Lecture Urinary System Part 1

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
• 

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

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

• 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

Renal cortex: a granular superficial region

Renal medulla: the cone-shaped medulla (renal) pyramids separated by renal columns

Lobe: a medullary pyramis and its surrounding cortical tissue

Papilla: tip of the pyramid; releases urin into the minor calyx

Renal pelvis: the funnel-shaped tube withint he renal sinus that is continuous witht he ureter leaving the hilum

Major calyces: 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
• 

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)
• 

• 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

Renal corpuscle: 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)

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

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
• 

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
• 

Distal Convoluted Tubule

cuboidal cells with very few microvilli

functinos more in secretion than reabsoprtion

• confined to the cortex
• 

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
• 

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
• 

• 1. Glomerulus
• 2. Peritubular capillaries
• 3. Vasa Recta

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

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

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

• 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

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

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

Tall, closely packed cells of the ascending limb of the loop of henle

• act as chemoreceptors that sence NaCl content for filtrate
• 

Part of the JGA

interconnected with gap junctions

may pass ginals betweent he macula densa and granular cells

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

The kidney's filter the body's entire plasma volume 60x each day

filtrate: blood plasms without protiens

• Urine:
• -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
• 

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

NFP is the pressure responsible for the filtrate formation (10mm Hg)

• Determined by:
• -Glomerular hydrostatic pressure (HP_g) 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 (OP_g)
• 2. Capsular hydrostatic pressure (HP_c)

• 
• NFP= 55 mmHg - (30 mm Hg + 15 mm Hg)
• NFP= 10 mm Hg
• 

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
• -NFP

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

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

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

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
• 

• 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

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

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
• 

• 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

• Adenosine
• -a vasoconstrictor of the renal vasculature