physiology

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.

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