ANATOMY AND PHYSIOLOGY OF NEPHRONS - ENGLISH
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ANATOMY AND PHYSIOLOYG OF NEPHRONS -
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.
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.
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.
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
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.
• 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.
RENAL TUBULE-
• The
remaining part of nephron is renal tubule which consists of three parts-
•
(1) 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
•
(2) loop of Henle (nephron loop)- 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.
•
(3) 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.
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: calices). 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.
PHYSIOLOGY OF NEPHRONS-
• There
are three processes involved in the formation of urine:
• 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.
• 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.
• 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.
• 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
• 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.
• 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.
• 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.
• 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
• 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.
RENIN ANGIOTENIN 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.
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.
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