Functional Anatomy and Renal Blood Flow Flashcards
Overview of Kidney Functions (6)
- regulation of extracellular fluid volume and blood pressure
- regulation of osmolality of fluid
- maintenance of ion balance of fluid (Na+, K+, Cl-, Ca2+, PO43- – primarily regulate loss of Na+ and K+)
- homeostatic regulation of pH (acid-base balance) – by variable excretion of H+ and HCO3-
- excretion of wastes (ie. creatinine, urea, hormone metabolites, urobilinogen)
- production of hormones (ie. erythropoietin: stimulates RBC synthesis; renin: important regulator of blood pressure)
What is a nephron?
one functional unit of the kidney – exchange of substances from tubule back into vascular system
How are functions of the kidney accomplished?
- three processes: filtration, reabsorption, secretion (specific movements from blood to tubular fluid, or from tubular fluid to blood – depends on where it is occurring along the nephron)
- axial specialization
- coordinate function
What is the axial specialization of kidneys?
each segment receives product from previous segment
- composition of fluid in tubule changes as we move through tubule
- fluid gets exchanged into the next segment, where composition changes again
- some parts don’t change, depending on conditions
- some parts are very highly regulatable, and can change in response to different conditions
What is the coordinate function of kidney? (2)
coordination between multiple segments of one nephron or adjacent nephrons
- kidney tubule folds back on itself, and has some sections where it can communicate with earlier segments
coordination between multiple organs
- ie. communication between kidney and lungs – helps acid-base balance
- ie. communication between kidney and liver – when you have higher acid load over sustained period of time
Gross Anatomy – What are the main components of the renal system?
- kidney (2)
- ureter (2)
- urinary bladder
- urethra
What are ureters?
tubes that connect kidney to urinary bladder
What does the urinary bladder do?
houses urine
What is the urethra?
where urine exits into environment
Where do changes in composition of fluid to-be-excreted occur?
all changes occur in outer portion of kidney
Kidney Anatomy – What is the renal capsule?
thin, tough fibrous capsule that covers surface of kidney
Kidney Anatomy – What is the renal cortex?
non-striated outer region underneath capsule
Kidney Anatomy – What is the medulla?
anything that sits under the line that divides the base of that section
- includes columns
- more inner portion, but still part of main section of kidney
Kidney Anatomy – What are renal pyramids? What do they contain?
striated pyramidal sections
have many nephrons that change composition of fluid
Kidney Anatomy – What is the renal pelvis?
at end of nephron, fluid gets drained into region that connects to renal pelvis, which is an open section inside kidney where fluid sits until it moves down through ureter
When can the composition of fluid to-be-excreted no longer be changed?
once fluid gets to renal pelvis – this fluid is housed in bladder and excreted through urethra
How many nephrons do you have?
2 million (1 million per kidney)
Where are nephrons located?
- some sit with their glomerulus close to border between cortex and medulla
- some sit with their glomerulus further away
What are the functional units of nephrons? (4)
- filtration
- reabsorption/secretion
- regulation
- excretion
Where does filtration occur?
renal corpuslce
Where does reabsorption/secretion occur?
renal tubule
What is reabsorption?
movement of substances from tubular fluid back into blood
What is secretion?
movement of substances from blood to be secreted into tubule
Where does regulation occur?
juxtaglomerular apparatus
What is the juxtaglomerular apparatus?
interface between renal tubule that is just approaching distal tubule, and glomerulus
Where does excretion occur?
collecting ducts
When does excretion occur?
when composition of fluid can no longer be changed
What are the 4 tubule segments of the nephron? What parts does each segment consist of?
- proximal tubule (PCT)
- loop of Henle (PST, tDLH, tALH, TAL)
- distal tubule (DCT, CNT)
- collecting duct (CCD, OMCD, IMCD)
What is the structure of the proximal tubule? (3)
- Bowman’s capsule: bulbous section containing capillaries
- glomerulus: capillary bed found within renal corpuscle
- lots of bending and folding
What is the structure of the loop of Henle? (3)
- forms hair-pinned turn as it enters more deeply into medulla of kidney
- tube is thinner in descending section
- tube is thicker after turn, when ascending and returning closer to starting point (glomerulus)
What is the structure of the distal tubule? (3)
- convoluted/folded
- tubule is any tubing past interface between nephron and glomerulus (at beginning of tubule)
- convergence of many different nephrons at end of distal tubule – drain into collecting duct
What does the collecting duct do? (3)
- important site in terms of regulation
- expels fluid into renal pelvis
- convergence of other nephrons
What is the renal corpuscle?
site of filtration, composed of Bowman’s capsule and glomerulus
What is the macula densa? Where is it?
cells on portion of tubule that interfaces with glomerular region (where efferent and afferent arteriole enter and exit)
where thick ascending limb (TAL) transitions into distal convoluted tubule
What is Bowman’s capsule?
space that fluid (or different substances coming from blood) will be filtered into
What are some structural characteristics of Bowman’s capsule? (2)
- basement membrane
- lined by parietal epithelium
What are some structural characteristics of glomerular capillary tufts?
- visceral epithelium (podocytes)
- glomerular basement membrane
- inside of capillary lined by fenestrated endothelium
What does visceral epithelium (podocytes) do?
- adds structural integrity – no interstitial tissue, therefore support is needed
- very little selectivity that plays role in filtration
- mesangial cells help change amount of space between podocytes, which impacts surface area available for filtration to occur
What happens when mesangial cells are contracted?
pull podocytes together → reduced surface area
What happens when mesangial cells are relaxed?
podocytes move apart → more surface area
What does the glomerular basement membrane do?
filters – more selective, primary decider about what can pass (ie. size, charge)
What is the fenestrated endothelium and what does it do?
epithelium glomerular capillary tufts that has large openings between cells
helps make large transport of different substances across membrane possible
Where does blood enter and get filtered?
enters through afferent arteriole, and filtered once it gets into capillary tuft
What happens to substances during filtration in capillary tuft?
- some substances and fluids will cross barrier and enter Bowman’s capsule
- some substances remain inside vessel, and leave via efferent arteriole and go to vasa recta or peritubular capillaries
In later regions of the nephron, why are there principal cells and intercalated cells?
allows more diverse and integrated response when fine tuning fluid composition, and regulating pH homeostasis
Where in the nephron are cells thinner? Why?
descending thin limb and ascending thin limb (as we’re moving into the loop of Henle)
have less mitochondria – not much active transport occurring, therefore cells don’t need them
What reabsorption occurs in the proximal straight tubule?
67% of Na+ and H2O we reabsorb – bulk movement
Do epithelial cells remain the same as we move through the tubule?
no – composition of epithelial cells change as we move through tubule
What is an important feature of the proximal straight tubule? Why?
brush border (microvilli-covered surface)
allows for huge expansion of surface area, which is critical for the large amount of transport occurring in this region
What are cells in tubules connected by?
tight junctions
What do tight junctions help do?
determine permeability between cells, across barrier
What is the apical membrane?
side of cell with tight junctions, and in contact with lumen (continuous with external environment)
What is the other side (opposite to apical membrane)?
basolateral membrane
How do tight junctions change throughout the tubule?
- starts with relatively loose tight junctions, which helps with huge amount of transport occurring
- very tight junctions in other parts of tubule allow limited (or almost none) movement of substances between cells
When do we start seeing more than one cell type in the nephron?
starting at the connecting tubule
What is filtration?
Where does it occur?
What does it depend on?
movement of substances (fluid, glucose, ions, etc.) from blood into tubule
occurs in glomerular capillaries → Bowman’s capsule
highly depends on glomerular basement membrane selectivity
What is secretion?
What are other mechanisms along the tubule through which substances can get secreted?
movement of substances from blood into tubule after beginning of proximal tubule
substances can still get secreted by active transport, and different types of channels – some places are highly regulated, some places have bulk movement
What is reabsorption?
movement of substances from tubule back into blood
How can fluid or substances be modified? (3)
- by being taken back into blood by reabsorption
- by adding things to fluid by secretion
- fluid loss through evaporation, sweating, and other sources
Which artery supplies the kidney?
renal artery
What does all the blood delivered to the kidney pass through?
glomerular capillaries
What do efferent arterioles supply?
depends on where you are in the kidney
- peritubular capillaries – found superficially in cortical region
- vasa recta – deeper penetrating vasculature
Where does blood exit the kidney?
via renal vein
What is the filtration fraction?
percent of total plasma volume that filters into tubule
Describe the general pathway of blood through the kidney.
enters kidney → glomerulus → continues on path through kidney → exits kidney
Describe how the plasma volume changes along the nephron.
- 100% when entering afferent arteriole
- 20% filters into nephron (from glomerulus into Bowman’s capsule), and 80% continues through glomerulus
- > 19% of fluid in nephron is reabsorbed into blood
- > 99% of plasma entering kidney returns to systemic circulation via efferent arteriole
- < 1% of volume is excreted to external environment
What determines the amount of solute excreted?
amount filtered - amount reabsorbed + amount secreted
What are 3 properties that will allow for assessment of renal blood flow (RBF)?
Is it easy or hard to collect these data?
- urine output – easy
- blood that gets to kidney – easy, blood sample will closely match composition of blood being delivered to kidney
- renal vein output – hard, sample needs to be collected before it get mixed back in with the rest of our blood
Describe the input = output equation of renal blood flow.
input equation: one way in
- renal artery
output equation: two ways out
- taken back into blood, and leave through renal vein
- stay in nephron and leave through ureter to be excreted in urine
What are clearance values?
how well or how much kidney is able to clear substance from blood and excrete it into external environment
What are clearance values used for?
to estimate RPF – focusing on plasma because it is the fluid part of blood
What are the 3 criteria for a substance to be able to be used to estimate RPF?
Why?
largely excreted by nephron
- filtered and secreted, but NOT reabsorbed
- if it’s going to be excreted, we want it to be completely excreted so that we can know the values between our blood that is being delivered and the urine being excreted
not metabolized/produced by kidney
- can change amounts – by increases in production or break down
not toxic to body
Describe para-aminohippuric acid.
- 20% filtered
- extensively secreted – some PAH goes from blood into Bowman’s capsule
- < 10% remaining in renal venous blood
What is excretion largely dependent on?
movement from blood into tubule through secretion mechanism
What is RPF used for?
to estimate what RBF is
How can we assess RPF?
by using molecule that is going to be fully excreted – important that it is relying on secretion
substance enters through artery, and is largely going to be cleared by kidneys so that it is essentially 100% excreted
What does RPF tell us?
how much plasma is delivered to kidney
Can the substance used to assess RPF be filtered?
can be partially filtered, but it is critical that any non-filtered substance is COMPLETELY SECRETED
Why is para-aminohippuric acid (PAH) used to assess RPF?
it is partially filtered, but completely secreted (~100% excreted) – relies on secretion for its excretion
ie. if there’s damage to glomerulus and filtration is impacted (ie. 10% being filtered – when normally 20% being filtered, 80% being secreted), because we rely on secretion, we will just secrete more of it
- 90% leftover will still be secreted, and ultimately we will excrete everything
- gives good indication of overall picture of how much plasma is actually being delivered to kidney
Does RPF change if filtration is impaired?
NO – secretion will still allow for complete excretion, even if filtration is reduced
What does RPF and GFR together tell us?
allows for assessment of renal function (filtration fraction = GFR/RPF)
What is glomerular filtration rate (GFR)?
how much substance is crossing filtration barrier in glomerulus – gives overall idea of kidney function (filtration fraction)
Does filtration matter for RPF?
Does filtration matter for GFR?
amount of filtration doesn’t matter for RPF, but really important for GFR
