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Renal System Urinary System

Anatomy and Physiology of Renal System

The renal system is composed of the following four structures

  • Kidneys
  • Ureters
  • Bladder
  • Urethra

The kidneys balance the urinary excretion of substances against the accumulation within the body through ingestion or production. Consequently, they are a major controller of fluid and electrolyte homeostasis. The kidneys also have several non-excretory metabolic and endocrine functions, including blood pressure regulation, erythropoietin production, insulin degradation, prostaglandin synthesis, calcium and phosphorus regulation, and vitamin D metabolism.

Filtration at the renal glomerulus is the first step in urine formation. Noramlly, a volume equal to plasma volume is filtered every 45 minutes, and a volume equal to total body water is filtered every 6 hours.

Glomerular filtrate is similar to plasma but lacks cell and large-malecular-weight proteins. The glomerular filtrate is modified by active transport, diffusion and osmosis as it passes through the renal tubules.

Reabsorption of filtrate components, enhances conservation of glucose, peptides, electrolytes and water.

Secretion of plasma components enhances elimination of organic acids, and bases (and some drugs). The remnants of the glomerular filtrate exit the kidney through the Ureters.

The Ureters conduct urine from the kidneys to the bladder by the peristaltic contraction.

The Bladder is a distensible chamber that stores urine until it is eliminated.

The Urethra is the exit passageway from the bladder and it carries urine for elimination from the body.

Renal System

Kidneys

The kidneys are a pair of bean-shaped organs, which is found along the posterior wall of the abdominal cavity. The left kidney is located slightly higher than the right kidney because the right side of the liver is situated  The kidneys are located in the abdominal cavity, posterior to the peritoneum on each side of the vertebral column, at about the level of the twelfth rib and touch the muscles of the back. Adult kidneys average 11 cm in length, 5 to 7.5 cm in width and 2.5 cm in thickness.

Affixing the kidneys in position behind the parietal peritoneum is a mass of perirenal fat (Adipose capsule) and connective tissue called Gerota’s (subserosa) fascia. A fibrous capsule,(renal capsule) forms the external covering of the kidney itself, except the hilum. The kidney is further protected by layers of muscle of the back, flank and abdomen as well as by layers of fat, subcutaneous tissue and skin.

Each kidney is divided into three major areas…

  • Cortex
  • Medulla
  • Pelvis

Cortex

The cortex of the kidney lies just under the fibrous capsule and portions of it extend down into the medullary layer to form the renal columns (columns of Bertin) or cortical tissue that separates the pyramids.

Medulla

The medulla is divided into 8 to 18 cone-shaped masses of collecting ducts called renal pyramids. The bases of the pyramids are positioned on the corticomedullary boundary. Their apices extend toward the renal pelvis, forming papillae, Each papilla has 10 to 25 openings on the surface, through which the urine empties into the renal pelvis. Eight or more groups of papillae are present in each pyramid; each empties into a minor calyx and several minor calices join to form a major calyx, The two to three major calices are outpouchings of the renal pelvis. They channel the urine from the pyramids to the renal pelvis.

Pelvis

The renal pelvis cavity is lined with transitional epithelium. The combined volume of the pelvis and calices is about 8 ml. Volumes in excess of this amount damage the renal parenchymal tissue. The renal pelvis narrows as it reaches the hilus and becomes the proximal end of the ureter.

Within the cortex lies the nephron, the functional unit of the kidney, which consists of both vascular and tubular elements. each kidney contains about 1.3 million of them

The nephron is the unit of the kidney responsible for ultrafiltration of the blood and reabsorption or excretion of products in the subsequent filtrate. Each nephron is made up of…

A filtering unit…

  • The glomurulus 125 ml/min of filtrate is formed by the kidneys as blood is filtered through this sieve-like structure. This filtration is uncontrolled.
  • The Proximal Convoluted Tubule Controlled absorption of glucose, sodium, and other solutes goes on in this region.
  • The Loop of Henle, this region is responsible for concentration and dilution of urine by utilising a counter-current multiplying mechanism- basically,water is impermeable but can pump sodium out, which in turn affects the osmolarity of the surrounding tissues and will affect the subsequent movement of water in or out of the water-permeable collecting duct.

The distal convoluted tubule, This region is responsible, along with the collecting duct that it joins, for absorbing water back into the body- simple maths is that the kidney doesn’t produce 125ml of urine every minute. 99% of the water is normally reabsorbed, leaving highly concentrated urine to flow into the collecting duct

Each nephron consists of glomerulus and renal tubule. Each nephron is capable of forming urine. Urine is formed by three basic processes…

  • Filtration
  • Reabsorption
  • Secretion

Filtration

Glomerulus is a tiny filter. It contains tuft of glomerular capillaries. This capillary tuft is present in a double layered Bowman’s capsule. Capillaries are thin walled and pressure in them is high as compared to capillaries present at other sites in the body. This allows fluid and water products to be filtered into the Bowman’s capsule. Bowman’s capsule continues as renal tubule. So glomerular filtration is 125 ml/minute (1.80 liters/day) The amount of urine formed is approximately 1 to 1.5 mi/minute this meansthat large portion of filtrate is absorbed as it passes through the tubule of the nephron.

Reabsorption

It is the important function of the tubule. It prevents loss of fluid, salts and other important substances such as glucose, amino acids, protein. Waste products are not absorbed therefore they are allowed to retain in the tubule for excretion.

Secretion

Certain substances are added to the filtrate while it is passing through the tubule so these substances are excreted.

The blood supply to the kidney directly through from the aorta via the renal arteries; Renal veins take blood away from the kidneys into the inferior vena cava.

Ureters

The Ureters form the medial tapering of the renal pelvis at the hilus of the kidney.
The ureters are a pair of tubes which carry urine from the kidneys to the urinary bladder. The ureters are about 10 to 12 inches long and run on the left and right sides of the body parallel to the vertebral column. The Ureters lie in the extraperitoneal connective tissue and descend vertically along the psoas muscle towards the pelvic cavity.

There are three points of potential obstruction exist…

  • The ureteropelvic junction
  • The pelvic brim ( where Ureters cross iliac arteries)
  • The ureeterovesical junction

Each ureter has elastic characteristics and is made of three tissue layers…

  • An inner mucosa (transitional epithelial membrane) lining the lumen
  • A muscular layer
  • A fibrous outer layer

Blood is supplied to the Ureters by one or more vessels that run longitudinally along the tube. The number and assortment of arteries anastomosing with the ureteric vessles vary with each individual. Because the Ureters travel through several anatomic areas, the ureteral vessles are fed by several of the following arteries.

  • Renal (frequently)
  • Testicular or ovarian
  • Aorta and common iliac
  • Internal iliac (frequently)
  • Vesical
  • Umbilical
  • Uterine

Gravity and peristalsis of smooth muscle tissue in the walls of the ureters move urine toward the urinary bladder. The ends of the ureters extend slightly into the urinary bladder and are sealed at the point of entry to the bladder by the ureterovesical valves. These valves prevent urine from flowing back towards the kidneys.

Bladder

The urinary bladder is a hollow organ located in the anterior half of the pelvis behind the symphysis pubis.  The space between the bladder and the symphysis pubis is filled with a loose connective tissue that allows the bladder to stretch cranially as it fills. The walls of the bladder allow it to stretch to hold anywhere from 600 to 800 milliliters of urine. The peritoneum covers the top border of the bladder and base is held loosely in place by the true ligaments.

There are three ill-defined muscle layers:

  • The inner and outer layer (longitudinal)
  • The middle layer (circular)

The fibers from these layers interweave to form a mesh-like muscle layer called the detrusor muscle. This arrangement allows the bladder wall to be elastic while maintaining strength.

The superior and lateral aspects of the bladder are served by the superior vesical artery, which branches from the umbilical artery and internal iliac artery. The inferior vesical artery, which supplies the underside of the bladder, may arise independently or in common with the middle rectal artery. The veins draining the bladder pass to the internal iliac trunk.

Innervation for the bladder comes from the hypogastric sympathetic, pelvic parasympathetic and prudendal somatic nerves.

Urethra

The urethra is the tube through which urine passes from the bladder to the exterior of the body. The female and male urethras differ greatly. The female urethra is around 2 inches long and ends inferior to the clitoris and superior to the vaginal opening. In males, the urethra is around 8 to 10 inches long and ends at the tip of the penis. The urethra is also an organ of the male reproductive system as it carries sperm out of the body through the penis.

The urine flow through the urethra is controlled by the internal and external urethral sphincter muscles. The internal urethral sphincter is made of smooth muscle and opens involuntarily when the bladder reaches a certain set level of distention. The opening of the internal sphincter results in the sensation of needing to urinate. The external urethral sphincter is made of skeletal muscle and may be opened to allow urine to pass through the urethra or may be held closed to delay urination.