Urinary system

Urinary system

The urinary system consists of the 2 kidneys, two ureters, bladder, and urethra.
It contributes to the maintenance of the homeostasis by producing the urine, in which various metabolic waste-products are eliminated, these kidneys also regulate the fluid balance of the body and are the site of production of rennin and erythropoietin.

Kidney

Position: -
The kidneys located high on the posterior wall of the abdomen beneath the peritoneum. Right kidney is lowered than left due to the presence of liver. The kidneys occasionally congenitally located in the lower part of the abdomen. Ectopia of the kidneys or Ectopic kidneys.

Structure :

The kidneys are solid, bean shaped organs. It has a convex and concave border which faces the midline. The concave area show a depression called hilum through which the ureter desend and the renal vein leave, and the renal artery enter and lymphatic vessel and nerve.

Size: -

The size of the kidney is (5 inches) length, (3 inches) width, and (1 inch) thickness. the average weight (170 gram).

On section: -

The kidneys show two areas an outer area called cortex and inner area called medulla which surround a cavity called Renal Sinus, which opens at the hilum.
Renal Sinus: -
It contains: -
The upper expanded portion of the ureter (pelvis of the ureter)
Two major calyces and about eight minor calyces.
Branches of the renal arteries veins and nerves.
Loose connective tissue and fat.
In fresh unfixed kidneys the cortex is dark brown in color and granular in appearance.
It contains many convoluted tubules and numerous rounded, reddish bodies, Renal Corpuscles or Malpighian Corpuscles. Which are not visible by the necked eye.
The inner Medullary Zone of the parenchyma is radially striated in appearance because of the tubular and vascular elements that run in parallel radial lines.
The cortical tissue pulges inside the medulla penetrating the whole depth of the medulla and is called Columns of Bertini or Renal columns
The medullas consist of 8-18 medullary pyramids the broad base of each pyramid is in contact with the cortex, and the round apex project into the minor calyx. The two or three pyramids unite to form a single papilla hence the number of the papillae is less than the number of the pyramids. The tips of the papillae are perforated in a form of a sieve-like appearance (area cribrosa).
10-25 pores are present in the papilla, which are the openings of the papillary ducts of uriniferous ducts.
The cortical region is subdivided into many radiating columns composed of straight tubules alternating with regions containing glomeruli and convoluted tubules, the columns or straight tubules radiate outwards from the medulla and hence are named Pars Radiata or Medullary Rays. The regions between the rays contain glomeruli and convoluted tubules called Pars Convolute or Cortical Labyrinths.


Renal lobule :-
Constitute a medullary ray and the portions of the adjacent labyrinth whose tubules drain into the collecting tubules.

Uriniferous tubules
The parenchyma of the kidney consists of closely packed uriniferous tubules, between which are blood vessels and scanty amount of interstitial tissue.
Two types of uriniferous, tubules are distinguished :
Secretory tubules (nephron): Which function in the formation of urine.
Excretory tubules: Which are ducts that collect urine and carry it to the pelvis.
Include the colleting ducts and papillary ducts.

Nephrons

Each nephron consists of:
1. Renal corpuscle (Malpighian corpuscle).
2. Proximal convoluted tubule.
3. Loop of Helene.
4. Distal convoluted tubule.

Renal corpuscle

The renal corpuscle is spherical in shape consist of two parts: -
1. Glomerulus.
2. Bowman's capsule


Each renal corpuscle has got two poles: -
Vascular pole: - the point where the afferent arteriole enter and the efferent arteriole leave.
Urinary pole: - where the proximal convoluted tubule.

Glomerulus

The Afferent arteriole as it enters Bowman's capsule divide into five large groups of capillaries. Each capillary divide into smaller capillaries, that follow an irregular looped coarse and form the efferent arteriole.
The looped capillaries arising from each main branch of the afferent arteriole tend to be grouped together giving the glomerulus a lobulated appearance or Segmental appearance so that the glomerular capillaries are divided into five segments. Which is demonstrated by a disease affecting only one segment e.g. (segmental glomerulonephritis).
The efferent arteriole has a smaller caliber than the afferent arteriole because the arteriole due to that a considerable quantity of fluid is filtered from the blood while it flows through the glomerular capillaries.

Glomerular capillaries

Are the most fenestrated type of capillaries in the body.
The endothelial cells of the gl. Capillaries are very thin.
The fenestrate Lacks diaphragms.
Large molecules are readily passed through the pores such as ferritin molecules.
Basal lamina of the glomerular endothelial cells unites with the basal lamina of the visceral layer of the Bowmans capsule to form basement.
The basement membrane lacks lamina reticularis.

Bowmans capsule

Formed of 2 layers
Parietal layer: consist of simple squamous epithelium lying on a basal lamina and a thin layer of reticular fiber which form a network with the reticular tissue around the renal tubules.
Visceral layer: lined by a special type of cells called (Podocytes)
The space between these two layers is called bowman's space which receive the filtration and continuous with the lumen of PCT.


Podocytes
The podcytes are branched cells with a body from which arise primary processes that give secondary processes which embrace the glomerular capillaries the cytoplasm of the podocyte contains by E/M ibosomes, microtubules, microfilaments, the secondary processes reach the gl. Capillary and surrounding it at a regular distance from each other. They lie in a direct control with the basal lamina. With a space between these secondary processes filtration slits It has an important function in the blood renal barrier and in regeneration of basement membrane.

Glomerular filtration barrier:

The barrier between the blood inside the glomerular capillaries and the urinary space is:
1. Capillary endothelial cells
2. Thick glomerular basement membrane,
3. Basement membrane of the podocytes
4. Podocytes of the visceral layer.

Beside the endothelial cells and podocytes. The glomerular capillaries have mesangial that support the glomerular capillaries.

The mesangium is formed of 2 parts:

1. Mesangial cells
2. Mesangial matrix

Mesangial cells

Irregular in shape with processes run in haphazard fashion through the matrix.
Nucleus is rounded or oval with dense rim of nuclear chromatin inside the nuclear membrane &small chromatin clumps are scattered throughout the neoplasm.
Cytoplasm contains myosin like filaments & bear angiotensin type 2 receptors
The capillary wall is deficient in basement membrane at the site of the attachment to the mesangium endothelial cells lying directly on the mesangial matrix.
Mesangium lies between two capillaries & enclosed by their own basement membrane.


Mesangial matrix
A cellular material produced by the mesangial cells.
Functions of the mesangium:
Support of the glomerular capillary loop system
Control blood flow through the glomerular loop by myosin - angiotensin mechanism
phagocytosis.
Maintenance of the glomerular basement membrane

Proximal convoluted tubule :- L| M & E|M

1. It is the initial tortous segment close to the renal corpuscles .
2. It is the larger & longer than distal convoluted tubules .
3. It is found in the cortex usually in cross section .
4. It is lined with cuboidal epithelium .The cytoplasm is strongly acidophilic due to abundance of elongated mitochondria. The luminal surface show microvilli of about 1 Mm in length which from the so called Brush border canalicul &small vesicles are seen near the luminar surface . The basal portion of these cells presents lateral extensions that interdigitate with analogous extensions of the neighboring cells which increase the contact between the cells.
5. Mitochondria concentrated at the base of the cell & arrange parallel to the long axis of the cell. This arrangement is typical for the cells engaged in Na pump &ion transport.

Functions of proximal convoluted tubule

1. reabsorption of 85%of Na, Cl &water . Na is reabsorbed by active process. Cl, & water, reabsorbed by passive process.
2. reabsorption of glucose. When the amount of glucose exceeds the absorbing capacity of the proximal tubule it appears in urine as in case of diabetes.
3. reabsorption of protein & amino acid.
4. Excretion of any foreign substances
Loop of Henle
Formed of the following:
Thick descending segment: which is the straight continuation of the proximal convoluted tubules that descend in the cortex &have the same structure of the proximal convoluted tubule.
Function: the descend part is quite permeable. Permitting the free passage of water & Na. since the interstitial fluid of the kidney medulla is hypertonic, Na enter and water leaves the glomerular filtrate in the descending part of the loop.
Thick ascending segment: The ascending part is impermeable to water & highly active in transporting sodium to the interstitial fluid. Thus, it is directly responsible for the hypertonicity of the interstitial fluid of the medullary region. As a consequence of the loss of sodium, the filtrate which reaches the distal convoluted tubules is hypotonic.
Thin segment of loop of Henle: These associated with juxta medullary glomeruli are long & extend deep into the medulla, where as those associated with the mid cortical or subcapsular glomeruli extend only short pathway into the medulla. Thus, thin Henle loop may be found in the cortex & in the medulla.
The microscopic appearance of the thin segment
1. Have flat squamous lining epithelium.
2. Resemble dilated capillary.
3. Lining cells show short microvilli in electron microscope.
4. Some basal & lateral interdigitations.


Distal convoluted tubule:
When the thick part of loop of Henle penetrate the medulla, it preserves its histological structure but become tortuous & then called distal convoluted tubules which is the last segment of the nephron. The distal convoluted tubule found in the cortex.
The histology of the distal convoluted tubule: L|M &E|M.
It appears in slid in cross section because it is convoluted.
It is lined by cuboidal epithelium.
Number of lining cells of the distal convoluted tubules is more than that of the proximal convoluted tubules.
Cells are less acidophilic than the proximal convoluted tubules due to decreased number of mitochondria.
Lumen is wider.
Microvilli on the laminar surface are not well formed & comparatively scanty.
Mitochondria are present but less than that of proximal convoluted tubules.
Boundary between the cells are present while in the proximal convoluted tubules it is absent.
Extensive Basel & lateral extensions.
Functions:
Maintain acid - base balance.
Important in urine concentration.
Reabsorption of Na ions from the lumen of the distal convoluted tubules to the blood while K is excreted in the urine.
Bicarbonate ions are reabsorbed back to the blood & hydrogen ions are excreted so the urine is acidic.
Its function is under the effect of aldosterone secreted by the adrenal cortex.
ADH secreted by the posterior pituitary acts on the last part of the distal convoluted tubules & increase the permeability of the tubule to water rendering the urine more concentrated. ADH also act on the collecting duct in a similar way.

Collecting tubules

The arched collecting tubules connect the distal convoluted tubules with the collecting duct. they are still in the cortex & empty in the straight collecting tubules. these tubules are lined with a single layer of cuboidal epithelium which show a distinct cell borders which are more obvious than in any of the other portions of the uriniferous tubules. Tow types of cells can be distinguished in the lining epithelium of the tubules:
Clear cells: characterized by
Cuboidal light cytoplasm.
Few cytoplasmic organelles.
Microvilli are short & scanty.
Basal membrane enfolding is present in the proximal part of the tubules.
2. Dark cells:
Dark cytoplasm
Rich cytoplasmic organelles.
Numerous well-developed microvilli with vesicles.
No basal enfolding.
Numerous mitochondria.
The initial part of the collecting duct is similar lining epithelium to that of the collecting tubules. As the duct pass down the medullary rays & into the medulla, the dark is increased & the clear cells increased& become taller. Basement membrane become thicker as the duct comes nearer to the papillary duct.

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Urinary System Dr. wasan waadallah
Lecture 1

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