Anus, Rectum, and Prostate

Anatomy And Physiology

The rectum and anus form the terminal portions of the gastrointestinal (GI) tract ( Fig. 21.1 ). The anal canal is approximately 2.5 to 4 cm long and opens onto the perineum. The tissue visible at the external margin of the anus is moist, hairless mucosa. Juncture with the perianal skin is characterized by increased pigmentation and, in the adult, the presence of hair.

The anal canal is normally kept securely closed by concentric rings of muscle, the internal and external sphincters. The internal ring of smooth muscle is under involuntary autonomic control. The urge to defecate occurs when the rectum fills with feces, which causes reflexive stimulation that relaxes the internal sphincter. Defecation is controlled by the striated external sphincter, which is under voluntary control. The lower half of the canal is supplied with somatic sensory nerves, making it sensitive to painful stimuli, whereas the upper half is under autonomic control and is relatively insensitive. Therefore, conditions of the lower anus may cause pain, whereas those of the upper anus usually will not.

Internally, the anal canal is lined by columns of mucosal tissue (columns of Morgagni) that fuse to form the anorectal junction. The spaces between the columns are called crypts, into which anal glands empty. Inflammation of the crypts can result in fistula or fissure formation. Anastomosing veins cross the columns, forming a ring called the zona hemorrhoidalis. Internal hemorrhoids result from dilation of these veins. The lower segment of the anal canal contains a venous plexus that drains into the inferior rectal veins. Dilation of this plexus results in external hemorrhoids.

The rectum lies superior to the anus and is approximately 12 cm long. Its proximal end is continuous with the sigmoid colon. The distal end, called the anorectal junc

tion, is visible on proctoscopic examination as a sawtooth-like edge, but it is not palpable. Above the anorectal junction, the rectum dilates and turns posteriorly into the hollow of the coccyx and sacrum, forming the rectal ampulla, which stores flatus and feces. The rectal wall contains three semilunar transverse folds (Houston valves), the lowest of which can be palpated by the examiner.

The prostate gland is located at the base of the bladder and surrounds the urethra. It is composed of muscular and glandular tissue and is approximately 4 × 3 × 2 cm. The posterior surface of the prostate gland is in close contact with the anterior rectal wall and is accessible by digital examination. It is convex and divided by a shallow median sulcus into right and left lateral lobes. A third or median lobe, not palpable on examination, is composed of glandular tissue and lies between the ejaculatory duct and the urethra. It contains active secretory alveoli that contribute to ejaculatory fluid. The seminal vesicles extend outward from the prostate ( Fig. 21.2 ).

The vagina lies in contact with the anterior rectal wall of the rectum and is separated from it by the rectovaginal septum. See Chapter 19 for a more detailed discussion.

Review of Related History

For each of the symptoms or conditions discussed in this section, targeted topics to include in the history of the present illness are listed. Responses to questions about these topics provide clues for focusing the physical examination and the development of an appropriate diagnostic evaluation. Questions regarding medication use (prescription and over-the-counter preparations) as well as complementary and alternative therapies are relevant for each.

History of Present Illness

Past Medical History

• Gender identity: e.g., female, male, non-binary, transgender woman, transgender man; gender assigned at birth

• Organ inventory: prostate gland; penis (natal or surgically constructed), scrotum and testicles, (natal or surgically constructed), prostate gland, clitoris (natal or surgically constructed) vagina (natal or surgically constructed), cervix, uterus, fallopian tubes, ovaries

• Hemorrhoids

• Spinal cord injury

• Bowel habits and characteristics: timing, frequency, number, consistency, shape, color, odor

• Males, transgender women, and non-binary individuals assigned male at birth: prostatic hypertrophy or carcinoma

• Females, transgender men, and non-binary individuals assigned female at birth: episiotomy or fourth-degree laceration during delivery

• Colorectal cancer or related cancers: breast, ovarian, endometrial

• Anal, rectal, prostate surgeries

Family History

• Rectal polyps

• Colon cancer or familial cancer syndromes (see risk Factors box, Chapter 18 )

• Prostate cancer (see Risk Factors box)

Personal and Social History

• Travel history: areas with high incidence of parasitic infestation, including zones in the United States

• Diet: inclusion of fiber foods (cereals, breads, nuts, fruits, vegetables) and concentrated high-fiber foods; amount of animal fat

• Risk factors for colorectal, prostate, or anal cancer (see Risk Factors Box in Chapter 18 )

• Sexual practices at risk for anal HPV infection (see Patient Safety, “Sexually Transmitted Infections” and Box 21.1 )

  BOX 21.1

  Screening for Sexually Transmitted Infections in Special Populations

  From Centers for Disease Control (2015).

  The Centers for Disease Control and Prevention (CDC) has identified populations who are at risk for sexually transmitted infections based on sexual practices or, in the case of fetuses, are at risk for debilitating effects of intrauterine or perinatally transmitted infections. Adolescents, persons in correctional facilities, men who have sex with men, women who have sex with women, and transgender men and women may engage in practices that expose them to a variety of sexually transmitted Diseases. The CDC has specific screening and prevention recommendations for each group which are available at http://www.cdc.gov/std/tg2015/specialpops.htm .

• Vaccination status for the human papilloma virus (HPV)

• Use of alcohol

Examination And Findings