The urinary system comprises the kidney, ureter, urinary bladder, and urethra. The kidney filters blood to produce urine, which contains excess water, electrolytes and waste products of the body. Urine flows from the kidney through the ureter into the bladder where it is temporarily stored. The bladder is then emptied via the urethra.
The kidney performs several important functions:
The kidney performs many of the above functions by filtering blood to remove ions and small molecules and then selectively reabsorbing ions and small molecules depending on the current conditions of the body. The main functional unit of the kidney is the nephron which combines a filtration unit (corpuscle) and a long tube lined by epithelial cells that vary along its length in their properties of reabsorption and passive diffusion. The segments of the tube are divided into proximal convoluted tubule, loop of Henle and distal convoluted tubule. Each nephron is fed by an artery (afferent) that supplies blood to be filtered. The nephron can control the rate at which it filters blood by dilating or constricting the afferent artery.
The renal corpuscle is responsible for the filtration of blood. It contains two structures: the glormerulus and Bowman's capsule. The glomerulus is a cluster of capillaries surrounded by a thick basement membrane and a layer of epithelial cells called podocytes. Blood flows into the capillaries from an afferent arteriole and leaves the corpuscle via an efferent arteriole. Blood in the efferent arteriole enters a second capillary bed that perfuses the region of the kidney that house the nephron tubules.
The hydrostatic pressure in the glomerular capillaries is significantly higher than oncotic pressure which pushes fluid out of the capillaries into Bowman's space and the rest of the nephron. The capillaries contain a fenestrated endothelia to facilitate passage of fluid and are surrounded by a thick basement membrane and a layer of epithelial cells called podocytes. As discussed below, the endothelial cell, basement membrane and podocyte define the filtration barrier of the glomerulus.
Bowman's capsule surrounds the glomerulus and is defined by a layer of epithelial cells that are continuous with the podocytes that surround the capillaries. The capsule is composed of a layer of epithelial cells.
Mesangial cells are also found in the glomerulus, and they secrete the proteins and glycoproteins that compose the matrix that supports the other cells in the glomerulus.
Podocytes are specialized epithelial cells that separate the network of capillaries in the glomerulus from Bowman’s space. Podocytes extend processes that surround the capillaries. These processes form secondary processes called foot processes. The foot processes associate with the basement membrane opposite from the endothelial cells of the capillaries.
The filtration barrier of the glomerulus is a size and charge selective barrier. The structural components of the barrier start with a fenestrated capillary epithelium with a negatively charged surface that repels many plasma proteins. Beneath the capillary epithelium is a thick basement membrane that is also negatively charged. The foot processes of the pododcytes form the final barrier. Note the slit diaphragm between the foot processes. These play a role in filtration of plasma as mutations in genes that encode proteins of slit diaphragms lead to proteinuria. The strong negative charge on the surface of the endothlelium and within the basement membrane repel negatively charged proteins in plasma, notably albumin.
The proximal convoluted tubule is the first and longest segment of renal tubule and begins at the urinary pole of the glomerulus. The proximal convoluted tubule is the site where the majority (65%) of ions and water in the urinary space is reabsorbed back into the body. The cells of the proximal convoluted tubule have a deeply stained, eosinophilic cytoplasm. The cells are large so that in cross section not every nucleus will be visible, making it appear that the proximal convoluted tubule has fewer nuclei than other tubules. The cells also have an apical brush border to increase their surface area. The convoluted portion of the tubule leads into a straight segment that descends into the medulla within a medullary ray and becomes the loop of Henle.
This is an electron micrograph of the proximal convoluted tubule. Note the microvilli forming the brush border on the apical surface. The basal striations in these cells are due to the parallel alignment of mitochondria alongside the basolateral regions of the plasma membrane. These mitochondria support active ion transport by sodium-potassium pumps on the basal plasma membrane by providing an abundant amount of ATP. The ion gradient created by these pumps in turn supports ion and water reabsorption on the apical side of the proximal convoluted tubule cells. Basal striations with mitochondria and sodium-potassium pumps are also found in cells in the distal convoluted tubules.
The loop of Henle forms a hair-pin structure that dips down into the medulla. It contains four segments: the thick descending limb, which extends from the proximal convoluted tubule, followed by the thin descending limb, the thin ascending limb, and the thick ascending limb. The turn of the loop of Henle usually occurs in the thin segment within the medulla, and the tubule then ascends toward the cortex parallel to the descending limb. The end of the loop of Henle becomes the distal convoluted tubule near its original glomerulus. The loops of Henle run in parallel to capillary loops known as the vasa recta. Recall from Physiology that the loop of Henle serves to create high osmotic pressure in the renal medulla via the counter-current multiplier system. Such high osmotic pressure is important for the reabsorption of water in the later segments of the renal tubule.
Periodic acid-Schiff Staining (PAS). The thick descending limbs of the loop of Henle look similar to the proximal tubule, with apical brush borders. The thick ascending limbs are composed of cuboidal cells, but unlike the proximal convoluted tubule, they do not have apical brush borders. Collecting ducts can also be seen on this slide. They can be easily distinguished by the presence of prominent lateral borders between adjacent cells.
The distal convoluted tubule follows the loop of Henle. Further reabsorption and secretion of ions occur in this segment. The initial segment of the distal convoluted tubule lies right next to the glomerulus and forms the juxtaglomerular apparatus.
The cells of the distal convoluted tubule are smaller and more lightly stained than those of the proximal convoluted tubule. Consequently, more nuclei are apparent in a cross section of distal convoluted tubule compared to proximal convoluted tubule. Distal convoluted tubules also lack a brush border on their apical surface. Note that in any given section of the kidney cortex, much less space is occupied by distal convoluted tubules as compared to proximal convoluted tubules. This is simply because the distal convoluted tubule is shorter and less convoluted.
The juxtaglomerular apparatus is a specialized structure formed by the distal convoluted tubule and the glomerular afferent arteriole near the vascular pole of the glomerulus. Its main functions are to regulate blood pressure and the filtration rate of the glomerulus. The complex secretes renin, which regulates systemic blood pressure via the renin-angiotensin-alodosterone system. The complex also regulates pressure in the afferent arteriole in response to changes in the rate of filtration. The complex is composed of the following cells:
The terminal portion of the distal tubule empties through collecting tubules into a straight collecting duct. The collecting duct system is under the control of antidiuretic hormone (ADH). When ADH is present, the collecting duct becomes permeable to water. The high osmotic pressure in the medulla (generated by the counter-current multiplier system/loop of Henle) then draws out water from the renal tubule, back into the surrounding blood vessels. ADH causes the epithelial cells that line collecting ducts to increase the number of aquaporin 2 channels in their apical plasma membrane. Aquoporins allow the diffusion of water across the plasma membrane. In the absence of ADH, aquaporin 2 localizes to storage vesicles in the cytoplasm of collecting duct epithelial cells. ADH stimulates the transport of aquaporin 2 from storage vesicles to the plasma membrane.
Collecting ducts can be differentiated from other tubules by the prominent lateral borders of the epithelial cells.
This is a low power view of a cross section through the kidney shows the outer cortex and inner medulla of the kidney. The cortical section contains corpuscles, tubules, and blood vessels. The medullary primarily contains loops of Henle, collecting ducts and blood vessels. Note the large blood vessels at the border of the cortex and medulla. These are the arcuate arteries and are often used to distinguish cortex from medulla.
Numerous collecting ducts merge into the renal pelvis, which then becomes the ureter. The ureter connects the kidney and the urinary bladder.
The ureter is a muscular tube, composed of an inner longitudinal layer and an outer circular layer of smooth muscle. The lumen of the ureter is covered by transitional epithelium (also called urothelium). Recall that the transitional epithelium is unique to the conducting passages of the urinary system. Its ability to stretch allows the dilation of the conducting passages when necessary.
The ureter empties the urine into the bladder. The transitional epithelium continues over the surface of this organ. The thickened muscular layers become interwoven and cannot be clearly identified at this point.
The urinary bladder is lined by transitional epithelium, underneath which are thick layers of smooth muscle interwoven in various directions. This image shows a relaxed bladder where the epithelial cells appear cuboidal. In a distended bladder the epithelial cells are stretched and become more squamous.
The urethra carries the urine away from the bladder to the outside of the body. In the male, it is joined by the genital system. The epithelium changes from transitional to stratified or pseudostratified columnar in the urethra, and to stratified squamous in the distal end of the urethra.