Test 1 (Renal Blood Flow Lecture) Flashcards
(39 cards)
Renal Functional Unit
Nephron:
- Number DECREASES with Age
Composed of:
a) Glomerulus (Glomerular Capillaries): Filters Blood
b) Tubule: Converts Filtrate into Urine
- Both are surrounded by the Blind end of the Tubular Epithelium … BOWMAN’s CAPSULE (Glomerular Capsule)
- Space between Capsule and Glomerulus … BOWMAN’s SPACE
- Filtrate passes from Blood to Tubular System through this Space
Types of Nephrons
1) Superficial (Cortical)
- SHORT loop of Henle
- Surrounded by Peritubular Capillaries
2) Juxtamedullary
- LONG loop of Henle
- LONG EFFERENT ARTERIOLES which divided into specialized peritubular Capillaries called the VASA RECTA!!!!!
- The Vasa Recta function to CONCENTRATE URINE
Renal Blood Supply
- Approximately 20% of CARDIAC OUTPUT
- High Pressure is Glomerular Capillaries (about 60 m Hg) causes FILTRATION of Blood
- 1100 - 1300 mL filtered/ min which produces 125- 130 mL of fluid termed the GLOMERULAR FILTRATE
- Lower Pressure in the PERITUBULAR CAPILLARIES (about 13 mm Hg) permits FLUID REABSORPTION
- Pressure in BOTH Capillary beds can be regulated by RESISTANCE changes in AFFERENT and EFFERENT Arterioles
Renal Blood Flow
- Important in Regulation of Body Fluid Volume and solute concentration
- Oxygen consumption of Renal Tissue is HIGHER than that of the BRAIN (Related to the HIGH RATE of Active Na+ Reabsorption which drives the Na+/ K+ ATPase)
- Cortex receives most RBF, Medulla receives LESS via VASA RECTA
- RBF determined by Pressure Gradient between Renal Artery and Renal Vein divided by the VASCULAR RESISTANCE
Glomerular Filtration: First step in Urine Formation
- Plasma is filtered under pressure from Glomerular Capillaries into Bowman’s Capsule
- Normally, Glomerular Filtrate is essentially FREE of BLOOD CELLS and PROTEINS, but otherwise Identical to Plasma
- Glomerular Filtrate is HEAVILY MODIFIED as it passes down the Nephron
- Urine is very different from GLOMERULAR FILTRATE
Clinical Application of GFR
- Estimation of GFR is essential in assessment of Renal Function
- TOTAL GFR is the sum of the Filtration Rate in all functioning Nephrons and therefore is an INDEX of FUNCTIONING RENAL MASS
- GFR estimate can be used to evaluate the severity and course of RENAL DISEASE
- A fall in GFR indicates disease Progression (Most often resulting from a DECREASE in NET PERMEABILITY due to LOSS of FILTRATION SURFACE AREA)
- A Rise in GFR is indicative of at least PARTIAL RECOVERY
Glomerular Membrane: A Molecular Sieve
- Free passage of Water, Small Solutes (Glucose, Amino Acids, Electrolytes): concentration are the same on BOTH SIDES of MEMBRANE
- Passage of LARGE Molecules (Proteins) and formed elements is impeded
- Normally, only VERY SMALL amounts of Protein are filtered into BOWMAN’S Capsule
Structure of Glomerular Membrane
Three Distinctive Layers:
1) FENESRTATED CAPILLARY ENDOTHELIUM:
- Highly permeable to WATER, Dissolved Solutes
2) GLOMERULAR BASEMENT MEMBRANE:
- Collagen, Proteoglycans contain Anionic (NEGATIVE) Charges
3) PODOCYTE EPITHELIUM:
- Slit pores between Podocytes RESTRICT LARGE Molecules
Proteins of Glomerular Slit Diaphragm CD2AP and CD2- associated Protein
- CD2AP
- NEPHRIN Molecules
Mechanism of Filtration
- Based on size and Charge of Macromolecules:
a) LARGE Molecules are RESTRICTED more than Smaller
b) ANIONIC Molecules are RESTRICTED more than Neutral or Cationic
Clinical Application of Filtration
- Some kidney disease cause LOSS of NEGATIVE CHARGE on the basement membrane before noticeable changes in RENAL STRUCTURE
Minimal Change Disease or Neuphropathy:
- Results in FILTRATION of Proteins (Especially ALBUMIN) and their appearance in Urine (Albuminuria or Proteinuria)
- More extensive Renal Injury often results in LARGE amount of PROTEIN in URINE
Physical Forces affecting Glomerular Filtration
- GFR is remarkably HIGH (125 ml/ min, 180 L/ day)
GFR is a product of 3 Physical Factors:
1) HYDRAULIC CONDUCTIVITY (Lp) of Glomerular Membrane
- Permeability or Porosity of Capillary Wall
2) SURFACE AREA for Filtration (2m^2)
- Product of 1 and 2 is ULTRAFILTRATION COEFFICIENT Kf
3) Capillary Ultrafiltration Pressure (Puf)
GFR = Kf x Puf
Mechanisms for Altering GFR
GFR = Kf x Puf
1) Altered Kf:
- MESANGIAL CELL Contraction
2) Alrered Puf:
- Changes in P(Gc)
P(Gc) Determined by 3 Factors:
1) Renal Arterial Blood Pressure
2) Afferent Arteriolar Resistance
3) Efferent Arteriolar Resistance
Glomerular Mesangial Cells can alter Kf
GLOMERULAR MESANGIAL CELLS (M) located within Glomerular Capillary Loops
- Contraction of Mesangial Cells SHORTENS CAPILLARY LOOPS, LOWERS Kf, and thus LOWERS GFR!!!!!!!!
Ultrafiltration Pressure: Driving force for Glomerular Filtration
- Puf is determined by HYDROSTATIC and COLLOID OSMOTIC PRESSURES in Glomerular Capillaries
Bowmans’ Capsule:
Puf = P(Gc) - (P(Bc) + p(Gc))
Glomerular Filtration
- Depends on NET FILTRATION PRESSURE!!!!!
Puf = P(Gc) - (P(Bc) + p(Gc))
Glomerular and Peritubular Capillary Starling Forces
1) Glamerular Capillary:
- At the AFFERENT Arteriole the Capillary Hydrostatic pressure is GREATER than the Plasma Oncotic Pressure and Capsular Hydrostatic pressure combined which means there is a NET FILTRATION!
- At the EFFERENT Arteriole the Capillary Hydrostatic pressure is EQUAL to the Plasma Oncotic Pressure (It has INCREASED) and the Capsular Hydrostatic Pressure combined which means there is NO NET MOVEMENT
2) There will be a HIGHER GFR when the RBF is HIGHER!!!
3) Peritubular Capillary:
- At the EFFERENT Arteriole the Plasma Oncotic Pressure is much HIGHER than the Capillary Hydrostatic Pressure which means there is a NET REABSORPTION!!!
- At the Venous End, the Plasma Oncotic Pressure is only slightly higher than the Capillary Hydrostatic Pressure which means there is still a NET REABSORPTION but it is not as great as the Efferent Arteriole
Glomerular and Peritubular Capillary Starling Forces Explination
1) In normal individuals, GFR is primarily regulated by ALTERATION in PGc
2) PGc is determined by changes in SYSTEMIC Arterial Pressure (PA), Afferent Arteriolar Resistance (RA), and Efferent Arteriolar Resistance (RE)
Key Concepts of GFR
Control of GFR by adjusting RESISTANCE of Afferent and Efferent Arterioles:
1) AFFERENT Arteriolar Constriction:
- Greater Pressure drop UPSTREAM of GLOM Capillaries
- PGc falls, which LOWERS GFR
- Renal Blood flow falls due to INCREASE in Resistance
2) EFFERENT Arteirolar Constriction:
- Pooling of Blood in Glomerular Capillaries
- Increase PGc INCREASES GFR
- Renal Blood Flow would DECREASE!!!!
Garden Hose Analogy
1) Increased Systemic Arterial Pressure
- Causes an INCREASE in GFR!!!
2) Afferent Arteriolar Constriction
- Decreases the GFR and the RBF
3) Efferent Arteriolar Constriction
- The GFR will INCREASE and the RBF will go DOWN!!!
Effects on Afferent and Efferent Arteriolar Constriction on RPF and GFR
1) AFFERENT Constriction
- GFR will Decrease
- RPF will Decrease
- PGc will Decrease
**GFR Decreases because both Glomerular Capillary Pressure and Renal Plasma Flow fall
2) EFFERENT Constriction
- GFR will Increase and then plates
- RPF will Decrease
- PGc will INCREASE
***GFR INCREASES because rising Capillary pressure dominates
***GFR DECREASES because falling RPF Dominates
3) Dilation of Efferent Arteriole:
- Decrease the GFR
4) Dilation of Afferent Arteriole:
- Increase the GFR
Hydrostatic Pressures in Renal Microcirculation: Effects of Arteriles Constriction
1) The largest Decrease in Hydrostatic Pressure when there is Afferent Arteriolar constriction is in the AFFERENT Arteriole
2) The largest Decrease in Hydrostatic Pressure when there is Efferent Arteriole constriction is in the EFFERENT Arteriole
Renal Blood Flow
- Regulated by several mechanisms in order to CONTROL GFR
- Vascular Resistance to RBF is primarily determined by AFFERENT and EFFERENT Arterioles
- Controlled y SYMPATHETIC Nervous System (NE) and various Hormones along with INTERNAL (Myogenic, T-G Feedback) control mechanisms
- GFR is strictly regulated over a MAP range between 80 to170 mm Hg via a process called AUTOREGUALTION!!!!!!!
Effects of Sympathetic Stimulation
1) Constriction of Afferent and, to a lesser extent, Efferent Arterioles:
- DECREASE in RBF and GFR
- Diverts the Renal Fraction to VITAL ORGANS
2) Increased RENIN Secretion by Granular Cells
3) Angiotensin II thus produced restores BLOOD PRESSURE (Systemic Vasoconstriction)
4) Angiotensin II promotes Arteriolar Constriction (Efferent > Afferent):
- Raises Blood Pressure
- May stabilize GFR (Moderate Ang II)
5) Stimulate Na+ Reabsorption in Proximal Tubule, Thick Ascending limb of Henle’s loop, Distal Convoluted Tubule, Collecting Duct