Premed · Premed · Anatomy Physiology 2
Lecture 18: The Urinary System — Kidney Anatomy
Anatomy and Physiology II
Learning Objectives
By the end of this lecture, students will be able to:
- Identify the organs of the urinary system and their general functions
- Describe the gross anatomy and internal structure of the kidney
- Describe the blood supply to the kidney and its unique vascular arrangement
- Identify the structural components of the nephron and collecting duct system
- Distinguish between cortical and juxtamedullary nephrons
- Describe the anatomy and function of the juxtaglomerular apparatus
- Outline the anatomy of the ureters, urinary bladder, and urethra
Lecture Content
I. Overview of the Urinary System
The urinary system consists of four principal organs. The two kidneys produce urine by filtering the blood. The two ureters transport urine from the kidneys to the bladder. The urinary bladder stores urine temporarily, and the urethra conducts urine from the bladder to the exterior.
The kidneys perform a remarkably diverse set of functions. They regulate blood volume and blood pressure, maintain plasma osmolarity and electrolyte concentrations (including sodium, potassium, calcium, chloride, and phosphate), and regulate blood pH by excreting hydrogen ions and reabsorbing bicarbonate. They excrete metabolic wastes such as urea, creatinine, uric acid, and bilirubin, as well as foreign substances including drugs and toxins. The kidneys also serve important endocrine functions, producing erythropoietin (EPO) to stimulate red blood cell production in bone marrow, calcitriol (active vitamin D, or 1,25-dihydroxycholecalciferol) to promote intestinal calcium absorption, and renin to activate the renin-angiotensin-aldosterone system (RAAS). During prolonged fasting, the kidneys contribute modestly to gluconeogenesis.
II. Gross Anatomy of the Kidney
The kidneys are retroperitoneal organs situated against the posterior abdominal wall at the level of the T12 to L3 vertebrae. The right kidney sits slightly lower than the left, displaced by the liver. Each kidney is approximately 12 cm long, 6 cm wide, and 3 cm thick, weighing about 150 grams. They are bean-shaped, with a medial concavity called the hilum that serves as the entry and exit point for the renal artery, renal vein, ureter, lymphatics, and nerves.
Three protective layers surround each kidney, listed from superficial to deep. The renal fascia is a layer of dense connective tissue that anchors the kidney to surrounding structures. The perirenal fat capsule (adipose capsule) cushions and insulates the kidney. The renal capsule (fibrous capsule) is a tough, transparent layer applied directly to the kidney surface that serves as a barrier against infection.
Internal Structure
The renal cortex is the outer region, appearing granular due to the nephron components it contains, including renal corpuscles and convoluted tubules. Cortical tissue extends between the medullary pyramids as renal columns (of Bertin). The renal medulla is the inner region and contains 8 to 18 cone-shaped renal (medullary) pyramids that have a striated appearance due to the parallel arrangement of collecting ducts and loops of Henle. The base of each pyramid faces the cortex while the apex, called the renal papilla, points toward the renal pelvis, with each papilla draining into a minor calyx. The renal pelvis is a funnel-shaped expansion of the upper ureter within the hilum. The 8 to 18 minor calyces merge into 2 to 3 major calyces, which in turn merge into the renal pelvis and then the ureter, collecting urine and channeling it downward. A renal lobe consists of one medullary pyramid plus its surrounding cortical tissue and half of each adjacent renal column.
<image>A multi-panel figure of kidney gross anatomy. Panel A: Anterior view of the urinary system in situ showing both kidneys, the abdominal aorta, inferior vena cava, renal arteries and veins, ureters descending to the urinary bladder, and the urethra. The retroperitoneal position is indicated relative to the posterior abdominal wall. Panel B: A coronal section through a single kidney showing the three protective layers (renal fascia, perirenal fat capsule, renal capsule), the renal cortex (outer, granular), the renal medulla containing cone-shaped medullary pyramids with striations, renal columns of Bertin between pyramids, renal papillae at the tips of pyramids, minor calyces receiving urine from papillae, major calyces, the renal pelvis at the hilum, and the ureter exiting at the hilum alongside the renal artery and renal vein. Labels clearly identify each structure.</image>
III. Renal Blood Supply
The kidneys receive approximately 20 to 25% of cardiac output at rest, amounting to about 1,200 mL per minute. The arterial pathway follows a sequential branching pattern: the abdominal aorta gives rise to the renal artery, which divides into five segmental arteries (each supplying a distinct segment), then interlobar arteries (which travel in the renal columns between pyramids), then arcuate arteries (which arch along the corticomedullary junction), then cortical radiate (interlobular) arteries (which radiate outward through the cortex), and finally afferent arterioles that feed into the glomerular capillaries, a fenestrated capillary tuft within the renal corpuscle. Blood exits the glomerulus through the efferent arterioles.
A defining feature of the renal vasculature is its arrangement of two capillary beds in series, forming a portal system. The glomerular capillaries constitute a high-pressure bed dedicated to filtration. The peritubular capillaries form a low-pressure bed surrounding the proximal and distal convoluted tubules in the cortex and serve reabsorption and secretion. The vasa recta are long, straight capillary loops that descend alongside the loops of Henle of juxtamedullary nephrons into the medulla and play a critical role in maintaining the medullary osmotic gradient. The venous drainage mirrors the arterial pathway but without segmental veins: peritubular capillaries and vasa recta drain into cortical radiate (interlobular) veins, then arcuate veins, interlobar veins, the renal vein, and finally the inferior vena cava.
IV. The Nephron — Functional Unit of the Kidney
Each kidney contains approximately 1 million nephrons. A nephron consists of a renal corpuscle and a renal tubule.
A. Renal Corpuscle (in the cortex)
The renal corpuscle is composed of the glomerulus and Bowman's capsule (glomerular capsule). The glomerulus is a tuft of fenestrated capillaries supplied by the afferent arteriole and drained by the efferent arteriole, with fenestrations of approximately 70 nm that allow passage of most plasma components but not blood cells. Bowman's capsule is a double-walled epithelial cup surrounding the glomerulus. Its visceral layer is composed of podocytes that wrap around the glomerular capillaries. Podocytes extend foot processes (pedicels) that interdigitate, forming filtration slits of approximately 25 nm, spanned by filtration slit diaphragms that provide size- and charge-selective filtration. The parietal layer consists of simple squamous epithelium forming the outer wall, and the capsular space (Bowman's space) between the layers receives the filtrate.
The glomerular filtration membrane consists of three layers: the fenestrated endothelium of the glomerular capillary, the glomerular basement membrane (GBM) which is a thick basal lamina rich in collagen IV and proteoglycans and serves as the major size and charge barrier, and the filtration slits of the podocyte foot processes. This membrane freely filters water, electrolytes, glucose, amino acids, urea, and small proteins while excluding blood cells, platelets, and most plasma proteins, especially albumin (approximately 69 kDa).
B. Renal Tubule
The proximal convoluted tubule (PCT) begins at the renal corpuscle and is located in the cortex. It is lined by simple cuboidal epithelium with prominent microvilli (brush border) that dramatically increase surface area, and its cells contain abundant mitochondria reflecting the high metabolic activity required for active transport. The PCT is the site of most reabsorption, handling approximately 65% of filtrate volume.
The loop of Henle (nephron loop) is a U-shaped structure that dips into the medulla. Its descending limb is a thin segment with simple squamous epithelium that is permeable to water but impermeable to solutes. The ascending limb has two portions: the thin ascending limb, where passive solute movement occurs, and the thick ascending limb, which has cuboidal epithelium and actively transports sodium, potassium, and chloride out of the tubular fluid via the Na-K-2Cl cotransporter while remaining impermeable to water. This arrangement creates and maintains the medullary osmotic gradient through the countercurrent multiplier mechanism.
The distal convoluted tubule (DCT) is located in the cortex and is lined with cuboidal epithelium that has fewer microvilli than the PCT. It serves as a site of regulated reabsorption and secretion under hormonal control. Aldosterone enhances sodium reabsorption and potassium secretion here, while parathyroid hormone (PTH) enhances calcium reabsorption.
The collecting duct system receives fluid from multiple nephrons and descends through the cortex and medulla toward the renal papilla. Its principal cells respond to aldosterone for sodium and potassium balance and to ADH (vasopressin) for water reabsorption via aquaporin-2 channels. Intercalated cells are involved in acid-base balance by secreting hydrogen ions or bicarbonate. The collecting duct is the site of final regulation of urine concentration.
<image>A detailed multi-panel figure of the nephron. Panel A: A single nephron and its associated blood supply shown in context within the kidney — the renal corpuscle in the cortex, the PCT winding through the cortex, the loop of Henle descending into the medulla and ascending back to the cortex, the DCT returning to the cortex near the renal corpuscle, and the collecting duct descending through the medulla to the papilla. The afferent arteriole, glomerulus, efferent arteriole, peritubular capillaries, and vasa recta are all labeled. Panel B: An enlarged cross-section of the renal corpuscle showing the afferent and efferent arterioles, the glomerular capillary tuft, the visceral layer (podocytes with foot processes), the parietal layer (simple squamous epithelium), the capsular space (Bowman's space), and the origin of the PCT. Panel C: A magnified view of the glomerular filtration membrane showing the three layers — fenestrated capillary endothelium with pores, the glomerular basement membrane (GBM), and podocyte foot processes with filtration slits and slit diaphragms. Arrows show the direction of filtrate flow.</image>
V. Types of Nephrons
Cortical nephrons make up approximately 85% of all nephrons. Their renal corpuscles are located in the outer cortex, and they have short loops of Henle that penetrate only slightly into the medulla. They are surrounded by peritubular capillaries without significant vasa recta and serve primarily in routine reabsorption and secretion.
Juxtamedullary nephrons account for the remaining 15%. Their renal corpuscles are located near the corticomedullary junction, and they possess long loops of Henle that extend deep into the medulla, with some reaching the papilla. Their efferent arterioles give rise to vasa recta that parallel the loops of Henle. These nephrons are critical for producing concentrated urine because they establish the medullary osmotic gradient.
VI. Juxtaglomerular Apparatus (JGA)
The juxtaglomerular apparatus is located where the thick ascending limb or early DCT contacts the afferent arteriole of the same nephron and consists of three components. Juxtaglomerular (JG) cells, also called granular cells, are modified smooth muscle cells in the wall of the afferent arteriole that contain renin-filled granules. They function as mechanoreceptors (baroreceptors) that sense blood pressure in the afferent arteriole and also respond to sympathetic stimulation via beta-1 receptors. The macula densa consists of specialized epithelial cells in the wall of the thick ascending limb or early DCT that serve as chemoreceptors monitoring NaCl concentration in the tubular fluid; when NaCl is low, they signal JG cells to release renin. Extraglomerular mesangial cells (lacis cells), situated between the arterioles and the macula densa, play a signaling role.
The JGA regulates glomerular filtration rate and blood pressure through two mechanisms. Renin release activates the renin-angiotensin-aldosterone system. Tubuloglomerular feedback allows the macula densa to sense NaCl delivery and adjust afferent arteriolar tone to stabilize GFR.
<image>A detailed diagram of the juxtaglomerular apparatus. The image shows a nephron with the thick ascending limb of the loop of Henle passing adjacent to the afferent and efferent arterioles of its own renal corpuscle. The macula densa cells are highlighted in the tubular wall at the point of contact, shown as tall, densely packed epithelial cells monitoring tubular NaCl. The juxtaglomerular (granular) cells are highlighted in the wall of the afferent arteriole, shown as modified smooth muscle cells containing renin granules. Extraglomerular mesangial cells fill the space between the arterioles and macula densa. Arrows indicate the tubuloglomerular feedback loop: low NaCl at the macula densa signals JG cells to release renin, which enters the bloodstream and activates the RAAS cascade (angiotensinogen → angiotensin I → angiotensin II → aldosterone), ultimately increasing blood pressure and Na+ reabsorption.</image>
VII. Ureters, Urinary Bladder, and Urethra
Ureters
The ureters are paired muscular tubes approximately 25 to 30 cm long that connect each kidney to the bladder. They are retroperitoneal and enter the bladder at its posteroinferior surface. Their wall consists of three layers: a mucosa lined with transitional epithelium, a muscularis of smooth muscle with an inner longitudinal and outer circular arrangement, and an adventitia. Urine is propelled through the ureters by peristalsis rather than gravity. The ureters enter the bladder at an oblique angle, creating a physiological valve that prevents the backflow of urine.
Urinary Bladder
The urinary bladder is a hollow, distensible muscular organ located in the pelvic cavity. The trigone is a smooth triangular area on the posterior wall defined by the two ureteral openings and the internal urethral orifice and is clinically significant as a common site for urinary tract infections. The bladder wall consists of a mucosa lined with transitional epithelium that stretches to accommodate filling, a submucosa, the detrusor muscle comprising three layers of smooth muscle, and an adventitia or serosa. The bladder has a capacity of approximately 500 to 600 mL, though the urge to void typically arises at about 200 mL.
Micturition (urination) involves the coordinated contraction of the detrusor muscle and relaxation of both sphincters. The internal urethral sphincter is composed of smooth muscle under involuntary parasympathetic control, while the external urethral sphincter is composed of skeletal muscle under voluntary control via the pudendal nerve.
Urethra
The female urethra is approximately 3 to 4 cm long and opens anterior to the vaginal opening. The male urethra is approximately 18 to 20 cm long and traverses three regions: the prostatic urethra, which passes through the prostate gland; the membranous urethra, which passes through the external urethral sphincter and urogenital diaphragm; and the spongy (penile) urethra, which passes through the corpus spongiosum of the penis.
VIII. Clinical Correlations
Renal calculi (kidney stones) are crystallized salts, most commonly calcium oxalate, that form in the renal pelvis or ureter and cause severe flank pain known as renal colic. Urinary tract infections (UTIs) are more common in females due to the shorter urethra. Polycystic kidney disease (PKD) is an inherited condition characterized by progressive cyst formation that can lead to renal failure. Renal cell carcinoma is the most common kidney cancer in adults. Nephroptosis (floating kidney) occurs when the kidney drops as the patient stands, resulting from loss of the perirenal fat that normally supports it.
