1. UNIT IX- Excretory System
Prepared by,
Mr. Vipin Chandran

2. INTRODUCTION:
As a result of activities of cells, lot of metabolic
wastes, i.e. unwanted by-products are formed. The
excess of these substances must be removed from
the body so as to maintain a proper homeostasis.
Excretion is thus defined as a process by which
metabolic wastes are removed from the body and an
osmotic balance is maintained.
The major waste products include: Nitrogenous
wastes like ammonia, urea, oxalate, creatinine &
uric acid.

3. INTRODUCTION:
The urinary system is one of the
excretory systems of the body. It
consists of the following structures
1) Two kidneys
2) Two ureters
3) One urinary bladder
4) One urethra
Nephrology is the scientific study of
anatomy, physiology & pathology of
the kidneys.
Branch of medicine that deals with
male & female urinary systems & male
reproductive system is called Urology.

4. Functions of Urinary system:
1) Kidneys regulate blood volume & composition;
help regulate blood pressure, pH, glucose level,
produce two hormone (calcitriol & erythropoietin)
& excrete waste in urine.
2) Ureter transport urine from kidney to urinary
bladder.
3) Urinary bladder stores urine and expels it into
urethra.
4) Urethra discharge urine from body.

5. KIDNEYS:
Gross Structure:
1) 2 kidney, each weight is 150 g in adults, located in
retroperitoneally in the lumbar region of
abdominal cavity on either side of vertebral
column.
2) Kidney is bean-shaped organs, approx. 10cm
long, 5cm wide & 2.5cm thick. The right kidney
is slightly lower than the left because of liver.
3) Vertical section of kidney shows:
Outer cortex- reddish in colour

6. KIDNEYS:
Gross Structure:
Inner medulla- pale in colour, it contains 10-15
renal pyramids which terminate medially in renal
papillae. Papillae projects into calyces. Such 10-15
minor calyces join to form two major calyces which
come out through pelvis of kidney to ureter.
4) The ureter exit from the hilus of the kidney &
pass to the bladder. The blood vessels, lymphatics
& nerves enter into or exit from the kidney via
hilus.

7. Structure of Kidney:

8. KIDNEYS:
Microscopic Structure:
The basic functional unit of kidney is Nephron. There
are approximately 1 to 1.3 million nephrons in each
kidney which drain into renal pelvis. Total length of
nephron ranges from 45 to 65 mm.
Partsof nephronare:
1) Bowman’s capsule
2) Glomerulus
3) Proximal convoluted tubule (PCT)
4) Loop of Henle
5) Distal convoluted tubule (DCT)
6) Collecting tubules.

9. KIDNEYS:
Microscopic Structure:
Nephron consist of 2 parts:
a) Renal corpuscle, where blood plasma is filtered.
b) Renal tubule, where filtered fluid passes.
1) Bowman’s capsule is the initial dilated part of the
nephron.
2) Glomerulus: It is formed by the invagination of a
tuft of capillaries into the Bowman’s capsule. The
capillaries are supplied by afferent arteriole & the
blood leaves from the tuft by efferent arteriole.
Bowman’s capsule & glomerulus together constitute
Malpighian corpuscle.

10. Renal Corpuscle consist of Bowman’s Capsule
& Glomerulus:

11. KIDNEYS:
Microscopic Structure:
Bowman’s capsule has two layers:
Visceral cell layer is very closely applied to the loop
capillaries so as to surround each loop on all sides. It is
continuous at the site of entrance of afferent & efferent
arterioles with parietal layer.
Parietal cell layer is applied to Bowman’s capsule &
forms the outer lining of the glomerulus. It is
continuous with the PCT.
A space is present between visceral and parietal layers
of the bowman’s capsule called Bowman’s space.

12. KIDNEYS:
Microscopic Structure:
Glomerular membrane: Glomerular
membrane is an extremely thin
membrane, each glomerulus contains six
lobules & each of these consists 3 to 6
capillary loops.
Arrangement of afferent & efferent
arterioles within glomerulus allows
maintenance of higher pressure (about 45
mmHg).
The major function of glomerular
membrane is to produce an ultrafiltrate,
i.e. the glomerular filtrate will contain all
the constituents of plasma except proteins.

13. KIDNEYS:
Microscopic Structure:
3) Proximal Convoluted Tubule: PCT lumen is
continuous with that of Bowman’s capsule. It consists
of singular layer of cells with curved outline & brush
border formed by numerous microvilli which
markedly increase the surface area for absorption.
4) Loop of Henle: It consist of descending limb which
arises in continuity with the terminal part of PCT.
Descending limb continues into the thick ascending
limb which is formed by low cuboidal epithelium.
5) Distal Convoluted Tubule: Thick ascending limb of
Henle is continued as DCT, this tubule come very
close to its own glomerulus & establishes close
proximity to the afferent & efferent arterioles of the
glomerulus.

14. KIDNEYS:
Microscopic Structure:
6) Collecting Tubules: DCT join to form collecting
tubules. It passes through the renal cortex & medulla
to empty into pelvis of the kidney.
The whole kidney is enveloped by a thin but tough
fibrous membrane called renal capsule. It limits the
swelling, if the kidney becomes oedematous.

16. BLOOD SUPPLY OF KIDNEYS:

17. FUNCTIONS OF KIDNEY:
The kidney perform the following main
functions:
a) Formation of urine
b) Regulation of water balance
c) Regulation of electrolyte balance
d) Maintenance of acid-base balance
e) Endocrine functions.

18. A. Formation of Urine:
As blood passes through the kidneys, the nephrons clear
the plasma of some substances e.g. urea, while
simultaneously retaining other essential substances such
as water. There are three linked mechanisms in the
formation of urine:
1) Substance to be excreted are removed by glomerular
filtrations & renal tubular secretion and passed into
the urine.
2) Substances that the body needs, e.g. Na+, HCO3- are
returned to the body by reabsorption processes.

19.  Glomerular Filtration:
It is the initial step in urine formation. The plasma that
traverses the glomerular capillaries is filtered by highly
permeable glomerular membrane & the resultant fluid,
the glomerular filtrate, is passed into bowman’s
capsule.
Glomerular filtrate is called ultrafiltrate of plasma. It
practically contains no protein & no cells.
Glomerular Filtration Rate: It refers to the volume of
glomerular filtrate formed each minute by all nephron
in both the kidneys.
Normal Value: 125 mL/ minute= 170-180 L per day.

20.  Glomerular Filtration:
Mechanism of Glomerular Filtration: For each
nephron, Effective (Net) filtration pressure is given as:
EFP = (Pcap- Pbow) - (COPcap-COPbow)
Where,
Pcap: Hydrostatic pressure in glomerular capillaries
= 45 mmHG
Pbow: Hydrostatic pressure in the Bowman’s capsule
= 10 mmHG
COPcap: Colloidal osmotic pressure of plasma in
glomerular capillaries= 25 mmHG.
COPbow: Colloidal osmotic pressure of filtrate in
bowman’s capsule. Normally it is zero.

21.  Reabsorption & Secretion in
Renal Tubules:
The terms renal tubular secretion & renal tubular
reabsorption refer to the ‘direction of transport’. It is
the process by which composition & volume of
glomerular filtrate is changed during its passage
through the tubules.
i. Secretion refers to the transport of solutes from
peritubular capillaries into tubular lumen, i.e. it is the
addition of a substance to the filtrate.
ii. Reabsorption denotes the active transport of solutes
& the passive movement of water from tubular lumen
into peritubular capillaries, i.e. it is the removal of a
substance from the filtrate.

22.  Reabsorption & Secretion in
Renal Tubules:
Renal tubular transport maximum (Tm): The renal
tubular maximum refers to the maximum amount of a
given solute that can be transported (reabsorbed or
secreted) per minute by the renal tubules.
Transport of individual substances in different
segments of the renal tubule:
i. The PCT reabsorbs 70% to 85% of the filtered Na+;
Cl-; HCO3
- & water; & almost 100% of the filtered
K+; HPO4
2- ; amino acids & glucose.
ii. Reabsorption of water is passive, while reabsorption
of solutes can be passive or active. Solute
reabsorption generates an osmotic gradient which
causes passive reabsorption of water (via osmosis).

23.  Composition of Urine:
1) Colour: Pale yellow due to presence of pigments
(Urochrome & Urobilin).
2) Salts: Inorganic (Na+, K+, Ca2+ & phosphate),
Organic (Urea, uric acid & creatinine)
3) Volume: 1-2.5 L/day (average 1.5 L/day)
4) Specific gravity: 1.005-1.030
5) Reaction: pH ranges from 4.5 to 8 (average 6-6.5)
6) Microscopic examination (WBC, pus cells, hyaline
casts).

24. B. Regulation of Water Balance:
Since GFR is 125 mL/ minute, i.e. about 180 L/day of filtrate
is formed, whereas normal urine volume is 1- 1.5 L/day.
Thus, more than 99% of the filtrate (water) is normally
reabsorbed.
1) Reabsorption from PCT: Passive reabsorption of 75- 80%
of water occurs.
2) Reabsorption from Loop of Henle: In passing through the
Loop of Henle, another 5-10% of the filtered water is
removed.
3) Reabsorption in DCT & Collecting duct:
In the early part of DCT approx. 5-8% of filtered water is
removed passively, secondary to sodium reabsorption under
the influence of aldosterone.
In the terminal DCT & Collecting duct, ADH hormone
facilitates another 10-12% of water reabsorbed.

25. B. Regulation of Water Balance:
When ADH is present in high concentration, kidney
excrete small volume of concentrated urine, but volume
of urine cannot be less than 500 mL/ day because some
solute require water for their excretion called obligatory
volume of urine. This is the minimum urine volume
required to excrete the solutes, otherwise it will result
in accumulation of waste products in the body.

26. C. Regulation of Electrolyte Balance:
1) Kidney forms urine which varies widely in its solute
concentration according to the need of the body, e.g.
in overhydration, kidney can produce urine of 50
mosm/L (normal plasma osmol conc. Is 300 mosm/L)
& during dehydration kidneys can produce urine of
1200 mosm/L.
2) Kidney regulate the concentration of water &
electrolytes in blood. Thus it keeps the blood pressure
under control.

27.  Renin-Angiotensin-Aldosterone System (RAAS):
1) When blood volume & blood pressure decreases,
walls of afferent arterioles are stretched less &
juxtaglomerular cells secrete enzyme renin into the
blood.
2) Renin clip off a 10- amino acid peptide called
angiotensin I from angiotensinogen, which is
synthesized by hepatocytes.
3) By clipping off two more amino acids, ACE converts
angiotensin I to angiotensin II, which is the active
form of the hormone.
Angiotensin II affects renal physiology in 3 ways:

28. 1) It decreases GFR causing
vasoconstriction of afferent
arterioles.
2) It enhances reabsorption of
Na+, Cl- & water in the PCT.
3) It stimulates the adrenal cortex
to release aldosterone, it
stimulates cells in collecting
duct to reabsorb more Na+, Cl,
which causes increase in blood
volume & blood pressure.
 Renin-Angiotensin-Aldosterone System (RAAS):

29. D. Regulation of Acid- Base Balance:
1) Kidneys are responsible for clearing the body
metabolically produced H+.
2) For all electrical neutrality of body fluids excretion of
excess anion in urine is very important, this situation
is prevented due to manufacture of two important
cations, H+ & NH4+.
3) Major sites of urine acidification are DCT &
Collecting tubules.
4) Three important reactions (buffer systems) in tubular
fluid that remove free H+:
Bicarbonate system
Dibasic phosphate system
Ammonia system

30. E. Endocrine Function:
Kidneys are endocrine organs which secrete:
1) Renin: Renin is a major component of the renin-
angiotensin-aldosterone mechanism & is useful to
regulate blood pressure.
2) Renal erythropoietic factor (REF, Erythropoietin) it
increases the number of circulating erythrocytes.
3) 1,25 DHCC (Dihydrocholecalciferol).

31. URETERS:
Urinary passages include renal calyces & pelvis,
ureter, urinary bladder & urethra, the urine
produced by the nephrons continuously trickles
from the renal papillae into the calyces & the
papillae which extends through the hilum to
continue as the ureter.
1) Ureters are 25 to 30 cm long tubes with a
diameter of 3 mm, extend from hilum to
urinary bladder.
2) They are located retroperitoneally but at the
level of sacral bone, they pass obliquely
through the posterior wall of urinary bladder
to open up into its bottom.
3) Function: ureters convey urine from the
kidneys to the urinary bladder by peristaltic
contractions of the smooth muscular wall.

32. URINARY BLADDER:
1) Urinary bladder is mainly a smooth muscle
hollow vesicle & is composed of following:
i. The body comprises detrusor muscle.
ii. The trigone, a small triangular area near the
mouth of urinary bladder, through which both
ureters & urethra pass.
iii. Internal sphincter: The trigone muscle fibres
get interlaced around the opening of urethra
forming internal sphincter. Its main function
is to maintain tonic closure of the urethral
opening.
iv. External sphincter: It is a voluntary skeletal
muscle. Normally this sphincter remains
tonically contracted which prevents constant
dribbling of urine.

33. URINARY BLADDER:
v. Urinary bladder wall is made
of many layers, including:
Urothelium or transitional
epithelium
Lamina propria, its connective
tissue
Detrusor muscle, present
outside the lamina propria
Fatty connective tissue, it
present outside of bladder &
separates it from other organs.

34. URINARY BLADDER:
2) Physiological capacity of urinary bladder varies with age:
At birth – 20-50 ml
At 1 year – 200 ml
Adults – 600 ml
3) Urine stored in the bladder remains unchanged in the
chemical composition.
4) Urinary bladder is innervated by efferent & afferent nerves,
it indicate degree of distention & convey pain sensibility.
5) Micturition reflex: As the urinary bladder fills with urine,
the wall stretches, impulses are initiated by stretch
receptors in the bladder wall causing sensory signal
through parasympathetic & sympathetic nerve, this whole
constitute the micturition reflex.

35. URINARY BLADDER:
6) Mechanism of voluntary micturition:
i. In response to desire to micturate, urinary bladder can
be voluntarily emptied. Impulses from cerebral cortex
motor area pass down to sacral segment causing
stimulation of parasympathetic fibres. Then impulse
pass to cause contraction of body of urinary bladder
wall & relaxation of trigone & internal sphincter, this
finally result in emptying of bladder.
ii. First urge to pass urine is felt at urinary bladder
volume of approx. 150 ml.
iii. A marked sense of fullness or discomfort is felt at
about 400 ml, which normally results in initiation of
micturition reflex.

36. URINARY BLADDER:
iv. If inconvenient to micturate or desire to hold urine,
impulses from cerebral cortex relaxes the detrusor
muscle.
v. By constant practice urinary bladder can be trained
to accommodate very large volume of urine before
uncontrollable & unbearable rise of intra- vesicle
pressure occurs.