Laboratory Exercise 6: Anatomy of the Urinary System
The urinary system preserves homeostasis of body fluids. Through the processes of glomerular
filtration, selective tubular reabsorption and tubular secretion, the kidneys clear metabolic waste
product from the blood, balance water, organic solute and salt (inorganic solute) composition and
concentration and maintain normal acid-to-base ratio. Water, salts and N-waste products form urine.
Urine passes from the kidneys through the ureters to the bladder, where it is stored. Urine is excreted
from the body through the urethra.
A. Gross Anatomy of the Urinary System
Kidneys – are red-brown bean shaped organs in retroperitoneal position. They are situated at T12-L2.
This position offers partial protection. Further protection is provided by a pad of adipose tissue and a
tough outer connective tissue covering, the renal capsule.
Deep to the capsule is the outer layer of the kidney, the cortex. The cortex contains the renal tubules
(nephrons) and blood vessels.
The deeper part of the kidney is the medulla. The medulla has triangular masses, the renal pyramids,
which contain straight tubules, the collecting ducts. The tips of the renal pyramids form papillae which
drain urine into the cuplike renal calyces. The renal calyces merge to create a large collecting basin,
the renal pelvis. The renal pelvis narrows into the ureter, which conveys urine to the urinary bladder.
Urine flow out of the bladder is controlled by the internal and external sphincters. The internal
sphincter is smooth muscle that relaxes when the bladder wall contracts. This activity is initiated when
urine volume reaches about 300 mL in the bladder, which triggers the parasympathetic micturition
reflex. The external sphincter is striated muscle; it can be voluntarily relaxed to advance urine through
the urethra. By contracting this muscle, urine outflow can be suppressed.
On the medial side, the kidney is indented. This region, the hilum contains the renal artery, vein and
ureter.
B. Histology of the Urinary System
The structural and functional (physiological) unit of the kidney is the nephron. A nephron contains a
complex convoluted tubule and an attached blood vessel network. The nephron extracts N-waste
product from the blood and adjusts water, salt and organic solute levels as it form the urine. There are
about one million nephrons/kidney.
The nephron begins in the cortex as the renal corpuscle. Each corpuscle has capillary loops, the
glomerulus which is surrounded by the thin-walled glomerular (Bowman’s) capsule. The capsule
continues as the highly convoluted proximal tubule, U-shaped loop of Henle and distal convoluted
tubule. Each nephron merges with a straight collecting duct. The collecting ducts converge to form
the renal pyramids.
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Nephron’s Blood Supply
The glomerulus’s blood supply is from the afferent arteriole. The afferent arteriole is derived from the
interlobular artery, a branch of the renal artery. The glomerulus is drained by an efferent arteriole.
From there the blood enters the peritubular capillary network that surrounds the convoluted tubules.
(A-a-c-a-c-v-V).
Urine formation begins when water and dissolved ions and molecules of the plasma are filtered
through the glomeruli. The convoluted tubules of the nephrons reclaim vital organic and inorganic
solutes and water from the tubular lumen by tubular reabsorption into the peritubular capillary blood.
The convoluted tubules excrete by tubular secretion organic and inorganic solutes from the peritubular
capillary blood into the tubule lumen. Thus the concentration of urine solutes is adjusted to the needs
of the body. Under normal conditions, the concentration of urine at the end of the collecting duct is
hypertonic and slightly acidic (pH5-6) to the plasma entering the glomeruli.
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