Nephron Structure and Function, Juxtaglomerular Complex Flashcards
What is the renal corpuscle?
The renal corpuscle is the filtration unit of the nephron in the kidney.
What is the renal corpuscle composed of?
- glomerulus
- Bowman’s Capsule
What happens in the renal corpuscle of the nephron?
Filtration
What is the function of the proximal convoluted tubule in waste excretion?
- reabsorption of water, ions, and nutrients
- secretion of drugs and toxins, as well as H+
What happens in the nephron loop during waste excretion?
- descending limb of the nephron loop reabsorbs water
- ascending limb reabsorbs ions e.g. Na+ & Cl-
What is the function of the distal convoluted tubule in waste excretion?
Regulated reabsorption and secretion
What is the function of the collecting duct in waste excretion?
Regulates the reabsorption of water and ions, such as Na+, Cl-, K+, H+, and HCO3-
What does the papillary duct deliver?
The papillary duct delivers urine to the minor calyx
Summarise the function of the tubules of the tubular portion in the Nephron
Renal corpuscle = filtration
Proximal Convoluted Tubule (PCT) = Reabsorption and Secretion
Loop of Henle = Reabsorption
Distal Convoluted Tubule (DCT) = Reabsorption and Secretion
Collecting duct = Reabsorption
(slide 12)
What is the glomerulus?
A ball of narrow capillaries that is the filtering part of the renal corpuscle in the kidney.
How many loops does the glomerulus have, and in how many lobes is it found?
The glomerulus has 20-30 loops and is found in approximately 5 lobes
How is the glomerulus fed?
The glomerulus is fed by the afferent arteriole
What is the pressure like in the glomerulus?
The pressure in the glomerulus is high
What is Bowman’s capsule?
Bowman’s capsule is a cup-shaped structure that surrounds the glomerulus in the renal corpuscle
Where does the glomerulus feed into?
The glomerulus feeds into the efferent arteriole
Where is the renal corpuscle located?
The kidney
What is the cellular structure of the glomerulus?
The glomerulus has:
- a fenestrated endothelium
- surrounded by a basement membrane
- podocytes
(slide 23 & 25)
What is fenestrated endothelium?
A type of endothelium that has fenestrations (pores or openings), which filters molecules based on their size
What are the characteristics of the fenestrated endothelium?
- squamous endothelial cells
- modified capillary endothelium
- fenestrations
What is squamous?
Squamous refers to a flat and scale-like shape of cells or tissues
What is modified capillary endothelium?
A type of endothelium found in the glomerulus
- has fenestrations that allow for the filtration of small molecules & fluids
What is the basement membrane?
The basement membrane is a thin layer of extracellular matrix
What does the basement membrane contain?
Contains heparin sulphate, collagen, laminin, and other proteins (negatively charged)
What are podocytes?
Podocytes are modified epithelial cells that form the cellular structure of the glomerulus in the kidney.
(slide 24)
What is the function of podocytes in the glomerulus?
Podocytes form foot processes that
- create filtration slits; allows for filtration of blood in kidney
- provide strength to cellular structure of glomerulus
(slide 24)
What are mesangial cells?
Cells located in the glomerulus of the kidney that have three main functions: they are structural, contractile, and phagocytic.
What is the Glomerular Filtration Membrane?
A specialized filter in the kidney that is responsible for filtering blood to form urine
What does the Glomerular Filtration Membrane allow to pass from the blood into the kidneys?
- water
- electrolytes
- glucose
- amino acids
- fatty acids
- vitamins
- urea
- uric acid
- creatinine
What doesn’t the Glomerular Filtration Membrane allow to pass from the blood into the kidneys?
- blood cells
- plasma proteins
- large anions
- protein-bound minerals & hormones
- most molecules > 8 nm in diamete
What is the function of the Basement Membrane in the Glomerular Filtration Membrane?
Filters molecules based on their size and charge
- negatively charged, so allows positively charged molecules to pass through more easily
What is the effective pore size of the Glomerular Filtration Membrane?
The effective pore size is approximately 8 nanometers
What is Glomerular Filtration Rate (GFR)?
GFR is a measure of the rate at which blood is filtered through the glomerular capillaries in the kidney.
What is Capillary Hydrostatic Pressure (Pc)?
Pc is the pressure exerted by the blood within the glomerular capillaries
What is Capillary Osmotic Pressure (πc)?
πc is the pressure exerted by the proteins in the blood within the glomerular capillaries
What is Tubular Hydrostatic Pressure (Pt)?
Pt is the pressure within the tubules of the kidney
What is Tubular Osmotic Pressure (πt)?
πt is the pressure exerted by the solutes within the tubules of the kidney
What is the equation for Glomerular Filtration Rate (GFR)?
GFR = Kf x net filtration pressure
Kf = kidney permeability constant
net filtration pressure = difference between the forces favoring filtration (Pc & πt) and those opposing filtration (Pt & πc)
(slide 28)
What factors determine Kf in the GFR equation?
- membrane permeability
- filtration area
- ml min-1 mmHg-1
What is the GFR equation including pressure constants?
GFR = Kf[(Pc–Pt) – (πt–πc)]
What is the average GFR?
120ml min-1
What is the clinical relevance of GFR and how is it clinically determined?
GFR is the best indicator of renal function and is clinically determined by plasma clearance rate
What is plasma clearance rate?
Rate at which a substance is cleared from plasma
What is the formula for the plasma clearance rate (Cx)?
Cx = (Ux x V) / Px
What do the variables Ux, V, and Px represent in the clearance formula?
Cx = clearance of x in ml min-1
Ux = concentration of x in the urine in mg ml-1
V = volume of urine in ml min-1
Px = concentration of x in the plasma in mg ml-1
What are the requirements for a substance to be used in the calculation of plasma clearance rate?
The substance must be:
- freely filtered by the glomerulus
- not reabsorbed or secreted by the kidney
- not be produced or metabolised by the kidney
- not alter GFR
What is the gold standard test for measuring GFR and what substance is used in this test
The inulin clearance test, and inulin is the substance used in this test
What is inulin?
Inulin is a plant polysaccharide that is not secreted or reabsorbed by the kidney
How is inulin infused during the inulin clearance test, and what measurements are taken during the test?
Inulin is infused at a rate that maintains constant plasma concentrations
- urine inulin concentration and urine flow are measured over time
What is the usual clinical test for estimating GFR, and what substance is used in this test?
Serum creatinine or 24-hour urine collection
What is the substance used in the serum creatinine / 24-hour urine collection?
serum creatinine (a muscle metabolite)
What are the key features of the cellular structure of the Proximal Convoluted Tubule (PCT)?
- cuboidal epithelium with microvilli brush border for absorption
- lots of mitochondria
- highly permeable to water & solutes
(slide 42)
How do the ascending loop (AL) and descending loop (DL) of the loop of Henle differ in permeability?
- DL: passive transport of water leaving
- thin part of AL: passive reabsorption of NaCl
- thick part of AL: active reabsorption of only NaCl
What does peritubular capillary arrangement depend on?
Depends on the nephron type
How is the peritubular capillary arrangement in cortical nephrons?
The arrangement is the same as other tissues
What is the vasa recta?
Blood supply that runs in parallel to the loop of Henle and has branches going around it
How is the peritubular capillary arrangement in juxtamedullary nephrons?
Efferent arteriole leads to the vasa recta
What is the counter-current multiplier in the Loop of Henle?
It refers to the counter-current flow of filtrate and blood in opposite directions in the Loop of Henle.
How does the structure of the Loop of Henle contribute to the counter-current multiplier?
- blood & filtrate want same osmolarity as surrounding medulla
- vasa recta is permeable to water & NaCl
- descending limb = permeable to water only
- thin ascending limb = passive reabsorption of NaCl
- thick ascending limb = actively reabsorbs NaCl
What is the effect of the counter-current multiplier in the Loop of Henle?
The water diffuses out of the descending limb
- concentrating the filtrate
- rehydrating the blood
NaCl is moved out of the ascending limb
- concentrating the blood going into the medulla
- making filtrate exiting the loop hypo-osmotic
What is the cellular structure of the thin descending limb of the Loop of Henle?
Composed of:
- squamous epithelial cells
- highly permeable to water but not solutes
- connected by tight junctions - prevent movement of solutes between cells
What is the cellular structure of the thin ascending limb of the Loop of Henle?
Has passive NaCl transporters
What is the cellular structure of the thick ascending limb of the Loop of Henle?
Made up of:
- cuboidal epithelial cells
- connected by tight junctions
- have a glycoprotein layer
- highly permeable to solutes but not water
- have lots of mitochondria to power the K+/Na+ ATPase that drives NaCl reabsorption
What are the key cellular features of the Distal Convoluted Tubule (DCT) in the kidney?
- cuboidal epithelium
- arranged in a single layer
- few microvilli - no brush border
- not as impermeable to water as ascending limb
What is the mechanism of water reabsorption in the Collecting Duct of the kidney?
Controlled by the following mechanisms:
- filtrate from the distal convoluted tubule is hypo-osmotic
- water diffuses out of the filtrate (tubular fluid) into the salty medulla
- urea reabsorption – helps generate medulla concentration gradient
What is the role of hormones in the regulation of water reabsorption in the Collecting Duct of the kidney?
The amount of water reabsorbed in the Collecting Duct of the kidney is under hormonal (ADH) control to maintain homeostasis.
What are the 2 cell types in the upper collecting duct?
- principal cells
- intercalated cells
What is the cellular structure of the Collecting Duct in the upper cortex and medulla?
Principal cells:
- cuboidal cells with short microvilli
- permeability to water that is hormonally regulated
Intercalated cells:
- cuboidal cells with microvilli
- involved in the secretion of H ions for acid-base balance in the body
- lots of mitochondria
What is the cellular structure of the Medullary Collecting Duct in the upper cortex and medulla?
- positioned in the renal medulla
- principal cells
What is auto regulation in the context of GFR control?
Ability of the kidneys to maintain a constant glomerular filtration rate (GFR) despite fluctuations in blood pressure
What is the myogenic mechanism in GFR control?
Type of auto regulation that involves smooth muscle cells in the afferent arteriole responding to changes in renal blood vessel pressure.
- BP increases = arteriole constricts to prevent excessive increase in GFR
- BP decreases = the arteriole dilates to maintain GFR
What are the baroreceptors involved in the myogenic mechanism of GFR control?
Stretch receptors located in the afferent arteriole
- detect changes in BP and send signals to the smooth muscle cells to constrict or dilate the arteriole accordingly
- increased stretch = increased BP
What is the tubuloglomerular mechanism in GFR control?
Type of auto regulation, macula densa cells sense and respond to changes in nephron tubular flow.
- detects Na & Cl in the distal convoluted tubule
- low Na & Cl absorption = high fluid flow = high pressure
- high Na & Cl absorption = low fluid flow = low pressure
What do levels of Na correlate with tubular flow?
- high Na+ = high tubular flow
- low Na+ = low tubular flow
What is the Juxtaglomerular Apparatus?
A specialized structure located in the kidney, specifically at the junction of the afferent arteriole and the distal convoluted tubule (DCT)
What are the components of the Juxtaglomerular Apparatus?
The JGA is composed of three main cell types:
- juxtaglomerular (JG) cells
- macula densa cells
- extraglomerular mesangial cells
What are JG cells?
- modified smooth muscle cells
- located in walls of afferent arteriole
- ability to constrict or dilate arterioles
- release of enzyme renin
What are macula densa cells?
- modified epithelial cells located in wall of DCT
- sense NaCl in tubular fluid
- release paracrine signaling molecules that regulate JG cell function
How is blood flow in the glomerulus regulated in response to low GFR?
detects:
- baroreceptors sense low arteriole pressure
- macula densa cells sense low Na concentration in DCT (low tubular flow)
action:
- dilate afferent arteriole
- constrict efferent arteriole
How is blood flow in the glomerulus regulated in response to high GFR?
detects:
- baroreceptors sense high arteriole pressure
- macula densa cells sense high Na in DCT (due to high tubular flow)
action:
- constrict afferent arteriole
- dilate efferent arteriole
What are mesangial cells?
Contractile cells in the glomerulus:
- provide structural support
- contribute to auto regulation
What is the role of smooth muscle cells in the glomerulus?
regulating blood flow through the afferent & efferent arterioles
