The Urinary System: Renal Physiology Flashcards
During the formation of urine
20-25% of resting cardiac output is delivered to the kidney
◦Every minute ~ 1.25 L of blood passes through
the kidneys (roughly half is plasma)
◦From this, 125 ml is filtered across the
glomerulus (~20% of plasma) and becomes filtrate (most is reabsorbed into the body)
Filtrate contains
Almost everything found in plasma (excluding proteins)
Of the 180 L of filtrate that is formed each day:
◦< 1% (1.5 L) leaves the body as urine
◦>99% is reabsorbed
The first process in urine formation & adjustment of blood composition is
- Glomerular filtration
◦ Produces cell- & protein-free filtrate
The 2nd process in urine formation & adjustment of blood composition is
- Tubular Reabsorption
◦ Selectively returns 99% of substances from filtrate → blood in renal tubules & collecting ducts
The 3rd process in urine formation & adjustment of blood composition is
- Tubular secretion
◦ Selectively moves substances from blood → filtrate in renal tubules & collecting ducts
A passive process in which hydrostatic pressure forces fluids & solutes through filtration membrane
Glomerular filtration
◦ Not energy requiring
◦ Non-selective (water & any solutes smaller than plasma proteins pass through)
The filtering out of blood cells is prevented by
The fenestrated endothelium of glomerular capillaries
The filtering out of negatively charged molecules (e.g., plasma proteins) is inhibited using
Negatively charged glycoproteins in the basement membrane (forces like opposing magnets)
Filtration of medium sized proteins is inhibited by
Filtration slits formed by the podocytes in the visceral layer of glomerular capsule
Pressure that is
◦Essentially glomerular blood pressure
◦The primary force pushing water & solutes out of
blood and across the filtration membrane
Glomerular hydrostatic pressure
Pressures that oppose GHP; try to move fluid back
into the glomerulus
Capsular hydrostatic pressure and blood colloid osmotic pressure
Positive net filtration pressure (NFP) is formed from
Fluid (blood plasma) moving from glomerulus → glomerular capsule
Glomerular filtration rate (GFR) is
The volume of filtrate formed each minute by all the glomeruli of the kidneys combined
Glomerular filtration rate (GFR) is directly proportional to
- Net filtration pressure – sensitive to hydrostatic pressure in glomerulus (controlled by the diameter of the afferent arteriole)
- Total surface area available for filtration
- Permeability of the filtration membrane
GFR is tightly regulated for two crucial, sometimes opposing needs:
◦ the need for a constant GFR to maintain extracellular homeostasis
◦ the regulation of blood pressure (↑GFR = ↓BP; ↓GFR =↑BP)
GFR is controlled by
changing glomerular hydrostatic pressure (GHP is controlled by the diameter of the afferent arteriole & renal blood flow)
◦↑afferent arteriole diameter =↑blood flow =↑GFR
◦↓afferent arteriole diameter =↓blood flow =↓GFR
Intrinsic controls regulating GFR
◦are renal autoregulation
◦act locally within kidney to maintain GFR
Extrinsic controls regulating GFR
◦are neural & hormonal regulation
◦nervous and endocrine systems maintaining BP
◦Take precedence over intrinsic controls when extreme changes in BP occur
The afferent arteriole smooth muscle is
stretch-sensitive & responds to changes in systemic blood pressure
During increased systemic blood pressure the intrinsic myogenic response is
◦Vascular smooth muscle stretches → contraction → afferent arteriole constriction →↓blood flow into nephron
◦ Reduced blood flow into nephron protects the glomeruli from damaging high blood pressure
During decreased systemic blood pressure the intrinsic myogenic response is
Vascular smooth muscle less stretched → relaxation → afferent arteriole dilation →↑blood flow into nephron
◦Increased blood flow into nephron helps maintain GFR
In tubuloglomerular feedback (intrinsic) the response of the macula densa cells of juxtaglomerular complex to filtrate NaCl concentration reflects
GFR
◦ As GFR↑, NaCl in filtrate↑
In tubuloglomerular feedback (intrinsic) high filtrate NaCl:
Macula densa cells release vasoconstrictor chemicals → afferent arteriole constrict →↓blood flow to nephron & GFR
In tubuloglomerular feedback (intrinsic) low filtrate NaCl:
Inhibited release of vasoconstrictor chemicals from macula densa cells → afferent arteriole dilate →↑blood flow to nephron & GFR
The baroreflex acting to increase BP when blood pressure is low is
extrinsic regulation via sympathetic nervous system controls
In SNS extrinsic regulation increased sympathetic activity via increased blood pressure leads to
increased norepinephrine & epinephrine causing vasoconstriction of systemic arterioles
◦ ↑ total peripheral resistance to ↑ systemic blood pressure
In the kidneys increased sympathetic activity (extrinsic) leads to
◦ vasoconstriction of afferent arterioles →
↓ blood flow to nephron → ↓ GFR ◦ ↓ urine output to ↑ blood volume → ↑ systemic BP ◦ blood flow redirected to other vital organs
Extrinsic regulation
Renin-angiotensin- aldosterone mechanism is:
hormones released in response to decreased systemic BP/blood volume in effort to increase
in regulation via the (extrinsic)
Renin-angiotensin- aldosterone mechanism aldosterone increases BP/volume via
increasing Na + & H2O reabsorption
in regulation via the (extrinsic)
Renin-angiotensin- aldosterone mechanism angiotensin II increases BP/volume via
peripheral vasoconstriction
The three pathways that activate the renin that is released from
juxtaglomerular cells in reponse to low systemic BP
◦ Sympathetic nervous system – as part of baroreflex
◦ Activated macula densa cells (low Na+ in filtrate
representing low GFR)
◦ Reduced stretch (due to low BP)
Different regions of the nephron tubules & collecting ducts secrete or absorb different molecules because there are
different specific transport proteins & channels in
the epithelial cell membranes at different parts of the nephron
Excretion is
the solutes & fluid that drain into the
minor & major calyces & renal pelvis and are excreted
as urine
Tubular reabsorption begins when
The filtrate enters the proximal convoluted tubule
In which tubule is nearly all organic nutrients (e.g., glucose & amino acids) reabsorbed
proximal convoluted tubule
processes included in tubular reabsorption in the PCT are:
◦ Transcellular (across/through) or paracellular (through spaces between cells) routes
◦ Active & passive tubular reabsorption
healthy urine does not contain
amino acids, proteins, glucose
◦ Transport across apical membrane → diffusion
through cytosol → transport across basolateral membrane
Transcellular (through cells) route in tubular reabsorption
◦ Solute moves between tubule cells through
leaky tight junctions
◦ Particularly in PCT some charged molecules that can’t get across charged cell membrane (H2O, Ca2+, Mg2+, K+, & some Na+)
Paracellular (in between cells) route in tubular reabsorption
Passive tubular reabsorption is
◦ Not energy requiring
◦ Molecules move down their electrochemical gradients via diffusion, facilitated diffusion, or osmosis
Active tubular reabsorption is
◦ primary active transport: Direct use of ATP
◦ secondary active transport: Indirect use of ATP
the most abundant cation in filtrate; most energy is used to actively reabsorb it via transcellular mechanism
Na+
Na+ is transported via
◦ primary active transport across basolateral membrane with Na+-K+-ATPase
◦ Bulk flow of H2O sweeps it into the adjacent peritubular capillary
Active pumping of Na+ out of cell creates a gradient for
Na+ to diffuse into cell across apical membrane
◦ secondary active transport for cotransport of glucose, amino acids, some ions, & vitamins
H2O is reabsorbed via
◦ osmosis (gradients created by reabsorption of Na+ & other solutes)
◦ passive tubular reabsorption
Aquaporin water channels for H2O reabsorption are abundant in the
PCT
◦ Obligatory water reabsorption
The collecting duct contains inactive aquaporins stored in cytoplasm that are activated by
Antidiuretic hormone (ADH)
◦ Facultative water reabsorption
Passive tubular reabsorption of solute occurs due to
higher solute content as H2O is reabsorbed
◦ Creates concentration gradients for solutes to
move into tubule cell & peritubular capillaries (fat-solubles, ions, & urea)
◦ allows fat-soluble drugs & environmental pollutants
to be reabsorbed as well
Reabsorption via transport proteins is limited by
Transport maximums: when concentration of solute in urine exceeds the saturation point of the transporters, excess is lost in the urine
◦ E.g., water soluble vitamins – taking more doesn’t mean you “get” more
Tubule most active in tubular reabsorption:
PCT
◦ Nearly all glucose, amino acids, & vitamins; 65% of
H2O & Na+
In the nephron loop/loop of Henle absorption of H2O takes place in the
Descending limb
◦ permeable to H2O but not ions
In the nephron loop/loop of Henle absorption of ions takes place in the
Ascending limb
◦ impermeable to H2O but passive &
active transport of ions
In the DCT & collecting duct – Na+ & H2O permeability is regulated by
Hormones
◦ aldosterone: monitors Na+ content
◦ ADH: adds/removes aquaporins
Tubular secretion (reabsorption in reverse) is most active in the
PCT (also occurs in other areas of nephron)
Purpose of tubular secretion is to dispose of
Unwanted solutes and reabsorbed solutes
◦ Protein-bound substances (such as drugs &
metabolites)
◦ Urea & uric acid
◦ Excess K +
◦ Excess H + or HCO3- to control the blood pH
