URINARY SYSTEM

The urinary system consists of:

KIDNEYS

Functions of the kidneys include :

Renal Morphology (Macroscopic)

The kidneys are bean-shaped retroperitoneal structures. The area of the concave surface of the kidney where nerves, blood and lymph vessels enter and leave is known as the hilum (or hilus). The kidney is covered with a capsule composed of dense connective tissue. If a whole kidney is sliced vertically it can be seen to be composed of an outer cortex and an inner medulla. The main structures seen in the medulla are 10-18 medullary pyramids. The areas of connective tissue between the pyramids are known as columns of Bertin. (This is cortical tissue situated in the medulla).

The upper expanded area of each ureter is known as the renal pelvis. This further divides into 3 or 4 major calyces, each of which subdivides into 2-3 minor calyces. The apex of each medullary pyramid fits into a minor calyx and has 10-25 openings through which the urine produced in the kidney is excreted before passing through the ureter to accumulate in the urinary bladder.

From the base of each pyramid, 400-500 medullary rays extend radially into the cortex. These medullary rays are straight collecting ducts into which nephrons open.

Renal lobes are defined as a medullary pyramid and the surrounding connective tissue. The human kidney is described as being multilobar (10-18 lobes).

Renal lobules are defined as single medullary rays and surrounding tissue i.e. all the structural elements that drain into a common collecting duct.

The morphofunctional unit of the kidney is the nephron. There are an estimated 1.3 million nephrons in each kidney. Each nephron is about 50-55 mm long and it is estimated that the total length of all the nephrons of both kidneys is about 100 km.

The nephron

Each nephron consists of:

All the components of the nephron are epithelial structures, with a continuous basal lamina and surrounded by a delicate reticular fiber network. The renal corpuscles are round or ovoid structures (about 150-200mm diameter) located in the cortex. They have a double-walled epithelial capsule (Bowman's capsule). The outer or parietal layer of Bowman’s capsule is composed of very flattened (squamous) epithelial cells. The inner or visceral layer of Bowman's capsule is composed of specialized epithelial cells known as podocytes. An afferent arteriole enters the renal corpuscle at the vascular pole and forms a complex capillary bed (glomerular tuft), before leaving the vascular pole as an efferent arteriole. The podocytes are associated with and surround the capillaries of the glomerular tuft and form part of the filtering apparatus.

The structure of the podocytes can only be properly visualized by electron microscopy. The podocytes are specialized epithelial cells with a large cell body, the site of the nucleus, and cytoplasmic processes. The primary processes divide into smaller secondary processes. The processes are known as pedicels, which contain abundant microfilaments and microtubules. The secondary processes have numerous fingerlike projections that interdigitate with similar projections from adjacent processes to form filtration slits (about 25mm). A thin diaphragm (similar to that of fenestrated endothelium) is found between the slits. The secondary processes lie on a thickened basal lamina (about 0.1mm), which is common with that of the underlying fenestrated capillaries (large fenestrae 70-90nm without diaphragms). The basal lamina contains Type IV collagen (non-fibrous) and laminin (lamina densa) with associated heparan sulfate (glycosaminoglycan). During filtration, particles larger than 10nm or negatively-charged proteins larger than albumin (MW 69,000) do not pass the filter. The heparan sulfate impedes the passage of negatively charged proteins across the filter.

The fluid that passes from the capillaries through the filter into the capsular space is known as ultrafiltrate, which enters the start of the proximal convoluted tubule at the urinary pole.

Mesangial cells are cells associated with the capillaries of the glomerulus. They are found in sites where the basal lamina forms a common sheath shared by two or more capillaries. It is thought that mesangial cells may function as a sort of pericyte, or may function as macrophages, which engulf particulate matter by phagocytosis and help keep the filter clean. In recent years mesangial cells have also been considered to be contractile and to be specialized vascular smooth muscle rather than connective tissue cells.

Glomeruli are commonly classified into two categories according to their location in the cortex :

These tubules originate at the urinary pole of the renal bodies. They consist of simple cuboidal epithelium, which is very acidophilic (owing to the activity of the numerous mitochondria). These epithelial cells are relatively large. Their nuclei are regular and fairly central. The apical membrane of each cell is covered in large numbers of microvilli to form a distinct "brush border", which is coated with a PAS-positive glycocalyx. The lateral membranes of the cells are modified to form numerous lateral processes, which interdigitate with similar processes from adjacent cells. The basal region of the cells have many invaginations of the basal plasmalemma. Numerous large mitochondria are situated between the basal invaginations. These mitochondria and invaginations result in basal striations, which are very well seen in histological preparations stained with iron hematoxylin. Lysosomes and peroxisomes arcommon in the apical regions of the cells.

The cells of the proximal convoluted tubules are involved in active ion-transport, with large energy demands. In particular they are involved in reabsorption of water and other components of the ultrafiltrate.

Each proximal convoluted tubule continues as the descending thick segment of the Loop of Henle, which continues as the thin segment. The thin segment of the loop is characterized by its extremely flattened epithelial cells, which possess relatively few organelles. The site of the U-shaped loop is an area of extremely concentrated urine. The loop of Henle continues as an ascending thick segment. The epithelial cells of the distal convoluted tubule are smaller and less acidophilic (fewer mitochondria) than the proximal convoluted tubule cells. They lack basal striations and also lack brush borders on their apical surfaces.

Juxtaglomerular Apparatus (JGA)

Each distal convoluted tubule comes into contact with the vascular pole of the renal corpuscle of its own nephron and with the afferent arterioles. At this site the distal tubule cells become modified to form a macula densa. The macula densa consists of tall (columnar) epithelial cells with the most prominent feature being their tall and elongated nuclei. The function of the macula densa is unknown though it is thought to transfer data concerning the osmolality of the fluid in the distal convoluted duct.

At the site of contact with the macula densa the smooth muscle cells of the Tunica media of the afferent arterioles have been modified and become rounded and packed with cytoplasmic granules. Tcells, known as juxtaglomerular cells, are described as epithelioid (epithelium-like) and are the site of synthesis and accumulation of the peptide hormone, renin. Renin plays an important role in control of sodium and blood pressure. Sodium deficiency (low salt) causes renin secretion (part of the Renin-Angiotensin-Aldosterone hormonal pathway).

Extraglomerular mesangial cells (Lacis cells) are also found in the juxtaglomerular region, though their function is still unknown.

Collecting tubules are the main component of the medullary rays of the cortex, however the collecting tubules dominate in the medulla (in the pyramids). They consist of straight tubules of cuboidal epithelium with central nuclei. The cells stain weakly and the borders between adjacent cells are distinct. Near the apex of the pyramid the cuboidal epithelium becomes a columnar epithelium and the ducts are much larger and typically ovoid. These are known as the papillary ducts of Bellini, and open into the minor calyces. 
 

BLOOD CIRCULATION OF THE KIDNEY

Arterial system :

Venous system :

The venous vessels accompany and parallel the arterial vessels.

The pyramids of the medulla are composed mainly of straight collecting tubules. Each tubule is accompanied by straight arterioles and venules (known as vasa recta) that originate in the arcuate arteries and return to the arcuate veins. 
 

THE URETER

The ureters are fibromuscular tubes that conduct the urine to the urinary bladder.
They are composed of three layers:

After fixation, the mucosa is usually thrown into longitudinal folds giving a star-like appearance when seen in transverse sections.

URINARY BLADDER

URETHRA

The urethra is the urinary tube leading from the bladder to the external orifice (in the male via the penis, in the female it is a fairly straight short muscular tube, 2-6cm)

 

                                                                       

                    Kidney                               Renal cortex                        Renal corpuscles                    Renal corpuscles

                                                                        

            Renal corpuscle                         Vascular pole                         Vascular pole                          Vascular pole

                                                                        

               Urinary pole                          Medullarr rays                          Macula densa                      Apex of pyramid

                                                                        

            Apex of pyramid                       Urinary tubules                        Urinary tubules                       Urinary tubules

                                                      

                  Ureter                                       Ureter                               Urinary bladder

 

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