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Biology
Summary
Chapter 15
Homeostasis
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1 Urinary System
This system serves the purpose of excretion in humans.
Components of Urinary System
• A pair of kidneys
• A pair of ureters
• Urinary bladder
• Urethra
Components of Urinary System
The right kidney is
slightly lower in position
than the left one.
Do you know why?
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Structure of Kidneys
•Shape
Kidneys are bean-shaped organs
•Regions
Two distinct regions on cross-section i.e. renal cortex and renal medulla.
•Nephron
A pair of kidneys consist of millions of functional units called nephrons.
•Blood Supply
The nephrons have extensive blood supply via the renal arteries, which leave each kidney via the renal vein.
•Function and Fact
The function of kidney and blood in clearing wastes is very evident from the fact that the weight of kidneys account for less
than 1% of total body weight while they receive 20% of blood with each cardiac beat.
•Pelvis
Following filtration of blood and further processing through the tubular system, urine is collected in a central cavity of the
kidney called the pelvis.
•Ureters
Urine leaves the kidney through a duct called the ureter. The ureters of both the kidneys drain into the urinary bladder
through ureteral orifices
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•Urinary bladder
It stores urine for some time.
•Urethra
During urination, urine leaves the body
from the bladder through a tube called
the urethra, which empties near the
vagina in females or through the penis in
males.
•Urethral Sphincter
Sphincter muscles near the junction of
the urethra and bladder control the urine
in the bladder.
Structure of Kidney
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MCQs
Short Questions
how much blood?
2. What is the role of the urinary bladder?
1. With one cardiac beat, kidneys receive
a) 10%
b) 20%
c) 40%
d) 50%
1. What are the components of the urinary system?
3. Name the two regions of the kidney.
Long Question
1. Explain the structure of the kidney.
2. The central cavity of the kidney where
urine is collected after processing is:
a) Renal pelvis
b) Renal pyramid
c) Renal cortex
MCQ Key
1
B
3. On cross-section, there are how many
2
A
a) 2
3
A
d) Renal medulla
distinct regions of the kidney?
b) 3
c) 4
d) 5
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2 Nephron
Nephrons are functional units of kidneys and are arranged along distinct regions, an outer cortex and inner medulla.
Based on their properties, nephrons are classified as:
Properties
Cortical Nephrons
Juxtamedullary Nephrons
Location
Along the cortex
Along the border of the cortex and medulla
Loop of Henle
Short Loop of Henle
Long loop of Henle extending deep into the
inner medulla
Percentage
About 80%
About 20%
Function
Formation of Dilute Urine
Formation of Concentrated urine
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Types of Nephrons
The word “nephron”
literally means kidney?
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Structure of nephron
A nephron has the following major components
•Glomerulus
•Bowman’s capsule
•Proximal convoluted tubule
•Loop of Henle
•Distal convoluted tubule
In each nephron, the inner end forms a cup-shaped swelling, called
Bowman's capsule and it is around a ball of capillaries called the
glomerulus.
Glomerulus circulates blood through the capsule as it arrives through the
afferent arteriole and leaves the capsule by the efferent arteriole.
Efferent arteriole subdivides again into another network of capillaries, the
peritubular capillaries.
Bowman’s capsule continues as an extensively convoluted proximal
tubule, a loop of Henle and the distal tubule, which empties into collecting
tubules.
The collecting tubules open into
the pelvis. The filtrate from the
glomerulus passes through these
structures and is processed
ultimately for urine formation.
The peritubular capillaries
intermingle with proximal and
distal tubules of the nephron
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Vasa recta
In juxtamedullary nephrons, additional capillaries extend down to form a loop of vessels called vasa recta.
Nephron
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MCQs
Short Questions:
a) Cortical nephrons
2. What is glomerular filtrate?
1. Concentrated urine is produced by
b) Juxta-medullary nephrons
c) Both
d) Renal pyramids
2. Blood leaves the glomerulus through
a) Afferent arterioles
1. What are cortical and juxta-medullary nephrons?
3. What are the major components of a nephron?
4. What are vasa recta?
Long Question:
1. Write a note on the structure of nephrons.
b) Efferent arterioles
c) Peri-tubular capillaries
d) Vasa recta
MCQ Key
3. Additional loop of vessels around the loop of Henle
1
B
a) Peri-tubular capillaries
2
B
3
B
of juxta-medullary nephrons are called
b) Vasa recta
c) Afferent arterioles
d) Efferent arterioles
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3 Functions of Kidney/ Process of
Urine Formation
The whole process of urine formation can be divided into three distinct steps:
• Filtration
• Selective reabsorption
• Tubular secretion
Filtration
*Blood passing through the glomerulus is filtered into Bowman's capsule.
*It is specifically filtered here, unlike at the other parts of the vessels, because
glomerulus walls are porous, and the fraction of the blood pressure reaching here
provides the filtration pressure.
*The filtrate appearing in Bowman’s capsule is called glomerular filtrate, which
contains numerous useful substances such as glucose, amino acids, salts, etc. in an
aqueous solution.
Reabsorption
• All the useful constituents of the glomerular filtrate are reabsorbed in proximal
tubules and when filtrate leaves proximal tubules, it mostly contains nitrogenous
wastes.
Secretion
• The tubular epithelium also secretes substances into the lumen.
This secretion is very selective and is mainly of hydrogen ions to balance the pH
value of the filtrate passing through the tubule.
!
E
M
O
S
AWE
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Reabsorption of Useful Substances
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MCQs
1. Filtration of blood in glomerulus occurs due to
a) Filtration pressure
b) Porous walls of glomerular capillaries
c) Both
d) None of these
2. Which is not present in glomerular filtrate?
a) Glucose
b) Amino acids
c) Blood cells
d) Salts
3. Which ions are secreted by tubules to maintain the pH of urine?
a) Calcium ions
b) Hydrogen ions
c) Potassium ions
d) Sodium ions
Short Questions:
1. What is the process of filtration in urine formation?
2. What is the process of reabsorption in urine formation?
3. What is the process of secretion in urine formation?
Long Question:
1. Explain the process of urine formation.
MCQ Key
1
C
2
C
3
B
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Osmoregulatory
Function
of
Kidney
4
Restricted Supply of Water
The conservation of water is the principal function of the body.
Two mechanisms for conservation
Countercurrent mechanism
Hormonal mechanism
Sufficient or Excess Supply of Water
* Reabsorbtion of water from the filtrate is reduced.
* Inhibition of release of antidiuretic hormone (ADH) in the presence of
hypoosmotic body fluids.
* The reduction in reabsorption causes large volumes of diluted urine.
Mammalian Kidney
* Mammalian kidneys including those of humans
are adapted to conserve water by over 99.5%
reabsorption of glomerular filtrate.
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Countercurrent Mechanism
* The interstitial fluid of the kidney is gradually concentrated from the
cortical to the medullary part.
* The inner medulla is highly concentrated with the presence of
urea and through a mechanism of counter-current multiplier.
* This mechanism causes gradual osmotic outflow of water from the
filtrate back to the kidney as it passes downward in the descending
loop of Henle.
* Furthermore, ascending loop of Henle does not allow outflow of
water from its filtrate, instead actively transports Na+ ions into
kidney interstitium to sustain high concentration in the kidney.
Do you know why we produce large
quantities of dilute urine in winter
and small amount of concentrated
urine in summer?
Counter Flow of Blood and Urine
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Hormonal mechanism
Following hormones play role in the concentration of urine
Aldosterone
• It is secreted from the adrenal cortex.
• It acts upon ascending limb or thick limb of the loop of Henle.
• It causes active uptake of sodium.
Anti-diuretic hormone/Vasopressin
• ADH is released from the posterior pituitary lobe.
• It acts on the distal convoluted tubule and collecting tubule.
• It causes active transport of water from the filtrate in the distal
tubules and collecting tubules back to the kidney.
• Gradually increasing osmotic concentration from cortex to inner
medulla is the main factor for the production of hypertonic
(concentrated) urine in mammals including humans.
Kidney as Osmoregulatory Organ
• The production of varied concentrations of urine depending on the
availability of water.
•It exhibits clearly that the kidney functions as an osmoregulatory
organ along with its excretory role of nitrogenous wastes.
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MCQs
Short Questions:
a) Decreased
2. What is the role of ADH and aldosterone?
1. In excess supply of water, ADH is
b) Increased
c) Remains the same
d) Variable
2. Aldosterone is secreted by
a) Adrenal medulla
b) Adrenal cortex
1. What is a counter-current multiplier?
3. How concentrated urine is produced?
4. How diluted urine is produced?
Long Question:
1. Write a note on the osmoregulatory
function of kidneys.
c) Renal medulla
d) Renal cortex
3. Inner medulla is highly concentrated
due to the presence of
a) Sodium ions
b) Urea
c) Water
d) Potassium
MCQ Key
1
A
2
B
3
B
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