Chapter 25 - The Urinary System/Renal Physiology

prefix and suffix for urinary system / renal physiology

nephro- or -uria

how many liters of fluid is filtered from blood by the kidneys every day?

~200 liters

main function of the kidneys

maintaining the composition of the body’s extracellular fluids by filtering the blood

5 things involved in the functioning of the kidneys

1.) Regulate total body water volume + concentration of solutes in water

2.) Regulate concentration of ions (Na and K) in ECF

3.) Acid-base balance (bicarbonate)

4.) Remove toxins, metabolic wastes, + other foreign substances (nitrogenous base waste)

5.) Hormone production of EPO (regulates hematocrit) + renin (regulates/increases BP)

filtration

the mass movement of solutes and water from the plasma into the renal corpuscle and renal tubules (the kidneys filter blood)

reabsorption

the process by which nephrons remove water and solutes from the filtrate formed from filtration and return it to the blood

secretion

the process by which excess ions (K⁺, H⁺, etc) and waste products are pumped back into the filtrate after it has been reabsorbed

each kidney lies between the

parietal peritoneum and dorsal body wall

kidneys are what kind of organ?

retroperitoneal organs

retroperitoneal organs

don’t have a visceral peritoneum because they are found between parietal peritoneum and the body wall itself

the medial portion of the kidney is

concave

the medial portion of the kidney is concave → it contains a

renal hilum

renal hilum

ureters, renal blood vessels, lymphatics, and renal nerve supply enter here

what is a hilum in general?

any area of an organ that is indented because of blood supply, nervous system supply, etc.

ureter

tube that extends from kidney to urinary bladder that allows urine to drain out of kidneys as it is produced

adrenal gland sits

immediately superior to each kidney (don’t directly contribute to renal function)

3 supporting external structures of the kidneys

1.) Renal fascia

2.) Perirenal fat capsule

3.) Fibrous capsule

renal fascia and function

dense connective tissue

• function = anchors kidneys to surrounding structures

perirenal fat capsule and function

fat mass surrounding kidneys

• function = cushions kidneys from physical trauma (adipose tissue is a shock absorber)

fibrous capsule and function

thin, transparent capsule

• function = prevents disease from spreading to kidneys from other parts of body (fibrous means it’s tough)

3 major internal regions of kidneys

1.) Renal cortex

2.) Renal medulla

3.) Renal pelvis

renal cortex function

provides area for glomerular capillaries and blood vessel passage; EPO and renin are produced here

**where blood filtration actually occurs**

what is a cortex in general?

the outermost region of an organ

renal medulla contains

several renal pyramids

renal pyramids

packed with capillaries and urine-collecting tubules

smallest number and largest number of pyramids found in 1 kidney

• smallest = 7

• largest = 18

function of renal medulla

allows for some water reabsorption, electrolyte balance, disposal of waste and H⁺ ions

**where reabsorption and secretion occur**

what is a medulla in general?

inner region of an organ

renal pelvis

open space in center of each kidney

what is a pelvis in general?

an open space in the inside of an organ

renal pelvis branches to form

major calyces (calyx)

major calyces lead into

minor calyces at the tip of each renal pyramid

function of calyces and renal pelvis

urine collection from renal medulla

5 arteries of the kidneys

1.) Renal arteries

2.) Segmental arteries

3.) Interlobar arteries

4.) Arcuate arteries

5.) Cortical radiate arteries

renal arteries

deliver blood to kidneys → divide into smaller blood vessels to serve major regions of the kidney

segmental arteries

5 branches, ensure blood reaches all regions of the kidney

• come from renal arteries, branch into interlobar arteries

interlobar arteries

travel between renal pyramids

• come from segmental arteries, branch into arcuate arteries

arcuate arteries

arc over bases of pyramids

• come from interlobar arteries, branch into cortical radiate arteries

cortical radiate arteries

supply renal cortex with blood

4 veins of the kidneys

1.) Cortical radiate veins

2.) Arcuate veins

3.) Interlobar veins

4.) Renal veins

cortical radiate veins

allow blood to drain away from the cortex

arcuate veins

arc over base of each renal pyramid, sit next to arcuate arteries

• come from cortical radiate veins, form interlobar veins

interlobar veins

in between each renal pyramid

• come from arcuate veins, form renal veins

which blood vessel is an artery and NOT a vein?

segmental artery → there is NO segmental vein

nerve supply to the kidneys via the

renal plexus

renal plexus

autonomic nerve fibers and ganglia, enters each kidney at renal hilum

renal plexus - sympathetic vasomotor fibers regulate

blood supply to each kidney

why do sympathetic vasomotor fibers regulate renal blood supply and not parasympathetic?

because the sympathetic division controls the diameter of blood vessels more than the parasympathetic division does

function of sympathetic vasomotor fibers of renal plexus

adjusts diameter of renal arterioles to adjust blood flow to glomeruli

what is the importance of changing blood flow to the kidneys?

it changes total volume of urine produced, which changes overall blood volume and BP for the entire body

nephron

the functional unit of the kidney (smallest structure that can carry out all 3 functions: filtration, reabsorption, and secretion)

function of the nephron

responsible for forming filtrate and eventually urine in the kidneys

each nephron contains 2 structures

1.) Renal corpuscle

2.) Renal tubule

renal corpuscle (general function)

filters blood to form filtrate (filtration)

renal tubule (general function)

reabsorbs some substances from the filtrate and secretes other substances into the filtrate (reabsorption and secretion)

what happens to anything that is secreted into filtrate OR not reabsorbed from filtrate?

it forms urine and leaves the body

where is renal corpuscle located?

entirely within the renal cortex

2 subdivisions of the renal corpuscle

1.) Glomerulus

2.) Glomerular capsule

glomerulus

cluster of fenestrated capillaries

blood enters glomerulus via ___ and exits via ___

enters via afferent arteriole, exits via efferent arteriole

what is the significance of afferent and efferent arterioles?

by having the glomerulus fed and drained by arterioles, it keeps pressure in glomerulus higher, and this pressure is what drives fluid out of the capillary bed to form filtrate

the capillaries of the glomerulus are very porous, meaning

some fluid and substance in the blood are easily filtered out of the capillary - this maximizes the amount of fluid the kidneys can filter

filtrate

raw material used to produce urine (**FILTRATE IS NOT URINE**)

glomerular capsule

double-layered structure that completely surrounds glomerular capillaries

the inner layer of the glomerular capsule has

podocytes with foot processes

what is the function and importance of podocytes and its foot processes?

the foot processes form a filter

renal tubules and collecting duct - pathway through the kidney

begins in renal cortex, extends into renal medulla, then returns to renal cortex

what is the benefit of the hairpin-like structure of the renal tubules and collecting duct?

it provides a large amount of surface area → renal tubule is where you get most reabsorption and most secretion

4 subdivisions of renal tubules

1.) Proximal convoluted tubule (PCT)

2.) Nephron loop (Loop of Henle)

3.) Distal convoluted tubule (DCT)

4.) Collecting ducts

proximal convoluted tubule (PCT)

leads immediately off from glomerulus, located in renal cortex, large cuboidal epithelial cells with dense microvilli

nephron loop (Loop of Henle)

travels between renal cortex and renal medulla

2 components of nephron loop

1.) Descending limb

2.) Ascending limb

descending limb of nephron loop

leads off from PCT, high permeability to H₂O, impermeable to solutes

ascending limb of nephron loop

continuous with DCT, high permeability to solutes, impermeable to H₂O

function of the nephron loop

allows the kidneys to vary the concentration of urine according to how much water is reabsorbed at nephron loop

distal convoluted tubule (DCT)

located in cortex, composed of small cuboidal epithelia, smaller diameter than PCT, contain NO microvilli

what does the microanatomy of the DCT indicate?

DCT has less surface area overall than PCT, meaning by the time the filtrate reaches the DCT, reabsorption and secretion are almost complete

reabsorption and secretion in DCT is mostly controlled by

hormones

2 important cell types in collecting ducts

1.) Principal cells

2.) Intercalated cells

principal cells

maintain Na⁺ balance in the body

would the maintenance of Na⁺ balance in the body by principal cells influence absorption of other substances?

yes → maintaining Na⁺ helps to ensure correct water balance of the body because water follows solutes

intercalated cells

help maintain acid-base balance (responsible for secretion/reabsorption of hydrogen or bicarbonate ions)

each collecting duct receives

filtrate from tubules of multiple nephrons → collecting ducts fuse together and dump urine into minor calyces

2 types of nephrons

1.) Cortical nephrons

2.) Juxtamedullary nephrons

cortical nephrons

located almost entirely in the cortex, small portion of nephron loop found in renal medulla

juxtamedullary nephrons

nephron loops deeply invade renal medulla, important for forming highly concentrated urine

how does a change in nephron structure affect urine formation?

changing length of nephron loop = changing how you can concentrate urine

3 capillary beds of nephrons

1.) Glomerulus

2.) Peritubular capillaries

3.) Vasa recta

glomerulus

maintains high pressure to increase filtrate production

peritubular capillaries

low pressure capillaries arising from efferent arteriole that cling to proximal and distal tubules of cortical nephrons

function of peritubular capillaries

reabsorb water and solutes from tubule cells

peritubular capillaries empty into

radiate veins → filtered blood returns to circulation

vasa recta

found only on juxtamedullary nephrons, run parallel to long nephron loop, help form concentrated urine

juxtaglomerular complex (JGC)

portion of nephron where portion of ascending limb lies against afferent and efferent arterioles

overall function of JGC

regulate BP and filtration rate of the glomerulus

3 cellular modifications at JGC

1.) Macula densa

2.) Granular cells (juxtaglomerular cells)

3.) Extraglomerular mesangial cells

macula densa cells and function

chemoreceptor cells

• function = monitor NaCl content of filtrate entering distal convoluted tubule

how does the rate of filtrate formation affect NaCl concentration in the DCT? what happens to the afferent arteriole to “fix” this problem?

example:

• NaCl concentration in the ascending limb is high = it is NOT being reabsorbed from filtrate = filtrate is moving too quickly through the nephron

• too quickly = glomerular capillaries are forming too much filtrate = macula densa cells want to reverse this and decrease filtrate formation

• to adjust filtrate formation, you have to adjust how much blood is supplied to the glomerular capillaries

  • less blood = less filtrate formed, more blood = more filtrate formed

• less blood = afferent arteriole vasoconstricts

• more blood = afferent arteriole vasodilates

granular cells (juxtaglomerular cells)

specialized smooth muscle cells found in arteriolar walls of afferent arteriole - can sense BP in afferent arteriole (sensitive to stretch) and are stimulated by macula densa cells

what do the granular cells do if BP is high in afferent arteriole?

they respond by contracting, causing the afferent arteriole to vasoconstrict, which decreases pressure in glomerular capillaries