Basic Anatomy: Parts and Structure of the Urinary System

The urinary system consists of four main components working together to filter blood, produce urine, and eliminate waste from the body. The kidneys perform the primary filtration work, the ureters transport urine from kidneys to bladder, the bladder stores urine until elimination, and the urethra provides the exit pathway for urine.

Your kidneys, each about the size of a large bar of soap and weighing approximately 150 grams, are located on either side of your spine just below the rib cage. The right kidney sits slightly lower than the left due to the liver's position. These bean-shaped organs are surrounded by protective fat and fascia, helping cushion them against the movements and impacts of daily life. Despite their modest size, kidneys receive about 20-25% of cardiac output, making them among the most highly vascularized organs in your body.

Each kidney has three distinct regions visible to the naked eye. The outer renal cortex contains most of the nephrons' filtering apparatus and appears granular due to the dense packing of microscopic structures. The inner renal medulla consists of cone-shaped regions called pyramids, containing the collecting ducts and loops of Henle that concentrate urine. The innermost renal pelvis serves as a funnel-shaped collection chamber where urine from all nephrons converges before entering the ureter.

The nephron represents the kidney's functional unit, with each kidney containing approximately 1 million of these microscopic filtering systems. Each nephron consists of a renal corpuscle (where initial filtration occurs) and a renal tubule (where filtered fluid is processed into final urine). The renal corpuscle contains a glomerulus—a cluster of tiny capillaries surrounded by Bowman's capsule, a cup-shaped structure that collects the filtered fluid.

The glomerulus is a remarkable structure where blood pressure forces water and dissolved substances through capillary walls into Bowman's capsule. These capillaries have special properties that allow small molecules like water, glucose, amino acids, and waste products to pass through while retaining larger molecules like proteins and blood cells. The filtration barrier consists of three layers: capillary endothelium with fenestrations (pores), a basement membrane, and podocytes—specialized cells with finger-like projections that create filtration slits.

The renal tubule processes the filtered fluid (filtrate) through several distinct segments. The proximal convoluted tubule reabsorbs about 65% of filtered water and sodium, plus virtually all glucose and amino acids. The loop of Henle consists of descending and ascending limbs that create concentration gradients enabling urine concentration. The distal convoluted tubule provides fine-tuning of electrolyte balance, while the collecting duct makes final adjustments to urine concentration and volume.

Blood supply to the kidneys follows a unique pattern that enables efficient filtration. The renal artery branches into progressively smaller vessels, eventually forming the glomerular capillaries where filtration occurs. Blood leaving the glomerulus enters a second set of capillaries (peritubular capillaries) that surround the tubules, allowing reabsorption of filtered substances back into the bloodstream. This two-capillary system is unusual in the body and essential for kidney function.

The ureters are muscular tubes, each about 25 centimeters long, that transport urine from the renal pelvis to the bladder. These tubes use peristaltic contractions—wave-like muscle movements—to propel urine downward against gravity. The ureters enter the bladder at an angle that creates a one-way valve effect, preventing urine from backing up into the kidneys when the bladder fills or contracts.

The bladder is a hollow, muscular organ that stores urine until voluntary elimination occurs. When empty, the bladder appears collapsed with thick, folded walls. As it fills, the muscular walls (detrusor muscle) stretch while the internal sphincter remains closed to prevent leakage. The bladder can typically hold 400-600 milliliters of urine, though the urge to urinate usually begins around 150-200 milliliters.

The urethra is the final pathway for urine elimination, with significant anatomical differences between males and females. In females, the urethra is about 4 centimeters long and serves only urinary function. In males, the urethra is approximately 20 centimeters long and serves both urinary and reproductive functions. Both sexes have internal and external urethral sphincters—the internal sphincter operates involuntarily, while the external sphincter is under voluntary control.