it eases the load of concentration placed on the kidneys.
In blood plasma there is normally 0.03 percent of urea, while in the urine there is normally 67 times as much, or about 2 percent. This great increase is caused by the concentration of urea contained in a large kidney area in a relatively small quantity of urine.
Beside removing waste products normally found in the body, the kidneys also remove toxic substances, such as certain barbituric acid derivatives, mercury, alcohol, and other drugs.
One of the familiar diseases associated with the kidneys is glomerulonephritis, which is caused by protein loss from the body due to damaged glomeruli. Another disease, though not as prevalent, is uremia. This is caused by failure of the kidneys to remove the waste products from the blood, which then accumulate in high concentrations. This condition is serious and sometimes fatal.
The ureters are two membranous tubes 1 mm to 1 cm in diameter and about 28 to 34 cm in length. Their only function is to carry urine from each kidney to the urinary bladder.
The urinary bladder is a musculomembranous sac located in the pelvic girdle. It functions as a reservoir for urine until it empties through the urethra.
The urethra is the tube that carries the urine from the bladder to the exterior. The urinary meatus is the external urethral opening. In the male the urethra is common to the urinary and reproductive systems; in the female it belongs only to the urinary system.
The female urethra is about 4 cm long, extending from the bladder to the external orifice in the vestibule. It is embedded in the anterior wall of the vagina and surrounded by the sphincter urethrae.
The male urethra is about 20 cm long and is divided into three parts: the prostatic, membranous, and penile portions. The prostatic urethra is surrounded by the prostate gland; it contains the orifices of the prostatic and ejaculatory ducts. This portion of the male urethra is about 2.5 cm long. The membranous urethra is about 2 cm in length and is surrounded by the external sphincter. The penile urethra, the longest portion, is about 15 cm long. It lies in the ventral portion of the penis, extending to its external opening.
Figure 3-55 .The male reproductive system.
The male organs of reproduction are the penis and testes (testicles), and associated ducts and glands (fig. 3-55).
The SCROTUM is a cutaneous pouch containing the testes and part of the spermatic cord. Immediately beneath the skin is a thin layer of muscular fibers (cremaster), which is controlled by temperature and contracts or relaxes to lower or raise the testes in relation to the body. This muscular activity of the scrotum is necessary to regulate the temperature of the testes, which is important in the maturation of sperm cells.
The TESTES are oval glands suspended by the spermatic cord in a pouch. They perform two functions: production of spermatozoa (sperm)