Introduction
-The
important functions of kidney is:
1) To discard the body waste that are either
ingested or produced by metabolism.
2) To control the volume and composition of body
fluids.
The functional unit of the kidney is the nephron which
is responsible for the formation of urine.
The nephron
Each nephron has two major portions
1) Glomerulus (cluster of capillaries) [it is formed by a pair of
afferent and efferent arterioles surrounded by [Bowman′s
capsule].
2) Renal tubules
- which originates as a bulb called Bowman′s capsule.
The renal tubule is made up of:
i) Proximal (near) convoluted tubules
ii) loop of Henle (ascending and descending limb)
iii) Distal (far) convoluted tubule
iv) Collecting duct
formation of urine
The formation of urine involves three major
processes:
1) glomerular filtration, which takes place in
the renal corpuscles.
2) Tubular reabsorption which take place in
the renal tubules.
3) Tubular secretion which also take place in
the renal tubules.
1)Filtration process:
-The pressure of the blood within the glomerulus forces water and
dissolved solutes through the semipermeable capillary membrane and
into the Bowman′s space.
- Approximately 120 ml/min of the renal plasma is filtered
through the glomeruli forming ultrafiltrate which is further processed
as it travels through the nephron.
-The ultrafiltrate has the same composition as blood plasma but it is
normally free of proteins and also cellular-free.
-Some of the filtered products include water, glucose, electrolytes, amino
acids, urea, uric acid, creatinine, and ammonia.
-[The rate of filtration (the glomerular filtration rate) is an important
indicator of renal function] How?
2) Tubular reabsorption:
- As the ultrafiltrate (glomerular filtrate) passes through the proximal
tubules, a large portion of the water, sodium chloride, bicarbonate,
potassium, calcium, amino acids, glucose and other substances needed
by the body are reabsorbed and pass back into the bloodstream.
Like the proximal tubule, the descending limb of the loop of Henle
is very permeable to water, but the resorption of solutes does not
occur in this part of the loop. The ascending limb, however, is
nearly impermeable to water, but there is active reabsorption of
sodium chloride. In this section of the tubule and in the remaining
tubule, hydrogen ion and ammonia are secreted.
3) Tubular secretion:
It involves sending molecules from the blood capillaries into the tubular
filtrate for excretion. (out of the body)
- The tubular secretion process:
● removes unneeded foreign waste substances that are not filtered by
the glomerulus including various medications and toxins.
● promotes secretion of hydrogen ions and other ions to help regulate
acid – base and electrolytes balance.
-Various ions are also secreted including hydrogen ions, ammonium ions,
sodium ions, potassium ions, bicarbonate ions, uric acid and some
weak acids and bases.
- Hydrogen ions is secreted in the proximal and distal tubules.
- Tubular secretion is caused mainly by active transport.
Thus; by the selective reabsorption and
secretion action of the renal tubules, the
glomerular filterate modification leads
eventually to excretion of urine that has a
composition different from that of the
glomerular filterate .
-The
glomerular filtrate enters the proximal tubules in an isotonic
state.
-In the proximal tubules, the reabsorption process begins. Water and salt
( Nacl) is reabsorbed.
-At the descending limp of Henle, the water reabsorption continues
osmosis (75% of water reabsorption occurs here ) and the water becomes
more and more concentrated as it descends.
-At the hairpin of Henles loop the fluid reaches the maximum
concentration (becomes hypertonic) and the epithelium becomes water
impermable, so water reabsorption Ceases.
- the fluid continues ascending entering the ascending limb , which is
impermeable to water ,and becomes progressively less concentrated
due to Nacl reabsorption and will become isotonic then hypotonic at
the end of the thick ascending limp of Henles loop .
-In the distal tubules, active reabsorption of Nacl continues to a less
extent .
- both the distal convoluted tubule and the collecting tubules are
sensitive to stimulation by the ADH (anti diuretic hormone). Thus their
major function is to reabsorb water only to the extent required by the
body.
Role of kidneys in the regulation of acid – base
balance:
Body fluids are maintained within a narrow range that is slightly
alkaline.
-The normal PH of arterial blood is between 7.35 and 7.45
- If the blood PH rises above 7.45 , a person said to have alkalosis.
- If the blood PH decreases below 7.35 , a person said to have acidosis.
-Acids are continually produced during metabolism, so kidneys help
maintain a normal PH by either excreting or retaining acids and bases.
● If the blood is acidic, hydrogen ions are excreted ( out of the body) and
bicarbonate ions are reabsorbed.
● If the blood is basic, hydrogen ions are not excreted (reabsorbed) and
bicarbonate ions are not reabsorbed. ( excreted)
● The ammonia that is secreted combines with the hydrogen ions
to form ammaonium ions in the tubular lumen and this helps to
regulate the hydrogen ions conc. of the urine.
● Hydrogen ions are produced as waste from the metabolism and are
generally secreted.
-Bicarbonate can also be secreted but is more often reabsorbed
(usually up to 100%) to help maintain the proper blood PH.
[The urine is usually acidic, so it follows that an excess of
hydrogen ions is usually excreted.
- Ammonia provides another means of buffering and
removing the hydrogen ions in urine:
H+ +NH3
NH4+
- Ammonia is produced in tubule cells by the deamination
of amino acids.
Three basic renal processes
 The substance is freely filtered but is also partly
reabsorbed from the tubules back into the blood.
 For each substance in the plasma, a particular
combination of filtration, reabsorption, and secretion
occurs. The rate at which the substance is excreted in
the urine depends on the relative rates of these three
basic renal processes.
Filtration, Reabsorption, and Secretion of
Different Substances
 In general, tubular, reabsorption is quantitatively more
important than tubular secretion in the formation of urine,
but secretion plays an important role in determining the
amounts of potassium and hydrogen ions and a few other
substances that are excreted in the urine.
 Most substances that must be cleared from the blood,
especially the end products of metabolism such as urea,
creatinine, uric acid, and urates, are poorly reabsorbed and
are, therefore, excreted in large amounts in the urine.
 Certain foreign substances and drugs are also poorly
reabsorbed but, in addition, are secreted from the blood
into the tubules, so that their excretion rates are high.
Filtration, Reabsorption, and Secretion of
Different Substances
 Nutritional substances, such as amino acids and
glucose, are completely reabsorbed from the tubules
and do not appear in the urine even though large
amounts are filtered by the glomerular capillaries.
Each of the processes - glomerular filtration, tubular
reabsorption, and tubular secretion - is regulated
according to the needs of the body.
Glomerular filtration rate (GFR):
The glomerular filtration rate (GFR) is the amount
of renal filtrate formed by the kidneys in 1minute,
and averages 100 to 125 mL per minute.
● GFR:
will lead to the inefficient elimination and
accumulation of the body waste products and toxins.
● GFR:
can lead to insufficient time for proper reabsorption.
 Estimation of the GFR is used to assess the kidney
function.
Example of kidney function test:
creatinine clearance test.
What is creatinine?
The main storage compound of high energy
phosphate needed for muscle metabolism.
Creatine
H2O
Creatinine
(Waste product)
Creatinine: anhydride of creatine!
Jaffe´ reaction for measuring creatinine
Creatinine + alkaline picrate solution
Bright orange/red colored
complex absorbs light at
485nm
Normal creatinine clearance test
Blood creatinine:
Men: 0.6-1.2 milligrams per
deciliter (mg/dL) or 71-106
micromoles per liter (mcmol/L)
Women: 0.4-1.0 mg/dL or 36-90
mcmol/L
Creatinine clearance:
Men (younger than 40 years):
107-139 milliliters per minute
(mL/min) or 1.8-2.3 milliliters per
second (mL/sec)
Women (younger than 40 years):
87-107 mL/min or 1.5-1.8 mL/sec
Creatinine clearance values
normally go down as you get
older (normal values go down by
6.5 mL/min for every 10 years
past the age of 20).
Tubular reabsorption
Tubular secretion