The urinary system consists of two kidneys, two ureters, a urinary bladder, and a urethra. The kidneys filter waste from the blood to form urine. The ureters then carry urine from the kidneys to the bladder. The bladder stores urine temporarily until urination. During urination, urine exits the body through the urethra. The basic functional unit of the kidney is the nephron, which filters blood to remove wastes and regulate fluid levels.

1. U N I T - 3
Urinary system

2. Urinary system
 The urinary system's function is to filter blood and create urine as a waste by-product.
 The urinary system is also known as Excretory and renal system .
 The urinary system consists of a two kidneys, two ureter, one urinary bladder and one urethra.
(i) Two Kidneys : formation of urine
(ii) Two Ureter –transports the urine
(iii) One Urinary bladder – stores urine temporarily
(iv) One Urethra – carries urine out side the body .
I. Kidneys –
 The kidneys are reddish brown color , bean-shaped
 organs about 11 cm long, 5 cm wide and 3 cm thick, each weight about 150 gm in an adult
male and about 135 gm in adult female.
 Located below the ribs toward the middle of the back.
 The right kidney is positioned slightly lower than the left kidney to accommodate the liver.
 The kidney is divided into two major structure: renal cortex and renal medulla

4. Cont..
 The hilum is a notch located near the centre of the kidney's inner concave surface. It is the point where the
ureter, blood vessels, as well as nerves, all enter.
 Adrenal gland. Atop each kidney is an adrenal gland, which is part of the endocrine system is a distinctly
separate organ functionally.
 Fibrous capsule. A transparent fibrous capsule encloses each kidney and gives a fresh kidney a glistening
appearance.
 Perirenal fat capsule. A fatty mass, the perirenal fat capsule, surrounds each kidney and acts to cushion it
against blows.
 Renal fascia. The renal fascia, the outermost capsule, anchors the kidney and helps hold it in place against
the muscles of the trunk wall.
 Renal cortex. The outer region, which is light in color, is the renal cortex.
 Renal medulla. Deep to the cortex is a darker, reddish-brown area, the renal medulla. Renal pyramids. The
medulla has many basically triangular regions with a striped appearance, the renal, or medullary pyramids; the
broader base of each pyramid faces toward the cortex while its tip, the apex, points toward the inner region of
the kidney.
 Renal columns. The pyramids are separated by extensions of cortex-like tissue, the renal columns.
 Renal pelvis. Medial to the hilum is a flat, basinlike cavity, the renal pelvis, which is continuous with the
ureter leaving the hilum.
 Calyces. Extensions of the pelvis, calyces, form cup-shaped areas that enclose the tips of the pyramid and
collect urine, which continuously drains from the tips of the pyramids into the renal pelvis.


5.  Renal artery. The arterial supply of each kidney is the renal artery, which divides
into segmental arteries as it approaches the hilum, and each segmental artery
gives off several branches called interlobar arteries.
 Arcuate arteries. At the cortex-medulla junction, interlobar arteries give off
arcuate arteries, which curve over the medullary pyramids.
 Cortical radiate arteries. Small cortical radiate arteries then branch off the
arcuate arteries and run outward to supply the cortical tissue.
1. Perirenal fat: This covers the fibrous capsule.
2. Renal fascia: This is a connective tissue that lies outside the perirenal fat and
encloses the kidneys and suprarenal glands.
3. Pararenal fat: This lies external to the renal fascia and is often in large
quantities.

8.  Renal Cortex: The outer region, which is light in color.
 Renal Medulla: It is a darker reddish-brown area, deep
to the cortex.
Function
 Remove waste products and drugs from the body.
 Balance the body's fluids.
 Release hormones to regulate blood pressure.
 Control production of red blood cell

9. Characteristics of Urine
 Daily volume. In 24 hours, only about 1.0 to 1.8 liters of urine are produced.
 Components. Urine contains nitrogenous wastes and unneeded substances.
 Color. Freshly voided urine is generally clear and pale to deep yellow.
 Odor. When formed, urine is sterile and slightly aromatic, but if allowed to
stand, it takes on an ammonia odor caused by the action of bacteria on the
urine solutes.
 pH. Urine pH is usually slightly acidic (around 6), but changes in body
metabolism and certain foods may cause it to be much more acidic or basic.
 Specific gravity. Whereas the specific gravity of pure water is 1.0, the specific
gravity of urine usually ranges from 1.001 to 1.035.
 Solutes. Solutes normally found in urine include sodium and potassium ions,
urea, uric acid, creatinine, ammonia, bicarbonate ions, and various other ions.


10. Ureters
 Size. The ureters are two slender tubes each 25 to 30 cm (10 to
12 inches) long and 6 mm (1/4 inch) in diameter.
 Location. Each ureter runs behind the peritoneum from the
renal hilum to the posterior aspect of the bladder, which it
enters at a slight angle.
 Function. Essentially, the ureters are passageways that carry
urine from the kidneys to the bladder through contraction of
the smooth muscle layers in their walls that propel urine into
the bladder by peristalsis and is prevented from flowing back
by small valve-like folds of bladder mucosa that flap over the
ureter openings.


11. Urinary Bladder
 Location. It is located retroperitoneally in the pelvis just
posterior to the symphysis pubis.
 Function. The detrusor muscles and the transitional
epithelium both make the bladder uniquely suited for its
function of urine storage.
 Trigone. The smooth triangular region of the bladder base
outlined by these three openings is called the trigone, where
infections tend to persist.
 Detrusor muscles. The bladder wall contains three layers of
smooth muscle, collectively called the detrusor muscle, and its
mucosa is a special type of epithelium, transitional
epithelium.

12. urethra
 The urethra is a thin-walled tube that carries urine by peristalsis from the
bladder to the outside of the body.
 Internal urethral sphincter. At the bladder-urethral junction, a thickening
of the smooth muscle forms the internal urethral sphincter,
an involuntary sphincter that keeps the urethra closed when the urine is
not being passed.
 External urethral sphincter. A second sphincter, the external urethral
sphincter, is fashioned by skeletal muscle as the urethra passes through the
pelvic floor and is voluntarily controlled.
 Female urethra. The female urethra is about 3 to 4 cm (1 1/2 inches)
long, and its external orifice, or opening, lies anteriorly to the vaginal
opening.
 Male urethra. In me, the urethra is approximately 20 cm (8 inches) long
and has three named regions: the prostatic, membranous, and spongy
(penile) urethrae; it opens at the tip of the penis after traveling down its
length.

13. Nephron
 Nephron is a basic microscopic structural and functional unit of the Kidney
 The structure that actually produces urine in the process of removing waste
and excess substances from the blood.
 There are about 1 million nephrons in each human kidney.
 A nephron is used separate to water, ions and small molecules from the blood,
filter out wastes and toxins, and return needed molecules to the blood.
 The renal corpuscle and renal tubule are two main components of the nephron
 The nephron of the kidney in mammals is a tube that is approximately 30-55
mm in length.
 The nephron has an inflated and closed tube. The end of this tube is folded to a
cuplike shape structure. The nephron is composed mainly of two structures—
the renal corpuscle and a renal tubule.

14. There are three parts to nephrons:
 1- Renal corpuscular consists of the glomerulus and bowman's capsule and has a role in
blood filtration.
 2- Renal tubules consist of distal and proximal convoluted tube and loop of Henle and
have a role in reabsorption of essential nutrients back to the blood.
 3- Collecting duct collects urine and passes it to the ureters to be expelled later through
the urethra.
Function :-
 A nephron is responsible for removing waste products, stray ions, and excess water
from the blood.
 The blood travels through the glomerulus, which is surrounded by the glomerular
capsule.
 As the heart pumps the blood, the pressure created pushes small molecules through the
capillaries and into the glomerular capsule.
 Filter and reabsorb water molecules and other important solutes present in the blood.

16. There are three stages involved in the process of urine
formation. They are
1. Glomerular filtration or ultra-filtration
2. Selective reabsorption
3. Tubular secretion
Physiology of urine formation

19.  Glomerulus. One of the main structures of a nephron, a glomerulus is a knot of
capillaries.
 Renal tubule. Another one of the main structures in a nephron is the renal tubule.
 Bowman’s capsule. The closed end of the renal tubule is enlarged and cup-shaped and
completely surrounds the glomerulus, and it is called the glomerular or Bowman’s
capsule.
 Podocytes. The inner layer of the capsule is made up of highly modified octopus-
like cells called podocytes.
 Collecting duct. As the tubule extends from the glomerular capsule, it coils and twists
before forming a hairpin loop and then again becomes coiled and twisted before
entering a collecting tubule called the collecting duct, which receives urine from many
nephrons.
 Proximal convoluted tubule. This is the part of the tubule that is near to the
glomerular capsule.
 Loop of Henle. The loop of Henle is the hairpin loop following the proximal
convoluted tubule

20. .
Distal convoluted tubule. After the loop of Henle, the tubule continues to coil and
twist before the collecting duct, and this part is called the distal convoluted tubule.
Cortical nephrons. Most nephrons are called cortical nephrons because they are
located almost entirely within the cortex.
Juxtamedullary nephrons. In a few cases, the nephrons are called juxtamedullary
nephrons because they are situated next to the cortex-medullary junction, and their
loops of Henle dip deep into the medulla.
The afferent arteriole carries blood into the glomerulus, whereas the efferent
arteriole transports blood out.
Peritubular capillaries. They arise from the efferent arteriole that drains the
glomerulus.

21. Glomerular filtration
 This takes place through the semipermeable walls of the glomerular capillaries
and Bowman’s capsule.
 The afferent arterioles supplying blood to glomerular capsule carries useful as
well as harmful substances.
 The useful substances are glucose, aminoacids, vitamins, hormones,
electrolytes, ions etc and the harmful substances are metabolic wastes such as
urea, uric acids, creatinine, ions, etc.
 Due to this difference in diameter of arteries, blood leaving the glomerulus
creates the pressure known as hydrostatic pressure
 The glomerular hydrostatic pressure forces the blood to leaves the glomerulus
resulting in filtration of blood.
 On average, the kidney filters 1100 mL to 1200 mL of blood per minute.
 A capillary hydrostatic pressure of about 7.3 kPa (55 mmHg) builds up in the
glomerulus

22. Cont..
 However this pressure is opposed by the osmotic pressure of the blood, provided
mainly by plasma proteins, about 4 kPa (30 mmHg), and by filtrate hydrostatic pressure
of about 2 kPa (15 mmHg in the glomerular capsule
 By the net filtration pressure of 10mmHg, blood is filtered in the glomerular capsule.
 Water and other small molecules readily pass through the filtration slits but Blood cells,
plasma proteins and other large molecules are too large to filter through and therefore
remain in the capillaries.
 The filtrate containing large amount of water, glucose, aminoacids, uric acid, urea,
electrolytes etc in the glomerular capsule is known as nephric filtrate of glomerular
filtrate.
 The volume of filtrate formed by both kidneys each minute is called the glomerular
filtration rate (GFR).
 In a healthy adult the GFR is about 125 mL/min, i.e. 180 litres of filtrate are
formed each day by the two kidneys

23. Selective reabsorption
 As the filtrate passes to the renal tubules, useful substances including some water,
electrolytes and organic nutrients such as glucose, aminoacids, vitamins hormones etc
are selectively reabsorbed from the filtrate back into the blood in the proximal
convoluted tubule.
 Reabsorption of some substance is passive, while some substances are actively
transported. Major portion of water is reabsorbed by Osmosis.
 Only 60–70% of filtrate reaches the Henle loop. Much of this, especially water, sodium
and chloride, is reabsorbed in the loop, so that only 15–20% of the original filtrate
reaches the distal convoluted tubule, More electrolytes are reabsorbed here, especially
sodium, so the filtrate entering the collecting ducts is actually quite dilute.
 The main function of the collecting ducts is to reabsorb as much water as the body
needs. Nutrients such as glucose, amino acids, and vitamins are reabsorbed by active
transport.
 Positive charged ions ions are also reabsorbed by active transport while negative
charged ions are reabsorbed most often by passive transport.
 Water is reabsorbed by osmosis, and small proteins are reabsorbed by pinocytosis.

24. Tubular secretion
 Tubular secretion takes place from the blood in the peritubular capillaries to
the filtrate in the renal tubules and can ensure that wastes such as creatinine or
excess H+ or excess K+ ions are actively
 secreted into the filtrate to be excreted. Excess K+ ion is secreted in the
tubules and in exchange Na+ ion is reabsorbed otherwise it causes a clinical
condition called Hyperkalemia.
 Tubular secretion of hydrogen ions (H+) is very important in maintaining
normal blood pH.
 Substances such as , e.g. drugs including penicillin and aspirin, may not be
entirely filtered out of the blood because of the short time it remains in the
glomerulus.
 The tubular filtrate is finally known as urine. Human urine is usually
hypertonic