Birth, or Parturition Transition between prenatal and postnatal periods of life Stages of labor (Figure 33-18) –Stage one Period from onset of uterine contractions until cervical dilation is complete 6 to 24 hours –Stage two Period from maximal cervical dilation until the baby exits through the vagina Few minutes to an hour –Stage three Process of expulsion of the placenta through the vagina 15 minutes after birth Cesarean section –Baby is delivered through an incision in the abdomen and uterine wall

Birth, or Parturition Multiple births –Identical twins result from the splitting of embryonic tissue from the same zygote early in development Inner cell mass of blastocyst may divide into two masses Usually share the trophoblast and placenta Genetically identical, environmental factors and personal experiences lead to individuality –Fraternal twins result from the fertilization of two different ova by two different spermatozoa Production of multiple ova is often inherited Also influenced by fertility drugs

Postnatal Period Begins at birth and continues until death; commonly divided into a number of periods (Figure 33-20) Infancy begins at birth and lasts until approximately 18 months –Neonatal period First 4 weeks of infancy Dramatic changes occur at a rapid rate –Changes allow the infant to become totally self-supporting –Weight doubles in first 4 months, triples in 1 year –Length increases by 50% in first year –One spinal curvature in early infancy –Lumbar curvature appears after 1 year

Postnatal Period 2 months – Follow moving object with the eyes 3 months – Lift the head and raise the chest 4 months – Sit when well supported 10 months – Crawl 12 months – Stand alone 18 months – Run

Postnatal Period Childhood - Infancy to sexual maturity –Girls age 12-14, boys age –Early childhood—growth continues at a rapid pace but slows down –By age 6, child looks more like a preadolescent than an infant or toddler –Nervous and muscular systems develop rapidly during middle years of childhood –Deciduous teeth are lost during childhood, beginning at approximately 6 years of age –Permanent teeth have erupted by age 14, except for third molars (wisdom teeth)

Postnatal Period Adolescence and adulthood –Adolescence is considered to be the teenage years –Rapid and intense physical growth, resulting in sexual maturity Puberty –Stage of adolescence during which a person becomes sexually mature Changes triggered by increases in reproductive hormones Primary sexual characteristics –Maturity of gonads and reproductive tract Secondary sexual characteristics –Fat and hair distribution, skeletal changes, etc. (Figure 33-26) –Adulthood—characterized by maintenance of existing body tissues

Postnatal Period Older adulthood and senescence –As a person grows older, a gradual decline occurs in every major organ system in the body –Gerontologists theorize a number of different aging mechanisms, all of which may be involved in the processes of aging: Limit on cell reproduction Environmental factors Viruses Aging genes Degeneration of mitochondria—perhaps progressive damage by oxygen free-radicals

Effects of Aging Skeletal system –Bones decrease in bone mineral density (BMD ) and thus change in texture, degree of calcification, and shape –Lipping occurs, which can limit range of motion –Decreased bone size and density lead to increased risk of fracture –Decreased BMD can be avoided (at least partly) by exercise and adequate calcium intake

Integumentary system (skin) –Skin becomes dry, thin, and inelastic –Pigmentation changes and thinning hair are common problems associated with aging Effects of Aging

Urinary system –Number of nephron units in the kidney decreases by almost 50% between the ages of 30 and 75 –Decreased blood flow through kidneys reduces overall function and excretory capacity –Diminished muscle tone in bladder results in decreased capacity and inability to empty, or void, completely Effects of Aging

Respiratory system –Costal cartilages become calcified –Respiratory efficiency decreases –Decreased strength of respiratory muscles Effects of Aging

Cardiovascular system –Degenerative heart and blood vessel disease—one of the most common and serious effects of aging –Atherosclerosis—build-up of fatty deposits on blood vessel walls narrows the passageway for blood –Arteriosclerosis—“hardening” of the arteries –Hypertension—high blood pressure Effects of Aging

Special senses –Sense organs—gradual decline in performance and capacity with aging –Presbyopia—far-sightedness due to hardening of lens –Cataract—cloudy lens, which impairs vision –Glaucoma—increased pressure within the eyeball; if left untreated, often results in blindness –Decreased hearing –Decreased taste Effects of Aging

Reproductive systems –Mechanism of sexual response may change –Fertility decreases –In females, menopause occurs between ages of 45 and 60 Effects of Aging

Muscular system –Muscle mass decreases to about 90% by age 50 and around 50% by age 80 –The number of muscle fibers decreases as humans age but can be offset by an increase in muscle fiber size through exercise –Ratio of “fast” to “slow” functioning in muscle fibers decreases, slowing the function of muscle organs Effects of Aging

Causes of Death In developed countries such as the United States, heart disease, cancer, and stroke (CVA) are among the leading causes of death In developing countries, heart disease and stroke are also leading causes of death, along with infectious diseases such as HIV/AIDS, diarrheal disorders, and malaria