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Urinary System

Terms in this set (27)

There are two key components of the urinary system:
1.Kidneys A pair of organs that performs the primary functions of the kidneys.
2.Urinary tract The urinary tract includes paired ureters that drain urine from the kidneys into the urinary bladder where it is temporarily stored until it is voided through the urethra to the exterior.
The primary functions of the kidneys include:
1. Regulation of the plasma concentration of sodium, potassium, chloride, calcium and other ions.
2. Long term regulation of blood volume and pressure
3. Long term regulation of *plasma pH
4. Excretion of organic wastes
5. Assists the liver in detoxification of toxins and gluconeogenesis
Kidneys The kidneys are located lateral to the vertebral column between the last thoracic vertebra (T12) and the third lumbar vertebra (L3) with the right kidney slightly inferior to the left.
The kidneys are retroperitoneal in position with the kidneys and associated structures located between the muscles of the dorsal body wall and the parietal peritoneum
In addition to the parietal peritoneum and nearby organs, three layers of connective tissue stabilize and protect the kidneys:
1. Renal capsule- A fibrous layer on the surface of the kidney.
2. Adipose capsule- Adipose tissue that surrounds each kidney
3. Renal Fascia- A sheet of connective tissue that anchors the kidney to surrounding structures
Superficial Anatomy of the Kidney Each kidney has a prominent medial indentation called the hilus which serves as the entry point of blood vessels, nerves and ureter.
At the hilus an inner layer of the renal capsule extends into an internal cavity of the kidney called the renal sinus. This cavity is filled with blood vessels, nerves, and the tributaries of the collecting system that eventually form the ureters
Sectional Anatomy of the Kidneys When the kidney is cut open three distinct regions become visible:
1. Renal Cortex The renal cortex is the outermost layer that is contact with the capsule
2. Renal Medulla The renal medulla is internal to the cortex and contains 6-18 conical structures called renal pyramids. The base of each pyramid rests on the cortex and the tip, called the renal papilla, projects into the renal sinus. Adjacent pyramids are separated by bands of tissue called renal columns which are essentially extensions of cortical tissue.
Renal Lobe is a segment of the kidney centered around the renal pyramid. It includes the renal pyramid, the overlying renal cortex, and a share of the adjacent renal colums
3. Renal sinus Each renal papilla that projects into the renal sinus is surrounded by a cup-shaped drain called a minor calyx. Four or five minor calyces merge to form a major calyx. The major calyces fuse to form a large, funnel shaped chamber, the renal pelvis The renal pelvis tapers to form the ureter which exits at the hilus.
Blood Supply to the Kidneys Each kidney receives blood from the renal artery which enters at the hilus. The renal artery divides into the segmental arteries within the renal sinus. The segmental arteries divide into interlobar arteries that penetrate the renal columns.
These arteries then divide into the arcuate arteries that course along the boundary between the cortex and medulla. Each arcuate artery divides into cortical radiate arteries also called interlobular arteries that supply the adjacent renal cortex. Numerous afferent arterioles branch off the interlobular arteries.
Blood drains from the kidney in a sequence of veins that reverse the pathway of the arteries and includes,
cortical radiate veins also known as interlobular veins, arcuate veins, interlobar veins, and a renal vein*
Histology of the kidney Each kidney contains over a million microscopic tubules called nephrons which are the basic structural and functional units of the kidneys.
Each nephron begins at the *renal corpuscle* which contains a capillary network called the *glomerulus*. Blood arrives at the glomerulus via the *afferent arteriole* and leaves by an *efferent arteriole*. Filtration occurs at the renal corpuscleglomerulus begins at the *renal corpuscle* which contains a capillary network called the *glomerulus*. Blood arrives at the glomerulus via the afferent arteriole and leaves by an efferent arteriole. Filtration occurs at the renal corpuscle and the filtrate flows through the tubule which is divided into the proximal convoluted tubule, loop of Henle, and distal convoluted tubule
There are two types of nephrons:
1. Cortical Nephrons About 85% of nephrons are located almost entirely within the cortex. These nephrons perform most of the absorptive and secretory functions of the kidney.
2. Juxtamedullary Nephrons These nephrons are located close to the cortico-medullary border and have long loops of Henle that extend deep into the pyramids. These nephrons create the conditions necessary to produce a concentrated urine.
The filtrate formed by each nephron is finally collected by a shared system of tubules called the collecting system.
The collecting system consists of a connecting tubule that connects each nephron to a collecting duct; the collecting ducts that descend toward the renal papilla; and the papillary ducts that result from the fusion of the collecting ducts in the renal papillae.
Renal Corpuscle The renal corpuscle consists of the expanded, blind end of the renal tubule called Bowman's capsule and the capillaries of the glomerulus
The glomerulus extends into Bowman's capsule without entering into the tubule. As a result, two layers of tubular epithelium come to surround the capillaries and create a capsular space between them
The two layers are:
1. Parietal (capsular) epithelium- This is an outer layer of simple squamous epithelium
2.Visceral (glomerular) epithelium- The cells of this layer come into direct contact with the endothelial cells of the capillaries and are called podocytes
Filtration occurs in the renal corpuscle across the filtration membrane formed by:
1.Capillary Endothelium This layer is formed by the glomerular capillaries which are fenestrated
2.Basal lamina The basal lamina of the endothelial cells is distinct in having a lamina dense that is thicker than normal. The lamina densa often encircles more than one capillary with mesangial cells between adjacent capillaries.
The functions of the mesangial cells include:
1. Provide physical support for the capillaries
2. Engulf organic materials that could accumulate and clog the filtration membrane
3. Regulate the diameter of the capillaries
Glomerular epithelium The podocytes that form the glomerular epithelium have cellular processes that wrap around the lamina densa.
These processes have smaller finger-like processes called pedicles that interdigitate with those of a neighboring cell's process to form narrow filtration slits between them
Proximal Convoluted Tubule (PCT) The PCT begins at the tubular pole of the renal corpuscle which is opposite the vascular pole where the afferent and efferent arterioles enter and leave
The primary function of the PCT is absorption The simple cuboidal epithelium of the PCT has numerous microvilli on the apical surface which increases the surface area for absorption.
Over 60% of water, sodium and chloride is reabsorbed from the filtrate in the PCT. All the organic nutrients and plasma proteins are reabsorbed.
There is also active reabsorption of various ions including calcium, potassium, magnesium, and bicarbonate
Loop of Henle When the PCT straightens out it forms a tubule that descends toward the renal papilla of the pyramid.
This descending limb then turns in a hair-pin fashion and ascends toward the renal corpuscle. This loop is called loop of Henle
The loop of Henle is the portion of the renal tubule responsible for creating an osmotic gradient in the pyramid in which the osmotic pressure increases from the base of the pyramid towards the renal papilla
The osmotic pressure gradient enables the kidney to produce urine that is more concentrated than the body fluid
Distal Convoluted Tubule (DCT) When the ascending limb of the loop of Henle reaches the renal corpuscle it passes by the vascular pole in between the afferent and efferent arterioles and becomes the DCT.
The DCT performs both absorption and secretion and is the primary location where secretion occurs. The DCT is the part of the tubule where regulation of sodium, potassium, calcium and pH actually begins
Juxtaglomerular Apparatus The location where the ascending limb passes in between the afferent and efferent arterioles
The Juxtaglomerular apparatus consists of:
1.Mascula Densa (dense spot) At this location, which is next to the arterioles, the epithelial cells are taller and more concentrated
2.Juxtaglomerular Cells These are modified smooth muscle cells in the wall of the afferent arteriole
3. Extraglomerular Mesangial Cells These cells occupy the space between the glomerulus, afferent and efferent arterioles and the macula densa
The Juxtaglomerular apparatus monitors sodium concentration in the filtrate of the tubule at this point and uses this information to regulate the production of filtrate to maintain consistency
This is also the site of production of the hormones renin and erythropoietin which regulate blood pressure and red blood cell production, respectively.
Structures for Urine Transport, Storage, and Elimination
Ureters The ureters are muscular tubes that transport urine from the tapering tip of the renal pelvis to the urinary bladder.
The ureters are retroperitoneal in position like the kidneys
The wall of the *ureters consists of:
1. Transition epithelium- This epithelium accommodates the cycles of expansion and contraction without changes in the impermeability of the epithelium
2. Muscular layer -There is an inner longitudinal and an outer circular layer
3.Adventitia- Connective tissue that surrounds and stabilizes the position of the ureters
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