Urinary System

                                     URINARY SYSTEM

Introduction:-

Urinary system of the body is formed by the organs responsible for converting the excess fluid and other substances in to urine, its filtration from the blood and exertion from the body. Theses organs include kidneys, ureter, urinary incontinence bladder and urethra. Urine is formed by the kidneys, then sent to the urinary bladder for storage, and finally excreted through urethra. Urine extract excess minerals, vitamins, and blood cells from the body.

 Urinary system along with the other system of the body maintains homeostasis. Kidneys filter play a major role in homeostasis as they maintain the acid base and water salt balance in the blood.

 

Anatomy of Urinary Tract:-

The urinary tract is made up of the following organs:

1)     A pair of kidneys (forming urine)

2)     A pair of ureters (transporting urine)

3)     A urinary bladder (storing urine), and

4)     A urethra (carrying urine outside the body).

 

Kidneys: -

 

Kidneys are present in pair in vertebrates as well as some of the invertebrates. They are the major organs of urinary system. They produce urine through which waste materials such as urea and ammonium are excreted. They also reabsorb glucose and amino acids. Along with this kidneys perform secretory function, e.g., calcitriol, erythropoietin, and renin are some of the hormones produced by kidneys.

 

Kidneys also perform some other homeostatic functions, like:

1)     Regulate electrolyte balance,

2)     Maintain acid-base balance, and

3)     Control the blood pressure.

Anatomy

Kidneys are bean-shaped and positioned in a retroperitoneal position (i.e., between the dorsal body wall and the parietal peritoneum) in the superior lumbar regain. The right kidney is present slightly lower than the left. Mass of an adult kidney is about 150gm and is 12cm long, 6cm wide, and 3cm thick. The kidneys have a convex lateral surface and a concave medial surface. They also have a vertical cleft (the renal hilum) leading into an internal space within the kidney (the renal sinus). Hilum forms the area where the ureters, renal blood vessels, lymphatics, and nerves join the kidney.

Organs Associated with the kidneys

Since the kidneys are present on either side of the vertebral column, each has different associated structures.

Right kidney

1)     Superiorly: The superior wall is attached to the right adrenal gland.

2)     Anteriorly: The anterior wall is attached to the right lobe of the liver, duodenum, and the hepatic flexure of colon.

3)     Posteriorly: The posterior wall is attached to the diaphragm and muscles of the posterior abdominal wall.

Left kidney

1)     Superiorly: The superior wall is attached to the left adrenal gland.

2)     Anteriorly: The anterior wall is attached to the spleen, stomach, pancreas, jejunum, and the splenic flexure of colon.

3)     Posteriorly: The posterior wall is attached to the diaphragm and muscles of the posterior abdominal wall.

Kidneys are covered by the following three connective tissue layers from outer to innermost:

1)     Renal fascial: This outer layer of dense irregular connective tissue attaches the kidneys to the bladder peritoneum and to the fascia covering the muscles of posterior abdominal wall.

2)     Adipose capsule: this middle and thickest layer of adipose tissue holds the kidneys in place and protects them against physical trauma.

3)     Renal capsule: this innermost and this layer of dense irregular connective tissue covers the exterior of each kidney and protects them from infection and physical trauma called a renal tubule.

Functions

Kidneys perform the following functions:

1)     Regulation of blood ionic composition: They regulate the concentration of some ions (Na+, K+, Ca2+, Cl-, and HPO42-­ ) in blood.

 

2)     Regulation of blood pH: They regulate blood pH by excreting H+ ions in to the urine and conserving HCO3- ions (an important buffer of H+ ions in the blood).

 

3)     Regulation of blood volume: They regulate blood volume by conserving water or eliminating the excess in urine. Blood volume in turn regulates the blood pressure, since an increase in blood volume increases blood pressure and vice versa.

 

4)     Regulation of blood pressure: They regulate blood pressure by secreting renin enzyme, which activates the renin-angiotensin-aldosterone pathway. Renin in increased amount increases the blood pressure.

 

5)     Maintenance of blood osmolarity: They maintain a constant blood osmolarity [value of which is close to 300 milliosmoles per liter(mOsm/litre)] by regulating loss of water and solute in the urine.

 

6)     Productions of hormones: They produce calcitriol ( the active form of vitamin D) which regulates calcium homeostasis; and erythropoietin which stimulate red blood cells production.

 

7)     Regulation of blood Glucose level: They utilize glutamine amino acid in gluconeogenesis( synthesis of new glucose molecules), and then release the resultant glucose in to the blood for maintaining the glucose level.

 

 

Nephrons:-

Nephrons is the basic structural and functional unit of the kidney. filtering units called nephrons It filters the blood out of remove wastes materials, reabsorb the required ones, and excrete the remaining with urine; thus regulates the concentration of water and soluble substances like sodium salt in the blood.

 

Based on the location of the associated renal corpuscle, the small tube called nephrons are divided into:

1)     Cortical Nephrons: In these nephrons, the renal corpuscles are present in the superficial renal cortex.

 

2)     Juxtamedullary Nephrons: In these nephrons, the renal corpuscles are present near the renal medulla.

Ureters:-

ureters are paired tubes through which the urine flows from the kidneys to the urinary bladder. Both the tubes begin from the sinus of the corresponding kidney as calyces (short cup-shaped tubes) surrounding the renal papillae.

 

 

Anatomy:-

Ureters are 25-30cm long, thick-walled, narrow cylindrical tubes. They begin near the lower end of the kidney with the tapering extremity of the renal pelvis. They run downward and medially in front of the psoas major, enter the pelvic cavity, and terminate in the fundus of the urinary bladder.

 

 

 

The origin point of the right ureter is covered by the descending part of the duodenum; while running down it lies to the right of the inferior vena cava and is crossed by the right colic and ileocolic vessels; And near the pelvis it passes behind the lower part of the mesentery and the terminal part of the ileum. The left ureter crossed by the left colic vessels, passes behind the sigmoid colon and its mesentery near the pelvis.

 

1)      Fibrous coat or tunica adventitia: one end of this coat is continuous with the fibrous tunic of the kidney on the sinus floor; and the other end is somewhere within the fibrous bladder.

2)     Muscular coat or tunica muscularis: this coat in the renal pelvis is further made up of two layers:

i)                   Longitudinal Fibre: This layer lies on the sides of the papillae at the extremities of the calyces.

ii)                 Circular Fibre: This layer surrounds the medullary substance.

The muscular fibres of the ureters are differentiated into three layers, i.e., an external longitudinal, a middle circular, and an internal (less distinct than the other two, but runs longitudinally).

  

3)     Mucous Coat or Tunica Mucosa: This smooth coat has a few longitudinal folds which erode on distension. It joins the mucous membrane of the bladder, while it is prolonged over the papillae of the kidney. Its epithelial layer is similar to that of the bladder.

 

It is further made up of following cellular layer:

i)                   In the innermost layer, the cells are flat and concave on there deep surfaces into which the round ends of the second layer cells fit. The cells of this layer remain in direct contact with the urine.

 

ii)                 In the intermediate layer, the cells resemble that of columnar epithelium, are pear-shaped, have rounded internal extremities which fit into the concave surface of the first layer cells, and narrow external extremities wedged between the third layer cells.

 

 

iii)               In the external or third layer , the cells are conical or oval, vary in different parts, and carry processes extending down into the basement membrane. A dense layer of fibrous tissue(beneath the epithelium) containing elastic separated this layer from the muscular coat.

Functions:-

 

1)     They transport urine from the renal pelvis of the kidney to the urinary bladder.

 

2)     Since they pass beneath the urinary bladder, during urination when pressure in the bladder is high the ureters are compressed and back-flow of urine is prevented. Otherwise, cystitis (inflammation of the ureter/urinary bladder) may develop which may lead to a kidney infection.

 

 

Urinary Bladder:-

Urinary bladder is a hollow muscular, and distensible ( or elastic) organ, which rests on the pelvic floor. It receive urine from the kidney via the ureters, store it within, and expels it during urination via the urethra. It is a reservoir where urine is stored temporarily.

 

The bladder is somewhat spherical in shape, although its shape and size vary from individual to individual and also depends on the urine volume it stores. An empty bladder is about the size and shape of a pear. The normal capacity of the bladder is 400-600ml.

 

 

Anatomy:-

Urinary bladder  is in the pelvic capacity, situated posteriorly to the symphysis pubis, and inferiorly to the parietal peritoneum. It is made up of three layers:

 

1)     Mucous Membrane: It is the inner lining of the bladder, consisting of transitional epithelium continuous with that of the ureters. The mucosa of an empty bladder has numerous folds called rugae, which along with the transitional epithelium allow the bladder to expand when filled with urine.

 

2)     Submucosa: It is the second layer, consisting of connective tissue with elastic fibres and supporting the mucous membrane.

 

3)     Muscularis: It is the outer layer consisting of smoot muscle having fibres interwoven in all directions, collectively termed detrusor muscle. This muscle contracts to expel urine from the bladder. Parietal peritoneum is present on the superior surface of this layer. In the other regions, the outer layer is fibrous connective tissue.

 

 

A triangular area, called trigone formed by three openings is present on the urinary bladder floor. The trigone base is formed by two opening from the ureters. These openings are covered by small flap of mucosa which act as valves to allow the entry of urine In to the bladder and to prevent its back-flow into the ureters. The trigone apex is formed by the third opening into the urethra. This opening is covered with detrusor muscles which form an internal urethral sphincter.

 

Functions:-

 

Urinary bladder functions are:

1)     It is a reservoir for urine.

2)     It expels urine via urethra.

 

A urinary bladder filled with urine becomes distended. Urine stimulates the stretch receptors on the bladder wall, which in turn trigger a reflex contraction of the bladder wall muscles and relax the internal sphincter (a valve which remains closed so that the urines remains in the bladder till urination). Soon the external brain signals the sphincter relaxes and the bladder expels the urine. A parasympathetic nerve fibre transmits a signal that causes bladder contractions and internal sphincter relaxations.

 

Urethra:-

Urethra is a tube-like structure which transports urine from leaking the urinary bladder to the exterior of the body. It forms the “exit tube” of the body for liquid wastes. It is closed by the urethral sphincter(a muscular structure) which keeps the urine in the bladder till urination.

 

Mucous membranes from the inner lining of the urethra, and muscular layer forms the outer layer. The smooth muscle fibre direct longitudinally. The urethral walls have highly specialized urethra glands which constantly secrete mucous coating the urethral canal.

 

Anatomy:-

Urethra is made up of two separate urethral sphincter muscles. The internal urethral sphincter muscle consist of lower voluntary muscles. Detrusor muscle make up the internal sphincter external urethral orifice.

female urethra

 male urethra

 

 

The characteristic features of female and male urethra are:

1)     Female urethra: It is 20cm long and opens to the exterior via urethra orifice, located in the vestibule in the labia minora between the clitoris and the vaginal orifice. Female urethra transports urine from the bladder to the outside at the time of urination.

 

2)     Male urethra: It is 20cm long and S-shaped to follow the line of the penis. It transports urine(during urination) and semen (during ejaculation) to the outside.

 

Male urethra consists of the following three regions:

 

i)                   Prostatic urethra: This part of the male urethra after leaving the urinary bladder, passes through the prostate gland present below the bladder; thus named so.

 

ii)                 Membranous Urethra: This part of the male urethra is the shortest, and passes through the levator ani muscle.

 

iii)               Spongy Urethra: This part of the male urethra is the longest, and passes through the penis to the external urethral sphincter orifice. It is named so since it passes through corpus spongiosum (an erectile body of the penis).

 

 

Functions:-

Urethra performs the following functions:

1)     It is the passageway through which urine is expelled out of the body.

2)     In males, it is also the passageway through which semen is ejaculated.

 

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