Urinary System


Functions  Dialysis Patient Dialysis animation
      The functions of the urinary system include:
1. Regulation of ion concentrations
2. Regulation of blood volume and pressure
3. Stabilization of blood pH
4. Conservation of nutrients
5. Elimination of organic molecules and toxins
6. Synthesis of calcitriol
Kidneys http://www.jeremyswan.com/anatomy/204/html/12b.html 
    The kidneys are paired organs located lateral to the vertebral column between T12 and L3. The kidney is about 5 in. (12 cm) long, 2.5 in. (6 cm) wide and 1 in. (3 cm) thick. There is a deep indentation called the hilus on the medial side of each kidney from which the ureters and blood vessels enter and leave. The hilus leads to a space within the kidney called the renal sinus. 
    The kidneys are retroperitoneal in position and are held in position by and are supported by the following layers of connective tissue:
1. renal capsule - a layer of dense connective tissue on the outer surface of each kidney.
2. adipose capsule - a layer of adipose tissue surrounding the renal capsule that protects and supports the kidney. 
3. renal fascia - a dense outer layer of connective tissue that anchors the kidney to surrounding structures.

Sectional Anatomy 
    A longitudinal cut through the kidney reveals the following features:
Cortex - outer layer of kidney in contact with capsule.
Medulla - the tissue internal to the cortex that has the following features:
Renal pyramids  - conical (in three dimensions) or triangular (in cut section) structures with the base lying against the cortex and the tip, called the renal papilla, pointing into the renal sinus. 
Renal columns - this is cortical tissue extending between the renal pyramids. 
     Within the renal sinus, the renal papilla of each is surrounded by a minor calyx that drains the urine that drips from the papilla. The minor calyces fuse to form a funnel-shaped renal pelvis. The renal pelvis tapers to form the ureter. 
 
Nephron YouTube - Function of the Nephron (Nephron tutorial)
     Each kidney contains over a million blind-ended tubules called nephrons. Each nephron consists of the following parts:
   Glomerular (Bowman's) capsule
     This is the blind end of the tube that is indented by a tuft of capillaries called the glomerulus. Blood enters the capillaries through an afferent arteriole and blood drains from the capillaries through an efferent arteriole. The location where arterioles enter and leave the capsule is the vascular pole. 
    The capsule is lined on the outside and inside by the following epithelia:
Capsular epithelium - This is the outer wall of the capsule and consists of simple squamous epithelium. 
Glomerular epithelium - The cells that form this epithelium rest directly on the capillaries of the glomerulus and consists of octopus-shaped cells called podocytes. 
     The process of filtration occurs in the glomerular capsule. The blood that enters the glomerulus is filtered across three physical barriers :
1. Capillary endothelium - The glomerular capillaries are porous.
2. Lamina densa - This is extracellular material that supports both the capillary endothelium and the glomerular epithelium. 
3. Glomerular epitheliumThe podocytes that form this epithelium have processes called foot processes. The foot processes have smaller processes called pedicels that interdigitate with one another. The space between the interdigitating pedicels form filtration slits through which the filtrate passes. 
Proximal Convoluted Tubule   
    The glomerular capsule at the tubular pole continues as the proximal convoluted tubule (PCT). The PCT consists of simple cuboidal epithelium whose apical surface is lined by microvilli.
    The PCT is the "mass reabsorber". The bulk of the water, ions, organic nutrients and any protein is reabsorbed in this segment of the tubule. 
Loop of Henle  
     The PCT straightens out and forms the loop of Henle. The loop of Henle has a descending limb which is directed toward the tip of the pyramid, makes an hairpin turn, and then becomes an ascending limb that parallels the descending limb. 
     The loop of Henle creates an osmotic gradient within the medulla that enables the kidney to reabsorb water and produce a urine that is more concentrated than the body fluids. The loop of Henle also reabsorbs about 25% of the water and continues to reabsorb ions.
Distal Convoluted Tubule 
     When the ascending limb of the loop of Henle returns to the cortex it becomes the distal convoluted tubule (DCT). The DCT is lined by simple cuboidal epithelium that has fewer microvilli.
     The DCT further refines the filtrate by continuing to reabsorb ions but also by secreting ions including H+ and K+.
Collecting Ducts
     The nephrons are connected to collecting ducts by connecting tubules. The collecting ducts descend into the medulla and converge on the papilla. As the collecting ducts converge at the papilla they fuse to form larger ducts that are called papillary ducts. 
Blood Supply to Kidneys
     The nephrons are able to perform their functions of filtration, reabsorption and secretion because of their intimate and precise association with the blood vessels that enter the kidney. 
     The renal artery brings blood to the kidney and enters the kidney through the hilus. Within the renal sinus the renal artery branches into segmental arteries. The segmental arteries enter the medulla through the renal columns where the arteries are called the interlobar arteries. The interlobar arteries travel along the bases of the pyramids where they are called the arcuate arteries. The arcuate arteries give off branches that ascend in the cortex called cortical radiate arteries. The afferent arterioles branch off the cortical radiate arteries (a.k.a. interlobular arteries) and bring blood to the glomerulus. Blood drains from the glomerulus by the efferent arteriole. The efferent arterioles than form peritubular capillaries that wrap around the tubules of the nephrons. 
     Blood drains from the kidneys by way of veins that reverse the direction of blood flow into the kidneys in the arteries. Hence, blood drains through the cortical radiate vein (interlobular vein), arcuate vein, interlobar vein, segmental vein and renal vein. 
     To summarize:
Renal a. -> segmental a. -> interlobar a. -> arcuate a. -> cortical radiate a. -> afferent arteriole -> glomerulus -> efferent arteriole -> peritubular capillaries -> cortical radiate v. -> arcuate v. -> interlobar v. -> segmental v. -> renal v. 
Structures for Urine Transport, Storage and Elimination
Ureters
     The ureters are muscular tubes that transport urine to the urinary bladder.
Urinary Bladder
     The urinary bladder is a distensible hollow organ that stores the urine until elimination.
Urethra
     The urethra is a muscular tube that connects the urinary bladder to the exterior and permits elimination of the urine.