PSIO Final Exam Flashcards
Urinary System
- consists of the kidneys, ureters, urinary bladder, and urethra
- urine flows from each kidney, down its ureter to the bladder and to the outside via the urethra
- kidneys filter the blood of wastes and return most of water and solutes to the bloodstream
Kidney Functions
- reg. of blood ionic composition
- reg. of blood pH and osmolarity
- reg. of blood glucose (via glycogenesis)
- reg. of blood volume
- reg. of blood pressure
- release of erythropoietin and calcitriol
- excretion of wastes and forge in substances
How much blood plasma is filtered every minute?
~125 mL
How much blood plasma is filtered every day?
~180 L
Three major physiological functions kidneys perform on blood
- filtration
- reabsorption
- secretion
Reabsorption
~99% of the nutrients, electrolytes, etc. in filtered blood plasma is returned to blood
Renal Threshold
if the concentration of any component of blood plasma exceeds its renal threshold, only some of the component will be returned to blood
the rest will be lost in urine
Daily Renal Turnover in Adult
look at chart
How many nephrons does a kidney have?
1 million
the number remains constant from birth
Why does the size of the kidney increase?
the size increase of individual nephrons
What happens if kidneys are injured?
there is no nephron replacement
When is dysfunction in the kidney evident?
- until function declines to less than 25% of normal
- remaining nephrons have the capacity to handle larger work loads when necessary
- if one kidney is removed, remaining kidney nephrons will enlarge to handle additional work (remaining kidney will be able to filter at 80% of the normal rate of two kidneys)
What are nephrons composed of?
corpuscle and tubule
Renal Corpuscle
- site of plasma filtrate
- glomerulus is a knot of capillaries where filtration occurs
- glomerular (Bowman’s) capsule is a double walled epithelial cup that collects filtrate
Renal Tubule
-site of reabsorption from filtrate and secretion into filtrate
Contains:
- proximal convoluted tubule (PCT)
- loop of Henle
- distal convoluted tubule (DCT)
Where do DCTs drain?
collecting duct > papillary duct > renal pelvis > ureter
Where are glomerular capillaries formed?
between afferent and efferent arterioles
What do efferent arterioles give rise to?
The peritubular capillaries and vasa recta
Renal Blood Flow
renal artery segmental arteries interlobar arteries arcuate arteries cortical radiate arteries afferent arterioles glomerular capillaries efferent arterioles peritubular capillaries cortical radiate veins arcuate veins interlobar veins renal veins
Cortical Nephrons
- 80-85% of nephrons are cortical nephrons
- renal corpuscles are in outer cortex and loops of Henle lie mainly in cortex but can dip into the medulla
Flow of fluid through a cortical nephron
- glomerular (Bowman’s) capsule
- proximal convoluted tubule
- descending limb of the loop of Henle
- ascending limb of the loop of Henle
- distal convoluted tubule (drains into collecting duct)
Flow of fluid through a juxtamedullary nephron
- glomerular (Bowman’s) capsule
- proximal convoluted tubule
- descending limb of the loop of Henle
- thin ascending limb of the loop of Henle
- thick ascending limb of the loop of Henle
- distal convoluted tubule (drains into collecting duct)
Juxtamedullary Nephrons
- 15-20% of nephrons are juxtamedullary nephrons
- renal corpuscles close to medulla and long loops of Henle extend into deepest medulla enabling excretion of dilute or concentrated urine
How much blood does the kidney receive through it’s blood vessels?
25% of resting cardiac output via renal arteries
Glomerular Capillaries Function
where filtration of blood occurs
Peritubular Capillaries and Vasa Recta Function
carry away substances reabsorbed from filtrate
Sympathetic Vasomotor nerves
regulate blood flow and renal resistance by altering diameter of arterioles
Vasoconstriction and vasodilation of afferent and efferent arterioles
produces large changes in renal filtration
Histology of the Nephron and Collecting Duct
- single layer of epithelial cells forms walls of the entire tube
- distinctive features due to function of each region
Distinctive Regions of Nephrons and Collecting Duct
- microvilli
- cuboidal versus squamous
- hormone receptors
Microvilli are found on:
- PCT cells
- intercalated cells of the CD
Cuboidal is more common; Squamous cells are found in:
- parietal layer of glomerular capsule
- loop of Henle descending limb
- loop of Henle thin ascending limb
Hormone receptors are found on:
- DCT cells
- principal cells of the CD
Excretion
the process of eliminating waste products from body, or product(s) excreted from the body
Rate of excretion
rate of filtration plus rate of secretion minus rate of reabsorption
Glomerular Filtration
- blood pressure produces glomerular filtrate
- filtration fraction is 20% of plasma
- 180 liters/day of filtrate is reabsorbed to form 1-2 liters/day of urine
Filtering capacity is enhanced by:
- thinness of membrane
- large surface area of glomerular capillaries
- high glomerular capillary blood pressure (due to smaller size of efferent arteriole)
Filtration Barrier is composed of:
- fenestration pore
- basal lamina
- slit membrane between pedicels
Fenestration (pore) of glomerular endothelial cell
prevents filtration of blood cells but allows all components of blood plasma to pass through
Basal lamina of glomerulus
prevents filtration of larger proteins
Slit membrane between pedicels
prevents filtration of medium-sized proteins
Renal Corpuscle and ascending loop of Henle interaction
top of ascending limb contains macula densa cells which interact with cells lining the afferent arteriole (called juxtaglomerular cells)
Juxtaglomerular Apparatus
structure where afferent arteriole makes contact with ascending limb of loop of Henle
- macula densa is thickened part of ascending limb
- juxtaglomerular cells are modified muscle cells in arteriole
NFP
net filtration pressure
total pressure that promotes filtration
GBHP
glomerular blood hydrostatic pressure
CHP
capsular hydrostatic pressure
BCOP
blood colloid osmotic pressure
NFP equation
GBHP- (CHP+ BCOP) = 10mmHg
Glomerular Filtration Rate (GFR)
amount of filtrate formed in all renal corpuscles of both kidneys per minute
average adult rate is 125 mL/min
Why does homeostasis require a GFR that is constant?
- if GFR is too high, useful substances are lost due to the speed of fluid passage through a nephron
- if GFR is too low, sufficient waste products may not be removed from the body
What does NFP depend most heavily on?
glomerular blood hydrostatic pressure (GBHP)
How do changes in net filtration pressure affect GFR?
- filtration stops if GBHP drops from 55 mmHg to 45 mmHg
- GBHP is determined by mean arterial pressure (MAP)
- kidney functions normally with MAP between 80 mmHg and 180 mmHg because of GFP regulation
Autoregulation of GFR
Mechanisms that maintain a constant GFR despite changes in arterial blood pressure:
- myogenic mechanism
- tubuloglomerular feedback
Myogenic mechanism
(FAST)
- system increases in blood pressure stretch the afferent arteriole
- smooth muscle contraction reduces the diameter of the afferent arteriole returning the GFR to its previous level in seconds
Tubuloglomerular feedback
(SLOW)
- elevated systemic blood pressure raises the GFR so that fluid flows too rapidly through the renal tubule: Na+ and Cl- and water are not reabsorbed
- macula densa in ascending limb of loop of Henle detects increased Na+ and Cl-: inhibit release of Nitric Oxide (a vasodilator) from the juxtaglomerular apparatus
- afferent arterioles constrict: reduce GFR
Neural Regulation of GFR
-blood vessels of the kidney are supplied by sympathetic fibers that cause vasoconstriction of afferent arterioles
(norepinephrine release from sympathetic postganglionic neurons binds to alpha 1 adrenergic receptors and causes vasoconstriction of afferent arterioles)
-SNS will override renal autoregulation
-SNS will also stimulate renin release from the juxtaglomerular cells (see hormonal regulation)
-at rest, renal blood vessels are maximally dilated (sympathetic activity is minimal): renal autoregulation prevails
-with moderate sympathetic stimulation, both afferent and efferent arterioles constrict equally (GFR decreases slightly)
-with extreme sympathetic stimulation (exercise/hemorrhage), vasoconstriction of afferent arterioles predominates
(-GFR decreases substantially
-lowers uric output to maintain blood volume
-permits greater blood flow to other tissues ie. muscle)
Hormonal Regulation of GFR
two hormones contribute to the regulation of GFR:
- atrial natriuretic peptide (ANP)
- angiotensin II
Atrial natriuretic peptide (ANP)
- increases GFR
- stretching of the atria that occurs with an increase in blood volume causes hormonal release
- relaxes glomerular mesangial cells increasing glomerular capillary surface area and increasing GFR
Angiotensin II
- reduces GFR
- activated by ACE in the lungs following the release of renin from juxtaglomerular cells
- potent vasoconstrictor that narrows both afferent and efferent arterioles reducing GFR
Glomerular filtration rate in the Renal Corpuscle
105-125 mL/min of fluid that is isotonic to blood
Filtered substances in Renal Corpuscle
water and all solutes present in blood (except proteins) including ions, glucose, amino acids, creatine, urea, uric acid, etc.
Fluid in glomerular capsule is:
isotonic to blood 300 mOsm/liter
Tubular Reabsorption and Secretion
- nephron must absorb 99% of the filtrate
- tubular secretion transfers materials from blood into tubular fluid
What does the most reabsorption?
proximal convoluted tubule (PCT)
Fine-Tuning Functions of Nephron
- solutes reabsorbed by active and passive processes
- water follows solutes by osmosis
- small proteins move across into the blood by pinocytosis
Tubular Secretion
- transfers materials from blood into tubular fluid
- helps control blood pH through the secretion of H+
- help eliminate certain substances (NH4+, creatinine, K+)
Paracellular Reabsorption
50% of reabsorbed material moves between cells by diffusion in some parts of the tubule
Transcellular Reabsorption
material moves through both the apical and basal membranes of the tubules cell by active transport
Which membranes of tubule cells have different types of transport proteins?
apical and basolateral
Reabsorption of Na+
- several transport systems exist to reabsorb Na+
- Na+/K- ATPase pumps sodium from tubule cell cytosol through the basolateral membrane only
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 (ADH)
What is reabsorbed in the PCT?
Water Na+ K+ Glucose Amino Acids
How does reabsorption in the PCT work?
- Na+ symporters help reabsorb materials from the tubular filtrate (glucose, amino acids, lactic acid, water soluble vitamins)
- intracellular sodium levels are kept low due to Na+/K+ pumps on the basolateral side
isosmotic reabsorption (water follows solutes)
What is the renal threshold for glucose and what happens when that level is reached?
200 mg/mL
Renal symporters cannot reabsorb glucose fast enough if blood glucose levels exceed 200 mg/mL
(some glucose remains in the urine: glycosuria)
What is a common cause for glycosuria?
Diabetes mellitus where in insulin activity is deficient and blood glucose cannot enter cells as readily as it does for unaffected individuals
What is reabsorbed in the Loop of Henle?
Water
Na+
K+
Cl-
What is secreted in the Loop of Henle?
Urea
Symporters in the Loop of Henle
Thick ascending limb of loop of Henle has Na+/K+/2Cl- symporters that reabsorb these ions
- K+ moves back into filtrate through K+ leak channels
- Na+ is pumped out on basolateral side
- Cl- diffuses across cell
Cations passively move to the vasa recta
-drawn to negative charge in capillary
In juxtamedullary nephrons the thick ascending limb is impermeable to water
What is reabsorbed in the early DCT?
Water
Na+
Cl-
Reabsorption of Na+ and Cl- in Early DCT
- like thick ascending limb, early DCT reabsorbs ions but it is more permeable to water
- Na+ and Cl- are reabsorbed by different symporters than those in the loop of Henle
- fluid entering the early DCT is hypotonic (~150 mOsm/L) and becomes more hypotonic until it reaches the late DCT
- all water reabsorption until late DCT is obligatory (always permitted) This is obligatory reabsorption of water
What is reabsorbed in the late DCT and CD?
Water
Na+
Urea
What is secreted in the late DCT and CD?
K+
Reabsorption of Na+ and secretion of K+ in the late DCT and collecting duct
Two types of cells are in the late DCT and collecting duct
- principal cells
- intercalated cells
- Principal cells reabsorb Na_ (and water if ADH present) and secrete K+
- intercalated cells reabsorb K+ and HCO3- and secrete H+
Formation of dilute urine
- remove excess fluid from blood by producing dilute urine
- blood plasma has 300 mOsm/L concentration
- filtrate osmolarity increases as it moves down descending loop of henle
- filtrate osmolarity decreases as it moves up ascending loop of henle
- filtrate osmolarity decreases in collecting duct and leads to dilute urine
What happens to the DCT and CD without ADH?
they are impermeable to water
throughout the journey, the volume of tubular fluid is constantly decreasing