Chapter 25- The urinary system Flashcards
How much fluid does the kidneys filter per day?
200 liters of fluid filtered from blood by kidneys every single day. The kidneys are responsible for maintaining the composition of the body’s extracellular fluids by filtering the blood.
Functions of the kidneys (5)
- Regulate total body water volume and concentration of solutes in water
- Regulate concentration of ions in ECF
- Acid base balance
- Remove toxins, metabolic waste, and other foreign substances
- Hormone production- EPO and renin
Location of the kidneys
Each kidney lies between the parietal peritoneum and dorsal body wall. Kidneys are retroperitoneal organs- during fetal development, they move behind the parietal peritoneum. Extend from T12- L3
Medial portion of the kidneys
Medial portion is concave. Also contains the renal hilum- ureters, renal blood vessels, lymphatics, and renal nerve supply enter here
Where are the adrenal glands located?
Adrenal gland sits immediately superior to each kidney. Not associated with the function of the kidneys, however, if something affects the kidneys, it will usually affect the adrenal glands
Supporting external structures of the kidneys include (3)
- Renal fascia
- Perineal fat capsule
- Fibrous capsule
Renal fascia
Dense connective tissue. Function- anchors kidneys to surrounding structures
Perineal fat capsule
Fat mass surrounding kidneys. Function- cushions kidneys from physical trauma- kidneys not protected by rib cage like some other organs
Fibrous capsule
Thin, transparent capsule. Function- prevents disease from spreading to kidneys from other parts of the body
3 major internal regions of kidneys
- Renal cortex
- Renal medulla
- Renal pelvis
Renal cortex
Function- provides area for glomerular capillaries and blood vessel passage, EPO produced here. All cells responsible for EPO production are found in the cortex
Renal medulla
Contains renal pyramids- packed with capillaries and urine collecting tubules. There are 7 renal pyramids separated by renal columns
Kidney lobe
Consists of the renal pyramid and surrounding columns
Renal pelvis
Funnel shaped tube continuous with ureters. The pelvis branches to form major calyces (calyx), which lead into minor calyces. The function of renal calyces is urine collection and emptying into pelvis
How much blood goes to the kidney per minute
1200 ml
Renal arteries (4)
- Segmental arteries (5)
- Interlobar arteries- travel between kidney lobes
- Arcuate arteries- arch over bases of pyramids
- Cortical radiate arteries- supply cortical tissue
Renal veins (5)
Renal veins trace arterial blood supply.
- Cortical radiate veins
- Arcuate veins
- Interlobar veins
- Renal veins
- Segmental veins aren’t really referenced
Renal plexus
Autonomic nerve fibers and ganglia- sympathetic vasomotor fibers regulate blood supply to each kidney
Sympathetic nervous system function in the kidneys
Sympathetic vasomotor fibers regulate blood supply to each kidney. Function- adjusts diameter of renal arterioles, influences nephron activity. More blood is directed to kidneys in response to excess water- this is how blood vessel diameter affects urine formation. During times of stress, blood flow to the kidneys decreases so it can be diverted to other areas of the body- we don’t need to be making urine during a dangerous/stressful situation.
Nephron
The nephron is the functional unit of the kidney. Function- responsible for forming filtrate and eventually urine in the kidneys
Filtration definition
The mass movement of solutes and water from the plasma into the renal corpuscle and renal tubules. Remember- the kidneys filter blood
Reabsorption definition
The process by which nephrons remove water and solutes from the filtrate formed from filtration and return it to the blood. 99% of filtrate is reabsorbed by the body.
Secretion definition
Process by which excess ions and waste that have been reabsorbed are now put back into the filtrate. Filtration and reabsorption are passive processes, so some things are naturally reabsorbed- they need to be put back in the filtrate so they can actually be gotten rid of
Each nephron contains (2 structures)
- Renal corpuscle
2. Renal tubule
Renal corpuscle
Renal corpuscle filters blood to form filtrate
Renal tubule
Renal tubule reabsorbs what is needed by the body from the filtrate and secretes more substances into the filtrate. Filtrate is the raw material from which urine is formed
Where is the renal corpuscle located?
In the renal cortex
Subdivisions of the renal corpuscle (2)
- Glomerulus
2. Glomerular capsule
Glomerulus
Cluster of blood vessels, like a capillary bed- only capillary bed that is both fed and drained by arterioles. Glomerular capillaries are very porous, have large fenestrations. The part filtering out through the fenestrations is called the filtrate- only blood cells and large proteins are left
Glomerular capsule
Double layered structure that completely surrounds glomerular capillaries. Inner layer has podocytes with foot processes
Where are the renal tubules located?
Begins in renal cortex, extends into renal medulla, then returns to renal cortex
Renal tubules subdivisions (4)
- Proximal convoluted tubule (PCT)
- Nephron loop (loop of Henle)
- Distal convoluted tubule
- Collecting ducts
Proximal convoluted tubule (PCT)
Leads immediately off from glomerulus. Large cuboidal epithelial cells with dense microvilli- increases the surface area
Nephron loop (loop of Henle) descending loop
Portion continuous with PCT. High permeability to water, low permeability to solutes
Nephron loop (loop of Henle) ascending loop
Continuous with DCT. High permeability to solutes, low permeability to water. Water can’t move from inside the limb to surrounding space- having each side have different permeabilities lets us form concentrated urine if we need to
Distal convoluted tubule
Located in cortex, composed of small cuboidal epithelia. Smaller diameter than PCT, contain no microvilli
Collecting ducts
Ducts pass through cortex and medulla. Each collecting duct receives filtrate from tubules of multiple nephrons- filtrate becomes urine in the collecting ducts. Collecting ducts fuse together, dump urine into minor calyces
Important cell types in collecting ducts (2)
- Principal cells
2. Intercalated cells
Principal cells
Maintain sodium balance in the body
Intercalated cells
Helps maintain acid base balance- can secrete or reabsorb ions (hydrogen and bicarbonate)
Types of nephrons (2)
- Cortical nephrons
- Juxtamedullary nephrons
Length of nephron loop is the biggest difference
Cortical nephrons location
Located almost entirely in the cortex, a small portion of the nephron loop is found in renal medulla
Juxtamedullary nephrons
Nephron loops for these nephrons are very long, increasing the surface area, and deeply invade the renal medulla. During dehydration, these nephrons will form concentrated urine with low water content
Glomerulus capillaries pressure
Maintains high pressure to increase filtrate production
Peritubular capillaries
Low pressure capillaries arising from efferent arteriole. Cling to renal tubules- reabsorb water and solutes from tubule cells. Empty into venules- filtered blood returns to circulation
Vasa recta
Efferent arterioles of juxtamedullary nephrons- run parallel to long nephron loop. Help form concentrated urine
Juxtaglomerular complex
Portion of nephron where distal ascending limb lies against arterioles. Not directly involved in urine formation. Function- regulate blood pressure and filtration rate of the glomerulus. There are 3 cellular modifications at this point of contact
3 cellular modifications at this point of contact (juxtaglomerular complex)
- Macula densa
- Granular cells (juxtaglomerular cells)
- Extraglomerular mesangial cells
Macula densa
Chemoreceptor cells. Function- monitor NaCl content of filtrate entering distal convoluted tubule, and stimulate a change in diameter of afferent arteriole if sodium is too low or too high. High sodium- constriction of afferent tubules. Filtrate slows down so you maximize the amount of time you have to reabsorb sodium
Granular cells (juxtaglomerular cells)
Specialized smooth muscle cells found in arteriolar walls of afferent arteriole, and can sense blood pressure in the afferent arteriole. Also stimulated by macula densa cells. Contain granules that secrete renin
What increases renin release?
Low sodium concentration and low pressure in the arteriole. Low pressure= low filtrate formation= low urine production. With low pressure, you filter less blood- want to optimize filtrate formation
Extraglomerular mesangial cells
Packed between tubule and arterioles, function unclear
Main steps involved in urine formation (3)
- Glomerular filtration
- Reabsorption
- Secretion
Main function of glomerular filtration step in urine production
Production of a cell and protein- free filtrate that serves as the raw material for urine. High pressure in the capillaries forces fluid (mostly blood plasma) out of the capillary and into the glomerular capsule. The filtrate is what is forced out of the capillary. There does need to be filtration going on so not just anything is entering the capsule.
Filtration membrane
Allows passage of water, small solutes into glomerular capsule. 3 layers make up this membrane. Prevents certain things from entering the filtrate.
Layers of the filtration membrane (3)
- Fenestrated endothelium of capillaries
- Basement membrane
- Foot processes of podocytes
Fenestrated endothelium of capillaries
Pores allow all but large proteins and cells to pass through
Basement membrane
Negatively charged layer that allows only passage of small molecules & electrically repels other macromolecular anions
Foot processes of podocytes
Foot processes create filtration slits. Filtration slits contain slit diaphragms- membranes that act like a filter for macromolecules
Filtration pressures
Necessary for filtrate formation to occur. Includes outward pressure (promotes filtrate formation) and inward pressure (opposes filtrate formation).
Hydrostatic pressure in glomerular capillaries (HPgc)
Blood pressure of the glomerular capillaries that forces fluid into the surrounding space. Remember: pressure here is always high. This is a type of outward pressure.
Inward pressures (2)
- Hydrostatic pressure in capsular space (HPcs)
2. Colloid osmotic pressure in glomerular capillaries (OPgc)
Hydrostatic pressure in capsular space (HPcs)
Type of inward pressure exerted by filtrate that is already in the glomerular capsule.
Colloid osmotic pressure in glomerular capillaries (OPgc)
Proteins that are still in capillaries will “pull” water back in, type of inward pressure
Glomerular Filtration Rate (GFR)
The total volume of filtrate formed per minute for all nephrons in the kidneys
Factors affecting GFR (3)
- NFP
- Surface area of capillaries
- Filtration membrane permeability- filtration occurs along the entire glomerular capillary
Glomerular mesangial cells
Adjust surface area of capillaries
Why does GFR need to be regulated? (2)
- Kidneys need constant GFR to make filtrate and maintain extracellular homeostasis
- Regulating GFR regulates blood pressure in entire body. Ex: decreasing GFR will decrease urine output
Control of GFR can be (2)
Intrinsic (renal) or extrinsic (CNS)
What regulating GFR, what is the primary variable controlled?
HPgc. When HPgc increases, NFP & GFR also increase, when HPgc decreases, NFP & GFR decrease
Renal autoregulation
Intrinsic- kidneys adjust resistance to blood flow. Intrinsic controls can maintain GFR for blood pressures ranging 80-180 mm Hg
2 mechanisms of renal autoregulation
- Myogenic mechanism
2. Tubuloglomerular Feedback Mechanism
Myogenic mechanism
Smooth muscle contracts when stretched, rising systemic blood pressure stretches afferent arteriole so the afferent arteriole constricts. Blood flow into glomerulus restricted to maintain GFR at desirable rate
Tubuloglomerular Feedback Mechanism
Controlled by macula densa of juxtaglomerular complex (JGC), remember- macula densa monitor NaCl concentrations. Increasing GFR = decrease in reabsorption rate. Macula densa cause vasoconstriction of afferent arteriole and decreases blood flow into glomerulus, GFR decreases to increase reabsorption rate
Neural mechanisms (GFR control)
Extrinsic control. The sympathetic nervous system will override renal autoregulation. Norepinephrine released by sympathetic system in response to low blood pressure. Vascular smooth muscle contracts, this includes afferent arterioles.
Hormonal mechanisms (GFR control)
Type of extrinsic control. Renin-Angiotensin-Aldosterone mechanism- overall effect is to increase BP. Granular cells of JGC stimulated to release renin
Renin-Angiotensin-Aldosterone mechanism activation involves (3)
- Stimulation by sympathetic nervous system
- Activated macula densa cells. Macula densa sense low NaCl concentrations in response to decreased GFR
- Reduced stretch. Granular cells release renin in response to “slack” in plasma membrane walls
Reabsorbed substances can follow 2 routes
- Transcellular route
2. Paracellular route
Transcellular route
Transported substances move through the tubule cells from apical side to basal side, then pass into capillaries
Paracellular route
Transported substances move in between the tubule cells, then
pass into capillaries. Water and many ions pass easily via this route
Reabsorption of sodium (2 steps)
This is a transcellular, active process.
1. Apical membrane- secondary active transport from lumen of tubule into the cell
2. Basolateral membrane- Na+/K+ ATPase pump transports Na+
out of cell by primary active transport
Reabsorption of nutrients and ions
Can be transcellular or paracellular. Secondary active transport moves glucose, amino acids, ions, vitamins
Secondary active transport to move nutrients
Na+ transported into cell & cotransports another solute with it. Cotransported molecule moves across basolateral membrane via
facilitated diffusion & enters capillary
Reabsorption of water
This is a passive process. Transmembrane protein aquaporin allows water to cross plasma membrane of tubule cell. PCT has many aquaporins, and water is always absorbed here. Collecting ducts have no aquaporins until antidiuretic hormone (ADH) is present
Transport maximum (Tm)
Any pathway using a transport protein has a transport
maximum (Tm). The more transport proteins for a specific molecule, the higher the amount absorbed
Why are villi and microvilli important for reabsorption?
Located in PCT, increase surface area so reabsorption can occur more quickly.
Which substances are reabsorbed in the PCT? (4)
- Glucose, amino acids, most nutrients
- Most water and sodium (65%)
- Most electrolytes
- Uric acid and urea
Reabsorption in the nephron loop
Water reabsorption is not coupled to solute reabsorption here. Water can leave the descending limb, but not the ascending limb, and solutes can leave the ascending limb, but not the descending limb. Importance- the difference in permeability between the ascending limb and descending limb allows the nephron to form dilute or concentrated urine
Reabsorption in the DCT and collecting duct
Most water and solutes have already been reabsorbed by PCT and
nephron loop. Hormonally controlled by multiple hormones
Antidiuretic hormone (ADH)
Inhibits urine formation by increasing water reabsorption. Aquaporins inserted into collecting ducts- amount of ADH is directly proportional to number of aquaporins inserted
Aldosterone
Promotes Na+ reabsorption by principal cells of collecting ducts
Hormones controlling reabsorption in the DCT and collecting duct (4)
- ADH
- Aldosterone
- ANP
- PTH
Atrial natriuretic peptide (ANP)
Inhibits Na+ reabsorption in collecting ducts
Parathyroid hormone (PTH)
Increases reabsorption of Ca2+ in the DCT
Where does secretion usually occur?
PCT is the main site, but also occurs in the collecting duct
Functions of secretion (3)
- Eliminates waste/undesirable material that are passively reabsorbed. Ex- nitrogenous wastes urea & uric acid
- Rids body of excess K+ in DCT and collecting ducts
- Controls acid-base balance & blood pH
- Secretion of excess H+ or HCO3-
Normal solute concentration of body fluids
~300 mOsm. Intracellular osmolality is the same
When is osmolality high/low?
Osmolality is high during dehydration, low during overhydration
How do kidneys adjust osmotic concentration based on fluid intake?
Kidneys allow body to make constant adjustments based on intake
and loss of fluids to maintain this normal osmotic concentration. When fluid intake is low, the kidneys produce small amount of concentrated urine (has low water content). When fluid intake is high, kidneys produce large amount of dilute urine (has high water content)
Countercurrent exchange mechanism
Movement of fluids in the opposite direction through the nephron loop allows exchange of material. Countercurrent exchange mechanism establishes a medullary osmotic gradient- kidneys can vary urine concentration. Allows the body to maintain a consistent osmolality.
Countercurrent multiplier
Occurs in ascending and descending limb of juxtamedullary nephron loops. Establishes an osmotic gradient.
Countercurrent exchange
Flow of blood through the ascending and descending limb of the vasa recta. Maintains the osmotic gradient established in the multiplier by preventing removal of Na+ and Cl- from the interstitial space and removing reabsorbed water
How does the countercurrent multiplier occur? (5 steps)
- The ascending limb transports solutes (NaCl) out of the tubule, the descending limb transports water out of the tubule
- Filtrate entering the descending limb has same osmolality as blood plasma
- Solutes are pumped out of the ascending limb filtrate and into the interstitial space. This creates a gradient.
- Water will follow this gradient from the descending limb and into the interstitial space, and is reabsorbed by the vasa recta
- As you approach the hairpin turn, osmolality of the filtrate becomes highest
Urea recycling and the osmotic gradient
Urea enters filtrate in thin ascending limb and moves into interstitial fluid. The urea in interstitial fluid eventually moves back into ascending limb (“recycling”). Urea increases osmolality and strengthens osmotic gradient
How does overhydration lead to dilute urine?
ADH release decreases, osmolality of urine can be ~100 mOsm. Aldosterone release increases to reabsorb ions from filtrate, leaving mostly water. In order for this to work, there are no aquaporins. ~50 mOsm
How does dehydration lead to concentrated urine?
ADH release increasesaquaporins allow increased water reabsorption. Maximal reabsorption brings 99% of water back in from filtrate, and urine can reach 1200 mOsm to conserve water. ADH release also strengthens the countercurrent multiplier/urea recycling
Diuretics examples (4)
- Alcohol- inhibits ADH release by posterior pituitary
- Hypertension drugs
- Drugs to treat edema/congestion
- Osmotic diuretic- any substance that is not reabsorbed by the body and carries water with it. Ex- glucose in diabetes mellitus
Loop diuretics
Prevent formation of osmotic gradient by countercurrent multiplier
by acting on ascending limb. No Na+/Cl- leaves the ascending limb- no gradient is created
Chronic renal disease
GFR of <60 ml/min for 3+ months. Filtrate formation decreases, wastes build up, blood pH decreases. Caused by diabetes mellitus, hypertension, pyelonephritis, physical trauma
When does renal failure occur?
When GFR<15 ml/min. Uremia-”urine in the blood” (nausea, muscle cramps, mental changes, fatigue, etc. etc.). Multiple organ failure eventually occurs- EPO release stops, severe ion imbalances, metabolic abnormalities and accumulation of toxins in body
Treatments for renal failure (2)
Hemodialysis and kidney transplant
Hemodialysis
Patients blood passed through selectively permeable membrane
tubing. Urea, K+, other substances diffuse out of blood, substances to be added to body diffuse into blood
Which solutes are found in urine?
95% water, 5% solutes. Solutes- mostly urea, uric acid, and creatinine. High volumes of solutes, proteins, or WBCs indicates pathology
What causes the color of urine?
Color comes from presence of urochrome- product of hemoglobin destruction. Other colors can come from diet, drugs, presence of blood, vitamin supplements
pH of urine
Slightly acidic (~6). Diet alters this- acidic diet leads to acidic urine
Specific gravity
The ratio of a mass of a substance to the mass of an equal
volume of distilled H2O. Urine is mostly water BUT also contains solutes- has a higher specific gravity than water. Ranges 1.001-1.035
Ureters
Tubes that allow urine to pass from the kidneys to be stored in the bladder
3 layers of the ureters
- Mucosa- transitional epithelia
- Muscularis- two smooth muscle sheets
- Adventitia- fibrous connective tissue
Muscularis function
Contractions help push urine into bladder- contractions respond most to stretch. Sympathetic & parasympathetic fibers innervate, but have little effect
Renal calculi
Build up of calcium, magnesium salts and uric acid in kidneys, crystallizes to form a hard structure- crystallization forms “stones” in renal pelvis. Stones can remain lodged in renal pelvis (nephrolithiasis) or can become lodged in the ureter (urolithiasis). Ureters have a small diameter, so it causes a lot of pain. Symptoms- severe abdominal pain, nausea and vomiting, cloudy/foul smelling urine
Renal calculi are caused by
High blood calcium, obesity, diet
Treatment of renal calculi
Some stones can be passed without surgical procedure. If lodged in ureter- removed endoscopically or surgically. If in pelvis- lithotripsy -surgical procedure to break down a lodged stone. Use wave energy- person has to pass small fragments
Bladder
Found in abdominopelvic area, stores urine temporarily. Trigone contains three openings: 1 for each ureter, 1 for urethra
3 layers of the bladder wall
- Mucosa- of transitional epithelium- continuous with lower portion of each ureter
- Detrusor- muscle layer- has 3 layers. Outer and inner layers are longitudinal, middle layer is circular
- Fibrous adventitia
Urine storage in the bladder
Human bladder normally holds up to 400-500 ml of urine. A person usually feels the need to go to the bathroom at 200 ml so the bladder isn’t overfilled. Critical capacity- 1000 ml urine- the bladder can burst if not emptied. When nearly empty, walls are collapsed and folded into rugae. As it fills, folds disappear and the superior portion moves higher in the abdominal cavity
Urethra
Extends from bladder, leads out of body. Mucosa is pseudostratified. Near the bladder- transitional epithelia, near external opening- stratified squamous epithelia. Consists of the internal and external urethral sphincters.
Internal urethral sphincter
Thickening of detrusor muscle, closes urethra when urine is not being passed
External urethral sphincter
Skeletal muscle tissue. Levator ani muscle also helps close off urethra
Micturition definition
The act of emptying the bladder
For micturition, 3 events must occur simultaneously
- Detrusor contracts
- Internal sphincter opens
- External sphincter opens
Which areas of the brain control micturition?
Pons has pontine micturition center and pontine storage center. Impulses send faster if bladder isn’t emptied so the urge to go is stronger over time. Afferent impulses from stretch of bladder walls sent to the brain, and higher brain centers also involved here
Pontine storage center
Low storage volumes- pontine storage center is active. Micturition is inhibited
Pontine micturition center
High storage volumes- pontine micturition center is active. Need to void increases as bladder becomes more full (micturition reflex). External urethral sphincter can prevent micturition. About 10 ml in bladder after voiding, the bladder is never completely empty
