Physiology of blood pressure
This information allows us to understand the diagnostic and therapeutic options for the treatment of hypertension.
The pressure is determined by the ratio of two factors: flow and resistance.
Blood pressure values are obtained with the formula: P = F x R
P = pressure
F = flow
R = resistance
The flow depends on the cardiac output:
- Preload -> is the venous return in the diastolic phase, depends on several factors, but in particular the volume of blood,
- Cardiac activity (pump efficiency);
The resistance R is given by the formula:
η = blood viscosity, L = tube length, r is the mean arteriolar radius is raised to fourth, plays a very important role in pathophysiology.
The factors that influence blood pressure are:
- The volume of blood,
- Cardiac activity,
- The viscosity of the blood,
- The mean arteriolar radius.
Circulating Blood Volume
The fluid component is related to the sodium concentration.
If the amount of sodium increases:
- More water accumulates in the extracellular space and in the blood plasma,
- It increases the mass of circulating blood, thus also the pressure.
In the case of increased sodium concentration, the arterioles become more sensitive to vasoconstricting stimuli.
The regulation and concentration of sodium depends on:
- Mechanism of thirst -> the stimulation of thirst is influenced by the relative concentration of sodium,
- Secretion of HAD (ADH) or antidiuretic hormone that acts at the level of renal collecting ducts and causes water retention.
Vasopressin increases peripheral resistance, thus causing blood pressure to rise.
Relative variation in blood volume (total blood volume) may also occur due to:
- Accumulation of blood mass in some areas,
- Increased tone of the post-capillary venules that influence the venous return of blood to the heart.
The veins are the main deposit system of the circulatory system: they contain more than 50% of the blood.
If the muscle tone of the veins increases, the blood they can contain is reduced.
The consequence is a greater diastolic filling of the ventricles.
The smooth musculature of the heart is controlled by the autonomous vegetative system through:
- The vagus nerve,
- The sympathetic nervous system (which has the opposite effect).
The sympathetic activity consists of the release of neurohormonal substances: adrenaline and noradrenaline.
- The increase in heart rate ,
- A greater force of contraction of the heart.
The consequence is the increase in cardiac output and, therefore, pressure .
The vagus nerve (parasympathetic system) has the opposite effect because it has a vasodilatory action.
It is the most important factor (it is raised to the fourth power). Anything that affects the diameter of the small arteries has a dramatic effect on the pressure.
There are three fundamental factors that influence the median artery radius:
- Neurovegetative influences,
- Angiotensina II,
- Substances that act locally.
At the peripheral level, substances that have a vasoconstricting effect and cause an increase in pressure are:
Substances that reduce this effect are:
- Nitric oxide,
Viscosity of blood
The viscosity of the blood plays a minor role in the formula.
The viscosity increases in case of:
- Increased blood cell concentration (eg in polycythemia or multiple myeloma),
- Increased plasma protein.
High blood pressure may be:
1. Essential or primary if there is no right cause, but there are many risk factors. This type affects 95% of people suffering from hypertension.
2. Secondary if it is caused by a disease of an organ, for example, kidneys, adrenal glands or the heart. It affects only 5% of people with hypertension.
3. Malignant is not caused by a tumor, but occurs when the pressure is too high and causes damage to the organs. For example, at eye level can cause a swelling called papillary edema, it can also cause kidney failure and heart disease.
Cardiovascular risk factors
- Age (men> 55 years, females> 65 years);
- Smoking ;
- Lack of physical activity,
- Dyslipidemia (hyperlipidemia);
- Total cholesterol> 250 mg / dL or
- LDL > 155 mg / dL ou
- HDL <40 in men, <48 in women.
- Family history of initial cardiovascular disease
- 55 years in men,
- 65 years in women,
- Abdominal obesity ( abdominal circumference> 102 cm in men and> 88 cm in women).
Causes of High Blood Pressure
Hypertension can be caused by:
- Hyperthyroidism (excess of thyroid hormones),
- Hypothyroidism (reduction of production of thyroid hormones).
The parathyroid glands regulate the levels of calcium and phosphorus in the body.
If the production of parathyroid hormone is excessive, the amount of calcium in the blood increases.
The consequence is an increase in blood pressure.
It is characterized by an excessive production of aldosterone.
It is a benign adrenal adenoma that involves:
- Cells in the glomerular zone (producing aldosterone),
- In 20-25% bilateral nodular hyperplasia of cortical cells (slightly more difficult to see),
- In other cases, a real mass may be highlighted.
The aldosterone causes sodium reabsorption and potassium excretiontherefore concerned:
- An increase in extracellular fluid,
- Larger blood volume.
It also increases the sensitivity of the smooth muscle cells of the walls of the arteries to the vasoconstriction stimuli of the sympathetic system.
The reduction of blood flow to the kidney is felt by the renin-angiotensin-aldosterone system and causes an increase in aldosterone in the blood, this situation can be observed in the case of:
- Renal artery stenosis,
- Extracellular edema (accumulation of fluid)
- Reducing plasma sodium caused by diuretics causes a decrease in plasma volume. The consequence is the activation of the mechanisms of renal regulation.
It is a tumor that can be:
- In the adrenal medulla, in 90% of cases
- In 10% of cases, it is found at the level of:
- Paragânglios (groups of neuroendocrine cells),
- Ovaries (paraganglioma).
Chromaffin cells secrete adrenaline and noradrenaline that cause an increase in blood pressure.
The doctor suspects pheochromocytoma in a young person suffering from intermittent hypertensive crises, so hypertension is paroxysmal ..
Increased intra-abdominal pressure stimulates the release of catecholamines.
This occurs during:
- Lean forward,
- Strong laughs
- A hearty meal,
- Efforts for defecation or urination;
- The deep palpation of the abdomen.
The symptoms that may occur are:
- Headache ,
- Sweating ,
- Anxiety ,
- Chest and abdominal pain ,
- Nausea and vomiting ,
Characterized by excessive production of cortisol that has a function similar to that of aldosterone.
They include a group of congenital diseases characterized by changes in the metabolism of sex hormones that occur in the adrenal cortex:
- Androgens (cross-linked zone);
- Cortisolo (área fasciculada);
- Aldosterone (glomerular zone).
Adrenogenital syndrome is a condition that can be caused by the lack of one of the 5 enzymes involved in cortisol synthesis .
In this case, the hypothalamus and pituitary gland detect a low level of cortisol (negative feedback).
In this situation they stimulate the production of CRH (corticotropin releasing hormone) and ACTH (adrenocorticotrophic hormone).
The consequence is:
- Hyperstimulation of the adrenal gland cortex,
- A higher production of substances that act as aldosterone and cause hypertension.
This disease is characterized by an overproduction of somatotropic hormone (GH), usually caused by a pituitary adenoma.
Patients have a growth of bones in the skull and hands .
In adulthood, changes in body structure also affect soft tissues.
The hypertension found in 20-50% of patients with acromegaly is caused by:
- An increase in plasma volume,
- Higher vessel sensitivity to angiotensin II.
The most common type of secondary hypertension is nephrovascular.
Renal artery stenosis causes a reduction in blood pressure in the kidney.
The consequence is the activation of the renin-angiotensin-aldosterone mechanism that causes:
- An increase in plasma volume,
- Sodium retention.
- In 90% of cases, these are atherosclerotic plaques in elderly individuals , frequently found in both kidneys ;
- In 10% a thickening of the wall occurs in the most distal tract (the final part), we speak of fibromuscular dysplasia.
Hypertension caused by nephropathy
All nephropathies can cause hypertension:
- Diabetic nephropathy,
- Nephropathy caused by analgesics (tumors with renin secretion).
The development of hypertension may be the consequence of:
- Malfunction of the afferent arterioles in the juxtaglomerular apparatus;
- The kidney is unable to produce substances that reduce pressure, such as: prostaglandins and bradykinin or produce many substances with hypertensive action;
- Decreased sodium excretion capacity.
Other causes of hypertension
- Neurogenic hypertension : brain diseases such as: trauma or neoplasia that can cause increased intracranial pressure;
- Polycythemia : causes an increase in viscosity that can cause hypertension.
- Pre-eclampsia : The problem that occurs after the twentieth week of pregnancy is accompanied by the suffering of different organs and systems, including the kidney. Occurs proteinuria and formation of edema , convulsions (eclampsia in the case), disseminated intravascular coagulation (DIC), etc ..
- Sleep apnea . Breathing is often interrupted during sleep, the consequence is a reduction of the oxygen that enters the body.
- Aortic coarctation : a stenosis under the aortic arch, downstream of the arteries that carry blood to the brain and arms. On physical examination, the doctor observes the absence or a significant reduction of the femoral pulses;
- Secondary effect of drugs or other substances:
Most people with high blood pressure do not have signs or symptoms, even if blood pressure values reach high levels.
According to a study by Di Tullio et al. – ( Istituto di Richerche Farmacologiche Mario Negri, Università di Milano, Italy ), the following appears to be caused by high blood pressure or hypertension therapy
- Severe headache in the occipital area, can be pulsing. It is often more intense in the morning and not wake up,
- Shortness of breath ,
- Pulsatile tinnitus or tinnitus in the ears
- Blood from the nose (rare)
Complications and consequences of hypertension
Complications can be:
- At the level of the affected organs.
The most serious problems occur in the cardiovascular system:
- Arteriosclerosis – in medium and large caliber arteries, particularly the coronary arteries. The course becomes tortuous and the flow turbulent. The consequences are: endothelial lesions, aortic dissection, plaque rupture or thrombus .
- Arteriolosclerosis (in arterioles), with microhemorrhages , inflammation and fibrinoid necrosis (degeneration with accumulation of fibrinogen and other plasma proteins) in the affected organs.
Increased blood pressure levels cause organic changes:
- The work of the left ventricle of the heart increases. The consequence is ventricular hypertrophy, dilatation and heart failure . In addition, the heart works more and consumes more O2. Since coronary blood flow is reduced due to atherosclerosis, ischemic heart disease (risk of myocardial infarction ) occurs .
- In the brain they develop: thrombosis (for atherosclerosis ), hemorrhages(caused by the rupture of microaneurysms), these changes can lead to coma. Unfortunately, the lesions can not be seen on the tomography because they are small and are linked to the change in microcirculation.
- The kidney has a reduced blood supply due to atherosclerosis and arteriolosclerosis. This can cause irreversible damage to the kidney to kidney failure . A change in blood flow occurs in the glomeruli due to hypertension in the glomerular capillaries.
- The eye is one of the most affected organs. Over time, high blood pressurecan cause damage to the blood vessels of the retina. In the microcirculation of the eye:
- The arteries shrink,
- Arterio-venous crosses are formed (the vein has tortuosity and is perpendicular to the artery),
- Occlusion and micro-infarction, cotton wooly white spots (
hard exudates ), superficial retinal haemorrhages in candle flame,
- In the most advanced degree, there is edema of the optic disc with risk of thrombosis. At this stage, hypertensive retinopathy can cause vision problems and can also lead to blindness.
- Malignant (accelerated) hypertension means that the diastolic pressure is very high , greater than 120 / 130mmHg. Direct damage to the vascular endothelium is observed. The consequence is the release of endothelin-1 that causes vasoconstriction. In addition, renin production is not inhibited (as it should be by increasing arteriolar pressure), but increases.