Last edited: 9/13/2021
1. FILTRATION, REABSORPTION, AND SECRETION
Filtration, Reabsorption, and Secretion Medical Editor: Mariel Antoinette L. Perez
OUTLINE II) GLOMERULUS
I) NEPHRON OVERVIEW
II) GLOMERULUS
III) PROXIMAL CONVOLUTED TUBULE (PCT)
IV) LOOP OF HENLE
V) DISTAL CONVOLUTED TUBULE
VI) COLLECTING DUCT
VII) SUMMARY
VIII) APPENDIX
IX) REVIEW QUESTIONS
X) REFRENCES
AfraTafreeh.com
I) NEPHRON OVERVIEW
(A) DIFFERENT PARTS Figure 2. Structure of Glomerulus and Macula Densa
Glomerular Capillaries (A) NET FILTRATION PRESSURE (NFP)
Bowman’s Capsule
• There’s a mixture of pressures
o Visceral layer
• The NFP should be approximately 10 mmHg
o Parietal layer (1) Glomerular Hydrostatic Pressure (GHP)
Force that pushes plasma out of the glomerular capsule
into the bowman’s space
Proximal Convoluted Tubule (PCT) Directly dependent on systolic blood pressure
Loop of Henle o BP = GHP
Distal Convoluted Tubule o BP = GHP
Average value: 55 mmHg
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(2) Colloid Osmotic Pressure (COP)
Exerted by plasma proteins like albumin
Keeps water in the blood
Average value: 30 mmHg
(3) Capsular Hydrostatic Pressure
Due to the pressure build-up in the Bowman’s capsule
Average value: 15 mmHg
(4) Colloid Osmotic Pressure
As long as the filtration membrane is intact, there should
be no proteins in the Bowman’s capsule
Average value: 0 mmHg
(B) GLOMERULAR FILTRATION RATE
Amount of plasma fluid or volume being filtered across
Figure 1. Nephron Overview [Meltzer] this glomerular membrane into the bowman’s capsule per
minute
(B) DEFINITION
NFP α GFR
We have 1.2 million nephrons per kidney o NFP is directly proportional to the GFR
o We usually have two kidneys total of 2.4 million
On average, 125 mL/min
nephrons
o Per min., 1.2L goes to AA 625mL used in filtration
process only 20% (125mL) is filtered
(C) ARTERIOLES
The glomerulus is one of the only examples in the body
wherein the capillary bed is both being fed and drained
by an arteriole
(1) Afferent Arterioles
Filtration, Reabsorption, and Secretion RENAL PHYSIOLOGY: Note #6. 1 of 5
, AfraTafreeh.com
(2) Efferent Arterioles (1) Osmolality Classification
Hypertonic
o osmolality
Due to this mechanism, a lot of fluid and filtrate o solutes (e.g., Na+,Cl–), H2O
substances will accumulate in the Nowman’s capsule and o hypertonic
drained into the next structure (PCT) Hypotonic
o osmolality
III) PROXIMAL CONVOLUTED TUBULE (PCT) o solutes (e.g., Na+,Cl–), H2O
One of the most important sites of a nephron Isotonic
o solutes = H2O
(A) TUBULAR REABSORPTION
Movement of substances in filtrates from the kidney (D) RENAL PYRAMID
tubule into the blood (1) Renal Cortex
Depending upon the chemicals being reabsorbed, it could
o Outer layer
be active or passive
o Lighter granulated tissue
A lot of filtrates are reabsorbed here
o Where PCT, DCT, and glomerular capillaries are
o Sodium (Na+), Potassium (K+), Water (H2O), Chloride
located
(Cl–), Magnesium (Mg2+), Calcium (Ca2+), Bicarbonate
(HCO3–), small proteins, lipids (2) Renal Medulla
65% of Na+ are reabsorbed o Striated due to the kidney tubule
o Some substances (i.e., glucose, amino acids) will go
AfraTafreeh.com
with Na+ via a cotransport mechanism IV) LOOP OF HENLE
These will be reabsorbed depending on the
Osmolality value before entering Loop of Henle
presence of Na+
o 300 mosm
o 100% glucose will be ideally reabsorbed
Isotonic with blood plasma
Glucosuria: traces of glucose in the urine
• Identifiable in diabetic patients Na+/K+/2Cl– Transporter
Amount of Na+ being reabsorbed can increase (so o Pumps out Na+, K+, and 2 Cl– ions from the filtrate as
that glucose will follow) until blood glucose levels blood goes up the ascending limb of Loop of Henle
reach above 180 mg/dL Happens along the entire length of ascending limb
• Transporters will saturate when it reaches o 25% Na+ is reabsorbed
transport maximum solutes get lost in the o 30% of K+ is reabsorbed
urine o 30% of Cl– is reabsorbed
65% of H2O is also reabsorbed because it follows Na+ Some K+ gets pushed back in the lumen creates a
o Due to obligatory water reabsorption depolarization on the ascending limb’s inner membrane
o Causes Mg2+ and Ca2+ to undergo passive
85-95% of HCO3– are reabsorbed paracellular transport into the medullary space
Questionable amount of Mg2+ will be reabsorbed
o Various literature say different values The ions being pumped out of the ascending limb
contribute to the medullary gradient becomes saltier
60% of K+ will be reabsorbed down the gradient
50-60% of Cl– will be reabsorbed o Becomes hypotonic
60% of Ca2+ will be reabsorbed Solutes < H2O
50% of urea will be reabsorbed
Small proteins (insulin, albumin, a bit of hemoglobin) can Water going down the descending limb is inclined to go
get reabsorbed via endocytosis out into the medullary space where it is saltier
Lipids o Water leaks out via the aquaporin-I
o undergo passive diffusion through the phospholipid o Becomes hypertonic
bilayer Solutes > H2O
enables them to get reabsorbed in the PCT Counter-Current Multiplier Mechanism
o Water leaks out of the descending limb due to ions
(B) TUBULAR SECRETION
getting pumped out of the ascending limb
Movement of substances from blood into the filtrates in 15% of H2O is reabsorbed
the kidney tubule Hence, only 20% of H2O is left
active process: requires ATP
o Adenosine Triphosphate V) DISTAL CONVOLUTED TUBULE
o “Energy currency of the cell”
By the time blood reaches DCT, it’s 100-200 mosm
The body may need or want to secrete (excrete) certain o 20% H2O left
substances o 10% Na+ left
o Drugs, H+ or HCO3–, NH4+, traces of creatinine
(A) EARLY DISTAL TUBULE
(C) OSMOLALITY
(1) Sodium-Chloride Symporter
Blood Plasma Osmolality
o 300 mosm (milliosmoles) Specialized transporters on the lumina membrane closest
Isotonic (similar value) with PCT and start of to the urine
descending limb Na+ and Cl– both go into the cell via protein channels
Only possible due to Na+/K+ pump
General flow of Osmolality (Renal Medullary Gradient)
o 300 mosm 500 700 900 1200 (deep in (2) Sodium-Potassium Pump
renal medulla)
Specialized channels in basolateral membrane
Pumps 3 Na+ ions out and 2 K+ ions in the cell
2 of 5 RENAL PHYSIOLOGY: Note #6 Filtration, Reabsorption, and Secretion
1. FILTRATION, REABSORPTION, AND SECRETION
Filtration, Reabsorption, and Secretion Medical Editor: Mariel Antoinette L. Perez
OUTLINE II) GLOMERULUS
I) NEPHRON OVERVIEW
II) GLOMERULUS
III) PROXIMAL CONVOLUTED TUBULE (PCT)
IV) LOOP OF HENLE
V) DISTAL CONVOLUTED TUBULE
VI) COLLECTING DUCT
VII) SUMMARY
VIII) APPENDIX
IX) REVIEW QUESTIONS
X) REFRENCES
AfraTafreeh.com
I) NEPHRON OVERVIEW
(A) DIFFERENT PARTS Figure 2. Structure of Glomerulus and Macula Densa
Glomerular Capillaries (A) NET FILTRATION PRESSURE (NFP)
Bowman’s Capsule
• There’s a mixture of pressures
o Visceral layer
• The NFP should be approximately 10 mmHg
o Parietal layer (1) Glomerular Hydrostatic Pressure (GHP)
Force that pushes plasma out of the glomerular capsule
into the bowman’s space
Proximal Convoluted Tubule (PCT) Directly dependent on systolic blood pressure
Loop of Henle o BP = GHP
Distal Convoluted Tubule o BP = GHP
Average value: 55 mmHg
AfraTafreeh.com
(2) Colloid Osmotic Pressure (COP)
Exerted by plasma proteins like albumin
Keeps water in the blood
Average value: 30 mmHg
(3) Capsular Hydrostatic Pressure
Due to the pressure build-up in the Bowman’s capsule
Average value: 15 mmHg
(4) Colloid Osmotic Pressure
As long as the filtration membrane is intact, there should
be no proteins in the Bowman’s capsule
Average value: 0 mmHg
(B) GLOMERULAR FILTRATION RATE
Amount of plasma fluid or volume being filtered across
Figure 1. Nephron Overview [Meltzer] this glomerular membrane into the bowman’s capsule per
minute
(B) DEFINITION
NFP α GFR
We have 1.2 million nephrons per kidney o NFP is directly proportional to the GFR
o We usually have two kidneys total of 2.4 million
On average, 125 mL/min
nephrons
o Per min., 1.2L goes to AA 625mL used in filtration
process only 20% (125mL) is filtered
(C) ARTERIOLES
The glomerulus is one of the only examples in the body
wherein the capillary bed is both being fed and drained
by an arteriole
(1) Afferent Arterioles
Filtration, Reabsorption, and Secretion RENAL PHYSIOLOGY: Note #6. 1 of 5
, AfraTafreeh.com
(2) Efferent Arterioles (1) Osmolality Classification
Hypertonic
o osmolality
Due to this mechanism, a lot of fluid and filtrate o solutes (e.g., Na+,Cl–), H2O
substances will accumulate in the Nowman’s capsule and o hypertonic
drained into the next structure (PCT) Hypotonic
o osmolality
III) PROXIMAL CONVOLUTED TUBULE (PCT) o solutes (e.g., Na+,Cl–), H2O
One of the most important sites of a nephron Isotonic
o solutes = H2O
(A) TUBULAR REABSORPTION
Movement of substances in filtrates from the kidney (D) RENAL PYRAMID
tubule into the blood (1) Renal Cortex
Depending upon the chemicals being reabsorbed, it could
o Outer layer
be active or passive
o Lighter granulated tissue
A lot of filtrates are reabsorbed here
o Where PCT, DCT, and glomerular capillaries are
o Sodium (Na+), Potassium (K+), Water (H2O), Chloride
located
(Cl–), Magnesium (Mg2+), Calcium (Ca2+), Bicarbonate
(HCO3–), small proteins, lipids (2) Renal Medulla
65% of Na+ are reabsorbed o Striated due to the kidney tubule
o Some substances (i.e., glucose, amino acids) will go
AfraTafreeh.com
with Na+ via a cotransport mechanism IV) LOOP OF HENLE
These will be reabsorbed depending on the
Osmolality value before entering Loop of Henle
presence of Na+
o 300 mosm
o 100% glucose will be ideally reabsorbed
Isotonic with blood plasma
Glucosuria: traces of glucose in the urine
• Identifiable in diabetic patients Na+/K+/2Cl– Transporter
Amount of Na+ being reabsorbed can increase (so o Pumps out Na+, K+, and 2 Cl– ions from the filtrate as
that glucose will follow) until blood glucose levels blood goes up the ascending limb of Loop of Henle
reach above 180 mg/dL Happens along the entire length of ascending limb
• Transporters will saturate when it reaches o 25% Na+ is reabsorbed
transport maximum solutes get lost in the o 30% of K+ is reabsorbed
urine o 30% of Cl– is reabsorbed
65% of H2O is also reabsorbed because it follows Na+ Some K+ gets pushed back in the lumen creates a
o Due to obligatory water reabsorption depolarization on the ascending limb’s inner membrane
o Causes Mg2+ and Ca2+ to undergo passive
85-95% of HCO3– are reabsorbed paracellular transport into the medullary space
Questionable amount of Mg2+ will be reabsorbed
o Various literature say different values The ions being pumped out of the ascending limb
contribute to the medullary gradient becomes saltier
60% of K+ will be reabsorbed down the gradient
50-60% of Cl– will be reabsorbed o Becomes hypotonic
60% of Ca2+ will be reabsorbed Solutes < H2O
50% of urea will be reabsorbed
Small proteins (insulin, albumin, a bit of hemoglobin) can Water going down the descending limb is inclined to go
get reabsorbed via endocytosis out into the medullary space where it is saltier
Lipids o Water leaks out via the aquaporin-I
o undergo passive diffusion through the phospholipid o Becomes hypertonic
bilayer Solutes > H2O
enables them to get reabsorbed in the PCT Counter-Current Multiplier Mechanism
o Water leaks out of the descending limb due to ions
(B) TUBULAR SECRETION
getting pumped out of the ascending limb
Movement of substances from blood into the filtrates in 15% of H2O is reabsorbed
the kidney tubule Hence, only 20% of H2O is left
active process: requires ATP
o Adenosine Triphosphate V) DISTAL CONVOLUTED TUBULE
o “Energy currency of the cell”
By the time blood reaches DCT, it’s 100-200 mosm
The body may need or want to secrete (excrete) certain o 20% H2O left
substances o 10% Na+ left
o Drugs, H+ or HCO3–, NH4+, traces of creatinine
(A) EARLY DISTAL TUBULE
(C) OSMOLALITY
(1) Sodium-Chloride Symporter
Blood Plasma Osmolality
o 300 mosm (milliosmoles) Specialized transporters on the lumina membrane closest
Isotonic (similar value) with PCT and start of to the urine
descending limb Na+ and Cl– both go into the cell via protein channels
Only possible due to Na+/K+ pump
General flow of Osmolality (Renal Medullary Gradient)
o 300 mosm 500 700 900 1200 (deep in (2) Sodium-Potassium Pump
renal medulla)
Specialized channels in basolateral membrane
Pumps 3 Na+ ions out and 2 K+ ions in the cell
2 of 5 RENAL PHYSIOLOGY: Note #6 Filtration, Reabsorption, and Secretion
