Urinary System

     There are two key components of the urinary system:
  1. Kidneys
     A pair or organs that performs the primary functions of the kidneys.
  2. Urinary Tract
     The urinary tract includes paired ureters that drain urine from the kidneys into the urinary bladder where it is temporarily stored until it is voided through the urethra to the exterior.
     The primary functions of the kidneys include:
  1. Regulation of the plasma concentration of sodium, potassium, chloride, calcium and other ions.
  2. Long term regulation of blood volume and pressure.
  3. Long term regulation of plasma pH.
  4. Excretion of organic wastes.
  5. Assists the liver in detoxification of toxins and gluconeogenesis.
     The kidneys are located lateral to the vertebral column between the last thoracic vertebra (T12) and the third lumbar vertebra (L3) with the right kidney slightly inferior to the left. The kidneys are retroperitoneal in position with the kidneys and associated structures located between the muscles of the dorsal body wall and the parietal peritoneum.
     In addition to the parietal peritoneum and nearby organs, three layers of connective tissues stabilize and protect the kidneys:
  1. Renal capsule - A fibrous layer on the surface of the kidney.
  2. Adipose capsule - Adipose tissue that surrounds each kidney.
  3. Renal fascia - A  sheet of connective tissue that anchors the kidney to surrounding structures.
  Superficial Anatomy of the Kidney
     Each kidney has a prominent medial indentation called the hilus which serves as the entry point of blood vessels, nerves and ureter.
     At the hilus an inner layer of the renal capsule extends into an internal cavity of the kidney called the renal sinus. This cavity is filled with blood vessels, nerves, and the tributaries of the collecting system that eventually form the ureters.
  Sectional Anatomy of the Kidney
     When the kidney is cut open three distinct regions become visible:
  1. Renal Cortex
     The renal cortex is the outermost layer that is contact with the capsule.
  2. Renal Medulla
     The renal medulla is internal to the cortex and contains 6 - 18 conical structures called renal pyramids. The base of each pyramid rests on the cortex and the tip, called the renal papilla, projects into the renal sinus. Adjacent pyramids are separated by bands of tissue called renal columns which are essentially extensions of cortical tissue.
  Renal Lobe
     The renal lobe is a segment of the kidney centered around the renal pyramid. It includes the renal pyramid, the overlying renal cortex, and a share of the adjacent renal columns.
  3. Renal sinus
     Each renal papilla that projects into the renal sinus is surrounded by a cup-shaped drain called a minor calyx. Four or five minor calyces merge to form a major calyx. The major calyces fuse to form a large, funnel-shaped chamber, the renal pelvis. The renal pelvis tapers to form the ureter which exits at the hilus.
  Blood Supply to the Kidneys
     Each kidney receives blood from a renal artery which enters at the hilus. The renal artery divides into the segmental arteries within the renal sinus. The segmental arteries divide into interlobar arteries that penetrate the renal columns. These arteries then divide into the arcuate arteries that course along the boundary between the cortex and medulla. Each arcuate artery divides into cortical radiate arteries (interlobular arteries) that supply the adjacent renal cortex. Numerous afferent arterioles branch off the interlobular arteries.
     Blood drains from the kidney in a sequence of veins that reverse the pathway of the arteries and includes, cortical radiate veins (interlobular veins), arcuate veins, interlobar veins and a renal vein.
  Histology of the Kidney
     Each kidney contains over a million microscopic tubules called nephrons which are the basic structural and functional units of the kidneys.
     Each nephron begins at the renal corpuscle which contains a capillary network called the glomerulus. Blood arrives at the glomerulus via the afferent arteriole and leaves by an efferent arteriole. Filtration occurs at the renal corpuscle and the filtrate flows through the tubule which is divided into the proximal convoluted tubule, loop of Henle, and distal convoluted tubule.
     There are two types of nephrons:
  1. Cortical Nephrons
     About 85% of nephrons are located almost entirely within the cortex. These nephrons perform most of the absorptive and secretory functions of the kidney.
  2. Juxtamedullary Nephrons
     These nephrons are located close to the cortico-medullary border and have long loops of Henle that extend deep into the pyramids. These nephrons create the conditions necessary to produce a concentrated urine.
     The filtrate formed by each nephron is finally collected by a shared system of tubules called the collecting system. The collecting system consists of a connecting tubule that connects each nephron to a collecting duct; the collecting ducts that descend toward the renal papilla; and the papillary ducts that result from the fusion of the collecting ducts in the renal papillae.              
  Renal Corpuscle
     The renal corpuscle consists of the expanded, blind end of the renal tubule called Bowman's capsule and the capillaries of the glomerulus.
     The glomerulus extends into Bowman's capsule without entering into the tubule. As a result, two layers of tubular epithelium come to surround the capillaries and create a capsular space between them. The two layers are:
  1. Parietal (capsular) epithelium - This is an outer layer of simple squamous epithelium.
  2. Visceral (glomerular) epithelium - The cells of this layer come into direct contact with the endothelial cells of the capillaries and are called podocytes.
     Filtration occurs in the renal corpuscle across the filtration membrane formed by:
  1. Capillary Endothelium
     This layer is formed by the glomerular capillaries which are fenestrated.
  2. Basal Lamina
     The basal lamina of the endothelial cells is distinct in having a lamina dense that is thicker than normal. The lamina densa often encircles more than one capillary with mesangial cells between adjacent capillaries. The functions of the mesangial cells include:
  1. Provide physical support for the capillaries.
  2. Engulf organic materials that could accumulate and clog the filtration membrane.
  3. Regulate the diameter of the capillaries.
  3. Glomerular Epithelium
     The podocytes that form the glomerular epithelium have cellular processes that wrap around the lamina densa. These processes have smaller finger-like processes called pedicles that interdigitate with those of a neighboring cell's process to form narrow filtration slits between them.
  Proximal Convoluted Tubule (PCT)
     The PCT begins at the tubular pole of the renal corpuscle which is opposite the vascular pole where the afferent and efferent arterioles enter and leave.
     The primary function of the PCT is absorption. The simple cuboidal epithelium of the PCT has numerous microvilli on the apical surface which increases the surface area for absorption. Over 60% of the water, sodium and chloride is reabsorbed from the filtrate in the PCT. All the organic nutrients and plasma proteins are reabsorbed. There is also active reabsorption of various ions including calcium, potassium, magnesium and bicarbonate.
  Loop of Henle
     When the PCT straightens out it forms a tubule that descends toward the renal papilla of the pyramid. This descending limb then turns in a hair-pin fashion and ascends toward the renal corpuscle. This loop is called the loop of Henle.
     The loop of Henle is the portion of the renal tubule responsible for creating an osmotic gradient in the pyramid in which the osmotic pressure increases from the base of the pyramid towards the renal papilla. This osmotic pressure gradient enables the kidney to produce urine that is more concentrated than the body fluids.
  Distal Convoluted Tubule (DCT)
     When the ascending limb of the loop of Henle reaches the renal corpuscle it passes by the vascular pole in between the afferent and efferent arterioles and becomes the distal convoluted tubule. The distal convoluted tubule performs both absorption and secretion and is the primary location where secretion occurs. The DCT is the part of the tubule where regulation of sodium, potassium, calcium and pH actually begins.
      Juxtaglomerular Apparatus
    The location where the ascending limb passes in between the afferent and efferent arterioles is called the juxtaglomerular apparatus.
     The juxtaglomerular apparatus consists of:
  1. Macula Densa (dense spot)
     At this location, which is next to the arterioles, the epithelial cells are taller and more concentrated.
  2. Juxtaglomerular Cells
     These are modified smooth muscle cells in the wall of the afferent arteriole.
  3. Extraglomerular Mesangial Cells
     These cells occupy the space between the glomerulus, afferent and efferent arterioles and the macula densa.
    The juxtaglomerular apparatus monitors sodium concentration in the filtrate of the tubule at this point and uses this information to regulate the production of filtrate to maintain consistency. This is also the site of production of the hormones renin and erythropoietin which regulate blood pressure and red blood cell production, respectively.
Structures for Urine Transport, Storage and Elimination
     The ureters are muscular tubes that transport urine from the tapering tip of the renal pelvis to the urinary bladder. The ureters are retroperitoneal in position like the kidneys.
     The wall of the ureters consist of:
  1. Transition epithelium - This epithelium accommodates the cycles of expansion and contraction without changes in the impermeability of the epithelium.
  2. Muscular layer - There is an inner longitudinal and an outer circular layer.
  3. Adventitia - Connective tissue that surrounds and stabilizes the position of the ureters.
  Urinary Bladder
     The urinary bladder is a hollow muscular organ. The mucosa of the bladder is thrown into folds called rugae in the contracted state. The rugae are absent and the mucosa remains smooth in a triangular region between the openings of the ureters and the entrance to the urethra. This region is called the trigone and serves to funnel the urine into the urethra when the bladder contracts.
     The muscularis of the urinary bladder contains three layers of smooth muscle, a longitudinal layer sandwiched in between two circular layers. The muscularis layer is called the detrusor muscle.
     A layer of serosa covers only the superior surface of the urinary bladder.
     The urethra conducts the urine from the urinary bladder to the exterior. It is much longer in the male than in the female. In the male it is divided into three sections:
  1. Prostatic urethra - which passes through the prostate gland.
  2. Membranous urethra - which passes through the urogenital diaphragm.
  3. Spongy urethra - which passes through the corpus spongiosum of the penis.