In this article we will discuss about:- 1. Regulation of Blood Flow 2. Coronary Blood Flow 3. Factors 4. Auto-Regulation.
Regulation of Blood Flow:
1. Increased carbon dioxide tension (increased pCO2) is the most important factor.
CO2 is a power full vasodilator of the cerebral blood vessels. Increasing the CO2 content of the inspired air (3-5%) almost doubles the blood flow to the brain. Voluntary hyperventilation decreases the pCO2, and brings about vasoconstriction and decreases the cerebral blood flow. This gives rise to dizziness.
2. Increased H+ concentration of the CSF increases the cerebral blood flow.
3. Hypoxia (decreased pO2) also increases the cerebral blood flow.
4. A rise in the intracranial tension compresses the blood vessels supplying the brain. This decreases the cerebral blood flow (Monro-Kellie doctrine).
5. Stimulation of sympathetic/parasympathetic never fibers has very little effect on cerebral blood flow.
Monro-Kellie doctrine:
Since the three compartments are placed in rigid box (cranium) expansion of any one of the compartment can occur only at the expense of compromise of the other two compartments. When the CSF compartment expands (due to increased accumulation of CSF) the vascular compartment is pressed upon. This decreases cerebral blood flow.
Coronary Blood Flow:
i. Blood flow through the coronaries supplies the heart muscle (myocardium).
ii. The right and the left coronary arteries take their origin from the root of the aorta.
iii. Normal coronary arterial blood flow is about 250 ml/minute.
iv. Arteriovenous oxygen difference is highest even under resting conditions. It is about 14 ml (20-6 ml)/100 ml.
v. Therefore, whenever there is an increased demand for oxygen by the heart muscle it is met with only by increasing the coronary blood flow.
vi. The venous blood from the myocardium is drained into the coronary sinus and the anterior cardiac veins.
vii. There is variation in blood flow during cardiac cycle. More blood flows through the coronary vessels during ventricular diastole than during systole. This is more so with respect to left coronary artery (Fig. 3.45).
viii. The volume flow variation is more phasic in the endocardial region when compared to epicardial region.
Determination of Coronary Blood Flow:
i. By applying Fick’s principle.
ii. Nitrous oxide is the substance of choice. Radioisotope thalium (Tl-201) can also be used.
iii. The venous blood from the myocardium is drained into the coronary sinus and the anterior cardiac veins.
CBF (coronary blood flow) = Q/AC – VC ml/minute
Wherein
Q is the quantity of nitrous oxide taken up by brain tissue.
AC is the concentration of the substance in arterial blood.
VC is the concentration of substance in venous blood.
Factors Influencing the Blood Flow:
1. Coronary blood flow is subjected to an auto- regulation.
2. The pressure head (aortic pressure minus coronary sinus pressure)
3. Phasic blood flow
4. Chemical factors (blood gases), the most important one being the oxygen supply (hypoxia) and decreasing in oxygen tension (fall in pO2). Any hypoxic situation will be promptly followed by an increase in blood flow.
5. Sympathetic stimulation
Left Coronary Flow:
i. During isometric contraction phase as the intraventricular pressure is suddenly increasing, the blood vessels are compressed upon and, therefore, the blood flow decreases.
ii. During maximum and reduced ejection phase, the cardiac muscle fibers contract and the intraventricular pressure increases to 120 mm Hg. The endocardial blood vessels are compressed and hence blood flow decreases.
iii. During the same time epicardial blood vessels are not compressed to a great extent. The total blood flow remains low (about 40 ml/minute)
iv. During diastole, as the intraventricular pressure rapidly falls, the compressor force on the blood vessels decreases and this leads to an increase in blood flow.
Right coronary arterial blood flow remains high both during ventricular systole and diastole, because the blood vessels supplying the right heart are not subjected to greater compression. This is because the pressure changes in the right ventricle during a cardiac cycle remains low (10-25 mm Hg).
Chemical Factors Mechanism involved:
1. Hypoxia produced leads to production of adenosine an end product of anaerobic metabolism and adenosine is a powerful vasodilator substance to the coronary blood vessels. The effect of hypoxia is not direct one but it is through the production of adenosine.
2. An increase in pCO2 or an increase in H+ will also bring about coronary vasodilatation, and increase in the blood flow.
Sympathetic Stimulation (Fig. 3.46):
Coronary arteries contain both and receptors. Stimulation of receptors will bring about coronary vasoconstriction. Stimulation of receptor results in coronary vasodilatation. However, stimulation of sympathetic fibers to the heart is associated with coronary vasodilatation.
Sympathetic fiber stimulation to the heart increases the force of contraction and, therefore, metabolism of the cardiac musculature. Metabolic end products bring about coronary vasodilatation. Therefore, the net effect of sympathetic stimulation is coronary vasodilatation and increase in blood flow.
Acetylcholine (ACh):
Parasympathetic (ACh is the neurotransmitter liberated by these nerve terminals) nerve stimulation is associated with coronary vasodilatation and increase in the blood flow.
Other coronary vasodilators include:
i. Potassium
ii. Lactate
iii. Prostaglandin
iv. And NO (nitric oxide), nitroglycerin, nitrates are used clinically as coronary vasodilators.
Auto-Regulation of Blood Flow:
Mean arterial pressure determines the blood flow through the vascular region. At the level of organ or tissue, it is the perfusion pressure, which is nothing but pressure difference between the beginning of the flow (P1, arterial end pressure) and at the end of flow (P2, venous pressure).
There is a direct relationship between this perfusion pressure (P1 – P2) and the blood flow. In most of the tissues or organs, the pressure difference (P1 – P2) will be around 70 mm Hg.
Organs and tissues in which autoregulation of blood flow occurs are:
1. Coronary flow (blood flow through the myocardium)
2. Cerebral flow (blood flow through brain)
3. Renal flow (blood flow through kidney)
4. Skeletal blood flow, etc.
The above organs have well-developed auto- regulatory mechanism to maintain the flow constant within a particular range of pressure. This is termed as autoregulation of blood flow.
By definition, autoregulation of blood flow states that it is the ability of an organ or tissue to regulate its own blood flow despite a change in blood pressure.
Critical Closing Pressure/Critical Opening Pressure (Fig. 3.52):
Critical closing pressure is the minimum mean arterial pressure that is essential to keep the arteries in a distended state. If the pressure in the vessel is below the minimal value, the blood vessels collapse. Normally, the critical closing pressure is around 20 mm Hg. Below this pressure, since the blood vessels collapse, the blood flow through the organ stops completely.
As the pressure increases above the critical closing pressure, the volume of blood flow also increases proportionately till a limit. So they will have a direct relationship. However, when once the pressure exceeds a particular value, in spite of an increase in pressure, there will not be any further increase in blood flow. This is termed as autoregulatory ability of the organ to regulate the blood flow.
Most of the organs have the ability to autoregulate their flow between a pressure range of 60 and 180 mm Hg. Beyond 180 mm Hg, the autoregulatory mechanisms fail and hence there would be further increase of blood flow proportionate to the increase in pressure. Autoregulation of blood flow is seen even in a denervated isolated organ. This suggests that the nerve supply is not responsible for autoregulation mechanism.