1. Glomerular Filtration
As blood flows through the glomerulus protein-free plasma filters through the glomerular capillaries into Bowman’s capsule
Normally about 20% of the plasma that enters the glomerulus is filtered
This process is known as glomerular filtration which is the first step in urine formation

2. Glomerular Filtration Rate
The rate at which glomerular filtrate is formed

3. GFR is determined by
the balance of hydrostatic and colloid osmotic forces acting across the capillary membrane
(2) the capillary filtration coefficient (Kf)( the product of the permeability and filtering surface area of the capillaries)
The glomerular capillaries have a much higher rate of filtration than most other capillaries because of high glomerular hydrostatic pressure and a large Kf
In the average adult human the GFR is about 125 ml/min or 180 L/day

4. Filtration Fraction = GFR/Renal plasma flow
The fraction of the renal plasma flow that is filtered (the filtration fraction) averages about 0.2
This means that about 20 percent of the plasma flowing through the kidney is filtered through the glomerular capillaries
Filtration Fraction = GFR/Renal plasma flow

5. Determinants of the GFR

6. The net filtration pressure represents the sum of the hydrostatic and colloid osmotic forces that either favor or oppose filtration across the glomerular capillaries

8. Forces Favoring Filtration (mm Hg)
Glomerular hydrostatic pressure
Bowman's capsule colloid osmotic pressure
Forces Opposing Filtration (mm Hg)
Bowman's capsule hydrostatic pressure
Glomerular capillary colloid osmotic pressure

11. Increased Glomerular Capillary Filtration Coefficient Increases GFR and Decreased Glomerular Capillary Filtration Coefficient Decreases GFR
The Kf is a measure of the product of the hydraulic conductivity and surface area of the glomerular capillaries

12. Increased Bowman's Capsule Hydrostatic Pressure Decreases GFR
(obstruction caused by stones)

13. Increased Glomerular Capillary Colloid Osmotic Pressure Decreases GFR
Two factors that influence the glomerular capillary colloid osmotic pressure are
(1) the arterial plasma colloid osmotic pressure (2) the fraction of plasma filtered by the glomerular capillaries (filtration fraction)

14. Increased Glomerular Capillary Hydrostatic Pressure Increases GFR
Changes in glomerular hydrostatic pressure serve as the primary means for physiologic regulation of GFR
Increased resistance of afferent arterioles reduces glomerular hydrostatic pressure and decreases GFR and dilation of the afferent arterioles increases both glomerular hydrostatic pressure and GFR
Efferent arteriolar constriction has a biphasic effect on GFR

15. Renal Blood Flow
Renal blood flow is 1100ml/min (22% of the cardiac out put)

16. Determinants of Renal Blood Flow

17. Changes in arterial pressure influence renal blood flow but the kidneys have effective mechanisms for maintaining renal blood flow and GFR relatively constant over an arterial pressure range between 75 and 160 mm Hg, a process called autoregulation

18. Blood Flow in the Vasa Recta of the renal medulla is very low as compared with flow in the renal cortex
The outer part of the kidney the renal cortex receives most of the kidney's blood flow
Blood flow in the renal medulla accounts for only 1 to 2 percent of the total renal blood flow
Flow to the renal medulla is supplied by a specialized portion of the peritubular capillary system called the vasa recta

19. Physiologic Control of Glomerular Filtration and Renal Blood Flow
Sympathetic nervous system activation decreases GFR and the renal blood flow due to constriction of the renal arterioles
The renal sympathetic nerves are important in reducing GFR during severe acute disturbances lasting for a few minutes to a few hours such as severe hemorrhage

20. Hormonal and Autacoid Control of Renal Circulation
Norepinephrine, Epinephrine, and Endothelin constrict renal blood vessels and decrease GFR
Angiotensin II preferentially constricts efferent arterioles

21. In most physiologic conditions increased angiotensin II levels raise glomerular hydrostatic pressure while reducing renal blood flow
Increased angiotensin II formation usually occurs in circumstances associated with decreased arterial pressure or volume depletion which tend to decrease GFR
In these circumstances the increased level of angiotensin II by constricting efferent arterioles helps to prevent decrease in glomerular hydrostatic pressure and GFR

22. Endothelial-derived Nitric Oxide decreases renal vascular resistance and increases GFR
Prostaglandins and Bradykinin tend to Increase GFR

23. Autoregulation of GFR and Renal Blood Flow
Feedback mechanisms intrinsic to the kidneys normally keep the renal blood flow and GFR relatively constant despite marked changes in arterial blood pressure
This relative constancy of GFR and renal blood flow is referred to as autoregulation
The major function of autoregulation in the kidneys is to maintain a relatively constant GFR and to allow precise control of renal excretion of water and solutes

24. Importance of GFR Autoregulation in Preventing Extreme Changes in Renal Excretion
Normally GFR is about 180 L/day and tubular reabsorption is L/day leaving 1.5 L/day of fluid to be excreted in the urine
In the absence of autoregulation relatively small increase in blood pressure (from 100 to 125 mm Hg) would cause a 25 percent increase in GFR (from about 180 to 225 L/day)
If tubular reabsorption remained constant at L/day this would increase the urine flow to 46.5 L/day
Because the total plasma volume is only about 3 liters, such a change would quickly deplete the blood volume

25. Tubuloglomerular Feedback and Autoregulation of GFR
The kidneys have a feedback mechanism that links changes in sodium chloride concentration at the macula densa with the control of renal arteriolar resistance
This feedback helps ensure a relatively constant delivery of sodium chloride to the distal tubule and helps prevent fluctuations in renal excretion

26. The tubuloglomerular feedback mechanism has two components that act together to control GFR:
an afferent arteriolar feedback mechanism
(2) an efferent arteriolar feedback mechanism

27. Decreased GFR slows the flow rate in the loop of Henle causing increased reabsorption of sodium and chloride ions and reducing the concentration of sodium chloride at the macula densa cells
This decrease in sodium chloride concentration initiates a signal from the macula densa that has two effects
(1) It decreases resistance to blood flow in the afferent arterioles which raises glomerular hydrostatic pressure and helps return GFR toward normal
(2) It increases renin release from the juxtaglomerular cells of the afferent and efferent arterioles

28. Renin acts as an enzyme to convert angiotensinogen into angiotensin I
Angiotensin I is converted to angiotensin II by the angiotensin converting enzyme
The angiotensin II constricts the efferent arterioles, increasing glomerular hydrostatic pressure and helping to return GFR toward normal

30. Myogenic Autoregulation of Renal Blood Flow and GFR
The myogenic constrictor response in afferent arterioles occurs within seconds and prevents transmission of increased arterial pressure to the glomerular capillaries

31. High protein intake and increased blood glucose increase renal blood flow and GFR