Understanding Cardiovascular Hemodynamics and Blood Pressure Regulation
Auricular Sinus Node (NSA)
Located on the upper wall of the right atrium at the mouth of the superior vena cava. Its specialized cells fire, self-excited, an electrical impulse that spreads through the atria via Bachman’s bundle, so that the two atria contract simultaneously.
Atrioventricular Node
Located at the junction between the two ventricles, consisting of specialized fibers. Its role is to delay the action potential.
Hemodynamics
Basic Theory of Circulatory Function
Blood flow depends on the nutritional needs of the tissues (support for increased cardiac output). Cardiac output depends on the amount of blood flowing to all tissues. Blood pressure is controlled independently by local blood flow and cardiac output (does not affect the change to the body).
Blood Pressure
It is rhythmic, oscillating between maximum and minimum. The most precise measurement is performed with a catheter, but being an invasive technique, an indirect method (stethoscope) is generally used. What we hear through the stethoscope is the closure of the valve that generates turbulent passage of blood. If we increase pressure with the gauge, there comes a time when the artery is closed. As you decrease the pressure, there comes a time when the blood flows through, overcoming the pressure (systolic or maximum) in turbulent flow (arterial wall vibrates and makes the noise that we hear). If we reduce the pressure further, the obstruction disappears and the sound stops. This will be the minimum or diastolic blood pressure, the time at which blood passes in laminar flow through the artery.
Mean Arterial Pressure: MAP = DBP + ((SBP-DBP) / 3). Is divided by 3 because the diastole lasts twice as long as systole.
Double Product (DP): DP = SBP * heart rate. Reliably indicates the state of overload of the heart muscle, indicating the requirement level of exercise (exercise intensity).
Short-Term Regulatory Mechanisms
They act in seconds. They are based on the baroreceptors: pressure receptors of the aorta and carotid arteries. When we’re lying down, heart pressure equals the pressure of the cerebral arteries, but if we stand up, the pressure of the heart increases cerebral blood pressure. When this occurs, the baroreceptors send a signal to the medulla oblongata, and this, in response, stimulates the sympathetic nervous system, increasing the degree of contraction pressure and heart rate (to beat more powerfully).
Chemoreceptors
Although they do not always act, they regulate when the pressure is lower than 80 mmHg (for any cause). Chemical receptors are found in the aorta and carotid arteries and are also sensitive to hypercapnia (increased CO2), acidosis (pH reduction) and hypoxia.
The pressure sensors in the right atrium and pulmonary arteries have the same functions, but are less important than those in the aorta and carotids.
Regulatory Mechanisms in the Medium Term
They act in minutes.
Renin-Angiotensin-Aldosterone System: The most important characteristic of higher mammals. Allows for a varied diet in salt, therefore sodium regulation exists. When blood pressure is reduced from any cause (hemorrhage, dehydration, etc.), the juxtaglomerular cells (kidney cells derived from smooth muscle and are sensitive to changes in blood pressure) release renin (enzyme) into the bloodstream. In blood, renin acts on angiotensinogen (protein) and the result is the cleavage of angiotensinogen to angiotensin I. Angiotensin I circulates throughout the vascular system until reaching the pulmonary capillaries, where they attach to specific receptors, releasing angiotensin II. Angiotensin II is a potent vasoconstrictor that acts on the adrenal cortex to release aldosterone (hormone) that reabsorbs sodium. By retaining more Na, more fluids are retained, increasing blood pressure.
Relaxation of the vessels (by stress): When blood pressure increases, a phenomenon occurs reflecting the muscle fibers, which are stretched and diminish the pressure. By increasing blood pressure, intracapillary pressure increases. By increasing capillary pressure, fluid outflow increases and capillary pressure tends to decrease.
Long-Term Regulatory Mechanisms
Phylogenetically, this is the oldest mechanism. It works in days or hours.
Kidney Filtering: When blood pressure increases, blood volume increases. By increasing blood volume, the blood supply to the kidney increases, increasing diuresis (more filtering), increasing the amount of fluids lost in the urine, and lowering blood pressure.
Transport of O2
The bronchial arteries originate from the aorta and provide O2 to lung cells, where the blood is oxygen-rich. Once they have delivered O2 to the cells, bronchial veins are incorporated into the pulmonary veins (rich in O2) that have just recovered oxygen in the lung. As the bronchial veins are low in O2, the partial pressure of O2 in the circulatory system is 95 mmHg, less than the theoretical figure of 104 mmHg.
Transport of CO2
Unlike oxygen, CO2 is not transported exclusively in hemoglobin. While 2% of O2 is carried free in plasma, for CO2:
- 70% is transported as bicarbonate in plasma after processing in the erythrocyte (thanks to carbonic anhydrase).
- 23% is bound to hemoglobin, a consequence of the Bohr effect produced by the acidified pH and increased concentrations of CO2.
- The remaining 7% travels freely in plasma.