Voiding Urine

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Urination, or emptying of the bladder, is also called mic-turition32 (MIC-too-RISH-un). It is controlled in part by the micturition reflex shown in figure 23.21, which is numbered to correspond to the following description:

(1) When the bladder contains about 200 mL of urine, stretch receptors in the wall send afferent nerve impulses to the spinal cord by way of the pelvic nerves. (2) By way of a parasympathetic reflex arc through segments S2 to S3 of the cord, signals return to the bladder and stimulate contraction of the detrusor muscle (3) and relaxation of the internal urethral sphincter (4). This reflex is the predominant mechanism that voids the bladder in infants and young children.

30cyst = bladder + itis = inflammation

31pyel = pelvis

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

906 Part Four Regulation and Maintenance

Parasympathetic fibers of pelvic r

Stretch receptors

Motor fibers to— detrusor muscle

Internal urethral— sphincter (involuntary)

External urethral sphincter (voluntary)

Sacral segments of spinal cord

Parasympathetic fibers of pelvic r

Stretch receptors

Sacral segments of spinal cord

Voiding Reflex Arc

Motor fibers to— detrusor muscle

Internal urethral— sphincter (involuntary)

External urethral sphincter (voluntary)

Somatic motor fiber of pudendal nerve

Figure 23.21 Neural Control of Micturition. Circled numbers correspond to text description.

Somatic motor fiber of pudendal nerve

Figure 23.21 Neural Control of Micturition. Circled numbers correspond to text description.

As the brain and spinal cord mature, however, we acquire voluntary control over the external urethral sphincter, and emptying of the bladder is controlled predominantly by a micturition center in the pons. This center receives signals from the stretch receptors (5) and integrates this information with cortical input concerning the appropriateness of urinating at the moment. It sends back impulses (6) that excite the detrusor and relax the internal urethral sphincter. (7) At times when it is inappropriate to urinate, a steady train of nerve impulses travel from the brainstem through the pudendal nerve to the external urethral sphincter, thus keeping it contracted. When you wish to urinate, these impulses are inhibited, the external sphincter relaxes (8), and contractions of the detrusor muscle expel the urine. The Valsalva maneuver (p. 855) also aids in expulsion of urine by increasing pressure on the bladder. Males voluntarily contract the bulbocavernosus muscle encircling the base of the penis to expel the last few milliliters of urine.

When it is desirable to urinate (for example, before a long trip) but the urge does not yet exist because the blad der is not full enough, the Valsalva maneuver can activate the micturition reflex. Contraction of the abdominal muscles compresses the bladder and may excite the stretch receptors even if there is less than 200 mL of urine in the bladder.

The effects of aging on the urinary system are discussed on pages 1111 to 1112. Some disorders of this system are briefly described in table 23.3.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

21. Describe the location and function of the detrusor muscle.

22. Compare and contrast the functions of the internal and external urethral sphincters.

23. How would micturition be affected by a spinal cord lesion that prevented voluntary nerve impulses from reaching the sacral part of the cord?

Saladin: Anatomy & I 23. The Urinary System I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 23 The Urinary System 907

Table 23.3 Some Disorders of the Urinary System

Acute glomerulonephritis

An autoimmune inflammation of the glomeruli, often following a streptococcus infection. Results in destruction of glomeruli leading to hematuria, proteinuria, edema, reduced glomerular filtration, and hypertension. Can progress to chronic glomerulonephritis and renal failure, but most individuals recover from acute glomerulonephritis without lasting effect.

Acute renal failure

An abrupt decline in renal function, often due to traumatic damage to the nephrons or a loss of blood flow stemming from hemorrhage or thrombosis.

Chronic renal failure

Long-term, progressive, irreversible loss of nephrons; see insight 23.4 for a variety of causes. Requires a kidney transplant or hemodialysis.


Increase in fluid pressure in the renal pelvis and calices owing to obstruction of the ureter by kidney stones, nephroptosis, or other causes. Can progress to complete cessation of glomerular filtration and atrophy of nephrons.

Nephroptosis34 (NEFF-rop-TOE-sis)

Slippage of the kidney to an abnormally low position (floating kidney). Occurs in people with too little body fat to hold the kidney in place and in people who subject the kidneys to prolonged vibration, such as truck drivers, equestrians, and motorcyclists. Can twist or kink the ureter, which causes pain, obstructs urine flow, and potentially leads to hydronephrosis.

Nephrotic syndrome

Excretion of large amounts of protein in the urine (a 3.5 g/day) due to glomerular injury. Can result from trauma, drugs, infections, cancer, diabetes mellitus, lupus erythematosus, and other diseases. Loss of plasma protein leads to edema, ascites, hypotension, and susceptibility to infection (because of immunoglobulin loss).

Urinary incontinence

Inability to hold the urine; involuntary leakage from the bladder. Can result from incompetence of the urinary sphincters; bladder irritation; pressure on the bladder in pregnancy; an obstructed urinary outlet so that the bladder is constantly full and dribbles urine (overflow incontinence); uncontrollable urination due to brief surges in bladder pressure, as in laughing or coughing (stress incontinence); and neurological disorders such as spinal cord injuries.

Disorders described elsewhere

Azotemia 881

Oliguria 901 Renal diabetes 902

Hematuria 887

Proteinuria 887 Uremia 881

Kidney stones 904

Pyuria 900 Urinary tract infection 904

Nephrosclerosis 889

33hydro = water + nephr = kidney + osis = medical condition 34nephro = kidney + ptosis = sagging, falling

33hydro = water + nephr = kidney + osis = medical condition 34nephro = kidney + ptosis = sagging, falling

Insight 23.4 Clinical Application

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  • Diane
    How detrusor muscle work with cerebral innervation?
    8 years ago

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