4. Functions of the Kidneys
Kidneys perform a variety of functions for homeostasis.
1. Filter 180 liters of blood daily to eliminate toxins & wastes
2. Excretion of metabolic waste products (urea, uric acid & creatinine)
3. Regulate Blood volume & Blood pressure by control water balance
4. Regulate RBC formation by producing Erythropoietin (EPO)
5. Regulate electrolyte balance (Na+, K+, Ca2+)
6. Regulate acid-base balance (by control H+ and HCO3 level).
7. Regulate body fluid Osmolarity (280 mOsm/l) through water balance
8. Enzyme and Hormone production: Renin, Erythropoietin, ….
9. Drug metabolism and chemical Detoxification
10. Glucose production during starvation (gluconeogenesis)
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5. 1. Cortex- outer layer which contain
major portion of nephrons
(functional units)
2. Medulla- inner layer which
contains
Pyramids-triangular shaped
structures,
Renal papilla: is tip of pyramid
Renal calyx – minor and major
Renal pelvis- large area (cavity)
which collects urine from calyx
(minor & major) and drain it in
to Ureters.
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Internal structure of Kidneys…
6. Renal Blood Circulation
The renal arteries deliver about
21% (1200 ml/min) of the
resting cardiac output to the
kidneys (Approx. one-fourth)
90% blood flow to cortex
Renal artery branches
extensively within the kidney
into smaller arteries called
afferent arterioles.
Renal blood flow has mainly
two purposes:
1. Filtration
2. Renal tissue oxygenation 6
7. Characteristics of Renal Blood Flow
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In kidney, Two capillary beds
1. Glomerular capillary: High
hydrostatic pressure (60 mmHg)→
important for Filtration process
2. Peritubular capillaries (vasa
recta): Low hydrostatic pressure
(13 mmHg) → importantant for
Reabsorption process
Peritubular capillaries
8. Nephrons
Nephrons: are structural & functional units of kidneys
• There are One million nephron in each kidney
• Only 25% of kidney nephrons is needed to adequately maintain all the essential
renal excretory and regulatory functions.
Each nephron consists of two parts:
1. Renal Corpuscle - contains glomerular capillaries & Bowman’s capsule
- Function: Site of blood plasma filtration.
2. Renal Tubules-Contains tubular structures that extends beyond corpuscle
-Function: Reabsorption & Secretion purpose
These tubular structures include; PCT- Proximal Convoluted Tubule, LOH- Loop of Henle,
DCT- Distal Convoluted Tubule, CD- Collecting Duct
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11. Renal Tubule
Proximal convoluted tubule (PCT) – PCT reabsorbs water &
solutes from filtrate & secretes substances into it.
Loop of Henle – is U-shaped loop of the renal tubule
• Proximal part is similar to the proximal convoluted tubule.
• Proximal part is followed by the thin segment & thick segment
• Distal convoluted tubule (DCT) – continuation of the loop of
Henle and function more in secretion than reabsorption. 11
12. Renal Tubule
Connecting Tubules: The distal portion of the distal
convoluted tubule nearer to the collecting ducts.
Two important cell types are found here:
1. Intercalated cells
• Cuboidal cells with microvilli.
• Function in maintaining the acid-base balance of the body.
2. Principal cells
• Cuboidal cells without microvilli.
• Help maintain the body’s water and salt balance
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18. Glomerular Filtration
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Glomerular filtration- is the filtration of fluid through the
glomerular capillaries in to bowman's capsule
The Glomerular Filtrate contains all plasma components
except plasma proteins (albumin, globulin & fibrinogen) and
formed elements (blood cells) -RBC, WBC Platelets.
The kidneys filter the body’s entire plasma volume 60 times
each day.(3L plasma x 60 times/day= 180L/day).
Plasma proteins (albumin) are not filtered and are used to
maintain colloid osmotic pressure of the blood.
Filtration is selective in terms of size but non-selective in
terms of usefulness (contains both useful & non-useful).
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Q. What derives filtration across the walls of glomerular capillaries?
Capillary blood pressure favors filtration process.
Colloid osmotic pressure & Capsular pressure oppose filtration .
Diameter of A.A > E.A→ blood
damming up in capillary → ↑CBP
20. Filtration Membrane
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Fluid filtered from glomerulus into
Bowman’s capsule pass through the 3 layers
that make up filtration membrane.
1. Endothelial cells with fenestrated
capillaries (pores)
2. Basement Membrane
3. Podocyte Cells with filtration slits in
inner layer of Bowman’s capsule
This allows huge filtration in the
glomerular capillaries except blood cells
and plasma proteins.
22. Filtration Membrane…
• Filtration membrane is highly permeable to water and small
molecules (less than 10.000 MW)
• Large molecules (more than 70.000 MW especially proteins
are not filtered due to their large size and negative electrical
charge because their passage is repulsed by negatively
charged glycoproteins present on endothelial pores,
basement membrane and podocytes.
Factors that determining the glumerular filterability
Molecular weight
Charges of the molecule
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24. Tubular Reabsorption
Reabsorption-is the process of taking useful substances from
the filtrate in to the blood (not to be lost in the urine)
The kidneys produces 180 liters of filtrate per day and normal
urinary output in per day 1 to 2 liters.
Approximately 99% of the filtrate is reabsorbed back into the
blood in the peritubular capillaries & Only 1% forms urine.
Reabsorbed substances are carried to the venous system →
heart
Some reabsorption is passive & most is active transport
Majority of reabsorption takes place in the Proximal tubule.
36. 36
Secretion – is the process of adding unwanted substance from
blood (peritubular) in to the filtrate for excretion with the urine.
Only 20% of the plasma entering the kidneys is filtered.
Secretion :allows for rapid elimination of substances from the
plasma via extraction of the 80% of unfiltered plasma in peritubular
capillaries and adding it to the substances already in tubule as result
of filtration
Tubular secretion is the reverse of reabsorption process.
Some materials move from the blood (peritubular capillaries) into the
renal tubules are: H+, K+ , Creatinine & some drugs like penicilin.
Tubular secretion is an active process.
Tubular Secretion
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Dilute & Concentrated Urine
The kidneys can excrete urine of varying concentrations depending on the
body’s state of hydration (overhydration and dehydration state).
39. Plasma Clearance
Plasma clearance: The volume of plasma which is cleared
from a particular substance by the kidney (in urine).
Plasma clearance expresses the kidneys’ effectiveness in
removing various substances from blood to the environment
Clearance is non-invasive way to measure GFR
Inulin clearance and Creatinine clearance: important to
measure kidney function by estimating GFR.
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40. Plasma Clearance…
GFR is an index of kidney function
Measurement of GFR: can be measured by means of the inulin
clearance and estimated by means of the creatinine clearance
1. Inulin: a polymer of fructose, precisely measure GFR (Standard)
Freely filtered into the Bowman’s capsule
Not reabsorbed, secreted or metabolized by the nephron
Non-endogenous, has to be infused intravenously
2. Creatinine: endogenously released into plasma by skeletal muscle
metabolism, is used to measure GFR (accurate measure GFR)
Not as accurate as inulin as a small quantity is secreted into the proximal
tubule.
amount excreted > amount filtered
Reasonably accurate measurement of GFR (easy to monitor)
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42. Excretion
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Excretion refers removal of a
substance from the body.
All plasma constituents filtered
or secreted but not reabsorbed
remain in the tubules and pass
into the renal pelvis to be
excreted as urine & eliminated
from body.
Urine output=1.5 liters/day).
44. Regulation of Acid-Base Balance (Buffer System)
Buffer is substance that
stabilizes [H+] when
H+ ions are added or
removed from a solution.
They do not eliminate
H+ from body –
reversibly bind
H+ until balance is re-
established and form
conjugated base or acid.
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45. Ureters: Retroperitoneal (behind
peritoneum, like kidney)
• Tubular organ 25cm long each
• Extends from the renal pelvis to
the bladder
• The smooth muscle in the wall of
the ureter contracts in peristaltic
waves to propel urine toward the
urinary bladder
• Gravity also assist to propel urine
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Transportation, Storage & Elimination of Urine…
46. Urinary bladder
• The urinary bladder is a
muscular sac which is found
behind the pubic bones.
• In women, the bladder is
inferior to the uterus;
• In men, the bladder is superior
to the prostate gland.
• The bladder is a reservoir for
accumulating urine, and it
contracts to eliminate urine.
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Transportation, Storage & Elimination of Urine…
47. 47
• On the floor of the bladder is a triangular area called
the trigone, and it does not expand during urinary
filling.
• The points of the triangle are the openings of the
two ureters and that of the urethra
• The smooth muscle layer in the wall of the bladder
is called the detrusor muscle.
• Contraction of dertusor muscles causes urination
Transportation, Storage & Elimination of Urine…
48. Urethra
• Transports urine from urinary
bladder to outside
• Its wall contains mucus secreting
urethral glands
Parts of male urethra
1. Prostatic urethra
2. Membranous urethra
3. Penile urethra
• Urethral orifice is on glans penis. 48
Transportation, Storage & Elimination of Urine…
Q. What is prostate hypertrophy &
its effect on urination?
Impede urine flow or ↑urine flow ?
50. Renal Failure
1. Acute: Sudden onset, rapid reduction in urine output
- usually reversible
2. Chronic: Progressive, not reversible
Up to 75% function can be lost before it is noticeable.
Generalized Site of Disease:
1. Prerenal: Inadequate renal blood flow
2. Intrarenal: Nephron damage
3. Postrenal: Obstruction, Structural defects
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