Urinary System

Written By Anjani Mishra

The urinary system consists of the paired kidneys and the urinary tract which include ureters, urinary bladder and urethra.

Kidneys

The kidneys are paired organs situated retro-peritonially on the posterior wall of the abdominal cavity.

Fig: Urinary system of dog

Fig: Urinary system of human

Structure

Capsule- the kidneys are covered by a connective tissue capsule consists of a collagen fiber network with small amount of elastic fibers.

A longitudinal Sections of kidney present;

1) A peripheral darker zone- the cortex,

2) An inner lighter zone- the medulla.

The cortex consists of renal corpuscles, proximal and distal convoluted tubules. The medulla consists of loop of henle's (both descending & ascending) and straight tubule which converge towards the apex called medullary rays.

Fig: Schematic diagram of the kidneys of cow

Fig: Section of kidney of ox

Fig: Schematic diagram of uriniferous tubule

Fig: Kidney, cortex and portion of medulla, dog

3. Capsule, 7. Medulla, 8. Pars convoluta, 9. Pars radiata, 11. Renal corpuscle

Uriniferous tubules

The functional unit of the kidney is the uriniferous tubules, which is highly convoluted structure involved in urine formation. Each uriniferous tubules consists of a secretory part, called the nephron, and a collecting tubule.

Nephrons

Nephrons are the basic functional subunits of the kidney. Each nephron consists of renal corpuscle, proximal convoluted tubule, thick and thin limbs of Henle's Loop, and distal convoluted tubule.

Renal corpuscle

It consists of glomerulus and the Bowman's capsule. It is oval to round structure about 200mm in diameter, where filtration of blood occurs.

Glomerulus

It is a tuft of capillaries that extends into the Bowman's capsule. It is formed by the branches of the afferent arteriole.
It is lined with glomerular endothelial cells that form the inner layer of the capillary wall. They have large fenestrae (gap).
A special type of cell, called mesangial cells lie between the capillary loops. These cells can contract and there by decreasing the surface area available for filtration.

Fig: Kidney, renal corpuscle, puppy

2. Capsular epithelium, 5. Distal convoluted tubule, 6. Glomerular epithelium,

10. Proximal convoluted tubule, 14. Urinary space

Bowman's (renal) capsule

It is a double walled epithelial capsule, composed of parietal and visceral layer. In between the parietal and visceral layer, there is a narrow space called Bowman's Space (or urinary space).
The parietal layer is composed of simple squamous epithelium, forming the outer limit of the renal corpuscle.
The visceral layer is composed of modified simple Squamous epithelium, the podocytes, that envelops the capillaries of the glomerulus.
Podocytes have a cell body from which arises several primary processes. Each primary process gives rise to numerous secondary processes, called, pedicels. Pedicels that embrace the capillaries of the glomerulus.

Proximal Convoluted tubule

An epithelial tube that begins at the urinary pole of the renal corpuscle.
It is lined with simple cuboidal to columnar epithelium
The living cells have abundant long microvilli. Together they form brush border that partly obscures the lumen and increase the surface area available for absorption.

Henle's loop

It is U-shaped epithelial tube consisting of a thick descending limb, a thin descending limb, a thin ascending limb, and a thick ascending limb.

Descending loop of henle (Thick)

It is lined by sim. Cuboidal epithelium that has a prominent brush border.
It is also known as the straight portion of the proximal tubule.

Descending loop of henle (thin)

It is lined with simple Squamous epithelium.
They have few short microvilli.

Ascending loop of henle (thin)

Same as above

Ascending loop of henle (thick)

It is lined by simple Cuboidal epithelium cells that posses a few microvilli.
It is also known as the straight portion of the distal tubule.

Fig: Kidney, cortex, dog

1. Adipose tissue, 3. Capsule, 8. Pars convoluta, 9. Pars radiata, 11. Renal corpuscle

Distal convoluted tubule

It is much shorter and has a wider lumen than the proximal convoluted tubule.
It is lined by simple Cuboidal epithelium cells.
They do not contain brush border.
It establishes contact with the vascular pole of renal corpuscle. At this point of close contact, the wall of the distal tubule is modified, forming macula densa, which is the part of juxtaglomerular apparatus(JGA).

Fig: Diagrams of uriniferous tubule showing the location and structure of juxtaglomerular apparatus(JGA)

Juxtaglomerular apparatus(JGA)

It is located at the vascular pole of the renal corpuscle. It consists of following structures;

Macula densa of the distal tubule,
Juxtaglomerular cells of afferent and occassionally efferent glomerular arterioles, and
Juxtaglomerular mesangial cells, also known as polkissen/lacis cell

Collecting tubules

They have the segments in both the cortex and medulla.
It is lined by simple Cuboidal epithelium cells (cortex)
They don not have brush border.
Medullary collecting tubules are lined by simple columnar cells.
They have larger lumen than the convoluted tubules.

(Note:Figure for practical note book)

Ureter

The ureter conveys urine from the kidney to the urinary bladder.

Structure

The wall of ureter has 3 layers;

1 1. Mucosa

a. Lamina epithelia

b. Lamina propria

2 2. Muscularis externa
3. Adventitia

Fig: Ureter, x.s., cat

1. Adventitia, 2. Lamina propria, 4. Muscularis, 7. Transitional epithelium

1. Mucosa

a. Lamina epithelia- It is lined with transitional epithelium with a wide variety of cell forms. In the distended ureter all cell appear in section to be narrow spindle.

b. Lamina propria- It consists mainly of collagen fiber, blood vessels, lymphatics. In horse, it contains tubulo-alveolar mucous glands with wide lumina. The lumen has a star-shaped appearance, that is due to the longitudinal folds of the mucosa.

Muscularis mucosa and sub-mucosa- absent

2. Mucularis externa

It consists of thin bundle and forms;

- An inner longitudinal,

- A middle circular, and

- A thin outer longitudinal layer, which may occasionally be interrupted by collagenous bundles. Near the opening into the bladder, the ureter contains only longitudinal muscle fiber.

3. Adventitia

External to the muscularis, a loose adventitia composed of collagen and reticular fiber contains the larger blood vessels and nerves

Fig: Ureter, x.s., horse

2. Lamina propria, 3. Mucous gland, 7. Transitional epithelium

Urinary bladder

Fig: Urinary bladder, cow

2. Lamina propria, 4. Muscularis, 5. Muscularis mucosae, 6. Sub-mucosa,

7. Transitional epithelium

Structure
The wall of urinary bladder consists of ;

1.Mucosa

a. Lamina epithelia – lined with transitional epithelium.

b. Lamina propria – the collagenous propria contains few elastic fibers.

c. Lamina muscularis mucosa – there is usually a thin layer of longitudinal muscle in between the

lamina propria and a loose sub-mucosa.

2. Sub-mucosa – Consists of loose connective tissue

3. Muscularis externa – It consists of coarse bundle of fibers and are arranged in :

- An inner longitudinal layer

- A middle circular (thickest layer)

- An outer longitudinal layer

The middle circular layer forms the sphincter around the internal urethral orifice, where as outer longitudinal may be modified or absent in some places.

4. Serosa/ adventitia

A serosa is present on the apex and body of the bladder. At the neck, the muscularis is covered by an adventitia of the same structure as ureter.

Fig: Urinary bladder, goat

1. Capillary, 5. Lamina propria, 9. Transitional epithelium

Urethra

Fig: Genital organs of bull

Fig: Genital organs of cow

Fig: Genital organs of bull

1. Ureter, 2. Prostate gland, 3. Seminal vesicle, 4. Urethral muscle, 5. Bulbourethral gland

Fig: Pelvic urethra

The male urethra

The urethra is divided into pelvic portion and an extra-pelvic portion or penile portion.

1. Pelvic urethra

Structure

The wall of penile urethra consist of following layers;

a. Lamina epithelia – The pelvic urethra is lined by a longitudinally folded mucosa which bears transitional epithelium showing evidence of secretory activity. The surface of the epithelium is irregular.

b. Lamina propria – The thin propria is collagenous and poorly vascularized.

Corpus cavernosum urethrae – Next to the mucosa lies the stratum vasculare or cavernosum, which is present in all species. It is composed of dense plexus of veins and have the character of erectile tissue. The trabeculae between the veins are rich in smooth musculature. The adjoining glandular layer contains the prostatic tissue (or urethral glands).

c. Lamina muscularis mucosa – The next layer is a sheet of thin bundles of smooth muscles, followed by the striated urethral muscle.

Fig: Penile urethra, x.s., ram

2. Cavernosus space, 10. Tunica albuginea, 11. Urethra, lumen

2. Extra pelvic/ Penile urethra

Structure

The penile urethra has fewer glands but more erectile tissue than the pelvic portion. The folded mucosa bears transitional epithelium, which changes at the urethral opening to stratified squamous epithelium on a papillary body.
The lamina propria varies in thickenss. In the bull it has the character of lymphatic tissue, but in the stallion and boar, it contains the small scattered 'glands of Littre.'
The cavernosus tissue adjoining the mucosa is composed of connective tissue trabeculae which are continuous peripherally with the tunica albuginea that limits the erectile tissue. The trabeculae contain many elastic fibers and blood vessels, and in the horse and bull, longitudinal smooth muscle fibers.
Between the trabeculae there are spaces increasing in size towards the albuginea, which are lined with endothelium and usually filled with blood. Towards the mucosa these spaces pass into an extensive venous plexus which occupies the deeper layers of the mucosa.
The cavernosus spaces receive their blood from these veins and from the capillaries of the trabecular system. Here the artery of the urethral bulb opens directly into the cavernosus sinuses.

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Urinary System | Histology | Anjani Mishra