Renal Physiology
By
Dr. Sadat A. Aziz
The Urinary system:
The urinary system consists of
• Two kidneys
• Two ureters
• The urinary bladder
• Urethra.
The formation of urine is the function of the
kidneys, and the rest of the system is responsible
for elimination of the urine.
Main functions of the kidneys:
1.Excretion of metabolic wastes like creatinine and uric acid before being
accumulated to toxic levels.
2. Regulation of the blood volume
3. Maintenance of water-salt balance
4. Maintenance of acid-base balance
5. Secretion of some biologically active molecules such as erythropoietin
Renal vasculature:
• The kidneys receive about 25% of the cardiac
output.
• Blood enters each kidney via the renal artery then
it divides into
1. Afferent arterioles which deliver blood to the
glomerular capillaries.
2. Efferent arterioles which drain blood from the
glomerular capillaries.
• deliver blood to the peritubular capillaries
• then into small veins
• finally into the renal vein.
The nephrons: they are the functional units of the kidneys. Each kidney contains approximately
1 million nephrons. A nephron consists of:
1. Renal corpuscle: which is the site where the blood plasma is
filtered, consisting of a glomerular capillary network which is
surrounded by the bowman`s capsule.
2. Renal tubule which is made of a single epithelial layer that
process the filtrate to urine via process of reabsorption and
secretion.
a. Proximal convoluted tubule
b. The loop of Henle which is a U-shaped tube consisting of a thin
descending limb, thin ascending limb and thick ascending limb
c. The distal convoluted tubule
d. Collecting ducts
Nephrons functions:
Nephrons have three main functions which include; glomerular filtration, tubular reabsorption and
secretion.
I. Glomerular filtration:
• It is a process in which water and some solutes in the blood plasma pass from the capillaries of the
glomerulus into the capsular space of the nephron
Permeation of solutes through glomerular capillaries depend on:
1. the solute`s size: Molecules with a molecular weight less than 6 kDa can be filtered freely. However,
molecules with a molecular weight more than 70 kDa are impermeable.
2. The solute`s charge: Negatively charged solutes are less likely to be filtered than neutral or positively
charged molecules due to the negative charges of the filtration barriers.
II. Tubular reabsorption:
It is the process of reclaiming of water and solutes from the
tubular fluid and returning them to the blood.
There is a tight coupling between sodium (Na+) and water
reabsorption which is called isosmotic reabsorption.
III. Tubular Secretion
It is a process in which the renal tubule extracts chemicals from
the capillary blood and secretes them into the tubular fluid. it
serves two purposes:
1. Waste removal
2. Acid-base balance
Glomerular Filtration Rate (GFR):
It is the amount of filtrate that can be formed per minute of
time by the two kidneys.
• Normally about 150-180 L/day which is about 50 to 60
times higher than the amount of blood plasma in the body.
However, about 99% of the filtrate can be reabsorbed and
only 1 to 2 L (per day) of filtrate is excreted as urine.
Urine Volume:
An average adult produces 1 to 2 L of urine per day.
Diuresis or polyuria: An increase in the urine output to more than 2
L/day, is called diuresis or polyuria.
Causes of polyuria:
1. An increase in the volume of fluid intake
2. Some drugs can increase urine output such as furosemide.
3. Chronic diseases such as diabetes.
a. Diabetes mellitus
b. Gestational diabetes
c. Diabetes insipidus
d. Renal diabetes
Oliguria: it is a reduction in the urine output to less than 500
mL/day.
Anuria: it is a clinical condition which is characterised by a
reduction in the urine volume to less than 100 mL/day.
• Low output can result from
1. Kidney disease
2. Dehydration
3. Circulatory shock
4. Prostate enlargement
• If urine output drops to less than 400 mL/day, the body cannot
maintain a safe, low concentration of wastes in the blood plasma
which leads to development of a condition Known as azotaemia.
Renin–Angiotensin System
• In response to reduced blood volume and blood
pressure, the juxtaglomerular cells secrete an enzyme
which is called renin into the blood.
• Renin stimulates conversion of angiotensinogen to
angiotensin I, then angiotensin converting enzyme (ACE)
converts angiotensin I to angiotensin II.
• Angiotensin II affects renal physiology in three main
ways:
1. It decreases the glomerular filtration rate by causing
vasoconstriction of the afferent arterioles.
2. It enhances reabsorption of Na, Cl, and water in the
proximal convoluted tubule.
3. It stimulates the adrenal cortex to release aldosterone.
HORMONES THAT INFLUENCE REABSORPTION OF
WATER
1. Aldosterone:
• It is secreted by the adrenal cortex in response to a
high blood potassium level, to a low blood sodium
level,
or
to a
decrease
in
blood
pressure
(hypotension).
• When aldosterone stimulates the reabsorption of Na
ions, water follows from the filtrate back to the
blood.
• This helps maintain normal blood volume and blood
pressure.
2. Atrial natriuretic peptide:
• It is released by the atria of the heart
in response to high blood pressure
(hypertension)
• In turn it stimulates a reduction in
the sodium ions reabsorption by the
kidneys
• which is followed by increasing the
excretion of water and lowering the
blood volume and blood pressure.
3. Antidiuretic hormone (ADH):
• Antidiuretic hormone (ADH or vasopressin) is released by
the posterior pituitary gland in response to reduced
volume of water in the blood and hypotension.
• It regulates facultative water reabsorption by the distal
convoluted tubules and throughout the collecting ducts.
• This helps maintain normal blood volume and blood
pressure.
• Hyposecretion of ADH leads to development of a condition
which is known as Diabetes insipidus that characterised by
a reduction in the water reabsorb and followed by an
increase in the volume of urine.