The Urinary System: Renal Physiology

Question 1

During the formation of urine

Answer 1

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)

Question 2

Filtrate contains

Answer 2

Almost everything found in plasma (excluding proteins)

Question 3

Of the 180 L of filtrate that is formed each day:

Answer 3

◦< 1% (1.5 L) leaves the body as urine
◦>99% is reabsorbed

Question 4

The first process in urine formation & adjustment of blood composition is

Answer 4

1. Glomerular filtration
   ◦ Produces cell- & protein-free filtrate

Question 5

The 2nd process in urine formation & adjustment of blood composition is

Answer 5

2. Tubular Reabsorption
   ◦ Selectively returns 99% of substances from filtrate → blood in renal tubules & collecting ducts

Question 6

The 3rd process in urine formation & adjustment of blood composition is

Answer 6

3. Tubular secretion
   ◦ Selectively moves substances from blood → filtrate in renal tubules & collecting ducts

Question 7

A passive process in which hydrostatic pressure forces fluids & solutes through filtration membrane

Answer 7

Glomerular filtration
◦ Not energy requiring
◦ Non-selective (water & any solutes smaller than plasma proteins pass through)

Question 8

The filtering out of blood cells is prevented by

Answer 8

The fenestrated endothelium of glomerular capillaries

Question 9

The filtering out of negatively charged molecules (e.g., plasma proteins) is inhibited using

Answer 9

Negatively charged glycoproteins in the basement membrane (forces like opposing magnets)

Question 10

Filtration of medium sized proteins is inhibited by

Answer 10

Filtration slits formed by the podocytes in the visceral layer of glomerular capsule

Question 11

Pressure that is
◦Essentially glomerular blood pressure
◦The primary force pushing water & solutes out of
blood and across the filtration membrane

Answer 11

Glomerular hydrostatic pressure

Question 12

Pressures that oppose GHP; try to move fluid back
into the glomerulus

Answer 12

Capsular hydrostatic pressure and blood colloid osmotic pressure

Question 13

Positive net filtration pressure (NFP) is formed from

Answer 13

Fluid (blood plasma) moving from glomerulus → glomerular capsule

Question 14

Glomerular filtration rate (GFR) is

Answer 14

The volume of filtrate formed each minute by all the glomeruli of the kidneys combined

Question 15

Glomerular filtration rate (GFR) is directly proportional to

Answer 15

1. Net filtration pressure – sensitive to hydrostatic pressure in glomerulus (controlled by the diameter of the afferent arteriole)
2. Total surface area available for filtration
3. Permeability of the filtration membrane

Question 16

GFR is tightly regulated for two crucial, sometimes opposing needs:

Answer 16

◦ the need for a constant GFR to maintain extracellular homeostasis
◦ the regulation of blood pressure (↑GFR = ↓BP; ↓GFR =↑BP)

Question 17

GFR is controlled by

Answer 17

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

Question 18

Intrinsic controls regulating GFR

Answer 18

◦are renal autoregulation
◦act locally within kidney to maintain GFR

Question 19

Extrinsic controls regulating GFR

Answer 19

◦are neural & hormonal regulation
◦nervous and endocrine systems maintaining BP
◦Take precedence over intrinsic controls when extreme changes in BP occur

Question 20

The afferent arteriole smooth muscle is

Answer 20

stretch-sensitive & responds to changes in systemic blood pressure

Question 21

During increased systemic blood pressure the intrinsic myogenic response is

Answer 21

◦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

Question 22

During decreased systemic blood pressure the intrinsic myogenic response is

Answer 22

Vascular smooth muscle less stretched → relaxation → afferent arteriole dilation →↑blood flow into nephron
◦Increased blood flow into nephron helps maintain GFR

Question 23

In tubuloglomerular feedback (intrinsic) the response of the macula densa cells of juxtaglomerular complex to filtrate NaCl concentration reflects

Answer 23

GFR
◦ As GFR↑, NaCl in filtrate↑

Question 24

In tubuloglomerular feedback (intrinsic) high filtrate NaCl:

Answer 24

Macula densa cells release vasoconstrictor chemicals → afferent arteriole constrict →↓blood flow to nephron & GFR

Question 25

In tubuloglomerular feedback (intrinsic) low filtrate NaCl:

Answer 25

Inhibited release of vasoconstrictor chemicals from macula densa cells → afferent arteriole dilate →↑blood flow to nephron & GFR

Question 26

The baroreflex acting to increase BP when blood pressure is low is

Answer 26

extrinsic regulation via sympathetic nervous system controls

Question 27

In SNS extrinsic regulation increased sympathetic activity via increased blood pressure leads to

Answer 27

increased norepinephrine & epinephrine causing vasoconstriction of systemic arterioles ◦ ↑ total peripheral resistance to ↑ systemic blood pressure

Question 28

In the kidneys increased sympathetic activity (extrinsic) leads to

Answer 28

◦ vasoconstriction of afferent arterioles → ↓ blood flow to nephron → ↓ GFR ◦ ↓ urine output to ↑ blood volume → ↑ systemic BP ◦ blood flow redirected to other vital organs

Question 29

Extrinsic regulation Renin-angiotensin- aldosterone mechanism is:

Answer 29

hormones released in response to decreased systemic BP/blood volume in effort to increase

Question 30

in regulation via the (extrinsic) Renin-angiotensin- aldosterone mechanism aldosterone increases BP/volume via

Answer 30

increasing Na + & H2O reabsorption

Question 31

in regulation via the (extrinsic) Renin-angiotensin- aldosterone mechanism angiotensin II increases BP/volume via

Answer 31

peripheral vasoconstriction

Question 32

The three pathways that activate the renin that is released from juxtaglomerular cells in reponse to low systemic BP

Answer 32

◦ Sympathetic nervous system – as part of baroreflex ◦ Activated macula densa cells (low Na+ in filtrate representing low GFR) ◦ Reduced stretch (due to low BP)

Question 33

Different regions of the nephron tubules & collecting ducts secrete or absorb different molecules because there are

Answer 33

different specific transport proteins & channels in the epithelial cell membranes at different parts of the nephron

Question 34

Excretion is

Answer 34

the solutes & fluid that drain into the minor & major calyces & renal pelvis and are excreted as urine

Question 35

Tubular reabsorption begins when

Answer 35

The filtrate enters the proximal convoluted tubule

Question 36

In which tubule is nearly all organic nutrients (e.g., glucose & amino acids) reabsorbed

Answer 36

proximal convoluted tubule

Question 37

processes included in tubular reabsorption in the PCT are:

Answer 37

◦ Transcellular (across/through) or paracellular (through spaces between cells) routes ◦ Active & passive tubular reabsorption

Question 38

healthy urine does not contain

Answer 38

amino acids, proteins, glucose

Question 39

◦ Transport across apical membrane → diffusion through cytosol → transport across basolateral membrane

Answer 39

Transcellular (through cells) route in tubular reabsorption

Question 40

◦ 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+)

Answer 40

Paracellular (in between cells) route in tubular reabsorption

Question 41

Passive tubular reabsorption is

Answer 41

◦ Not energy requiring ◦ Molecules move down their electrochemical gradients via diffusion, facilitated diffusion, or osmosis

Question 42

Active tubular reabsorption is

Answer 42

◦ primary active transport: Direct use of ATP ◦ secondary active transport: Indirect use of ATP

Question 43

the most abundant cation in filtrate; most energy is used to actively reabsorb it via transcellular mechanism

Answer 43

Na+

Question 44

Na+ is transported via

Answer 44

◦ primary active transport across basolateral membrane with Na+-K+-ATPase ◦ Bulk flow of H2O sweeps it into the adjacent peritubular capillary

Question 45

Active pumping of Na+ out of cell creates a gradient for

Answer 45

Na+ to diffuse into cell across apical membrane ◦ secondary active transport for cotransport of glucose, amino acids, some ions, & vitamins

Question 46

H2O is reabsorbed via

Answer 46

◦ osmosis (gradients created by reabsorption of Na+ & other solutes) ◦ passive tubular reabsorption

Question 47

Aquaporin water channels for H2O reabsorption are abundant in the

Answer 47

PCT ◦ Obligatory water reabsorption

Question 48

The collecting duct contains inactive aquaporins stored in cytoplasm that are activated by

Answer 48

Antidiuretic hormone (ADH) ◦ Facultative water reabsorption

Question 49

Passive tubular reabsorption of solute occurs due to

Answer 49

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

Question 50

Reabsorption via transport proteins is limited by

Answer 50

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

Question 51

Tubule most active in tubular reabsorption:

Answer 51

PCT ◦ Nearly all glucose, amino acids, & vitamins; 65% of H2O & Na+

Question 52

In the nephron loop/loop of Henle absorption of H2O takes place in the

Answer 52

Descending limb ◦ permeable to H2O but not ions

Question 53

In the nephron loop/loop of Henle absorption of ions takes place in the

Answer 53

Ascending limb ◦ impermeable to H2O but passive & active transport of ions

Question 54

In the DCT & collecting duct – Na+ & H2O permeability is regulated by

Answer 54

Hormones ◦ aldosterone: monitors Na+ content ◦ ADH: adds/removes aquaporins

Question 55

Tubular secretion (reabsorption in reverse) is most active in the

Answer 55

PCT (also occurs in other areas of nephron)

Question 56

Purpose of tubular secretion is to dispose of

Answer 56

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