The renal system functions to filter waste from the blood and regulate fluid balance. It contains two kidneys each composed of an outer cortex and inner medulla. The functional unit is the nephron, which filters blood in the glomerulus and reabsorbs nutrients in the proximal tubule. As fluid moves through the loop of Henlé and collecting duct, the countercurrent multiplier mechanism and ADH hormone regulate water reabsorption to produce a concentrated urine.

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3. Filtration
of the
blood
Reabsorption
Excretion
Homeostasis

4. left kidney
adrenal gland
ureters
The urethra carries urine
away from the bladder
renal artery
renal vein
vena cava aorta
bladder
The Renal
System

5. The Mammalian Kidney
The human kidneys are two bean-shaped
organs, located against the dorsal body
wall on either side of the backbone
The principal functions of the kidneys are:
• Excretion – the removal of toxic nitrogenous
waste products from the blood; these include
urea and creatine (a waste product of muscle
metabolism)
• Homeostasis – the kidneys regulate the water
content, ion composition and pH of the body
fluids; their role in the regulation of water
content is described as osmoregulation

6. Kidney Section
Pelvis
Pelvis
Cortex
Medulla
The kidney is composed of three distinct regions

7. Kidney Section
The outer cortex
Kidney tubule
(nephron) where
blood is filtered
and urine is
formed – about
one million in
each kidney
The medulla
consisting of
many cone-
shaped structures
called pyramids
Pyramids
The renal pelvis – a
large chamber that
collects urine from
the kidney tubules
and funnels it into
the ureter
Ureter

9. The Kidney Tubule (Nephron)
The kidney tubule or nephron is the
functional unit of the kidney
Each kidney tubule receives its own small supply of
blood from the extensive arteriole branches arising
from the renal artery; each human kidney contains
approximately 1 million tubules
Each kidney tubule is responsible for urine formation
and this involves three basic processes:
• Ultrafiltration of blood
• Selective tubular reabsorption of filtered
materials
• Tubular secretion; the secretion of
substances from the blood capillaries into
the kidney tubules

10. The Kidney
Tubule;
Nephron
Bowman’s
(renal) capsule
afferent
arteriole
efferent
arteriole
glomerulus
branch from
renal artery
proximal
convoluted
tubule
distal
convoluted
tubule
branch to
renal vein
loop of
Henlé
collecting
duct
peritubular
capillaries
There are approximately one million
kidney tubules in each human kidney

11. The Kidney
Tubule;
Nephron
Ultrafiltration of the blood
takes place at the glomerulus
The glomerular
filtrate flows into
the proximal
convoluted tubule
where useful
substances are
reabsorbed back
into the blood
The resulting fluid
flows into the loop of
Henlé where a
countercurrent
exchange mechanism
takes place
Water
reabsorption
occurs from the
distal convoluted
tubule and
collecting duct,
allowing for the
production of a
concentrated
urine
The walls of the distal tubules and collecting ducts
are variably permeable to water, depending upon the
presence or absence of antidiuretic hormone (ADH)

13. Urine formation begins with the filtration of
blood at the glomeruli of the renal corpuscles

14. Glomerular Filtration
The first step in the formation of urine is
glomerular filtration
The afferent arteriole, which provides the blood
supply to the glomerulus, branches to form a
network of capillaries which reunite to form the
outgoing or efferent arteriole
Ultrafiltration of the blood takes place at the
glomerulus producing a fluid that enters the cavity
of Bowman’s (renal) capsule; this filtered fluid is
called the glomerular filtrate
The driving force behind the filtering process is the
high blood pressure that builds up within the
glomerular capillaries

15. Blood pressure within the
glomerular capillaries
forces molecules through
the capillary/capsular
membranes
The capillary/capsular
membrane has three layers;
the capillary endothelium,
the basement membrane,
and the inner visceral layer
of Bowman’s capsule
(composed of podocyte cells)

16. The high blood pressure
(hydrostatic pressure) in
the glomerular capillaries
forces small molecules out
of the blood into the cavity
of Bowman’s capsule
The fluid that flows from the cavity of Bowman’s capsule into the
proximal convoluted tubule is isotonic to the blood plasma

18. Selective Reabsorption
The glomerular filtrate flows into the
proximal convoluted tubule, and at this
stage the fluid is isotonic with
respect to blood plasma
The principal role of the cells that line the
proximal tubule is the reabsorption of useful
materials back into the bloodstream
The epithelial cells lining the tubule bear
microvilli at their surface and possess numerous
mitochondria; these features facilitate the
reabsorption of water and solutes

19. lumen
epithelial
cells
microvilli
intercellular
spaces
many
mitochondria
interstitial
fluid

20. Fluid in the proximal convoluted tubules of the renal tubules contains
filtered molecules that are not intended for complete excretion
Glucose is the body’s main fuel; in healthy individuals, glucose is
completely reabsorbed from the proximal tubules back into the
surrounding blood capillaries
Salt, in the form of sodium and chloride ions, together with
amino acids, are partly reabsorbed into the blood from
the proximal tubule cells
Materials are reabsorbed by diffusion, facilitated diffusion and
active transport
The reabsorption of materials by the epithelial cells increases
their cytoplasmic concentration such that
water follows by osmosis
85% of the filtered fluid is reabsorbed from the proximal
convoluted tubules back into the blood stream

21. Facilitated
diffusion
Active
transport
Diffusion
Sodium ions in the
lumen of
the proximal
convoluted tubule

22. Reabsorption of Sodium Ions
• Sodium ions arc actively transported out of the cells lining the
proximal convoluted tubule into blood capillaries which carry them
away.
• The sodium ion concentration of these cells is therefore lowered.
• Sodium ions now diffuse down a concentration gradient from the
lumen of the proximal convoluted tubule into the epithelial lining
cells but only through special carrier proteins by facilitated
diffusion.
• These carrier proteins are of specific types, each of which carries
another molecule (glucose or amino acids or chloride ions, etc.)
along with the sodium ions. This is known as co-transport
• The molecules which have been co-transported into the cells of the
proximal convoluted tubule then diffuse into the blood. As a result,
all the glucose and most other valuable molecules are reabsorbed as
well as water.

23. Facilitated
diffusion
Facilitated
diffusion
Active
transport
and
diffusion
Active
transport
Diffusion
Glucose
molecules in the
lumen of
the proximal
convoluted tubule
Intercellular
space

24. The transport of glucose, amino acids and ions into the
epithelial cells creates a gradient of water potential across
the lining of the tubule; water is therefore withdrawn from
the tubule by osmosis, such that approximately 85%
of the filtered fluid is reabsorbed back into the blood

26. Countercurrent Multiplier
Mechanism
The ‘U’-shaped loop of the kidney tubule is the
loop of Henlé; it consists of a descending limb, a
hairpin bend and an ascending limb that runs in
the opposite direction (counter) to the
descending limb
The purpose of the loops of Henlé is to create a
gradient of hypertonicity (increasing salt
concentration) within the surrounding tissue of the
medulla that becomes increasingly concentrated on
moving from the cortex end towards the pelvis (i.e.
down the area surrounding the loop)

27. Countercurrent Multiplier
Mechanism
The collecting ducts of the kidney tubules pass
through the concentrated medulla and their walls
are variably permeable to water
The permeability of the collecting duct walls is
under hormonal control (antidiuretic hormone -
ADH); a high level of ADH in the blood renders the
collecting duct walls permeable to water
In the presence of ADH, water is reabsorbed from
the urine as it passes down the collecting ducts and
then enters the blood vessels within the medulla;
water is conserved

29. In response to the active
transport of ions from the
ascending limb, the
descending limb loses water
by osmosis and becomes
more concentrated; water
loss continues until the
concentrations of the
descending limb and the
interstitial tissue are equal
This process continues as
fluid moves along
the loop such that the
descending limb and
medullary interstitial tissue
become
increasingly concentrated
towards the tip