Urinary System: Anatomy and Function

• Chapter 6 covers the urinary system.

Objectives

• Define the urinary system.
• List the elements constituting the urinary system.
• Explain the structure of the nephron.
• Describe the process of urine formation.
• Illustrate the frontal section showing the structure of a kidney.
• Diagram the nephron.

Definition and composition

• The urinary system consists of anatomical formations that contribute to the secretion and excretion of urine.
• It includes the right and left kidneys, which secrete urine.
• It also consists of intrarenal urinary tracts, and extrarenal urinary tracts.
• Intrarenal structures consist of arched or straight tubules, collecting tubes, minor and major calyces, and the origin of the renal pelvis.
• Extrarenal structures consist of the renal pelvis body, right and left ureters, bladder, and urethra.

External configuration of the kidneys

• The kidney is a bean-shaped, retroperitoneal gland located on either side of the vertebral column between T12 and L3.
• It has two poles, two faces, and two borders.
• The superior pole is capped by the ipsilateral adrenal gland.
• The kidney has an inferior pole.
• The two faces of the kidney, ventrolateral and dorsomedial, are convex.
• The lateral border is arcuate and convex.
• The medial border is notched in its middle part by the renal hilum, a depression where the renal sinus opens and through which the renal pedicle and urinary tract pass.
• The axis of the kidney is oblique downwards, outwards, and backwards.
• The kidneys are approximately symmetrical and weigh 150g, measuring 12 cm long, 6 cm wide, and 3 cm thick.

Kidney structure

• A frontal section of the kidney shows a fibrous capsule and parenchyma from the surface to the renal sinus.
• The renal parenchyma consists of two zones of different colors.
• The cortical or cortex zone is reddish-yellow, made up of peripheral cortical lobules and renal columns that separate the pyramids from each other; it contains renal corpuscles and the initial part of collecting tubules.
• The medullary or medulla zone is dark red and is formed by renal pyramids that open at the renal papilla, containing collecting tubes.
• The renal lobe is formed by a renal pyramid and the surrounding cortical portion.
• The renal corpuscle consists of a renal capsule representing the invaginated tubular pole and a glomerulus, capillary tuft interposed between an afferent and an efferent and representing the arterial pole.

Nephron and kidney functions

• The nephron is the structural and functional unit of the kidney.
• It consists of the renal corpuscle, the proximal convoluted tubule, the loop of the nephron, and the distal convoluted tubule.
• The renal corpuscle consists of a capsule and a glomerulus.
• The proximal convoluted tubule is formed by a convoluted segment extended by a straight segment.
• The loop of the nephron has a descending and an ascending branch.
• The distal convoluted tubule has a straight segment extended by a terminal convoluted segment.
• Each nephron is connected to a collecting tube by an arched or straight tubule.

Urine formation process

• The process of urine formation by the nephrons, with the help of the appropriate hormonal factors, consists of:
• Glomerular filtration where arterial blood circulates from the afferent arteriole to the efferent arteriole.
• Blood serum filters abundantly as it passes through the glomerulus via the capillary wall and the capsular membrane of the renal corpuscle towards the proximal convoluted tubule and forms the primary urine.
• Concentration of primary urine occurs in the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule and reabsorbs water and useful electrolytes for the body while leaving toxic metabolites.
• The formation of definitive urine at the distal convoluted tubule outlet is taken over by the arched or straight tubules, the initial segments of the intrarenal urinary tract.

Intrarenal urinary tract

• Final urine is discharged by the arched or straight tubules into the collecting tubes.
• Each collecting tube runs through a pyramid from base to apex, opening at the papillary foramen.
• Urine leaving the renal papillae is drained by a minor calyx.
• Two or three minor calyces converge to form a major calyx.
• Major calyces release urine into the initial intra-sinus segment of the renal pelvis.

Extrarenal urinary tract

• The extra-sinus renal pelvis is the initial segment of the urinary tract or extrarenal excretory tract, a dilated segment that follows its intra-sinus portion and emerges at the hilum in contact with the elements of the renal pedicle.
• It gradually narrows, curves downwards, and continues with the ureter and has a right and left side.
• The ureter is a paired, contractile musculo-membranous duct that carries urine from the renal pelvis to the bladder.
• It has a descending path and is divided into abdominal and pelvic portions, measuring 24-32 cm long with a diameter of 3-5 mm.

Bladder

• The bladder is a single, median, musculo-membranous reservoir that holds urine between voiding and is located in the pelvic cavity behind the pubic bone, in front of the uterus and vagina in women, and the rectum and seminal vesicles in men.
• The physiological capacity of the bladder is 300 to 350 ml, but can hold 2 to 3 liters of urine if chronically distended.

Urethra

• The urethra is the excretory duct of the bladder and has notable differences between men and women.
• The male urethra has a dual urinary and genital function, starting at the internal urethral opening and ending at the external urethral opening at the top of the glans penis.
• It measures 16 cm long on average and in segments it is prostatic, membranous, and spongy.
• The female urethra is the excretory duct of the bladder in women, very short at 3 to 4 cm long, and extends from the internal urethral opening to the external urethral opening which opens at the top of the vulvar papilla.