Chapter 19: The Kidneys Flashcards
what are the functions of the urinary system?
- Regulate plasma ionic composition
- Regulate plasma volume
- Regulate plasma osmolarity
- Regulate plasma pH
- Remove metabolic waste products and foreign substances from plasma (creatinine, uric acid, urobilinogen, xenobiotics)
what are other functions of the urinary system?
- Secrete erythropoietin and renin
- Activate vitamin D3 to calcitriol
- Gluconeogenesis
-form urine
-located retroperitoneally
at the level of the lower ribs.
kidneys
transport urine from kidneys to bladder
ureters
store urine
bladder
excrete urine from bladder to outside of body
urethra
functional unit of kidney
nephron
consists of glomerulus and Bowman’s capsule
renal corpuscle
capillary network for filtration
glomerulus
- Receives the filtrate
- Inflow to renal tubules
Bowman’s capsule
what are the parts of renal tubules?
- proximal tuble
- loop of Henle
- distal convoluted tubule
- collecting duct
consists of descending limb, thin ascending limb, and thick ascending limb
Loop of Henle
divided into an outer cortex and an inner medulla
the kidney in cross section
flows into the renal pelvis prior to passing through the ureter into the bladder
urine leaving the nephrons
- enter the kidney at the hilus
- receive 20% of cardiac output at rest
- function is to filter blood
- account for 16% of ATP usage by the body
- take blood to the cortex
renal arteries
exit at the hilus
renal veins
- short loop of henle
- most numerous, 80-85%
- produce urine
cortical nephrons
- long loop of henle extends into medulla
- responsible for the medullary osmotic gradient
- produce urine
juxtamedullary nephron
contains all bowmans capsules, proximal and distal tububles
cortex
contains loops of henle and collecting ducts
medulla
has two arterioles and two sets of
capillaries that form a portal system
one nephron
from glomerulus to Bowman’s capsule
glomerular filtration
from tubules to peritubular capillaries
reabsorption
from peritubular capillaries to tubules
secretion
from tubules out of the body
excretion
-must cross three barriers to enter Bowman’s capsule
glomerular filtrate
Movement of protein-free plasma from glomerulus to Bowman’s capsule
GFR = 125 mL/min or 180 L/day
what are the 3 barriers glomerular filtrate must cross to enter bowman’s capsule?
- Capillary endothelial layer
- Surrounding epithelial layer
- Basement membrane sandwiched between these two layers
what are the filtration fractions?
1. Plasma volume entering afferent arteriole = 100%. 2. 20% of volume filters 3. >19% of fluid is reabsorbed 4. >99% of plasma entering kidney returns to systemic circulation. 5. <1% of volume is excreted to external environment
what are the forces favoring filtration?
- Glomerular capillary hydrostatic pressure
- Bowman’s capsule colloid osmotic pressure
- 55 mm Hg
- High due to resistance of efferent arteriole
Glomerular capillary hydrostatic pressure
- 0 mm Hg
- Low due to lack of protein in filtrate
Bowman’s capsule colloid osmotic pressure
what are the forces opposing glomerular filtration?
- Bowman’s capsule hydrostatic pressure
- Glomerular colloid osmotic pressure
- 15 mm Hg
- Relatively high due to large volume of filtrate in closed space
Bowman’s capsule hydrostatic pressure
- 30 mm Hg
- Higher than in systemic capillaries due to plasma proteins in smaller volume of plasma
Glomerular colloid osmotic pressure
= (PGC + πBC) – (PBC + πGC)
= (55 + 0) – (15 + 30) = 10 mm Hg
filtration pressure
what is the amount of renal plasma flow?
625 mL/min
what is the filtration pressure and rate in systemic capillaries?
- Filtration pressure = 2 mm Hg
- Filtration rate = 3 L/day
is the glomerular filtration rate constant?
relatively constant
what are the two factors that influence glomerular filtration rate (GFR)?
- net filtration pressure
- filtration coefficient
what is involved in net filtration pressure?
Hydrostatic pressure – colloid osmotic pressure – fluid pressure
what is involved in the filtration coefficient?
- Surface area of glomerular capillaries available for filtration
- Permeability of interface between the capillary and Bowman’s capsule
the epithelium around glomerular capillaries is modified into…..
podocytes
-surround each capillary, leaving slits through which filtration takes place
podocyte foot processes
- between the capillaries
- contract to alter blood flow
mesangial cells
-create a 3-layer filtration barrier
- glomerular capillary endothelium
- basal lamina
- Bowman’s capsule
pass through endothelial pores and filtration slits
filtered substances
what is involved in the intrinsic regulation of the glomerular filtration rate?
- myogenic reguation
- Tubuloglomerular feedback
- Smooth muscle in wall of afferent arteriole
- Contracts in response to stretch
myogenic regulation
- macula densa cells
- smooth muscles of arterioles contract in response to paracrines
Tubuloglomerular feedback
-secrete paracrine factors in response to an increase in flow of fluid past them
-sense distal tubule flow
and release paracrines that affect afferent
arteriole diameter.
Macula densa cells
- takes place over a wide range of blood pressures
- maintains a nearly constant GFR when mean arterial blood pressure is between 80 and 180 mmHG
autoregulation of glomerular filtration rate
loops back on itself so that the ascending limb of the loop of Henle passes
between the afferent and efferent arterioles
nephron
secrete renin, an enzyme
involved in salt and water balance.
granular cells
how does the tubuloglomerular feedback help GFR autoregulation?
- GFR increases
- flow through tubule increases
- flow past macula densa increases
- Paracrine from macula densa to afferent arteriole
- afferent arteriole constricts —-> resistance in afferent arteriole increases—-> hydrostatic pressure in glomerulus decreases—-> GFR decreases
how do hormones and autonomic neurons help with GFR regulation?
- By changing resistance in arterioles
- by altering the filtration coefficient
how is the resistance in arterioles altered?
- sympathetic neurons
- angiotensin II
- prostaglandins
- constrict afferent and efferent arteries
- decrease GFR
sympathetic neurons
vasoconstrictor
angiotensin II
vasodilators
prostaglandins
how is the filtration coefficient altered?
- podocytes filtration slits
- contraction of mesangial cells
Extrinsic control of GFR and renal vascular resistance during fluid loss due to…..
hemorrhage or sweating
- Movement from tubules into peritubular capillaries (returned to blood)
- occurs most in proximal tubules
- not regulated for the most part
reabsorption
Transepithelial transport (transcellular transport)
- Substances pass through the cell–cell junction between two adjacent cells
- other solutes move through junctions between epithelial cells
paracellular pathway
how does tubular reabsorption of solutes work?
- Na+ is reabsorbed by active transport
- Electrochemical gradient drives anion
reabsorption. - Water moves by osmosis, following solute reabsorption. Concentrations of
other solutes increase as fluid volume
in lumen decreases. - Permeable solutes are reabsorbed by
diffusion through membrane transporters
or by the paracellular pathway.
what is involved in re-absorption?
- active transport of sodium
- secondary active transport: symport with sodium
- passive reabsorption of urea
- plasma proteins and receptor mediated endocytosis
what are the steps of sodium reabsorption in the proximal tubule: active transport?
1. Na+ enters cell through various membrane proteins, moving down its electrochemical gradient. 2. Na+ is pumped out the basolateral side of cell by the Na+-K+-ATPase
Rate of transport when carriers are saturated
transport maximum
when can saturation of carriers occur?
When solute is transported across epithelium by carrier proteins
Solute in plasma that causes solute in filtrate to saturate carriers and spill over into urine
renal threshold
what happens if filtrate saturates carriers?
some solute is excreted in urine
- freely filtered at glomerulus
- normally 100% actively reabsorbed in proximal tubules
glucose reabsorption
should glucose appear in the urine?
no
what are the carrier proteins for glucose reabsorption?
- Apical membrane: secondary active transport
- Basolateral membrane: facilitated diffusion
what are the steps of sodium linked reabsorption: indirect (secondary) active transport?
1. Na+ moving down its electro- chemical gradient uses the SGLT protein to pull glucose into the cell against its concentration gradient. 2. Glucose diffuses out the basolateral side of the cell using the GLUT protein. 3. Na+ is pumped out by Na+-K+-ATPase.
shows the relationship between filtration, reabsorption, and excretion of glucose
composite graph
in secretion, in what direction does solute move?
from peritubular capillaries into tubules
what are some secretion substances?
- Potassium
- Hydrogen ions
- Choline
- Creatinine
- Penicillin
takes place in proximal tubules
nonregulated reabsorption
in the distal tubules and collecting ducts
Regulated reabsorption and secretion
where does water conservation take place?
loop of henle
- the mass reabsorber
- 70% sodium and water
- 100% glucose
proximal tubule
what are the features of the proximal tubule that allows it to be the mass reabsorber?
- Brush border provides for large surface area
- Large numbers of mitochondria
- Leaky tight junctions allow paracellular transport
how is transport regulated across the epithelium in the distal tubules and collecting ducts?
- Tight junctions limit paracellular transport
- Receptors for hormone that regulate the transport of water and several solutes
- establishes conditions necessary to concentrate urine
- Minimizes water loss
Loop of Henle
Amount of substance excreted =
amount filtered + amount secreted – amount reabsorbed
what are the 3 factors that the amount of substance secreted depends on?
- Filtered load (GFR x plasma concentration of x)
- Secretion rate
- Reabsorption rate
- Volume of plasma from which a substance has been removed by kidneys per unit time
- Noninvasive way to measure GFR
=excretion rate/plasma concentration
clearance
used to measure the GFR
inulin and creatinine
Amount of inulin excreted in urine =
amount that was filtered = filtered load
clearance of inulin=
excretion rate/ plasma concentration = GFR
excretion rate of inulin=
filtered load (filtration rate)= GFR x Pi
what is inulin clearance equal to?
GFR
what happens if filtration and excretion of inulin are the same?
then there is no net reabsorption or secretion, and the clearance of a substance equals the GFR
what is used for clearance calculations?
Urine excretion rates and plasma concentrations for solutes
how do you determine the fate of solute in renal tubules?
-
**If you know either inulin or creatinine clearance you can determine the renal handling of any compound
- If Cx > GFR, then substance was secreted
- If Cx < GFR, then substance was reabsorbed
- the process of urination
- urine does not change composition
micturition
what is the path of urine?
ureter—-> bladder —–> urethra
send signal to the spinal cord
Stretch receptors in bladder walls
override basic reflexes under parasympathetic control (during micturition)
Centers in the brain stem and cerebral cortex
found between the bladder and urethra
Internal sphincter (smooth muscle), external sphincter (skeletal muscle)
what are the steps of micturition?
- stretch receptors fire
- parasympathetic neurons fire; motor neurons stop firing
- smooth muscle contracts; internal sphincter is passively pulled open; external sphincter relaxes
