Download Cortical nephrons.

Renal system
Dr. Zainab H.H
Dept. of Physiology
Lec.1,2
Objectives
• List the functions of kidney
• Describe the morphology of the typical
nephron and its blood supply.
• Describe the concept of clearance
What is excretion?
Throwing out of waste product is known as excretion.
The organs through which excretion occurs.
1. Kidneys: Excrete water and water soluble waste
products.
2. Lungs: Excrete carbon dioxide, water vapour and
other volatile substances such as acetone.
3. Skin: Excretes water and salts mainly in the form of
sweat.
4. Gastrointestinal tract: Excretes undigested food.
The Role of the Kidneys in the Body Includes:
• Regulation of the volume and composition of the ECF, by
maintaining a balance between intake and output of water
and electrolytes in the body.
• Excretion and elimination of waste products of metabolism,
such as the excretion of urea, creatinine and uric acids; as
well as the excretion of various toxins such as drugs and
food additives.
• The kidneys act as endocrine glands producing hormones,
such as “erythropoietin hormone” and renin.
• Playing a dominant role in the long-term and
short-term regulation of arterial blood
pressure.
• Kidneys along with the respiratory system
contribute to acid-base regulation.
• Finally, kidneys synthesize glucose from amino
acids and other precursors
Metabolic waste products excreted by
kidneys are:
•
•
•
•
Urea from protein.
Uric acid from nucleic acid.
Creatinine from muscle creatine.
End products of haemoglobin breakdown.
STRUCTURE OF NEPHRON
• Nephron is a structural and functional unit of the
kidney.
• Each nephron is capable of forming urine.
• There are two types of nephrons:
• 1. Cortical nephrons. Glomeruli are present near
the surface of the kidneys.
– These nephrons constitute about 86% of total
nephrons.
– The main function of cortical nephrons is absorption
of sodium.
• 2. Juxtamedullary nephrons. Glomeruli lie at
the junction of cortex and medulla of the
kidney.
– These constitute 14% of the nephrons.
– The main role of juxtamedullary nephron is to
increase concentration of medullary interstitial
fluid.
Two kidneys together have two millions nephrons.
Nephron consists of two major parts:
– Glomerulus.
– A long renal tubule.
1. Glomerulus. It is made up of tuft of capillaries
which connect afferent arteriole with an efferent
arteriole.
• Capillaries have single layer of endothelial cells
attached to a basement membrane.
• Bowman’s capsule encloses the glomerulus
and is formed of two layers:
– inner layer which covers the glomerular
capillaries is called visceral layer,
– outer layer is called parietal layer.
 Space between visceral and parietal layers is
continued as the lumen of the tubular portion.
– fluid-filled space, Bowman’s space, is formed
within the capsule.
– Blood of capillary and fluid of Bowman’s space
are separated by the glomerular membrane.
– From the Bowman’s capsule, tubule of the
nephron extends, the lumen of which is
continuous with the Bowman’s space.
• 2. Renal tubule. It is mainly formed of three
parts:
(a) Proximal convoluted tubule.
(b) Loop of Henle consisting of:
– Thin segment :walls of descending limb and lower
end of ascending limb are very thin. Therefore,
they are termed thin segment.
– Hair pin bend.
– Thick ascending limb or segment
• (c) Distal convoluted tubules.
• open into initial arched collecting ducts called
cortical collecting ducts present in renal
cortex.
– Seven to ten such ducts form straight collecting
duct which passes into medulla forming medullary
collecting ducts.
• In the inner zone of medulla they form
papillary ducts or ducts of Bellini.
– These open into papilla of minor calyces.
– Three or four minor calyces unite to form one
major calyx.
– The major calyces open into pelvis of ureter.
– The pelvis is an expanded portion present in renal
sinus and it continues as ureter
BLOOD FLOW TO KIDNEYS
• Rate of blood flow to kidneys is 1200 ml/min.
• This is quite high as compared to their size.
• State peculiarities of renal circulation.
1.Very high blood supply, about 21% of cardiac
output.
2.Two sets of capillaries.
– The glomerular capillaries.
• These combine to form efferent arteriole which in turn
breaks into peritubular capillary network around the
tubules of cortical nephrons.
– vasa recta which are loop-shaped vessels in
juxtamedullary nephrons
• the efferent arterioles continue as these loops dip into the
medullary pyramids alongside the loops of Henle
3.Glomerular capillary bed has a high hydrostatic
pressure because efferent arteriole is of a smaller
diameter than afferent arteriole which offers
considerable resistance to blood flow.
4. Peritubular capillary bed is a low pressure
bed.
5. Only 1 to 2% of blood flows through vasa
recta. The flow is very sluggish.
6. Renal blood flow shows remarkable constancy
in face of blood pressure changes due to
autoregulation
• The kidneys regulate the hydrostatic pressure
in both capillary beds (glomerular &
peitubular ) , by adjusting resistance of the
afferent and efferent arterioles.
• High hydrostatic pressure in GC (60 mmHg)
causes rapid fluid filtration; whereas a much
lower pressure in the peritubular capillaries
(13 mmHg) permits rapid fluid reabsorption.
• The afferent arteriole is a short, straight
branch of the interlobular artery.
• The efferent arteriole, that drains the GC, has
a relatively high resistance than the afferent
arteriole.
The Process of Urine Formation:
• Urine formation begins with the
• Filtration of plasma through the GC into the
Bowman’s space.
• As the filtered fluid flows through the remaining
portions of the tubule, its composition is altered as a
result of two main processes:
1.
Tubular reabsorption.
2.
Tubular secretion.
and both processes will produce the final product,
urine.
Concept of Clearance:
• The renal clearance of a substance is the
volume of plasma that is completely cleared
or cleaned of that substance by the kidney
per unit of time. (usually expressed as mL/
minute).
• However, renal clearance provides a useful
way of quantifying renal excretory functions.
• It can be used to quantify the rate at which
blood flows through the kidneys, as well as
to measure the basic kidney functions such
as GFR.
Concept of Clearance (continues):
• Renal clearance of a substance (S) is calculated by
dividing the urinary excretion rate of (S) (US × V•) by
its plasma concentration (PS), as expressed below:
US × V•
CS =
PS
• Where: US = urine concentration of S.
V• = urine flow rate/ minute = (0.9 mL/
minute).
PS = plasma concentration of S.
• Clearance of any substance depends on the
behavior of the tubular cell towards that substance:
• If a substance like inulin, which is freely filtered at
the glomerulus level and is neither reabsorbed nor
secreted by the renal tubule, then its clearance
equals to GFR.
• On the other hand, if a substance is reabsorbed by
the renal tubule, its clearance is lower than the
GFR.
• Finally, if a substance that is in addition to filtration
is secreted by the renal tubule, then its clearance is
higher than the GFR.
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