Basics of Renal Physiology in context with Homeostasis

1. Muhammad Bashir
M.Phil. Student

2.  Urinary Organs
 The urinary system consists of the kidneys, ureters,
urinary bladder, and urethra.
 The bean-shaped kidneys are at the back of the
abdominal wall beneath the peritoneum, protected
by the lower rib cage.
 The renal artery and renal vein along with ureters
exit the kidney at the hilum.


3.  The kidneys produce urine which is conducted by
two muscular tubes called ureters to the urinary
bladder where it is stored before being released
through the urethra.
 Two urethral sphincters control the release of
urine.
 In females, the urethra is 4 cm long; in males, the
urethra is 20 cm long and conveys both urine and
sperm during ejaculation.

5.  As the bladder fills with urine, sensory impulses
travel to the spinal cord where motor nerve
impulses return and cause the bladder to contract
and sphincters to relax.
 With maturation, the brain controls this reflex and
delays urination, the release of urine, until a
suitable time.

7.  Excretion refers to the elimination of metabolic
wastes that were cell metabolites; this is the
function of the urinary system.
 Kidneys play a role in homeostasis of the blood by
excreting metabolic wastes, and by maintaining
the normal water-salt and acid-base balances of
blood.

8.  Kidneys excrete nitrogenous wastes, including
urea, uric acid, and creatinine.
 Urea is a by-product of amino acid metabolism.
 The metabolic breakdown of creatinine phosphate
in muscles releases creatinine.
 Uric acid is produced from breakdown of
nucleotides.
 Collection of uric acid in joints causes gout.

9.  Kidneys maintain the water-salt balance of the
body which, in turn, regulates blood pressure.
 Salts, such as NaCl, in the blood cause osmosis
into the blood; the more salts, the greater the blood
volume and also blood pressure.
 Kidneys also maintain correct levels of potassium,
bicarbonate, and calcium ions in blood.

10.  The kidneys regulate the acid-base balance of the
blood.
 Kidneys help keep the blood pH within normal
limits by excreting hydrogen ions (H+
) and
reabsorbing bicarbonate ions (HCO3
-
) as needed.
 Urine usually has a pH of 6 or lower because our
diet often contains acidic foods.

11.  Kidneys secrete or activate several hormones:
1) They secrete the hormone erythropoietin to
stimulate red blood cell production,
2) They activate vitamin D to the hormone calcitriol
needed for calcium reabsorption during digestion,
and
3) They release renin, a substance that leads to the
secretion of aldosterone.

12.  The kidneys filter wastes from the blood, and thus
the renal arteries branch extensively into smaller
arteries and then arterioles inside each kidney.
 Many venules unite to form small veins, which
merge to become the renal vein.

14.  There are three regions to a kidney: an outer renal
cortex, an inner renal medulla, and a central
space called the renal pelvis.
 Microscopically, each contains over one million
nephrons.
 The nephrons produce urine which flows into a
collecting duct; several collecting ducts merge and
drain urine into the renal pelvis.

16.  Each nephron has its own blood supply.
 An afferent arteriole approaches the glomerular
capsule and divides to become the glomerulus, a
knot of capillaries.
 The efferent arteriole leaves the capsule and
branches into the peritubular capillary network.

18.  The closed end of the nephron is a cuplike
glomerular capsule.
 Spaces between podocytes of the glomerular
capsule allow small molecules to enter the from the
glomerulus via glomerular filtration.
 The cuboidal epithelial cells of the proximal
convoluted tubule have many mitochondria and
microvilli to carry out active transport (following
passive transport) from the tubule to blood.

20.  The descending loop of the nephron allows water
to leave and the ascending portion extrudes salt.
 The cuboidal epithelial cells of the distal
convoluted tubule have numerous mitochondria
but lack microvilli.
 They carry out active transport from the blood to
the tubule or tubular secretion.
 Collecting ducts gather in the renal medulla and
form the renal pyramids.

21.  Glomerular Filtration
 During glomerular filtration, small molecules
including water, wastes, and nutrients are forced
from the blood inside the glomerulus to the inside
of the glomerular capsule.
 Blood cells, platelets, and large proteins do not
move across.
 About 180 liters of water are filtered daily.

22. Substance Amount
Filtered
Amount
Excreted
Reabsorp-
tion (%)
Water, L 180 1.8 99.0
Sodium, g 630 3.2 99.5
Glucose, g 180 0.0 100.0
Urea, g 54 30.0 44.0

23.  During tubular reabsorption, certain nutrients, water
and some urea moves from the proximal convoluted
tubule into the blood of the peritubular capillary
network.
 Tubular reabsorption is a selective process
because only molecules recognized by carrier
molecules are actively reabsorbed.
 The rate of this process is limited by the number of
carriers.

24.  During tubular secretion, specific substances such
as hydrogen ions, creatinine, and drugs such as
penicillin move from the blood into the distal
convoluted tubule.
 In the end, urine contains substances that have
undergone glomerular filtration but have not been
reabsorbed, and substances that have undergone
tubular secretion.

26.  The kidneys maintain the water-salt balance of the
blood within normal limits.
 By doing so, they also maintain blood volume and
blood pressure.
 Most of the water and salt (NaCl) present in the
filtrate is reabsorbed across the wall of the
proximal convoluted tubule.

27.  Salt passively diffuses out of the lower portion of
the ascending limb of the loop; the upper thick
portion actively extrudes salt into the tissue of the
outer renal medulla.
 Water is reabsorbed by osmosis from all parts of
the tubule.
 The ascending limb of loop of the nephron
establishes an osmotic gradient that draws water
from the descending limb of the nephron and the
collecting duct.

28.  The permeability of the collecting duct is under the
control of antidiuretic hormone (ADH).
 Diuresis is an increase in urine flow and
antidiuresis is a decrease.
 When ADH is present, more water is reabsorbed,
blood volume and blood pressure rise, and there is
a decreased amount of urine.
 If there is insufficient water intake, the posterior
pituitary releases ADH, causing more water to be
reabsorbed with a decreased urine output.

30.  Kidneys regulate salt balance by controlling excretion
and reabsorption of ions.
 Two hormones, aldosterone and atrial natriuretic
hormone (ANH), control the kidneys’ reabsorption of
sodium (Na).
 When the juxtaglomerular apparatus detects low blood
volume, it secretes renin that eventually results in the
adrenal cortex releasing aldosterone that restores
blood volume and pressure through reabsorption of
sodium ions.

32.  Reabsorption of salt increases blood volume and
pressure because more water is also reabsorbed.
 ANH is secreted by the atria of the heart when
cardiac cells are stretched by increased blood
volume.
 ANH inhibits secretion of renin; the resulting
excretion of sodium also causes excretion of
water and blood volume drops.

33.  Diuretics are chemicals that lower blood pressure
by increasing urine output.
 Alcohol inhibits secretion of ADH; dehydration
after drinking may contribute to the effects of a
hangover.
 Caffeine increases the glomerular filtration rate
and decreases tubular reabsorption of sodium.
 Diuretic drugs inhibit active transport of Na+
so a
decrease in water reabsorption follows.

34.  Kidneys rid the body of acidic and basic
substances.
 If the blood is acidic, hydrogen ions (H+
) are
excreted and bicarbonate ions (HCO3
-
) are
reabsorbed.
 If the blood is basic, H+
are not excreted and HCO3
-
are not reabsorbed.
 Breathing also ties up H+
when carbon dioxide is
exhaled.

36.  The urinary system has organs specialized to
produce, store, and rid the body of urine.
 Kidneys excrete nitrogenous wastes and maintain
the water-salt and the acid-base balance of the
blood within normal limits.

37.  Kidneys have a macroscopic anatomy and a
microscopic anatomy.
 Urine is produced by many microscopic tubules
called nephrons.
 Urine formation is a multistep process.
 Kidneys are under hormonal control as they
regulate the water-salt balance of blood.
 Kidneys excrete hydrogen ions and reabsorb
bicarbonate ions to regulate the pH of blood.