Functions of the urinary system
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Two types of nephrons
cortical (85%) and juxtamedullary (15%)
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peritubular capillaries
The network of tiny blood vessels that surrounds the proximal and distal tubules in the kidney
vasa recta
the capillary system in the kidney that serves the loop of Henle
Urine formation
1. Glomerular filtration
2. Tubular reabsorption
3. Tubular secretion
1. Glomerular Filtration
a passive process in which hydrostatic pressure forces fluids and solutes through a membrane
Filtration membrane
1. fenestrated endothelium
2. basement membrane
3. filtration slits - foot processes of podocytes
Pressures that affect filtration
1. outward pressures
2. inward pressures
Outward pressures
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Hydrostatic pressure in glomerular capillaries
HP g
chief force pushing water and solutes out of blood and across filtration membrane
55 mm Hg
Inward pressures
Two inward forces inhibit filtrate formation
-Hydrostatic pressure in the capsular space
- Colloid osmotic pressure in glomerular capillaries
Hydrostatic pressure in the capsular space
HP c
pressure exerted by filtrate in the glomerular capsule
15 mm Hg
Colloid osmotic pressure in glomerular capillaries
OP g
pressure exerted by the proteins in the blood.
30 mm Hg
Net Filtration Pressure
NFP
Forces fluid out of blood
Outward pressure - inward pressure
(55) - (15+30) =
10 mm Hg
Glomerular Filtration Rate
The volume of filtrate formed each minute by the combined activity of all glomeruli in the kidneys.
Directly proportional to:
-Net filtration pressure
-Total surface area available for filtration
-Filtration membrane permeability
Increased arterial blood pressure does what to GFR
It increases GFR
Vasoconstriction of afferent arteriole
decreases NFP and decreases GFR
Vasodilation of afferent arteriole
Increases NFP and increases GFR
Intrinsic control of GFR
Renal auto regulation:
1. Myogenic mechanism
2. Tubuloglomerular feedback mechanism
Goal: maintain GFR despite normal fluctuations in blood pressure
Myogenic mechanism
Smooth muscle cells in afferent arterioles contract in response to stretch and relax when not stretch
tubuloglomerular feedback
Macula dense cells detect NaCl concentration in the filtrate.
- If NaCl too high, GFR too high - macula dense stimulate vasoconstriction of afferent arteriole
- If NaCl too low, GFR too low- macula dense stimulate vasodilation of afferent arteriole
Extrinsic control of GFR
Neural control
- sympathetic nervous system controls, NE
Hormonal control
- Renin-angiotensin mechanism
Goal: Modify GFR to raise blood pressure
Sympathetic N.S control of GFR
Causes afferent arterioles to constrict, increasing peripheral resistance, and increasing blood pressure
NE is released during sever hemorrhage
Renin-angiotensin mechanism
The body's main mechanism to increase blood pressure. Regulates GFR indirectly
- Low blood pressure causes renin to be released by granular cells
- Renin leads to the production of angiotensin II
What does angiotensin II do?
Raises blood volume and blood pressure:
-Vasoconstriction
- Stimulates release of aldosterone
- Stimulates release of ADH
What does aldosterone reabsorb?
Na + and H2O
What does ADH reabsorb?
H20
2. Tubular reabsorption
Most solutes and water in filtrate are reabsorbed back into the blood via
-Paracellular
-Transcellular
Transcellular tubular reabsorption
Transported substances move through the apical membrane, the cytosol, and the basolateral membrane of the tubule cell and then the endothelium of the peritubular capillaries.
Paracellular tubular reabsorption
Movement of substances between the tubule cells, which is limited by the tight junctions connecting the cells. In the proximal nephron they are leaky and allow H2O and important ions to pass through.
Active tubular reabsorption
Requires ATP either directly (primary) or indirectly (secondary) for at least one of its steps.
Passive tubular reabsorption
encompasses diffusion, facilitated diffusion, and osmosis - processes in which substances move down their electrochemical gradients
Sodium transport across the basolateral membrane
Na+ is actively transported out of the tubule cell by primary active transport a Na+ K+ ATPase pump. This bulk flow of H2O sweeps Na+ into peritubular capillaries.
- Bulk flow of H2O and solutes in the cap. is very rapid due to low hydrostatic pressure and high osmotic pressure.
Sodium transport across the apical membrane
Active pumping of Na+ creates a strong electrochemical gradient that favors the entry at the apical face via secondary active transport.
- The pump maintains the intracellular Na+ concentrations at a low level
- The K+ pumped into the tubule cells diffuses out into the IF via leakage channels, leaving the interior of the tubule cell with a net negative charge.
Passive tubular reabsorption of water and anions
The movement of Na+ and other solutes establishes a strong osmotic gradient, and water moves into the peritubular capillaries by osmosis. Aquaporins aid in the process by acting as water channels across cell membranes.
Where are aquaporins always present?
PCT, their presence obliges the body to absorb water in the proximal nephron regardless of its state of over or under hydration.
Where are aquaporins virtually absent?
The apical membranes of the collecting duct unless ADH is present.
Secondary active transport
Glucose, amino acids, some ions, and vitamins. An apical carrier moves Na+ down its concentration gradient as it cotransports another solute. These cotransported solutes move across the basolateral membrane by facilitated diffusions via other transport proteins.
Transport maximum
Reflects the number of transport proteins in the renal tubules available to ferry a particular substance. When transporters are saturated the excess substances are excreted in urine.
Absorptive capabilities of renal tubules
PCT - most active reabsorbs and most events occur in this segment.
Loop - water absorption is not coupled to solute reabsorption
DCT & Collecting - dependent on hormones
3. Tubular secretion
Moves selected substances such as: H+, K+, metabolic wastes - ureas & uric acid, from the peritubular capillaries through the tubule cells into the filtrate.
- Disposing substances: drugs and metabolites that are bound to plasma proteins.
- Eliminating undesirable substances that were reabsorbed by passive processes.
- Riding the body of excess K+.
- Controlling blood pH
Osmolality
-Concentration of solutes particle per kg or H2O
- Reflects the solution's ability to cause osmosis
What controls the osmolality of body fluids?
The kidneys
Body fluids are maintained at
300 mOsm
Countercurrent Mechanism
Regulate urine volume/concentration
What creates the osmotic gradient in the kidneys
The long loop of Henle - juxtamedullary nephrons
-Acts as countercurrent multiplier
What preserves the gradient
Vasa recta
-Acts as the countercurrent exchangers
What uses the osmotic gradient to adjust urine osmolality?
Collecting ducts
countercurrent multiplier
The more NaCl the ascending limb extrudes, the more water diffuses out of the descending limb and the saltier the filtrate in the descending limb becomes.
The ascending limb then uses the increasingly salty filtrate left behind in the descending limb to raise the osmolality of the medullary interstitial fluid even further. Positive feedback.
The descending loop of Henle is
impermeable to solutes
permeable to water
The ascending loop of Henle is
permeable to solutes
impermeable to water
countercurrent multiplier cycle
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countercurrent exchangers
Vasa recta maintain osmotic gradient
-prevents removal of salt from the interstitial space
- removes reabsorbed water
Urine concentration and volume
collecting ducts use osmotic gradient to regulate filtrate concentration.
-variable water reabsorption is controlled by ADH
Formation of dilute urine
- if ADH no present ducts are impermeable to water and large volume of dilute urine is excreted
-if overhydrated, ADH production decreases and osmolality of urine falls as low as 100 mOsm
Formation of concentrated urine
ADH inserts aquaporins into cells of collecting ducts, water is reabsorbed due to a high osmotic gradient in the medulla
dehydrated, ADH production increases and osmolality of urine can rise to 1200 mOsm
Diuretics
Chemicals that enhance urinary output
- substances not adequately reabsorbed
-substances that inhibit ADH
-substance that inhibit Na+ reabsorption
renal clearance
-Volume of plasma that kidneys clear of substance in 1 min.
-used to determine GFR and asses kidney function
normal renal clearance
125 mL/min
Chronic renal disease
less than 60 mL/min
Renal failure
less than 15 mL /min
Characteristics of urine
-Color - clear; pale to deep yellow
-Transparency - cloudy urine can indicate infection
-Odor - slightly aromatic
pH of urine
slightly acidic 6
range 4.5-8.0
Specific gravity
measure of solute concentrations
1.001-1.035
Chemical composition of urine
95% water
5% solutes: (nitrogenous waste)
-urea
-uric acid
-creatinine
Micturation
the act of emptying the bladder
detrusor muscle
contracts and internal urethral sphincters open