Kidneys and Blood Pressure
PKGhatak, MD
A
Blood Pressure (BP) of 120 mm Hg systolic and 80 mmHg diastolic,
expressed as 120/80 mm Hg, is considered a normal BP in adults. When BP
is over 140/80, it is called hypertension. In about 80% of cases, no definite cause of
high BP will be detected. This is called Essential or Primary
Hypertension. About 5 to 20 % of cases of high BP are due to kidney
diseases.
Role
of Kidney in maintaining a normal BP.
There
are two systems that are operative. One is the Sympathetic nervous system and
the other is the Angiotensin-Renin-Aldosterone system. These two systems
are interconnected and act in a coordinated manner.
A. Sympathetic
systems.
The BP sensors are located in the baroreceptors of the Carotid
bodies and the Aortic bodies in the Aortic Arch. Any change in BP, high
or low, initiates an impulse in the carotid stretch receptors in the
carotid bodies and is carried by the Glossopharyngeal nerve and then
relayed to the Hypothalamus. The pressure receptors of the aortic
arch are sensitive only to falling BP. The Vagus nerve carries the
sensation to the center and then relays it to the hypothalamus. From the hypothalamus, the outgoing impulse goes to the Celiac Ganglia. The
postganglionic alpha fibers innervate all the systemic arteries and
the beta fibers supply the heart.
The
Hypothalamus also sends impulses to the adrenal glands and to the
Juxtaglomerular cells of the kidneys.
Venous
BP monitors: These are low-pressure monitors.
These monitors exist within large
veins, pulmonary vessels, and within the walls of the right atrium
and ventricle. Changes in volume influence the baroreceptors in the
venous system and lead to the secretion of antidiuretic hormone and renin.
Effect
of sympathetic alpha stimulation.
Arterial
walls respond to alpha stimulation by increasing the tone of the muscular
wall and the BP is raised. When the BP is high, the carotid body
receptors produce a negative response in the hypothalamus, it prevents the further rise in BP. When BP falls, the aortic body sends positive
impulses to the hypothalamus and BP is raised.
Effect
of sympathetic Beta stimulation. Beta fibers mainly supply the heart.
Stimulation of the beta fibers increases heart rate and force of
ventricular contraction and elevation of BP.
Effect
of sympathetic stimulation of the Adrenal glands. It increases the production
and release of Aldosterone.
Effect
of sympathetic stimulation of juxtaglomerular cells. It causes the secretion of Renin.
The action of the sympathetic system
Agent
|
Substrate
|
Response
|
Final effect
|
Alpha stimulation
|
Arterial wall
|
Vasoconstriction
|
BP elevation
|
Beta stimulation
|
Heart muscles
|
Rate and force of contraction
|
BP elevation
|
|
Adrenal Glands
|
Aldosterone release
|
Increased Na+ absorption in
exchange for K+
|
|
Juxtaglomerular cells
|
Renin release
|
Angiotensin I formation
|
|
Posterior Pituitary
|
ADH release
|
Blood volume increase
|
B. Angiotensin-Renin-Aldosterone system.
BP
sensors.
The outer wall of the afferent arteriole of the kidney contains BP sensors.
When blood flow/ pressure is low, these cells secrete
Paracrine molecules (paracrine are chemicals, have actions like
hormones but these chemicals are locally released and locally active).
These molecules stimulate Juxtaglomerular cells to produce and
release Renin.
Electrolyte
sensors.
The
Macula Densa cells of the distal convoluted tubules are specialized
endothelial cells. They continuously monitor Sodium and Chloride
concentrations of the glomerular fluid. Any fall in sodium chloride
concentration triggers the paracrine release. Paracrine increases the rate
of excretion of Potassium+ ion and H+ion production and release in
exchange for Sodium+ ion from the filtrate.
Angiotensinogen
is an alpha2 globulin produced by the liver and renal endothelial cells
and circulates in the blood and is a non-active molecule. Renin
converts angiotensinogen to angiotensin I. Angiotensin I is carried
by the renal veins to the inferior vena cava to the right side of the
heart and then to the lungs. In the pulmonary capillaries, the
endothelial cells secrete Angiotensin Converting Enzyme (ACE). ACE
converts angiotensin I to angiotensin II. In the blood, angiotensin
lasts about 30 seconds and in the tissues for about 15 minutes, then it is
degraded by ACE to Angiotensin III and Angiotensin IV. Both
angiotensin III & IV have variable effects but are in the same line as angiotensin II.
Angiotensinogen
is also produced by the fat cells, testicles, ovaries, brain, heart
and blood vessels. In these secondary locations, its functions are
limited at the local tissue levels.
Effect
of Angiotensin II:
Angiotensin
II has a multitude of functions. 1. It increases the tone of all systemic
arteries and veins. 2. Constricts afferent and efferent arterioles of the glomerulus. and thereby increases filtration pressure and the GFR (glomerular filtration rate) is increased. 3. Angiotensin II increases the rate of Na+ ion
reabsorption in the proximal tubules. 4. It accelerates Na+/K+ H+
exchange in distal convoluted tubules. 5. Angiotensin II causes the secretion of
aldosterone by the adrenal cortex. 6. It
releases an Antidiuretic hormone from the Posterior pituitary gland.
Na+/K+H+
ion exchange and water reabsorption produce an increase in total body
Sodium and a decrease in potassium. The total body intravascular volume
expansion takes place. The serum becomes alkalotic due to execs
HCO3- ions. This condition is called Hypokalemia alkalosis.
Effect
of Anti Diuretic Hormone (ADH).
ADH
increases water absorption in the collecting tubules from the filtrate,
thereby increasing blood volume. However, expansion of blood volume takes place only when both kidneys are
ischemic. If only one kidney remains normal, then increased glomerular filtrate and loss of Sodium and
water by the normal kidney in the urine keep the blood volume normal, but BP remains high.
Role of Kidneys in producing High
BP:
Any condition producing a decrease in blood flow in the Kidneys (ischemic) produces high BP.
Renal causes of hypertension are
discussed under
Renal- vascular
Renal parenchymal causes.
Of all the causes of Hypertension, renovascular hypertension has the most potential to be cured by
corrective surgery and usually, the patients are not required to take BP
medications. It is essential to find the cause of high BP before much damage
takes place.
1. Common
causes of Renovascular hypertension.
In
children:
Common
causes are coarctation of the aorta, Moyamoya disease. Kawasaki disease,
Takayasu arteritis, renal artery trauma, congenital renal artery
hypoplasia or renal artery stenosis and renal graft stenosis
following renal transplantation.
Moyamoya
disease is more prevalent in Japan and is probably inherited by autosomal
dominant inheritance. Cerebral arteries are deformed and various
degrees of neurological symptoms develop at a very early age.
Kawasaki disease is a multisystem inflammatory disease that usually
follows a viral infection and produces signs and symptoms involving the skin, mucous membrane and lymph nodes and internal organs. It is more
prevalent in China. Takayasu arteritis is an inflammatory disease of
the aorta and its main branches, producing narrowing of blood vessels
and aneurysms, producing severe ischemia to the involved areas like
arms, neck, brain and kidneys. There is no known cause and treatment
is not effective. It is often seen in Japan, Mexico, and India.
In
adults:
The
common pathology is renal artery stenosis due to atheromatous
plaques.
The
narrowing may be in one or both renal arteries. The plaques may
be only segmental or in multiple areas, commonly involve the proximal
third of the artery. The plaques develop the same way as in the
coronary arteries. The risk factors are diabetes, high BP, high
cholesterol, obesity, cigarette smoking, a sedentary lifestyle, and runs in a family.
Other
causes of renal artery diseases are Congenital stenosis, Renal
artery thrombosis, emboli, aortic dissection, A-V malformation, and aneurysm of the abdominal aorta.
In
young females:
Fibromuscular
dysplasia. This condition is largely confined to females of childbearing age. The condition may run in families. The medium size
arteries are involved. Common sites are renal, intracranial, face and
abdominal. The distal 2/3rd of the renal artery shows
dysplasia. The changes may be confined in one or all three layers
of the arterial wall. Bead-sized aneurysms are seen in cerebral
branches of carotid arteries.
Risk
factors are female hormones, the higher incidence of methysergide use in
migraine, cigarette smoking, alpha 1 antitrypsin deficiency, cystic
medial necrosis, neurofibromatosis, coarctation of the aorta and
Ehler-Danlos syndrome.
The
patients with fibromuscular hypoplasia are mostly symptoms free. A cerebrovascular episode like TIA, (transient ischemic attack) or subarachnoid hemorrhagic may lead
to a diagnosis of fibromuscular dysplasia of the renal artery.
2. Renal
parenchymal diseases cause hypertension.
Almost
all inflammatory diseases of the kidneys raise BP. A few common conditions are: -
Polycystic
disease of the kidney. Glomerulonephritis.
Polycystic disease of the kidney is the most common hereditary kidney disease in adults causing hypertension and other complications. It is inherited as a dominant mode, and the expression of the defective gene penetration is variable. In children, the disease is much less aggressive and provides a better prognosis.
Glomerulonephritis is
generally due to post-streptococcal pharyngitis, also seen in secondary to
Bacterial endocarditis, Hepatitis B, hepatitis C, HIV infection and
now COVID-19. Other less common causes are autoimmune
glomerulonephritis. Lupus, Goodpasture syndrome, IgA nephropathy,
Diabetic microvascular disease,
Investigation and Diagnosis:
The renal parenchymal disease is
evident by the presence of protein, RBC, red cell casts and granular casts
in the urine. If active infection is present, an increased
number of WBCs and WBC casts are detected in urine. Further tests,
including IgG, IgA and other serological tests are generally required
for the determination of the etiology in individual cases. In most cases, needle biopsy of the kidney and special immunological staining is required to properly direct medical therapy and evaluate prognosis.
Renovascular disease is detected
by an Ultrasonography of the abdomen. It is a noninvasive test and can
easily detect abdominal aortic aneurysm, dissection of the aorta,
atheromatous changes and fibromuscular dysplasia. The new generation
Doppler ultrasound study gives flow and degree of stenosis. In case the presence of gas in the bowel interferes with the ultrasound study, MRI is an
excellent alternative. The functional status of kidneys is
determined by serum creatinine, urinary creatinine and GFR,
electrolytes. Radio-opaque dye administration is held off till a
definite surgery is planned. In a situation that is likely to be
associated with vascular abnormalities of the brain and other vital
organs, then a whole body angiogram may be completed before surgery.
Treatment:
Renal artery stenosis is treated
by angioplasty and stent placement. This is the preferred treatment
for fibromuscular dysplasia and a stent often is not needed.
Resection, repair and Dacron
graft are used based on the nature of the pathology. In unilateral and
atrophic kidney nephrectomy is indicated. Post-surgery prognosis is very good for children and
young individuals.
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