The urinary system works to maintain homeostasis by filtering blood and producing urine. The basic functional unit of the kidney is the nephron, which consists of a renal corpuscle and renal tubule. In the renal corpuscle, blood is filtered through the glomerulus and glomerular capsule. The filtered fluid then passes into the renal tubule, where it undergoes selective reabsorption and secretion as it passes through the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting ducts. Hormones influence the selective reabsorption of water and solutes as urine is formed and concentrated.

1. By – SURESH KUMAR ( Nursing Tutor )
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2. Urinary System.
• The urinary system contributes to homeostasis by altering
blood composition, pH, volume, and pressure; maintaining
blood osmolarity; excreting wastes and foreign substances;
and producing hormones.
• The urinary system consists of two kidneys, two ureters,
one urinary bladder, and one urethra. In today’s class we will
study about anatomy and physiology of Nephrons.

3. Nephron
• These are small structures and they
form the functional units of the
kidney. Each nephron consists of
two parts: a renal corpuscle where
blood plasma is filtered, and a renal
tubule into which the filtered fluid
passes.

4. Renal corpuscle
• The two components of a renal corpuscle
are the glomerulus (capillary network) and
the glomerular (Bowman’s) capsule, a
double-walled epithelial cup that surrounds
the glomerular capillaries. Blood plasma is
filtered in the glomerular capsule, and then
the filtered fluid passes into the renal
tubule.

5. Glomerulus
• the afferent arteriole, enters each glomerular
capsule and then subdivides into a cluster of
tiny arterial capillaries, forming the
glomerulus. The blood vessel leading away
from the glomerulus is the efferent arteriole. The
afferent arteriole has a larger diameter than the
efferent arteriole, which increases pressure
inside the glomerulus and drives filtration
across the glomerular capillary walls

6. Glomerular Capsule
• The glomerular (Bowman’s) capsule
consists of visceral and parietal layers. The
visceral layer consists of modified simple
squamous epithelial cells called podocytes.
The parietal layer of the glomerular
capsule consists of simple squamous
epithelium and forms the outer wall of the
capsule.

7. Glomerular Capsule
• Filtration of blood takes place in this
portion of the nephron. Fluid filtered
from the glomerular capillaries enters
the capsular (Bowman’s) space,
the space between the two layers of
the glomerular capsule.

8. Renal tubule
• The remaining part of nephron is
renal tubule which consists of three
parts-
• (1) proximal convoluted tubule,
• (2) loop of Henle (nephron loop)
• (3) distal convoluted tubule

9. proximal convoluted tubule
• From Bowman’s capsule, the filtrate drains into the proximal
convoluted tubule. The surface of the epithelial cells of this
segment of the nephron is covered with densely packed
microvilli. The microvilli increase the surface area of the cells,
thus facilitating their resorptive function. The infolded
membranes forming the microvilli are the site of numerous
sodium pumps. Resorption of salt, water and glucose from
the glomerular filtrate occurs in this section of the tubule

10. Loop of Henle
• The proximal convoluted tubule then bends into a loop called
the loop of Henle. The loop of Henle is the part of the
tubule that dips or ‘loops’ from the cortex into the medulla
(descending limb), and then returns to the cortex (ascending
limb). The loop of Henle is divided into the descending and
ascending loops. The ascending loop of Henle is much
thicker than the descending portion.

11. Loop of Henle
• The descending loop of Henle is relatively impermeable to
solute but permeable to water, the fulid in tube becomes
hypertoinc. The ascending loop of Henle and early distal
tubule are impermeable to water. However, sodium and
chloride ions are actively transported out of the tubule,
making the tubular fluid very hypotonic.

12. Distal convoluted tubule
• The thick ascending portion of the loop of Henle leads into
the Distal Convoluted Tubule . The Distal Convoluted
Tubule is lined with simple cuboidal cells. it plays a part in the
regulation of calcium ions by excreting excess calcium ions in
response to calcitonin hormone. The final concentration of
urine, in this section, is dependent on a hormone called
antidiuretic hormone (ADH). If ADH is present, the distal
tubule and the collecting duct become permeable to water.

13. Collecting ducts
• The DCT then drains into the collecting ducts. Several
collecting ducts converge and drain into a larger system called
the papillary ducts, which in turn empty into the minor calyx
(plural: calyces). From here the filtrate, now called urine,
drains into the renal pelvis. This is the final stage where
sodium and water are reabsorbed. When a person is
dehydrated, approximately 25% of the water filtered is
reabsorbed in the collecting duct.

14. Physiology of nephrons
• There are three processes involved in the formation of urine:
•Filtration
•Selective reabsorption
•Secretion.

15. Filtration
• This takes place through the semipermeable walls of the
glomerulus and glomerular capsule. Water and other small
molecules pass through, although some are reabsorbed later.
Blood cells, plasma proteins and other large molecules are too
large to filter through and therefore remain in the capillaries.
The filtrate in the glomerulus is very similar in composition
to plasma with the important exceptions of plasma proteins
and blood cells.

16. Filtration
• Filtration takes place because there is a difference between the blood
pressure in the glomerulus and the pressure of the filtrate in the
glomerular capsule. Because the efferent arteriole is narrower than the
afferent arteriole, a capillary hydrostatic pressure of about 55 mmHg
builds up in the glomerulus. This pressure is opposed by the osmotic
pressure of the blood, provided mainly by plasma proteins, about 30
mmHg, and by filtrate hydrostatic pressure of about 15 mmHg) in the
glomerular capsule. The net filtration pressure is, therefore 10 mmHg.
55-(30+15)= 10 mmHg.

17. Filtration
• 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. Nearly all of the filtrate is later
reabsorbed from the kidney tubules with less than 1%, i.e. 1
to 1.5 litres, excreted as urine. The differences in volume and
concentration are due to selective reabsorption of some
filtrate constituents and tubular secretion.

18. Selective
reabsorption
• Most reabsorption from the filtrate back into the blood takes
place in the proximal convoluted tubule, whose walls are lined
with microvilli to increase surface area for absorption.
Materials essential to the body are reabsorbed here, including
some water, electrolytes and organic nutrients such as glucose.
Some reabsorption is passive, but some substances are
transported actively

19. Selective
reabsorption
• Only 60–70% of filtrate reaches the loop of the nephron. Much of
this, especially water, sodium and chloride, is reabsorbed in the loop,
so only 15–20% of the original filtrate reaches the distal convoluted
tubule, and the composition of the filtrate is now very different from
its starting values. 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 therefore is to
reabsorb as much water as the body needs.

20. Selective
reabsorption
• Active transport takes place at carrier sites in the epithelial membrane,
using chemical energy to transport substances against their
concentration gradients . Some ions, e.g. sodium and chloride, can be
absorbed by both active and passive mechanisms depending on the site
in the nephron. Some constituents of glomerular filtrate (e.g. glucose,
amino acids) do not normally appear in urine because they are
completely reabsorbed unless blood levels are excessive. Reabsorption
of nitrogenous waste products, such as urea, uric acid and creatinine is
very limited.

21. Selective
reabsorption
• in some cases reabsorption is regulated by hormones. Hormones that influence
selective reabsorption are-
• Parathyroid hormone- This comes from the parathyroid glands and together
with calcitonin from the thyroid gland regulates the reabsorption of calcium and
phosphate from the distal collecting tubules.
• Antidiuretic hormone- Also known as ADH, this is secreted by the posterior
lobe of the pituitary gland and increases the permeability of the distal convoluted
tubules and collecting tubules, increasing water reabsorption.

22. Selective
reabsorption
• Aldosterone- Secreted by the adrenal cortex, this hormone increases
the reabsorption of sodium and water, and the excretion of potassium.
• Atrial natriuretic peptide- Also known as ANP, this hormone is
secreted by the atria of the heart in response to stretching of the atrial
wall. It decreases reabsorption of sodium and water from the proximal
convoluted tubules and collecting ducts

23. Secretion
• Filtration occurs as the blood flows through the glomerulus.
Substances not required and foreign materials, e.g. drugs including
penicillin and aspirin, may not be cleared from the blood by filtration
because of the short time it remains in the glomerulus. Such
substances are cleared by secretion from the peritubular capillaries into
the convoluted tubules and excreted from the body in the urine.
Tubular secretion of hydrogen ions (H+) is important in maintaining
normal blood pH.

24. Renin–angiotensin–
aldosterone system
Sodium is a normal constituent of urine and the amount excreted is
regulated by the hormone aldosterone, secreted by the adrenal cortex. Cells
in the afferent arteriole of the nephron release the enzyme renin in
response to sympathetic stimulation, low blood volume or by low arterial
blood pressure. Renin converts the plasma protein angiotensinogen,
produced by the liver, to angiotensin 1. Angiotensin converting enzyme
(ACE), formed in small quantities in the lungs, proximal convoluted
tubules and other tissues, converts angiotensin 1 into angiotensin 2,
which is a very potent vasoconstrictor and increases blood pressure.

25. Renin–angiotensin–
aldosterone system
• Renin and raised blood potassium levels also stimulate the
adrenal gland to secrete aldosterone. Water is reabsorbed with
sodium and together they increase the blood volume, leading
to reduced renin secretion through the negative feedback
mechanism. When sodium reabsorption is increased
potassium excretion is increased, indirectly reducing
intracellular potassium.

26. By – SURESH KUMAR ( Nursing Tutor )