Lecture Urinary System Part 1
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What are the main components of the urinary system?
Kidneys are the major excretory organs
Urinary bladder is the temporary storage reservoir for urine
ureters transport urins from the kidneys to the bladder
- urethra transports urine out of the body
What are the main functions of the Kidney?
Removal of toxins, metabolic wastes, and excess ions from the blood
reglation of the blood volume, chemical composition, and pH
Gluconeogenesis during prolonged fasting
- Endocrine functions:
- -renin: regulation of blod pressure and kidney functions
- -erythropoietin: regulation of RBC production
Activation of Vitamin D
Describe the anatomy of the kidney
Retroperitoneal, in the superior lumbar region
right kidney is lower than the left due to crowding by liver
convex ;ateral surface, concave medial surface
renal hium leads to the renal sinus
ureters, renal blood vessels, lymphatics, and nerves enter and exit at the hilum
What are the three laters of supportive tissue surrouding each kidney?
- 1. Renal Fascia
- -the anchoring outer layer of dense fibrous connective tisse
- 2. Perirenal fat capsule
- -a fatty cushion
- -if kidney calls, called renal ptosis
- 3. Fibrous Capsule
- -prevents the spread of infection to the kidney
Describe the itnernal anatomy of the Kidney
a granular superficial region
the cone-shaped medulla (renal) pyramids separated by renal columns
a medullary pyramis and its surrounding cortical tissue
tip of the pyramid; releases urin into the minor calyx
the funnel-shaped tube withint he renal sinus that is continuous witht he ureter leaving the hilum
the branching channels of the renal pelvis that collect urine fromt he minor calyces and empty urine into the pelvis
Urine flows from the pelvis to the ureter to the bladder
- *walls of the calyces, pelvis, and ueter contain smooth muscle that contracts rythmically to propel urine by peristalsis
What is the blood and nerve supply for the kidneys?
Renal arteries deliver about 1/4 (1200ml) of cardiac output to the kidneys each minute
arterial flow into the venous flow out of the kidneys follow similar paths
nerve supply is via the sympahetic fibers nad the renal plexus
renal artery-->segmental-->interlobular-->arcuate-->cortical radiate arteries
- Cortical radiate veins-->arcuate-->interlobular--> renal veins (no segmental veins)
What are the two main parts of a nephron?
- Nephrons are the structural and functional untits that form urine
- ~1 million per kidney
- Two main parts:
- 1. Glomerulus: a tuft of capillaries
- 2. Renal tubules: begins as cupshaped glomerular (Bowman's capsule) surrounding the glomerulus
What is the renal corpuscle of a nephron and what is the epithelium?
gomerulus and its gomerular capsule (Bowman's capsule)
- Fenestrated glomerular endothelium
- -allows filtrate to pass from plasma into the glomerular capsules (which is simple squamous epithelium)
Where does the renal tubule start? What are the layers?
It starts at the glomerular capsule
- parietal layer: simple squamous epithelium
- visceral layer: branching epithelial podocytes
- -these are extensions that terminate in foot process that cling to the basement membrane
- -filtration slits between the foot processes allow filtrate to pass into teh capsular space inside the glomerular capsule
What is the PCT portion of the renal tubule?
Proximal Convoluted Tubule
made of cuboidal cells with dense microvilli (increased surface area) and large mitochondria
-function is reabsoprtion and secretion
- confined to the cortex of the kidney
What is the loop of henle porion of the renal tubule?
It has descending and ascending limbs
- Thin segment usually in the descending lumb
- -simple squamous epithelium
- -freely permeable to water
- Thick segment of ascending limb
- -cuboidal to columnar cells
What is the DCT portion of the renal tubule?
Distal Convoluted Tubule
cuboidal cells with very few microvilli
functinos more in secretion than reabsoprtion
- confined to the cortex
What are the collecting ducts? What are the two cells types?
They receive filtrate from many nephrons
they fuse together to deliver urine through papillae into minor calyces
- Cell types:
- 1. Intercalated cells
- -cuboidal cells with microvilli
- -function in maintaining the acid-base balance of the body
- 2. Principle cells
- -cuboidal cells without microvilli
- -help maintain the body's water and salt balance
What cortical and juxtamedullary nephrons?
Cortical nephrons are entirely in the cortex and account fot 85% of nephrons
- Juxtamedullary nephrons
- -long loops of henle deeply invade the medulla
- -extensive thin segments
- -important in the production of concentrated urine
What are the three parts of the nephron capillary bed?
- 1. Glomerulus
- 2. Peritubular capillaries
- 3. Vasa Recta
How does the Glomerulus function as a nephron capillary bed?
Afferent arteriole-->glomerulus-->efferent arteriole
Specilized for producing the filtrate
- Blood pressure is high because:
- -afferent artieroles are smaller in diameter than efferent arterioles
- -arterioles are high-resistance vessels
What are peritubular capillaries?
Part of the nephron capillary bed
Low-pressure, porous capillaries adapted for absorption of solutes and water
arise from the efferent arterioles leaving the glomerulus
cling to adjacent renal tubules in cortex
empty into venules
What are the vasa recta?
part of the nephron capillary bed
long vessels parallel to the long loops of henle
arise from the efferent arterioles of the juxtamedullary nephrons
function in formation of concentrated urine
What is vascular resistance in microcirculation?
- high resistance in afferent and efferent arterioles
- -causes blood pressure to decline from ~95mm to ~8mm Hg in kidneys
- Resistance in afferent arterioles:
- -protects glomeruli from fluctuations in systemic blood pressure
- Resistance in efferent arterioles:
- -reinforces high glomerular pressure
- -reduces hydrostatic pressure in peritubular capillaries
What is the Juxtaglomerular Apparatus (JGA)?
There is one per nephron
Important in regulation of filtrate formation and blood pressure
- Involves modified portions of the:
- -Distal portion fo the ascending limb of the loop of Henle
- -Afferent (sometimes efferent) arteriole
- Composed of:
- -granular cells (JG cells)
- -Macula densa
- -extraglomerular mesangial cells
What are Granular cells or JG cells?
part of the JGA in the arteriole wall
enlarged, smooth muscle cells of the arteriole
secretory granules contain renin
act as mechanoreceptors that sense blood pressure
What is the Macula Densa?
Tall, closely packed cells of the ascending limb of the loop of henle
- act as chemoreceptors that sence NaCl content for filtrate
What are extraglomerular mesangial cells?
Part of the JGA
interconnected with gap junctions
may pass ginals betweent he macula densa and granular cells
What is the filtration membrane?
Porour membrane between the blood and capsular space of the glomerular capsule
- Consists of 3 layers:
- 1. Fenestrated endothelium of the glomerular capullaries
- 2. Visceral membrane of the glomerular capsule (podopcytes with foot processes ad filtration slits)
- 3. Gel-like basement membrane (fused basal laminae of the other two layers)
- allows the passage of water and solute smaller than most plasma proteins
- -fenestrations prevent filtration of blood cells
- -negatively charges basement membrane repels large anions such as plasma proteins
- -slit diaphrams also help repel macromolecules
- Glomerular mesangial cells
- -engulf and degrade macromolecules caught in filtration membranne
- -can contract to change the total surface are available for filtration
What are the mechanisms of urine formation?
The kidney's filter the body's entire plasma volume 60x each day
filtrate: blood plasms without protiens
- -less than 1% total filtrate leaves the body as urine
- -contains metabolic wastes and undeeded substances
- 3 Major process of Urine Formation
- 1. Glomerular Filtration
- 2. Tubular Reabsorption
- -returns all glucose and amino acids, 99% of water, salt and other components to the blood
- 3. Tubular secretion
- -reverse of reabsorption- selective addition to urine
What is glomerular filtration?
Passive mechanical process driven by hdrostatic pressure (does not consume metabolic energy)
- The glomerulus is a very efficient filter becuase:
- -its filtration membrane is very permeable and it has a large surface area
- -glomerular blood pressure is higher (55mm Hg) than other capillaries and this results in much higher net filtration pressure
Molecules less than 5nm are not filtered (e.g. plasma proteins) and function to maintain colloid osmotic pressure of the blood
What is the Net filtration pressure and what is it determined by?
NFP is the pressure responsible for the filtrate formation (10mm Hg)
- Determined by:
- -Glomerular hydrostatic pressure (HPg) is the chief force pushing water and solutes out of the blood anda cross the filtration membrane
- -Two opposing forces
- 1. Colloid osmotic pressure of the glomerular blood (OPg)
- 2. Capsular hydrostatic pressure (HPc)
- NFP= 55 mmHg - (30 mm Hg + 15 mm Hg)
- NFP= 10 mm Hg
What is the Glomerular Filtration rate?
GFR is the volume of filtrate formed per minute by the kidneys (120-125 ml/min)
- Governed by (and directly proportionate to):
- -total surface area available for filtration
- -filtration membrane permeability
How is GFR regulated?
GFR is tightly controlled by two types of mechanisms
- 1. Intrinsic controls (Renal autoregulation)
- -act locally within the kidneys to maintain GFR
- 2. Extrinsic controls
- -nervous and endocrine mechanisms that maintain blood pressure, but affect kidney function
What are the intrisic controls for regulating GFR?
They maintain a nearly constant GFR when MAP is in the range of 80-180 mm Hg
- Two types of renal autoregulation (adjusts it own regulation of the blood flow)
- 1. Myogenic Mechanism
- 2. Tubuloglomerular feedback system, which senses changes in the JGA
How does the myogenic mechanism of the intrinsic controls work to regulate GFR?
This mechanism reflects the tendency of the vascular smooth muscle to contract when stretched
- When BP rises, constriction of the afferent arterioles
- -helps maintain the GFR
- -protects glomeruli from damagaing high BP
- When BP drops, dilation of the afferent arterioles
- -helps maintain normal GFR
How does the tubuloglomerular feedback mechanism of the intrinsic controls work to regulate GFR?
Flow dependent mechanism directed by the macula densa cells
if GFR increases, filtrate flow rate increases in the tubule
Filtrate NaCl concentration will be high because of insufficient time for reabsorption
Macula Densa cells of the JGA respons to rise in NaCl by releasing a vaso constricting checmical that acts on the afferent arteriole, thus lowering the GFR
- the opposite occurs if teh GFR decreases
How does the sympathetic of the extrinsic controls work to regulate GFR?
- Under normal circumstances at rest:
- -renal blood vessels are dialated
- -renal autoregulation mechanisms prevail
- Under extreme stress:
- -norepinephrine is released by the sympathetic nervous system
- -epinephrine is released by the adrenal medulla
- -both cause constriction of afferent artierioles, inhibiting filtration and triggering the release of renin
What is the Renin-angiotensin mechanism?
What are the effects of Angiotensin II in stabilizing systempic BP and extracellular fluid volume?
1. Contrsicts arteriolar smooth muscle, causing MAP to rise
- 2. Stimulates the reabsorption of Na+
- -water follows Na+ osmotically so BP and BV rise
- -acts directly on renal tubles
- -triggers the adrenal cortex to release adolsterone
3. Stimulates the hypothalamus to release ADH and activates the thirst center
4. Constricts efferent arterioles, decreasing peritubular capillary hydrostatic pressure and increasing fluid reabsorption
5. causes glomerular mesangial cells to contract, decreasing the surface are available for filtration
What are triggers for renin release by granular cells?
reduced stretch of granular cells (MAP belove 80 mm Hg)
stimulation of the granular cells by activated macula densa cells (Sense low NaCl)
- direct stimulation fo the granular cells via Beta1-adrenergiv receptors by renal nerves
What are some other factors affecting GRF?
- Prostaglandin E2
- -vasoialtor that counteracts vasoconstriction by norepinephrinee and agiotensin II
- -prevents renal damage when peripheal resistance is increased
- Intrarenal angiotensin II
- -reinfornces the effects of hormonal angiotensin II
- -a vasoconstrictor of the renal vasculature
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