Urinary System Flashcards
What does the urinary system consist of
The urinary system consists of the paired kidneys and ureters and the unpaired bladder and urethra, renal pelvis
The urinary tract is lined by transitional epithelium, except in the urethra where it may vary from stratified cuboidal to columnar (more about urethra in the reproductive systems).
Renal Pelvis
The lamina propria contains mucous glands in horses, which account for the frothy appearance of the urine in this species.
Ureter
Again, the initial part of the ureter contains mucous glands in horses; tunica muscularis consists of two to three layers, inner longitudinal and outer circular, or has another outer longitudinal layer; tunica adventitia.
Bladder
The morphology of transitional epithelium differs whether the bladder is relaxed (many layers, dome-shape surface of cells facing the lumen) or filled (fewer layers of flat to cuboidal cells). To prevent the diffusion of toxic chemicals back into the circulation, cells of the bladder epithelium facing the lumen are joined by tight junctions and have a special membrane of thick plates that is responsible for the osmotic barrier between the urine and tissue fluids. Muscle fibers in the bladder run in every direction, except near its neck, where three distinct layers may be identified.
What are the functions of the kidney
Functions of the kidney are to produce urine and thereby clearing the blood of waste products, regulate the electrolyte level and thereby maintaining acid base and electrolyte homeostasis, and regulate blood pressure and erythropoiesis.
How is the Kidney morphologically divided
Outer cortex
Inner medulla
What are the classifications of the kidney
Based upon the fusion of the cortical and medullary tissues, kidneys in domestic animals can be classified as follows:
Uni-pyramidal
Multi-pyramidal
Multi-lobar
Uni-pyramidal
only one pyramid:
Both cortical and medullary tissues are fused into one lobe (cat, dog, horse, and small ruminants).
Multi-pyramidal
Many pyramids:
Cortical tissue is fused, but medullary tissue is separate as pyramids within a lobe (pig and man); cortical tissue is partially separate and medullary tissue is separate as pyramids within a lobe (adult cattle).
Multi-lobar
Each lobe has a separate cortex and medulla (seal)
The parenchyma (functioning tissues) of the kidney consists of
The parenchyma of the kidney consists of two major parts:
Nephrons
Collecting Ducts
What is the nephron
The urine forming parts
What are the collecting ducts
The parts that further modify urine and then transport it to the bladder via the ureter
What is the nephron composed of
Is composed of four parts:
Renal Corpuscle(RC)
Proximal convoluted Tubule (PCT)
Loop of Henle (LH)
Distal convoluted Tubule (DCT)
What does the renal corpuscle consist of
Consists of a group of capillaries called glomerulus and a surrounding space called Bowman’s capsule.
What does the loop of Henle consist of
A descending limb made of thick and thin segments and an ascending limb made of thin and thick segments.
What are Collecting Ducts composed of
Composed of collecting tubules and papillary ducts
Collecting tubules
Impermeable to water, but water is absorbed under the effect of ADH. Eventually, the hypertonic urine leaves the kidney.
Describe the drainage of many nephrons
Many nephrons drain into one collecting tubule, and many collecting tubules drain into one papillary duct, which opens individually on the renal papilla.
The distribution of nephrons and collecting ducts in the cortex of the kidney
Contains Renal Corpuscle, Proximal Convoluted Tubule, Distal Convoluted Tubule , and the initial portion of collecting tubules (grossly called medullary rays)
The distribution of nephrons and collecting ducts in the outer medulla of the kidney
Contains thick segment of the descending limb
(also called straight part of the Proximal Convoluted Tubule since this part is structurally similar to the cortical part of the Proximal Convoluted Tubule),
Thin segments of the descending limb, thick segment of the ascending limb (also called straight part of the Distal Convoluted Tubule since this part is structurally similar to the cortical part of the Distal Convoluted Tubule) and collecting tubules
NO RENAL CORPUSCLE
The distribution of nephrons and collecting ducts in the inner medulla of the kidney
Contains thin segments of the descending and ascending limbs, collecting tubules, and papillary ducts.
What are the two types of Nephrons
Cortical
Juxtamedullary
Cortical Nephron
Renal Corpuscles are in the periphery of the cortex and have shorter LH, which extends only up to the outer medulla
Juxtamedullary Nephron
Renal Corpuscles are located near the cortico-medullary junction, have longer LH, which extends up to the tip of the papilla
How does length play a role in urine concentration
the longer the LH, the greater the urine concentration. Hence, fish have a very short LH; camel (desert’s ship), among domestic animals, has the longest LH; and birds have even longer.
What is a Renal Corpuscle
Blood filtration and renal ultrafiltrate formation
Each RC is about 150-200 microns in diameter, has two poles:
Vascular, where afferent arteriole enters and efferent arteriole leave
Urinary, where renal ultrafiltrate leaves the capsular space and enters the PCT.
What are the two main parts of the renal corpuscle
Glomerulus
Bowman’s Capsule
Glomerulus
Network of capillaries which are branches of an afferent arteriole; capillaries are sites for blood filtration.
Bowman’s Capsule
A double layer epithelial cup, the inner layer is called visceral layer, the outer layer is called parietal layer, both layers are continuous and enclose a space called the capsular space, which collects the renal ultrafiltrate that enters the PCT.
Visceral layer of the Bowman’s Capsule
Cells of the visceral layer are specialized called podocytes, which have feet-like primary processes (podocyte) and secondary processes (pedicles) and come in close contact with the endothelial cells of glomerular capillaries.
Parietal Layer of the Bowman’s Capsule
The parietal layer forms an outer boundary of the capsular space, and its epithelium is continuous with that of the PCT.
Blood-Urine Barrier
Consists of :
Fenestrated endothelium of glomerular capillaries
Thick basal lamina (0.15-0.5 um) formed by endothelial cells and
Podocytes, and a layer of pedicles separated by filtration slits.
Basal Lamina
Basal lamina is the main barrier, which serves both as a charge barrier and a size barrier.
Hence, it becomes obvious that changes in the thickness of basal lamina and numbers of charged particles, as well as changes in the permeability of capillaries, influence the glomerular filtration rate and the contents of the filtrate.
Basal Lamina (charge barrier component of the blood urine barrier)
On either side of the basal lamina are present negatively charged heparin sulphate particles, which, because of the negative charge, prevent negatively charged blood components (regardless of the size) from leaving the blood (it means that positively charged particles smaller than 65 K can freely pass through the basal lamina, for example, most cations).
Basal Lamina ( size barrier component of the blood urine barrier)
Because of fenestrations, most large molecules leave the bloodstream, but only molecules smaller than 65 kilodalton (K) can pass through the basal lamina (size barrier, albumin is 65-70 K, so cannot leave).
Why is glomerular filtrate formed
The glomerular filtrate is formed because glomerular capillaries, unlike other capillaries in the body, are interposed between two arterioles (afferent and efferent) and thus have a higher hydrostatic pressure.
It is noteworthy that a beagle dog of 10 kg will produce 37 ml of glomerular filtrate per minute (more than 14 gallons per day), but only 1% of this filtrate will be excreted as urine and the rest will be reabsorbed in the PCT, DCT, and collecting tubules.
Proximal convoluted tubule (PCT) and Distal convoluted tubule (DCT)
Both tubules lie next to each other in the cortex and thus are described together.
PCT are longer than DCT and thus more of them are present in the cortex.
Proximal convoluted tubule (PCT)
Reabsorbs 85-90% water, 100% glucose and amino acids, reabsorbs 80-90% ions, including Na+, K+, PO4–, Ca++, secrete organic acids (creatinine) and drugs (antibiotic, aspirin
PCT are lined by simple high cuboidal epithelial cells, which reabsorb from the renal ultrafiltrate all (100%) of the glucose, amino acids, and small proteins and at least 80% of sodium chloride and water.
Thus, it should not be surprising that these cells have all those features associated with fluid and solute absorption, including brush border, pinocytic vesicles, vacuoles, lysosomes (just to name some).
These cells also have extensive basal infoldings with mitochondria, which supply energy for the active transport of Na+ by the Na+, K+ pump located in the basolateral plasma membrane.
Cells of the PCT secrete into the renal filtrate certain organic acids (e.g., creatinine) and certain foreign substances (e.g., drugs, toxic chemicals).
Distal convoluted tubule (DCT)
Na+ is reabsorbed and K+ is secreted under the influence of aldosterone, H+ and NH4++ are also added to the filtrate, some water is reabsorbed, the net result is that the filtrate leaving the DCT is isotonic or slightly hypotonic.
Compared to the PCT, DCT are lined by simple low cuboidal epithelial cells, which lack brush border and thus have a limited absorptive function, but they have a well developed basal infoldings, which enable these cells to transport across the basal cell membrane.
In cells of the DCT, Na+ ions are actively removed from the filtrate (5-10%) and are replaced by K+, H+ and ammonia ions. In addition, K+, H+ and ammonia ions are actively secreted into the lumen. All these processes are stimulated by aldosterone.
Hence, DCT has an important role in maintaining acid-base balance and electrolyte balance, especially K+ balance (note, hyperkalemia can have fatal consequences).
Loop of Henle
Under the influence of renin-aldosterone and countercurrent multiplier mechanism, an increasing osmotic gradient from the base of the medulla (300 mOsmol) to the papilla (1200 mOsmol) is maintained in the medulla. Because of this osmotic gradient, water leaves and ions enter the descending limb; and ions enter the ascending limb (note, water cannot enter or leave in the ascending limb since this is impermeable to water). The net result is that the ultrafiltrate entering the DCT is hypotonic.
Has two segments
Descending and ascending segments
Thin parts of both segments are 20-25 um in diameter and are lined by simple squamous epithelium and thus are difficult to differentiate from the adjoining capillaries of the vasa recta.
Thin parts of both segments are involved in water retention, and only animals with thin segments can concentrate urine; however, there are differences between the descending and ascending thin segments in the manner they concentrate urine
Thick Descending Segment of the Loop of Henle
thick part of the descending limb is structurally and functionally like the PCT
Thin Descending Segment of the Loop of Henle
The thin descending limb is freely permeable to water
Thick Ascending Segment of the Loop of Henle
Thick part of the ascending limb is structurally and functionally like the DCT
Thin Ascending Segment of the loop of Henle
The entire ascending limb (both thin and thick) is impermeable to water.
Significance of the differences in the thin segments of the descending and ascending segments of the loop of henle
These differences enable the medulla to maintain an osmotic gradient of 300 mOsmol L near the corticomedullary junction to 1200 moms L near the tip of the papilla as follows:
As the descending limb descends deeper into the medulla, water moves out from the lumen into the medullary interstitial space (water then enters the surrounding capillaries of the vasa recta) and Na+ and chloride ions move into the lumen and thus gradually increasing the osmolality of the renal ultrafiltrate (again remember, the longer the LH, the greater the urine concentration).
Hence, the tip of the LH and the initial part of the ascending limb has a highly concentrated filtrate (note, not urine yet).
Since ascending limb is impermeable to water, water stays in the lumen, but ions, under the effect of aldosterone and osmotic gradient, leave the filtrate and thus making the filtrate gradually less and less hypertonic and, eventually, the filtrate becomes hypotonic (100-200 mOsmol L) by the time it reaches the distal convoluted tubules.
The descending and ascending limbs, as well as the arterial and venous vasa recta, in the medulla are parts of the countercurrent mechanism for urine concentration.
Collecting Duct System
Includes arched collecting tubules, straight collecting tubules, and papillary ducts.
The epithelium varies from simple cuboidal in the initial portion in the cortex and outer medulla to simple high columnar in the papillary ducts.
The cell boundaries are distinct, more so in the papillary duct.
Note, many nephrons empty into one collecting tubule located in the cortex (called medullary rays) and many collecting tubules empty into one papillary duct, which opens at the renal crest.
What are the two cell types in the collecting ducts
Two cell types: principal with light cytoplasm and intercalated with dark cytoplasm
What are the major parts of the urine concentrating mechanism and why
Collecting tubules are a major part of the urine-concentrating mechanism.
Their epithelial cells are impermeable to water but become permeable under the effect of ADH (antidiuretic hormone, also called vasopressin).
Also, cells of the initial part of the collecting tubules, under the influence of aldosterone, actively absorb Na+, which is followed passively by chloride and water absorption (similar in function to the DCT).
In the Absence of ADH what happens to the animal
In the absence of ADH, urine is hypotonic, and animal suffers from polyuria and polydipsia (increased thirst)
What stimulates ADH release
Dehydration and factors that lower the blood pressure will stimulate ADH release
What decreases ADH secretion
Alcohol decreases ADH secretion
What is the Function of the Juxtaglomerular Apparatus
Function of the JG apparatus is to maintain blood pressure by the following mechanism:
decrease in fluid volume is detected by macula densa cells, which then stimulate JG cells to secrete renin;
The latter acts on a plasma protein angiotensinogen, which is converted into angiotensin I;
which, in turn, is converted by a converting enzyme present in all endothelial cells, especially in the lung, into angiotensin II, a potent vasoconstrictor that stimulates the release of aldosterone in the adrenal cortex
( angiotensinogen to angiotensin I to angiotensin II, the latter stimulates aldosterone secretion in the adrenal cortex).
Aldosterone stimulates epithelial cells of the DCT to actively remove Na+ that is followed passively by absorption of chloride ions and water and thereby increasing the fluid volume in the extracellular space, which then leads to an increase in blood pressure.
What are the three components of the Juxtaglomerular Apparatus (JG apparatus)
Consists of three components:
J.G. cells,
macula densa
extra-mesangial cells
Juxtaglomerular cells ( JG cells)
JG cells are modified smooth muscle cells of the afferent arteriole, which become endocrine in function and secrete renin.
What are Macula Densa
Macula densa are modified cells of the DCT; as the DCT passes by the afferent arteriole, its cuboidal cells on the side that meets the afferent arteriole become taller and closely packed and monitor changes in the osmolality and volume of the fluid entering the DCT.
There is a close relation via the gap junctions between macula densa and JG cells
Extra-mesangial Cells
Extra-mesangial cells lie between the afferent and efferent arterioles
(note, intra-mesangial cells lie within the renal corpuscle, in close contact with the glomerular endothelial cells, where their function is to maintain the normal thickness of the basal lamina by phagocytosing foreign particles).
Blood Circulation of the kidneys
Extensive, all the body blood passes through both kidneys every 4-5 minutes.
Blood Circulation Pathway of the Kidney
- Renal artery branches to form interlobar arteries (between pyramids in the medulla), which branch and turn as
- arcuate artery (located at cortico-medullary junction), which branches to form
3.interlobular arteries, which branch to form afferent arterioles, which break up into
- glomerular capillaries, where the blood is filtered and then leaves via the efferent arterioles
- Efferent arterioles of the cortical nephrons break into
- cortical peritubular capillaries that surround the PCT, DCT, and initial part of the collecting tubules, where the fluid absorbed by these tubules enters back into the circulation. Conversely,
- efferent arterioles of the juxtamedullary nephrons break into straight vessels called
- vasa recta located in the medulla. Blood from the cortical peritubular capillaries and vasa recta drains into the
- venous system (interlobular vein, arcuate vein, interlobar vein, renal vein).
Blood supply on the arrangement of kidney
Blood supply delineates lobular arrangement of kidney, although these lobules cannot be delineated at the gross level.
Histologically, each lobule is demarcated laterally by interlobular arteries and centrally by medullary rays; all nephrons within a lobule empty into the collecting tubule of the same lobule.
The lobular arrangement is functionally important in that if blood vessels or collecting tubules are damaged or blocked within a part of the kidney, then, only that part of kidney is damaged, not the whole kidney
