Rather than being scattered throughout the body like the nephridia of earthworms, the excretory tubules of vertebrates, which are called nephrons, are collected into compact organs, the kidneys. Blood is cycled through the kidneys, which remove NITROGENOUS WASTES . and function in osmoregulation by adjusting the concentrations of various salts in the blood. The kidneys, the blood vessels that serve them, and the plumbing that carries urine formed in the kidneys out of the body are the components of the vertebrate excretory system. We shall focus first on the mammalian version of the system, and then compare the excretory systems of the various vertebrate classes.
Anatomy of the Excretory System
In humans, the kidneys are a pair of bean-shaped organs about 10 cm long (Figure 40.10). Blood enters the kidney via the renal artery, and leaves the kidney in the renal vein. Although the kidneys account for less than 1% of the weight of the human body, they receive about 20% of the blood pumped with each heartbeat. Urine, the waste fluid formed within the kidney, exits the organ through a duct called the ureter.
The ureters of both kidneys drain into a common urinary bladder. The bladder is periodically emptied by micturition (urination); this final excretion of urine from the body is through a tube called the urethra, which empties near the vagina of females or through the penis of males. Sphincter muscles near the junction of the urethra and the bladder control micturition.