Download Physiology د. نصير جواد المختار Lecture II: Function of Kidney

‫ نصير جواد المختار‬.‫د‬
Physiology
Lecture II: Function of Kidney, Functional Anatomy of the Kidney
Function of Kidney:
1. Excretion of metabolic waste products: the kidney eliminate waste
product of metabolism that are not needed by the body. These
products include urea (from the metabolism of amino acids),
creatinine (from muscle creatine), uric acid (from nucleic acids),
bilirubin (from hemoglobin breakdown) and metabolites of various
hormone the kidneys eliminate most toxins and other substances such
as drugs.
2. Regulation of water and electrolyte balance: The capacity of the
kidneys to alter sodium excretion in response to change in sodium
intake is 10 fold than normal. This also true for water and other
electrolytes such as potassium, calcium and hydrogen.
3. Regulation arterial pressure: By excreting variable amount of
sodium and water, and secretary vasoactive factors such as rennin,
that lead to formation of vasoactive products such as Angiotensin II.
4. Regulation acid-base balance: By excreting acid and by regulating
the body fluid buffer stores. Kidneys eliminate certain types of acid
such as sulfuric acid and phosphoric acid by metabolism of proteins.
5. Regulation of erythrocytes production: The kidneys secrete
erythropoietin which stimulates the production of red blood cells; one
important stimulus for erythropoietin secretion is hypoxia. The people
with severe kidney disease, severe anemia develops as result of
decreased erythropoietin production.
6. Regulation of 1, 25-Dihyroxy vitamin D3 production: the kidneys
produce the active form of D3 (1, 25-Dihydroxy vitamin D3)
calcitriol, which is essential for normal calcium deposition in bone
and calcium reabsorption by gastro-intestinal tract. It play role in
calcium and phosphate regulation.
7. Glucose synthesis: The kidneys synthesis glucose from amino acid
during prolonged fasting (gluconeogenesis).
Functional anatomy of the kidney:
General organization of the kidney and urinary tract:
Two kidneys lie on the posterior wall of the abdomen, outside the
peritoneal cavity. Each kidney of adult human weight about 150 grams.
Through hilum pass the renal artery, vein, lymphatic, nerve supply and
ureter that carry the urine from kidney to the bladder, where it is stored
1
‫ نصير جواد المختار‬.‫د‬
Physiology
urine until it is emptied by micturation. Kidney contain outer cortex and
inner medulla. The medulla is divided into multiple cone shaped masses
of tissue called renal pyramid, terminate into renal pelvis.
Renal blood flow:
Blood flow to the kidneys is normally about 22% of COP (1100
ml/minute). The renal artery enters kidney through the hilum from
interlobular arteries, arcuate arteries, interlobular arteries, afferent
arterioles which lead to glomerular capillaries where large amount of
fluid and solutes (except the plasma proteins) are filtered to formation of
urine. Distal end of capillaries of glomerulus coalesce to form efferent
arteriols then peritubular capillaries. Hydrostatic pressure in the
glomerular capillaries (60mmHG) causes rapid fluid filtration, where as a
much lower hydrostatic pressure in the peritubular capillaries (13 mmHg)
permit rapid fluid reabsorption. Peritubular capillaries empty into the
vessels of venous systems interlobular vein > arcuate vein > interlobular
vein > renal vein (Fig. 1).
Nerve supply:
The kidney has a rich adrenergic sympathetic nerve supply
distributed to the:
a. Vascular smooth muscle to cause vasoconstriction.
b. Juxtaglomerular cells to cause rennin sercretion.
c. Tubular cells to stimulate Na and water reabsorption.
Juxtaglomerular complex:
The juxtaglomerulus complex consists of macula densa cells in the
initial portion of the distal tubule and juxtaglomerular cells in the walls of
the afferent and efferent arterioles. The macula densa is a specialized
group of epithelial cells in the distal tubules that comes in close contact
with afferent and efferent arterioles. The macula densa cells contain
Golgi apparatus, which are intracellular secretary organelles, directed
toward the arterioles, suggesting that these cells may be secreting a
substance (rennin) toward the arterioles (Fig. 2).
Nephron:
It is basic functional unit of the kidney and capable of forming
urine. There are about million nephrons in each kidney in human body.
Kidneys cannot regenerate new nephron and their number decrease with
aging. Each nephron consist of:
2
‫ نصير جواد المختار‬.‫د‬
Physiology
1. Bowman’s capsule: It is the invaginated end of the tubule that
encased to glomerulus (branching capillaries). The pressure in the
glomerular capillaries is higher than that in other capillary beds. This
membrane is different from other capillary membrane by having three
layers instead of two. These three layers are endothelial layer, a
basement membrane and a layer of epithelial cells. Despite the
number of layers, the permeability of the glomerular membrane is
from 100-500 times as great as that the usual capillary due to
presence of thousands of small holes which are called fenestrate in the
endothelial cells by the presence of large spaces un the basement
membrane and by incontinuity of cells that form the epithelial layer.
2. Proximal convoluted tubule: They lie in the renal cortex along with
the glomerulus. The epithelial cells of them are highly metabolic
cells, with a large number of mitochondria to support rapid active
transport process. It contains a brush border due to the presence of
microvillus. Reabsorption in the proximal tubule us usotonic;
osmolarity of fluid in all parts of the proximal tubule is that of
plasma. Over all 65% of the filtered Na, and water, almost all the
filtered glucose, amino acids, organic acids, small amount of protein,
much of K, Ca, phosphate and urea are reabsorped in the proximate
tubule.
3. Loops of Henle: The epithelial cells of the thin descending segment
of the loop of Henle are very thin with no brush border and very few
mitochondria. They are highly permeable to water but much less
permeable to urea, sodium and most other ions. The epithelial cells of
ascending thin segment are less permeable to water but more
permeable to urea than descending portion. The epithelial cells of the
ascending thick segment are similar to those of proximal tubules
except that they have a rudimentary brush border and much tighter
tight junction. The cells adapted for strong active transport of sodium,
potassium and possibly chloride ions, impermeable to both water and
urea. The active reabsorption of Cl ions creates interluminal positivity
which enhances the movement of NA and other ions passively down
the electrical gradient from the lumen of the tubule to inside of the
tubular epithelial cell. This active transport of CL ions can be
inhibited by drugs such as diuretic (frusemide) (Fig. 3).
3
‫ نصير جواد المختار‬.‫د‬
Physiology
The thick ascending segment ascends back to the same glomerulus
through angle between the afferent and efferent arterioles. The cells
of this portion of thick ascending segment which are in complete
attachment with the epithelial cells of the afferent and efferent
arterioles that come in contact with the macula densa. The specialized
smooth muscle cells of afferent arterioles that come in contact with
the macula densa are called juxtaglomerular cells which contain
rennin granules. Macula densa and juxtaglomerular cells + few
granulated cells between them are known as juxtaglomerular comlex
or apparatus which has a dense adrenergic neural innervations (Fig.
4).
4. Distal convoluted tubule: they lie in the real cortex. The first half of
the distal tubule as the thick segment of ascending limb of the loop of
Henle. It absorbs most of ions but impermeable to both water and
urea. The second half of distal tubule and the cortical collecting
tubule are similar to each other and both impermeable to urea
reabsorb sodium ions in an exchange with K ions under the effect of
aldosterone, permeable to water only in the presence of antidiuretic
hormone. Its epithelium is similar to that of the proximal tubule but
they have distinct brush border.
5. Collecting tubules and ducts: About eight distal tubules coalesce to
form the collecting tubule which passes from cortex downward into
medulla, where it becomes the collecting ducts. The epithelium of the
collecting ducts made up of P cells which are involved in NA ions
reabsorption and vasopressin stimulated water reabsorption and
intercalated cells which are concerned with acid secretion and
bicarbonate ions transport.
4