Functions of the kidney
- Removal of metabolic waste products
- Water/ electrolyte balance
- Acid-base balance
- Regulation of arterial BP
- Secretion of hormones (erythropoietin, renin)
- Gluconeogenesis
Where are the kidneys in the body?
Lie on the posterior wall of the abdomen
how much does each kidney weigh?
150 grams (size of a clenched fist)
2 major regions of the kidney
• 2 major regions (KNOW THESE)
○ Renal cortex= outer
○ Renal medullary segment= inside
renal pyramids
medulla further divided into cone-shaped masses of tissue where nephrons are packed in
the renal artery branches into ___
interlobar arteries (Gives initial supply of oxygen to renal arteries)
hilium
indented region on the kidney
where the renal artery and renal vein enter in
renal blood supply
renal artery→ afferent arterioles→ glomerular capillaries→ distal end of glomerular capillaries then branches into efferent arterioles→ peritubular capillaries
2 capillary beds in the kidney
glomerular and peritubular capillaries
afferent arteriole
supplies glomerulus with blood
efferent arteriole
where it terminates from the first capillary bed of the kidney (glomerular) (High pressure capillary bed (60 mm Hg))
peritubular capillary bed
○ Next to the tubules
○ Low pressure compared to glomeruli (13 mm Hg)
nephron
functional unit of the kidney (each kidney is made up of 1 million nephrons)
renal corpuscle
consists of glomerular capillaries and Bowman’s capsule
Loop of Henle
composed of the straight part of the proximal tubule, the descending thin limb (which ends in a hairpin turn), the ascending thin limb, and the ascending thick limb
capillary endothelium
contains a number of large holes called fenestrae leading to the endothelium’s weak barrier characteristics
macula densa
goes right b/w afferent and efferent arteriole
basement membrane
contains a meshwork of collagen and proteoglycans which filter a large amt of water and small solutes
carries a negative charge
what is the primary restriction point for plasma proteins?
basement membrane
epithelial cells
not continuous and contain slit pores whereby the glomerular filtrate moves
what is the filterability of solutes determined by?
size and electrical charge
tubuloglomerular feedback
decreased sodium or vol will cause the macula densa cells to signal the juxtaglomerular cells to secrete renin
where is the juxtaglomerular complex found?
consists of the macula densa cells in the initial portion of the distal tubule and juxtaglomerular cells in the walls of the afferent and efferent arterioles (jux cells secrete renin)
renin
catalyzes the formation of Ang II
constricts the efferent arteriole → increases glomerular hydrostatic pressure and returning GFR to normal
what 3 processes does excretion involve?
- filtration
- reabsorption
- secretion
average GFR
125 mL/min
180 L/day
excretion equation
filtration rate - reabsorption rate + secretion
reabsorption
solute moving from tubular fluid back into the blood
Goes into peritubular capillary bed (low pressure)
secretion
in the direction of the blood back into the tubular fluid
Positive effect overall on excretion
what 2 barriers must a substance cross through to be reabsorbed?
- tubular epithelial memb (into renal interstitial fluid)
2. peritubular capillary memb (back into blood)
passive reabsorption
usually controlled by bulk
Has to go through apical membrane and basal membrane
paracellular diffusion
passive
diffusion b/w cells
transcellular diffusion
passive
diffusion across cell memb
osmosis
passive
from region of low solute conc to region of high solute conc
active transport
moving a solute against an electochemical gradient and requiring energy
primary active transport
Na+,K+-ATPase that functions w/in most parts of the renal tubule
(exists on the basolateral side of the tubular epithelial cell)
hydrolyzes ATP and uses the energy to transport Na out of the cell into the intersitium at the same time K+ is transported from the interstitium to the inside of the epithelial cell
secondary active transport
2 or more substances interact w/ a specific memb protein and are transported across the memb
co-transport
2’ active transport
Ex. movement of Na from the tubular lumen interior of the epithelial cell energizes the movement of glucose and AA (achieved by multiple types of Na+ cotransporters)
counter transport
2’ active transport
Ex. inward movement of Na+ providing energy for the outward movement of H+
proximal tubular reabsorption
65% of filtered load of Na+ and water
special cellular characeristics of proximal tubule
high metabolic capacity
extensive luminal brush border
large amt of co-transport proteins and counter transport protein
Loop of Henle 3 segments
- descending thin limb
- ascending thin and thick segments
- ascending thick limb
descending thin limb
highly permeable to water
ascending thin and thick limb
virtually impermeable to water (important for urine concentration!)
ascending thick limb
25% of filtered load of Na+, K+, Cl- (along with some Ca, bicarb, and Mg)
**Tubular fluid in ascending thick limb becomes very dilute
early distal tubule
first part of distal tubule forms the juxtaglomerular complex which porvides feedback control for GFR and blood flow
next part functions much like the ascending thick limb (reabsorbs most ions and impermeable to water)
late distal tubule and cortical collecting ducts cell types
- principle cells
2. intercalated cells
principle cells
reabsorb Na+ and water from the lumen
secrete K+ into the lumen
intercalated cells
reabsorb+ from the lumen and avidly secrete H+ into the lumen
H+ATPase= acid-base regulation
aldosterone
controls the reabsorption of Na+ and secretion of K+ by LDT and CCD
ADH
controls water permeability in LDT and CCD
(Normally segments are impermeable to water except when ADH is released)
helps water get reabsorbed
medullary collecting duct
- reabsorbs <10% of filtered water and sodium (fine tunes the filtrate)
- controlled by ADH
- permeable to urea
- can secrete H+ (acid-base balance)
excess water in body makes urine ___
dilute
osmolarity can go as low as 50 mOsmo/L
deficit of water in body makes urine ___
concentrated
osmolarity can go as high as 1200-1400 mOsm/L
ADH AKA ___
arginine-8 vasopressin (AVP)
secreted by the posterior pituitary
high levels of ADH
signal the kidneys to excrete concentrated urine
low levels of ADH
signal the kidneys to excrete dilute urine
aquaporins
exist as homotetramer with each subunit bearing an individual water channel
permeable to water but not to other solutes
ADH effect on aquaporins
increased ADH causes a luminal translocation of water channels in the medullary collecting ducts
MOA of ADH
increase in intracellular cAMP in principle cells of medullary collecting duct → Activation of Gs/adenylyl cyclase system and PKA → phos AQP-2 → fuze to the cell memb and function to move water down the osmotic gradient
what are the basic requirements for forming a concentrated urine?
- high level of ADH
2. high renal medullary interstitial fluid osmolarity
changes seen in dilute urine
descending Loop of Henle has water reabsorbed by osmosis, thus tubular fluid is conc → ascending loop of Henle Na+, K+, and Cl- are avidly transported so tubular fluid is dilute → distal tubule/ cortico and medullary collecting duct= with no ADH means water is impermeable which makes tubular fluid even more dilute