URINARY SYSTEM
PREPARED
BY MR. ABHIJIT DAS
INTRODUCTION
Ammonia, urea, uric acid, carbon dioxide, water and
ions are accumulated inside our body either by metabolic activities or by
excess ingestion. These substances have to be removed totally or partially from
our body.
Ammonia, urea and uric acid are the major forms of
nitrogenous wastes excreted by animals.
Humans along with many amphibians excrete urea and
are called ureotelic animals. The process of
elimination of urea is known as ureotelism.
The urinary system is the main excretory system in
humans. The urinary system plays a vital role in maintaining homeostasis of
water and electrolytes within the body.
The kidneys produce urine, which contains metabolic waste products, including the nitrogenous compounds such as urea and ammonia, excess ions and sometimes excreted drugs.
HUMAN URINARY SYSTEM
The urinary system is the main excretory system in
humans and consists of;
Ø Two
kidneys, which secrete urine
Ø Two
ureters that pass the urine from the kidneys to the urinary bladder
Ø One
urinary bladder, which collects and stores urine
Ø One urethra, through which urine leaves the body
STRUCTURE OF KIDNEY
Kidneys are bean-shaped
organs which are reddish brown in colour.
Each kidney of an adult human measures 10-12cm in length,
5-7cm in width and 2-3cm in thickness with an average weight of 150g.
The kidneys lie on the dorsal side of the abdominal
wall, one on each side of the vertebral column.
They extend from the level of 12th
thoracic vertebra to the 3rd lumbar vertebra.
There is a notch (depression) towards the centre of
the inner concave surface of the kidney called hilum
through which urethra and blood vessels enter.
The outer layer of the kidney is a tough capsule
made up of fats.
INTERNAL STRUCTURE OF KIDNEY
Inside the kidney, there are two zones, an outer cortex and inner medulla.
The medulla is divided into few conical masses
called medullary pyramids.
The cortex extends in between the pyramids called
renal columns (or column of Bertini).
Urine formed within the kidney passes into the
drainage system that begins at a minor calyx (plural:
calyces). Several minor calyces merge into a major
calyx, and two or three major calyces combine to form a renal pelvis. The renal pelvis is a hollow funnel
shaped structure which leaves the kidney as ureter.
FUNCTIONS OF KIDNEY
Ø Excretion
of waste products from blood.
Ø Formation
of urine, maintaining water, electrolytes and acid-base balance.
Ø Secretion
of Erythropoietin which stimulates formation of red blood cells.
Ø Secretion
of Renin which controls blood pressure.
THE NEPHRON
The structural and functional unit of kidney is
known as nephrons.
One kidney contains around 1
million nephrons.
The nephron is a tube of epithelial cells that is
closed at one end and opens into a collecting duct at the other end.
Each nephron has two parts; the glomerulus and the renal
tubule.
Glomerulus is a bunch of capillaries formed by the
afferent arteriole. Blood from the glomerulus is carried away by an efferent
arteriole.
The renal tubule begins with a double walled cup
like structure called glomerular capsule (or Bowman’s
capsule), which encloses the glomerulus.
Glomerulus along with the Bowman’s capsule is known
as the Malpighian body or renal capsule.
The Bowman’s capsule continues further to form a
highly coiled network called proximal convoluted
tubule (PCT). PCT is lined by simple cuboidal brush border epithelium
which increases the surface area for reabsorption.
Henle’s loop (or loop of Henle)
is the next part of the tubule which has a descending and an ascending limb.
The ascending limb continues as another highly
coiled tubular region called distal convoluted
tubule (DCT).
The DCT of many nephrons open into a straight tube
called collecting duct. The collecting ducts
unite, forming larger ducts that empty into the minor calyx.
TYPES OF NEPHRON
The malpighian body, PCT and DCT of the nephron are
situated in the cortical region of the kidney whereas the loop of Henle dips
into the medulla.
Nephrons are of two types; cortical
and juxta medullary.
Loop of Henle of cortical nephrons are very short
and extend very little into the medulla. Loop of Henle of juxta medullary
nephrons are very long and go deep into the medulla.
BLOOD VESSELS AROUND NEPHRON
The efferent arteriole exiting the glomerulus forms
a fine capillary network around the renal tubule called the peritubular capillaries.
Another capillary which covers the loop of Henle for
oxygen supply is known as vasa recta. Vasa
recta is absent or highly reduced in cortical nephron.
FORMATION OF URINE
There are three processes involved in the formation
of urine; glomerular filtration, tubular reabsorption and tubular secretion.
GLOMERULAR FILTRATION
The first step in urine formation is the filtration
of blood by glomerulus which is called glomerular filtration.
Kidneys filter 1100-1200ml of blood per minute.
The filtration membrane has three layers; the
endothelium of glomerular blood vessels, the epithelium of Bowman’s capsule and
a basement membrane between these two layers.
Almost all the constituents of the blood except the
proteins are filtered and passed onto the lumen of the Bowman’s capsule.
The rate at which glomerular filtrate is being
formed is called glomerular filtration rate (GFR).
GFR in a healthy individual is approximately 125ml/minute i.e. 180litres/day. Nearly all of the
filtrate is later reabsorbed from the kidney tubule to the blood vessels. So,
only 1-1.5 litres of filtrate are excreted
as urine.
The fluid filtered from the blood stream into the
glomerular capsule is now called filtrate and it’s composition will be adjusted
as it passes through the other parts of the renal tubule.
Constituents of glomerular filtrates;
Ø Water
Ø Mineral
salts
Ø Glucose
Ø Urea
Ø Creatinine
Ø Some
drugs
AUTOREGULATION OF GFR
CASE I
A fall in blood pressure,
which decreases the GFR, can activate the JG cells (juxta glomerular cells) of
the afferent arteriole to release renin.
Renin converts angiotensinogen in blood to
angiotensin I. Angiotensin I is converted to angiotensin II by ACE (angiotensin
converting enzyme).
ACTIONS OF ANGIOTENSIN II
1. Angiotensin
II being a powerful vasoconstrictor (which
constricts blood vessels) increases the glomerular blood pressure and thereby
GFR.
2. Angiotensin
II stimulates posterior pituitary gland to release vasopressin
which is an anti-diuretic hormone.
That means vasopressin facilitates water reabsorption from renal tubule by
forming some special proteins called aquaporins.
So an increase body fluid volume
increases the blood pressure and thereby GFR.
3. Angiotensin
II also activates the adrenal cortex to release Aldosterone.
Aldosterone causes reabsorption of Na+ and water from the DCT by
forming more Na+ channels. This also leads to an increase in blood
pressure and GFR.
The above complex
mechanism is generally known as Renin-Angiotensin-Aldosterone-System
(RAAS).
CASE II
When blood pressure rises, an increase in blood flow
to the atria of the heart occurs.
This causes the release of Atrial Natriuretic Factor
(ANF), which can causes vasodilation (dilation of blood vessels) and thereby
decreases the blood pressure and GFR. ANF also causes diuresis (excessive
production of urine).
TUBULAR REABSORPTION
In this stage the active transport of solutes and
passive movement of water from the tubular lumen into peritubular capillaries
occur.
So, reabsorption is the removal of substances which
has nutritive value such as glucose, amino acids, electrolytes (Na+,
K+, Cl-, HCo3-) and vitamins from
the glomerular filtrate.
TUBULAR SECRETION
It refers to the transport of substances from the
peritubular capillaries into the tubular lumen. That means, it is the addition
of substances to the glomerular filtrate.
Example of such substances: H+, NH3, K+, some drugs (Penicillin, Aspirin etc)
COUNTER CURRENT MECHANISM
Counter: opposite
Current: flow
The flow of filtrate in the two limbs of Henle’s
loop is in opposite directions and thus forms a counter current.
The flow of blood through the two limbs of vasa
recta is also a counter current.
SIGNIFICANCE OF COUNTER CURRENT
MECHANISM
The closeness between the loop of Henle and vasa
recta, as well as the counter current in them help in maintaining of an
increasing osmolarity (the concentration of filtrate expressed as the total
number of ions per litre) towards the inner medullary interstitium i.e. from
300mOsmol/L in the cortex to about 1200mOsmol/L in the inner medulla.
The concentration gradient is mainly caused by NaCl
and urea.
NaCl is transported by the ascending limb of loop of
Henle which is exchanged with the descending limb of vasa recta and NaCl is
returned to the interstitium of the medulla by the ascending portion of vasa
recta.
Similarly, small amount of urea enter the thin
segment of the ascending limb of loop of Henle which is transported back to the
interstitium of the medulla by the collecting tubule.
The above described mechanism is called the counter
current mechanism.
This mechanism helps to maintain a concentration
gradient in the interstitium of the medulla.
MICTURITION
The act of passing urine from body is known as
micturition.
Urine formed inside the nephron is ultimately
carried to the urinary bladder. Then urine is stored inside the bladder until a
signal is given by the central nervous system.
This signal is initiated by the stretching of the
urinary bladder as it gets filled with urine.
So the stretch receptors
present on the walls of the bladder send signals to the CNS.
Then the CNS sends motor messages to initiate the
contraction of smooth muscles of the bladder and simultaneous relaxation of the
urethral sphincter causing the release of urine.
SOME IMPORTANT TERMS
RENAL FAILURE
Renal failure is a condition when kidneys are not
able to remove urea or other wastes effectively from blood.
KIDNEY TRANSPLANT
Transplantation of a foreign kidney in the body is
known as kidney transplant.
In this case kidney is generally donated by
siblings, parents or close relatives to avoid graft rejection.
GRAFT REJECTION
Rejection of foreign organs by body’s immune system.
DIALYSIS
Removal of urea and other waste materials
artificially without the help of kidney is known as dialysis. It is done in
case of renal failure.
HEMATURIA
The presence of blood in a person’s urine is called
hematuria.
GLOMERULONEPHRITIS
Inflammation of the glomerulus is known as
glomerulonephritis.
KIDNEY STONES/ RENAL CALCULI
Kidney stones or renal calculi are hard deposits
made of minerals and crystallised salts formed within the kidney.