In this presentation, GFR (glomerular filtrate rate)has been discussed thoroughly.

1. Chapter 26
Gyton and Hall

2. Urinary excretion rate:
 The rates at which different substances are excreted
in the urine represent the sum of three renal
processes
Filtration rate + secretion rate – reabsorption rate.
 Urine formation begins with filtration of protein
free fluid from glomerular capillaries to bowman’s
capsule.
 Same concentration of fluids in plasma and
bowman’s capsule.
 Water and specific solutes reabsorbed.
 Secretion of other substances from peritubular
capillaries into tubules.

4. Filtration, Reabsorption, and
Secretion of Different Substances
 Tubular reabsorption > tubular secretion in urine formation.
 Secretion= Potassium and hydrogen ions and a few other
substances that are excreted in the urine.
 Urea, creatinine, uric acid and urates=poorly reabsorbed
 Foreign substances and drugs= poorly reabsorbed, secreted
from the blood into the tubules, so that their excretion rates
are high.

5.  Sodium ions, chloride ions, and bicarbonate ions, are
highly reabsorbed, so that only small amounts
appear in the urine
 Amino acids and glucose, are completely reabsorbed
from the tubules and do not appear in the urine.
 Glomerular filtration, tubular reabsorption, and
tubular secretion—regulated according to the needs of
the body

6. Large Amounts of Solutes Filtered
and Then Reabsorbed
 GFR allows the kidneys to rapidly remove waste products
from the body that depend primarily on glomerular
filtration for their excretion.
 Most waste products are poorly reabsorbed by the
tubules and depend on a high GFR for effective removal.
 Plasma volume is only about 3 liters, whereas the GFR is
about 180 L/day(125 ml/min), the entire plasma can be
filtered and processed about 60 times each day.
 This high GFR allows the kidneys control the volume and
composition of the body fluids.

7. Glomerular Filtration
1.Composition of the Glomerular Filtrate
 Glomerular capillaries are relatively impermeable to proteins.
 Glomerular filtrate is protein-free and devoid of cellular
elements, including red blood cells.
 Concentrations of most salts and organic molecules, are
similar to the concentrations in the plasma.
 Low-molecular-weight substances, calcium and fatty acids
are partially bound to the plasma proteins.

8. GFR Is About 20 Per Cent of the
Renal Plasma Flow
 GFR is determined by
(1) the balance of hydrostatic and colloid osmotic forces acting
across the capillary membrane and
(2) the capillary filtration coefficient (Kf),
It is 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 a high
glomerular hydrostatic pressure and a large Kf.

9.  In the average adult human, the GFR is about 125
ml/min, or 180 L/day.
 The fraction of the renal plasma flow that is filtered
(the filtration fraction) averages about 0.2;
this means that about 20 per cent of the plasma
flowing through the kidney is filtered through the
glomerular capillaries. The filtration fraction is
calculated as follows:
 Filtration fraction = GFR/Renal plasma flow

11. Glomerular Capillary Membrane
 Three major layers:
(1) the endothelium of the capillary,
(2) a basement membrane, and
(3) a layer of epithelial cells (podocytes) surrounding the
outer surface of the capillary basement membrane.
 These layers make up the filtration barrier
 Filter several hundred times as much water and solutes
as the usual capillary membrane.
 Prevents filtration of plasma proteins.

13. •The capillary endothelium have thousands of
fenestrae.
•Endothelial cells have fixed negative charges that
hinder the passage of plasma proteins.
•Surrounding endothelium is the basement
membrane that have large spaces through which large
amounts of water and small solutes can filter.
•The final part of the glomerular membrane is a layer
of epithelial cells that line the outer surface of the
glomerulus.
The foot processes are separated by gaps called slit
pores through which the glomerular filtrate moves.

14.  Protein urea??
 Albumin urea??

15. Determinants of the GFR
 The GFR is determined by
(1) the sum of the hydrostatic and colloid osmotic forces
across the glomerular membrane, which gives the net
filtration pressure.
(2) the glomerular capillary filtration coefficient, Kf.
 Expressed mathematically,
GFR =Kf x Net filtration pressure

16. Determinants of the GFR
 GFR =Kf x Net filtration pressure.
 Increased glomerular capillary filtration coefficient
increases GFR.
 Increased glomerular capillary hydrostatic pressure
increases GFR.
 Increased glomerular capillary colloid osmotic pressure
decreases GFR.
 Increased bowman’s capsule hydrostatic pressure
decreases GFR.

17. Sympathetic Nervous System
Activation Decreases GFR
Strong activation of the renal sympathetic nerves can
constrict the renal arterioles and decrease renal blood flow
and GFR.
For example, reflex activation of the sympathetic nervous
system resulting from moderate decreases in pressure at the
carotid sinus baroreceptors receptors has little influence on
renal blood flow or GFR.
Moreover, because the baroreceptors adapt within minutes or
hours to sustained changes in arterial pressure, it is unlikely
that these reflex mechanisms have an important role in long-
term control of renal blood flow and GFR.

18. Renal blood flow:
 In an average 70-kilogram man, the combined blood flow
through both kidneys is about 1100 ml/min, or about 20
per cent of the cardiac output.
 The mechanisms that regulate renal blood flow are closely
linked to the control of GFR and the excretory functions of
the kidneys.
 On a per gram weight basis, the kidneys normally consume
oxygen at twice the rate of the brain but have almost seven
times the blood flow of the brain.
 Thus, the oxygen delivered to the kidneys far exceeds their
metabolic needs, and the arterial-venous extraction of
oxygen is relatively low compared with that of most other
tissues.

19. Physiologic Control of Glomerular
Filtration and Renal Blood Flow
 Most of the renal vascular resistance resides in three
major segments:
interlobular arteries, afferent arterioles, and efferent
arterioles.
Resistance of these vessels is controlled by the
sympathetic nervous system, various hormones,
and local internal renal control mechanisms.

20. FILTERED EXCRETED REABSORBED
Fluid 180 lit 1.8 lit 99%
Na 630 g 3.2 g 99.5 %
Glucose 180 g Nil 100 %
Urea 54 g 44 g 30 %