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THE RENAL / URINARY SYSTEM

Definition/Description:

The Renal/Urinary system is the body system, which plays a vital role in the maintenance of

homeostasis by the following processes:

1. Production of urine through which toxic metabolic waste are eliminated from the body

2. Regulation of body fluids and electrolyte balance

3. Production and secretion of hormones such as

Renin (JG cells),

Erythropoietin (Cortical interstitial cells),

Epidermal growth factor (Intercalated cells of collecting tubules and ducts) and

Medullipin I(Medullary interstitial cells).

4. Synthesis of Prostaglandins and Prostacyclin by;

Intraglomerular Mesangial cells and

Medullary interstitial cells.

5. Regulation of Blood flow and Blood pressure.

6. Osmoregulation of body fluids.

7. Regulation of body pH

8. Haemopoiesis (Erythropoietin of the cortical interstitial cells)

9. Conversion of Vitamin D3 to its active form which is required for body calcium metabolism

Component Parts/Organs of the system: (See Fig. 1) These include:

1. Two kidneys (Formation of urine)

2. Two ureters (Collection and transport of urine)

3. One urinary bladder (Storage of urine)

4. One urethra (Transport/Elimination of urine)

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Figure: 1

RENAL MICROSTRUCTURE (Fig. 2)

MICROSTRUCTURE OF THE KIDNEY

The kidney is enveloped in three connective tissue sheaths, viz.

Pararenal fat: This is derived from extraperitoneal fat and separates the kidney from the peritoneum.

Renal fascia: This is a thin layer of fibroareolar connective tissue. It lies deep to the pararenal fat

layer

Perirenal capsule: This is a layer of adipose tissue beneath the renal fascia.

Renal capsule: This is a dense fibrous tissue layer which covers the surface of the kidney and passes

through the hilus (hilum) to line the renal sinus (A cavity in the medial aspect of the interior of the kidney

A frontal section taken through the substance of the kidney along the medial border reveals two

microscopically and functionally distinct areas, viz.

1. An outer deep-staining cortex and

2. An inner pale-staining medulla.

The two areas are separated by tangentially oriented blood vessels called the arcuate arteries and veins

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Figure: 2

Histological features of the Cortex:

a. It is deep-staining

b. Consists of:

Renal Corpuscles (Bowman’s capsule and Glomerulus)

Renal tubules

Collecting tubules and ducts

Medullary rays comprising collecting tubules and collecting ducts

Interstitial cells and connective tissue

Blood vessels

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Nerves

Lymphatics

Histological Features of the Medulla: a. It is pale-staining

b. Consists of:

Renal pyramids

Renal papillae

Renal columns of Bertin (Containing interlobar vessels)

Papillary ducts of Bellini

Interstitial cells and connective tissue

Lipid-laden interstitial cells implicated in the synthesis of Prostaglandins and Prostacyclins

Blood vessels

Lymphatics

Nerve fibers

THE NEPHRONS:

These are the functional units of the kidney, numbering between 1-4 millions per kidney and are made

up of:

1. The renal corpuscles and

2. The renal tubules (Note that the term Uriniferous tubules refers to the renal tubules, collecting

tubules and collecting ducts)

Renal Corpuscle: The renal corpuscle measures about 200µ in diameter and is responsible for the filtration of blood

plasma. It is composed of two parts, viz. Bowman’s capsule and Glomerulus

The Bowman’s capsule: This is the distended, funnel-shaped, blind, proximal end of the renal

tubule. Its characteristic features include:

1. It is lined by a simple squamous epithelium supported by a basal lamina and a thin layer

of reticular fibers.

2. It is subdivided into visceral layer of modified squamous cells called Podocytes and a

parietal layer of simple squamous cells.

The parts of the podocytes are:

a. Central part containing the nucleus and other organelles

b. Primary cytoplasmic processes

c. Secondary foot processes arising from the primary processes. These are called Pedicels.

3. The visceral and parietal layers of bowman capsule are continuous at the vascular pole

(See Diagram)

4. The squamous cell-lined parietal layer is continuous with the columnar cell-lined

proximal convoluted tubules at the urinary pole.

5. The visceral and parietal layers are separated by the Bowman’s space, which is

continuous with the lumen of the proximal tubules at the urinary pole (See diagram).

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The Glomerulus:

This is the globular network of densely packed anastomosing capillaries.

It occupies the concavity of the Bowman’s capsule surrounded by the podocytes of the capsule’s

visceral layer.

Other features of the glomerulus are: (See diagram).

1. It is supplied by a wide diameter afferent arteriole which enters it at the vascular pole

2. It is drained by smaller diameter efferent arteriole which leaves it at the vascular pole

3. The basement membrane of the glomerular capillaries fuse with that of the podocytes to form the

glomerular basement membrane which is extremely thick and serves as the filtration barrier.

4. It consists of supporting tissue called mesangium which consists of mesangial cells believed to

have supportive as well as phagocytic properties

5. The capillary endothelium has large and numerous fenestrations without diaphragms.

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Microscopy of the Proximal and Distal Convoluted Tubules The characteristic histological features of the proximal and distal convoluted tubules are best

appreciated when compared as in the table below:

S/N PROXIMAL TUBULE DISTAL TUBULE

1. Longer and more coiled Shorter and less coiled

2. Located in the cortex Located in the cortex

3. 75 – 85 % of re-absorption occurs here.

These include: Glucose, Amino acids,

Sodium Chloride, Water, Phosphate and

Calcium

Re-absorbs Sodium chloride mainly

and secretes Potassium and Hydrogen

ions.

4. Lined by large columnar cells with

abundant microvilli [brush border]

Lined by smaller cuboidal cells with

sparse microvilli

5. More acidophilic due to abundant

mitochondria

Less acidophilic due to fewer

mitochondria

6. Narrow lumen due to abundant microvilli Wide lumen due to fewer microvilli

7. Extensive basal invaginations Less extensive basal invaginations

8. Possesses basolateral cellular

interdigitations

Possesses basolateral cellular

interdigitations

9. Possesses junctional complexes towards

the luminal surface.

Possesses junctional complexes towards the

luminal surface.

10. Functions not controlled by

Aldosterone

Functions controlled by aldosterone

11. Contains more organelles Contains fewer organelles

12. Possesses apical canaliculi and

vesicles

No apical canaliculi and vesicles

13. Does not form macula densa Forms macula densa

14. Not involved in regulation of

filtration rate

Regulates filtration rate via Renin-Angiotensin-

Aldosterone mechanism

15. Does not regulate salt and body fluids Regulates salt and body fluids

16. Secretes Creatinine and foreign

materials hence used in assessing

renal function

Secretes Ammonium and hydrogen

ions for maintenance of acid-base

balance.

Under the influence of

Aldosterone, it facilitates re-absorption

of sodium chloride.

Also secretes potassium.

The sixteen features highlighted in the table are distinctive of the convoluted tubules

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The Loop of Henle: This is the U-shaped continuation of the proximal convoluted tubule located entirely in the renal

medulla. It is lined predominantly by squamous cells except for the parts of it adjacent to the proximal

and distal tubules. It has a wide lumen and it is subdivided into four parts viz.

1. Thick descending segment, which is similar histologically to the proximal tubule.

2. Thin descending limb, which is freely permeable to water and lined by squamous

epithelium.

3. Thin ascending limb, which is impermeable to water and lined by squamous epithelium.

4. Thick ascending segment, which is similar histologically to the distal convoluted tubule

and impermeable to water. Furthermore, sodium chloride is actively absorbed from this

segment into the medullary interstitium. This is a prerequisite process for urine

concentration

Classification of the Loop of Henle:

Based on the distribution of the loops of Henle within the substance of the kidney, two types of loops are

recognized viz.

1. Juxtamedullary Loops of Henle: These originate from the nephrons whose corpuscles are located

near the junction of the medulla and cortex of the kidney. They constitute 1/7 of the entire

population of nephrons and are characterized by longer loops than the other type. They are also

associated with Vasa recta (Straight Capillaries) of the medulla which originate from the efferent

arterioles of these nephrons.

Furthermore, these nephrons also play a principal role in establishing the gradient of hypertonicity in the

medullary interstitium, a requirement for production of hypertonic urine

2. Cortical Loop of Henle: These loops are shorter and originate from the nephrons whose

corpuscles are located in the peripheral and mid-cortical regions of the cortex. They account for

6/7 of the entire population of nephrons.

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Functions of the Loop of Henle:

The main function of the loops of Henle is to:

Generate a hypertonic gradient in the medullary interstitium by the process of Counter-Current

Multiplier System. thus facilitating

Water retention in the body

Production of hypertonic urine and

Maintenance of body fluids

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The Collecting Tubules and Ducts: These are component parts of the uriniferous tubules. The collecting tubules receive urine from the distal

convoluted tubules and drain into the straight collecting ducts in the medullary rays. Several collecting

tubules converge to form a collecting duct. The collecting ducts continue into the pyramid of the

medulla where they enlarge to become the papillary ducts of Bellini at the papillary end of the pyramid.

These ducts then open into orifices at the area cribrosa of the papillary summit.

Characteristic Histological Features of the Collecting tubules and Collecting ducts:

1. Both are lined by cuboidal cells, which become progressively taller in the duct where they

invariably become culumnar.

2. Their cytoplasm stains weakly due to paucity of organelles.

3. Both are sensitive to ADH, which enables them to re-absorb water passively from the filtrate

leading to production of hypertonic urine.

4. Urea is also reabsorbed from the collecting ducts

5. They lack basal invaginations (Folds) and basolateral interdigitations

6. They are characterized by irregular/occasional distribution of microvilli

7. Cells lining the duct have single central cilium

8. There is a sparse distribution of intercalated Dark cells with longer microvilli and mitochondria

amongst the cells of the tubules. These cells are believed to secrete a hormone called Epidermal

Growth Factor.

Juxtaglomerular Apparatus:

This is a functional unit of the kidney that is formed by segments of the afferent arteriole and the distal

convoluted tubule at the vascular pole of the corresponding Bowman’s capsule. Its basic functions are

the autoregulation of glomerular filtration rate and control of blood pressure. The characteristic

histological and functional features of the unit are:

1. Modification of smooth muscle cells of the tunica media of an afferent arteriole to form

Juxtaglomerular (JG) cells. These cells are PAS positive and secrete the hormone called Renin.

The modified smooth muscle of this segment of arteriole also lacks internal elastic lamina.

2. Modification of epithelial cells of adjoining distal convoluted tubule to form the Macula Densa,

which is responsive to the level of chloride ions in the filtrate.

3. Extraglomerular mesangial cells (Lacis cells). These cells form a conical mass at the centre of

this apparatus. The apex is directed towards the glomerulus, the side in contact with the afferent

and efferent arterioles and the base is in contact with the macula densa. Lacis cells are believed

to secrete erythropoietin. They are also involved in the Tubuloglomerular feedback mechanism

through which changes in macular sodium concentration is relayed to the glomerular capillary

network.

4. Implicated in the maintenance of blood pressure via the Renin/Angiotensin II/Aldosterone

mechanism.

THE URINARY TRACT

The Urinary tract is the system of ducts responsible for transport of urine from the kidney to the exterior.

It is subdivided into two parts viz.

1. Upper Urinary tract which consists of:

a. The minor calyx

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b. The major calyx

c. The renal pelvis and

d. The ureter.

2. Lower Urinary tract, which consists of:

a. The urinary bladder and

b. The urethra

In general, the wall of the urinary tract is made up of three histological layers viz.

Outer tunica adventitia of loose fibroelastic tissue.

Intermediate layer of smooth muscle.

Inner layer of mucous membrane which, is made up of an outer lamina propia of connective

tissue and an inner epithelium which is characteristic of the urinary system and transitional

in type in most parts of the tract. This urinary epithelium is conventionally referred to as the

Urothelium.

A Filtration Slit

B Primary process

C Secondary process (Pedicel)

D Podocyte cell body

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