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Together, the endothelial cells of glomerular capillaries and the podocytes, which completely encircle the capillaries, form a leaky barrier known as the filtration membrane
- Regulation of blood ionic composition
- Regulation of blood pH
- Regulation of blood volume
- Regulation of blood pressure
- Release of erythropoietin & calcitrol
- Excretion of wastes & foreign substances
- Production of hormones; calcitriol, erythropoietin
- Regulation of blood glucose level
The urinary system consist:
- two kidneys which perform the above functions and consequently produce urine
- two ureters which drain urine from the kidneys and transport it to...
- one urinary bladder which stores urine until it is ready to be expelled
- one urethra which transports urine from the urinary bladder to the exterior; in males, the urethra also transports semen
External Anatomy of Kidney
- The paired reddish kidneys are retroperitoneal organs attached to the posterior abdominal wall just above waist level
- 4-5 in long, 2-3 in wide, 1 in thick
- They are partially protected by the eleventh and twelfth pairs of ribs
- The right kidney is slightly lower than the left because of the presence of the liver on the right side
Near the center of the kidney’s concave medial border is a depression called the renal hilus through which travel ureter, blood, nerves vessels and lymphatic vessels.
Three barriers of the filtration membrane
- 2.Basal lamina
- 3.Slit membrane
Fenestration (pore) of glomerular endothelial cell prevents filtration of blood cells but allows all components of blood plasma to pass through
basa lamina of glomerulus prevents filtration of larger proteins
slit membrane between pedicles prevents filtration of medium-sized proteins
Three layers of tissue surround each kidney
- 1.Renal capsule (deep layer).Transparent membrane of dense irregular connective tissue,maintains organ shape,serves as a barrier against trauma
- 2.Adipose capsule (middle layer).Helps protect from trauma, holds it firmly in place within the abdominal cavity
- 3.Renal fascia ( the superficial layer).Dense, irregular connective tissue that anchors the kidney to the surrounding structures and to the abdominal wall
renal columns are portions of the cortex that extend between renal pyramids
nephrons are the functional units of the kidney which produce urine
nephrons drains into
Collecting duct into papillary ducts, then into minor calyces which merge to form major calyces that deliver the urine into a large cavity called the renal pelvis. From the renal to ureter to urinary bladder
nephron consists of two portions
- renal corpuscle, where plasma is filtered
- renal tubule, into which the filtered fluid passes
renal corpuscle has two components
- Network of capillaries called the glomerulus - capillaries where filtration occurs
- Surrounding the glomerulus is a double-walled epithelial cup called the glomerular Bowman’s capsule.Blood plasma is filtered in the glomerular capsule, and then the filtrated fluid passes into the renal tubule.
Blood & Nerve Supply of Kidney
Renal artery-segmental arteries-interlobar arteries-arcuate arteries-interlobular arteries-afferent arteries-glomerular capilarries-efferent arterioles-peritubular capillaries-interlobular veins-arcuate veins-interlobar veins-renal veins
Three main section of the renal tubule
- 1. Proximal convoluted tubule (PCT)
- 2. Loop of Henle.Connects the proximal and distal convoluted tubules;it loops into the renal medulla; it consists of a descending limb and an ascending thick and thin limb
- 3. Distal convoluted tubule (DCT)
Two types of nephrons
- Cortical nephron (80-85% of all nephrons), usually has its glomerulus in the outer part of the cortex.
- Juxtamedullary nephron (15 to 20% of all nephrons), usually has its glomerulus deep in the cortex
Histology of the Nephron
A single layer of epithelial cells forms the entire wall of the glomerular capsule, renal tube, and ducts. However each parts has different histological features.
Structure of glomerular capsule
The glomerular (Bowman’s) capsule consists of an inner visceral layer and an outer parietal layer.
The visceral layer of glomerular capsule consist
Modified simple squamous epithelial cells called podocytes whose footlike projections (pedicels) wrap around the single layer of endothelial cells of the glomerular capillaries and form the inner wall of the capsule
Parietal layer glomerular capsule consist
Parietal layer consists of simple squamous epithelial cells that form the outer wall of the capsule
Histology of Proximal convoluted tubule
Simple cuboidal epithelium with brush border of microvilli that increase surface area for reabsorbtion and secretion
Histology of descending and thin ascending limb
simple squamous epithelium
Histology of thick ascending limb
Simple cuboidal to low columnar epithelial cells
Histology of most of Distal convoluted tubule
Simple cuboidal epithelial cells which have microvilli
Last part of DCT and all of collecting duct
simple cuboidal epithelium consisting of pricipal cells and intercalated cells
The final portion of the ascending limb of the loop of Henle makes contact with the afferent arteriole serving the renal corpuscle; the tubule cells here are columnar and crowded together, and thus are known as the macula densa
Juxtaglomerular apparatus (JGA).
The wall of the afferent arteriole (and sometimes the efferent arteriole) contains modified smooth muscle fibers called juxtaglomerular (JG) cells; together with the macula densa, they constitute the juxtaglomerular apparatus (JGA).
Most are principal cells, which have receptors for both antidiuretic hormone (ADH) and aldosterone, two hormones that regulate their function
intercalated cells, which can secrete hydrogen ions and thus play a role in homeostasis of blood pH
To produce urine, nephrons and collecting ducts perform 3 basic processes
- 1.Glomerular filtration.Water and most solutes in blood plasma move across the wall of glomerular capillaries into the glomerular capsule and then into the renal tubule.
- 2.Tubular reabsorption.Water & useful substances are reabsorbed into the blood.
- 3.Tubular secretion. As fluid flows, the tubule and duct secrete other materials, such as wastes, drugs, and excess ions, into the fluid (tubular secretion removes a substance from the blood).
Function of glomerular capillaries
Glomerular capillaries where filtration of blood occurs
Function of peritubular capillaries
Peritubular capillaries that carry away reabsorbed substances from filtrate
Function of Vasa Recta
Vasa recta supplies nutrients to medulla without disrupting its osmolarity
Glomerular blood hydrostatic pressure (GBHP)
Glomerular blood hydrostatic pressure (GBHP) is the blood pressure in glomerular capillaries. It promotes filtration by forcing water and solutes in blood plasma through the filtration membrane
Capsular hydrostatic pressure (CHP)
Capsular hydrostatic pressure (CHP) is the hydrostatic pressure exerted against the filtration membrane by fluid already in the capsular space and renal tubule.
Blood colloid osmotic pressure (BCOP)
Blood colloid osmotic pressure (BCOP), which is due to the presence of proteins such as albumin, globulins, and fibrinogen in blood plasma, also opposes filtration
Renal autoregulation and consists of two mechanisms:
- 1. Myogenic mechanism
- 2. Tubuloglomerular feedback
What is Glomerular filtration rate?
Amount of filtrate formed in all renal corpuscles of both kidneys / minute is the glomerular filtration rate (GRF)
As BP rises, GFR also rises because renal blood flow increase. The elevated blood pressure stretches the wall of the afferent arterioles. In response, smooth muscle fibers in the wall of the afferent arteriole contract, which narrows the arteriole’s lumen. As result, renal blood flow decrees.
When GFR is above normal due to elevated systematic blood pressure, filtered fluid flows more rapidly along the renal tubes. As result, the proximal convoluted tubule and loop of Henle have less time to reabsorb NA+, CI— and water. Macula densa cells are thought to detect the increased delivery NA+, CI— and water and to inhibit release of nitric oxide (NO) from cells in the juxtaglomerular apparatus (JDA). NO causes vasodilatation afferent arterioles and constrict when the level of NO declines. As result, lees blood flows into the glomerular capillaries and GFR decreases.
Neural Regulation of GFR
Kidneys are supplied by sympathetic ANS fibers that release norepinephrine. Norepinephrine causes vasonstriction. At rest, sympathetic stimulation is moderately low, the afferent and efferent arterioles are dilated, and renal autoregulation of GFR prevails.With moderate sympathetic stimulation, both afferent & efferent arterioles constrict equally decreasing GFR equally.With extreme sympathetic stimulation (exercise or hemorrhage), vasoconstriction of afferent arterioles predominates. As result, blood flow into glomerular capillaries is greatly decreased and GFR drops lower urine output & permits blood flow to other tissues.
Two hormones contribute to regulation of GFR
- 1. Angiotensin II
- 2. Atrial natriuretic peptide (ANP)
Angiotensin II- reduces GFR; angiotensin II is vasoconstrictor that narrows both afferent and efferent arterioles and reduces renal blood flow, thereby decreasing GFR
Atrial natriuretic peptide (ANP)
Atrial natriuretic peptide (ANP)-Increase GFR. Stretching of the atria, as occurs when blood volume increase, stimulates secretion of ANP. ANP causes relaxation of mesangial cells in glomerulus increases capillary surface area available for filtration
Paracellular reabsorbtion. Fluid can leak between the cells in a passive process. 50% of reabsorbed material moves between cells by diffusion in some parts of tubule
Transcellular reabsorption material moves through both the apical and basal membranes of the tubule cell by active transport
Reabsorption of Na
sodium/potassium pumps are located in the basolateral membranes eject Na+ from the renal tubule cells
Reabsorption of water
- Obligatory water reabsorption occurs when water is “obliged” to follow the solutes being reabsorbed
- Facultative water reabsorption occurs in collecting duct under the control of antidiuretic hormone
When the blood concentration of glucose is above 200mg/mL, the renal symporters cannot work fast enough to reabsorb all the glucose that enters the glumerular filtrate. As result some glucose remains in the urine, a condition called glucosuria. Common cause: is diabetes mellitis because insulin activity is deficient and blood sugar is too high. Rare genetic disorder produces defect in symporter that reduces its effectiveness.
Reabsorption and secretion in the proximal convoluted tubule
The largest amount of solute and water reabsorption from filtered fluid occurs in the proximal convoluted tubules. This part reabsorb 65% of the filtered water. Glucose, amino acids, lactic acid, water-soluble vitamins and other nutrients are completely reabsorbed in the first half of the proximal convoluted tubule. Most solute reabsorbtion in the proximal convoluted tubule (PCT) involves Na+. Na+ transport occurs via symport and antiport mechanism in the proximal convoluted tubule. Na+ symporters help reabsorb materials from the tubular filtrate. Intracellular sodium levels are kept low due to Na+/K+ pump. In addition, proximal convoluted tubules secrete a variable amount of H+ ions, ammonium ions (NH4+) and urea.
Two Na+ and molecule glucose attach to the symporter protein, which carries them from the tubular fluid into the tubule cell
Na+/H+ antiporter carry filtered Na+ into a proximal convoluted tubule (CPT) cell as H+ is moved from cytosol into the lumen, causing Na+ to be reabsorbed into blood and H+ to be secreted into tubular fluid. Na+ leaves proximal convoluted tubule cells by sodium potassium pump
- This integral protein in the plasma membrane is a water channel that greatly increase the rate of water movement across the apical and basolateral membranes.
- Cells lining the proximal convoluted tubule and the descending limb of the loop of Henle are especially permeably to water because they have molecules of aquaporin-1.
Ammonia (NH3) is a poisonous waste product derived from the deamination (removal of an amino group) of various amino acids, a reaction that occurs mainly in hepatocytes (liver cell). Both ammonia and urea are filtered at the glomerus and secreted in the PCT cells into the tubular fluid.
reabsorption in the descending limb
Descending limb has a high water permeability, but has lower permeability to solutes such as NaCi. Water moves out by osmosis. The result osmolalities increase from 300mosm/liter in the outer medulla to 1200 mOsm/liter deep in the inner medulla.
Reabsorption in ascending limb
In the ascending limb water impermeable, therefore osmolalities decrease as Na+ moves out of the tubular fluid.
Countercurrent exchange is the process by which solutes and water are passively exchanged between the blood of the vasa recta and interstitial fluid of the renal medulla as a result of countercurrent flow.
Since countercurrent flow between the descending and ascending limbs of the vasa recta allows for exchange of solutes and water between the blood and interstitial fluid of the renal medulla, the vasa recta is said to function as a countercurrent exchanger.
When blood volume and blood pressure decrease, the walls of the afferent arterioles are stretched less, and the juxtaglomerular cells in the kidney secrete the enzyme rennin into the blood. Angiotensin from the liver comes together with rennin to form angiotensin I.In the lungs Angiotensin I converts to angiotensin II, which is active form of the hormone.
Angiotensin II affects renal physiology in three main ways: 1) Decreases GFR by vasoconstricting afferent arteriole. 2) Increase reabsorption of Na+, CI—and water in the proximal convoluted tubule by stimulating the activity of Na+/H+antiporters.3) It stimulates the adrenal cortex to release aldosterone, a hormone that in turn stimulates the principal cells in collecting ducts to reabsorb more Na+, and CI— and secrete more K+ into urine increases blood volume by increasing water reabsorption.
Antidiuretic hormone (ADH or vasopressin) is made in hypothalamus and released by the posterior pituitary. ADH stimulates the insertion of aquaporin-2(a water channel proteins) channels into the membrane. Water molecules move more rapidly from the tubular fluid into the cell. When ADH level declines, the aquaporin-2 channels are removed from the apical membrane. The kidney produce a large volume of dilute urine when ADH level is low.When osmolarity of plasma and interstitial fluid decreases, more ADH is secreted and facultative water reabsorption increases
Atria natriuretic peptide
A large increase in blood volume promotes release of atrial natriuretic peptide (ANP) from the heart. Stretching of atria heart stimulates secretion of ANP. ANP inhibits reabsorption of Na+ and water in PCT and suppresses secretion of aldosterone & ADH. ANP increase excretion of Na+ which increases urine output and decreases blood volume (vasodilation).
- Low level of Ca2+ in the blood stimulates the parathyroid gland to release parathyroid hormone (PTH). PTH in turn stimulates cells in the early distal convoluted tubules to reabsorb more Ca2+ into the blood.
Dueretics are substances that slow renal reabsorption of water and cause diuresis (increased urine flow rate), which in turn reduced blood volume. Diuretic drugs often are prescribed to treat hypertension (high blood pressure) because lowering blood volume usually reduces blood pressure. Naturally occurring diuretics include caffeine in coffee, tea, and sodas, which inhibits Na+ reabsorbtion. Alcohol which inhibits secretion of ADH prescription medicines can act on the PCT, loop of Henle or DCT.
An analysis of the volume and physical, chemical, and microscopic properties of urine, called a urinalysis.
- Water accounts for about 95% of the total volume of urine, 5% consist of electrolytes, solutes derived from cellular metabolism, exodenous substances such as drugs.
- Normal urine is protein free, but includes filtered and secreted electrolytes. it also consist urea, creatinine, uric acid, urobilinogen, fatty acids, enzymes & hormones.
Two types of b
- blood urea nitrogen (BUN) test
- plasma creatinine
Blood urea nitrogen (BUN) test
Blood urea nitrogen (BUN) is test measures the blood nitrogen that is part of the urea resulting from catabolism and deamination of amino acids. BUN rises when glomerular filtration rate decreases, that may occurs with renal diseases or obstruction of the urinary tract.
kidney function is measurement of plasma creatine result from catabolism of creatine phosphate is skeletal muscle a creative level above 1.5 mg/dl usually is an indication of poor renal function.
Renal plasma clearance
Renal plasma clearance is the volume of blood that is cleaned or cleared of a substance per unit of time, usually expressed in units of milliliters per unit. High renal plasma clearance indicates efficient excretion of substance in the urine, low clearance indicates inefficient excretion
If a person’s kidney are so impaired by diseases or injury that he or she is unable to function adequately, then blood must be cleansed artificially by dialysis, the separation of large solutes from smaller ones by diffusion through a selectively permeable membrane. One method of dialysis is hemodialysis, which directly filters the patient’s blood by removing wastes and excess electrolytes and fluid and then returning the cleansed blood to the patient. Blood removed from the body is delivered to a hemodialyzer (artificial kidney), where filtration is occurs.
10 to 12 in long, varies in diameter from 1-10 mm, extends from renal pelvis to bladder and enters posterior wall of bladder, retroperitoneal,there is no anatomical valve, but physiological valve only
Histology of ureters
- 1.The deepest coat is mucosa-mucous membrane with transitional epithelium and underlying lamina propria of areolar connective tissue.
- 2.The Intermediate coat is muscularis, compose of inner longitudinal and outer circular layers of smooth muscle fibers
- 3.The superficial coat of the uteres is the adventitia, a layer of areolar connective tissue containing blood vessels, lymphatic vessel, and nerves that serve the musclularis and mucose
The urinary bladder is a hollow, distensible muscular organ situated in the pelvic cavity posterior to the pubic symphysis. In females is anterior to vagina & inferior to uterus. In males lies anterior to rectum.
Histology of Urinary Bladder
- 1.The deepest is the mucosa, composed of transitional epithelium (since organ must inflate & deflate) and an underlying lamina propia. Mucus prevents the cells from being contacted by urine
- 2.Intermediate muscularis (also called detrusor muscle), which consist of three layers of smooth muscle fibers: 1) inner longitudinal, 2)middle circular 3) outer longitudinal. Circular smooth muscle fibers form internal urethral sphincter. Circular skeletal muscle form external urethral sphincter
- 3.The most superficial coat of the urinary bladder is the adventitia, a layer of areolar connective tissue that is continuous with that of the ureters.Over the superior surface of the urinary bladder is the serosa, a layer of visceral peritoneum
Discharge of urine from the urinary bladder, called micturition, is also known as urination or voiding
When volume exceeds 200-400 mL stretch receptors signal spinal cord and brain. These impulses propagate to the micturition center in sacral spinal cord and trigger a spinal reflex called the micturition reflex. In this reflex arc, parasympathetic impulses from the micturition center propagate to the urinary bladder wall and internal urethral sphincter. The nerve impulses cause contraction of the detrusor muscle to contract, external & internal sphincter muscles to relax
In the floor of the urinary bladder is a small triangular area called the trigone.
internal urethral orifice
in th eurinary bladder, the openings into the urethra is called internal urethral orifice.
External urethral orifice
The opening of the urethra to the exterior, the external urethral orifice, is located between the clitoris and the vagina openings. In males tube passes through prostate, deep muscles of perineum and penis.
Histology of female urethra
- 1) Deep mucosa.The mucosa is a mucous membrane composed of epithelium and lamina propria, near the urinary bladder the mucosa contains transitional epithelium that is continuous with that of the urinary bladder. Near the external urethral orifice the epithelium changed to nonkeratinized stratified squamous epithelium.
- 2) Superficial muscularis consist of circular arranged smooth muscle fibers.
Histology of male urethra
The male urethra, also consist of a deeper mucosa and superficial muscularis
Anatomical regions of male urethra
- 1.The prostatic urethra passes through the prostate.The epithelium of the prostatic urethra is continuous with that of the urinary bladder and consist of transitional epithelium that becomes stratified columnar or pseudostratified columnar epithelium more distally.
- 2.The membranous (intermediate ) urethra, passes through the deep muscles of the perineum.The mucosa contains stratified columnar or pseudostratified columnar epithelium.
- 3.Spongy urethra is the longest portion, passes through the penis.The epithelium of the spongy urethra is stratified columnar or pseudostratified columnar epithelium, except near the external urethral orifice. The lamina propria of the male urethra is areolar connective tissue
Lack of voluntary control over micturition is called Urinary Incontinence
A stone in the kidney (or lower down in the urinary tract). Also called a kidney stone. Renal stones are a common cause of blood in the urine and pain in the abdomen, flank, or groin. Kidney stones occur in 1 in 20 people at some time in their life.The development of the stones is related to decreased urine volume or increased excretion of stone-forming components such as calcium, oxalate, urate, cystine, xanthine, and phosphate. The stones form in the urine collecting area (the pelvis) of the kidney and may range in size from tiny to staghorn stones the size of the renal pelvis itself.
Urinary tract infections
Urinary tract infections is used to describe either an infection of a part of the urinary system or the presence of large numbers of microbes in urine.
Glomerulonephritis is an inflammation of the kidney that involves the glomeruli
Acute kidney failure is the sudden loss of your kidneys' ability to perform their main function — eliminate excess fluid and electrolytes as well as waste material from your blood. When your kidneys lose their filtering ability, dangerous levels of fluid, electrolytes and waste accumulate in your body. Symptoms include:decreased urine output, fluid retention, causing swelling in your legs, ankles or feet, drowsiness, shortness of breath, fatigue, confusion, seizures or coma in severe cases
Polycystic kidney disease
Polycystic kidney disease is a kidney disorder passed down through families in which multiple cysts form on the kidneys, causing them to become enlarged.Symptoms include: Abdominal pain or tenderness, blood in the urine, excessive urination at night,flank pain on one or both sides
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