CARDIO-VASCULARSYSTEM:

BLOOD FLOWIN SPECIAL AREAS (part – I)

SPECIALCIRCULAIONS

CORONARY CIRCULATION

CORONARY ARTERIES

• Coronary arteries are superficial vessels. The major coronary vessels travel in the epicardium of the heart and subdivide, sending penetrating branches through the myocardium.

• The penetrating branches subdivide into arcades that distribute blood to the myocardium.

NORMAL CORONARY BLOOD FLOW

CORONARY ARTERIES

Normally, the coronary arteries appear to function as end arteries. However, the presence of an arterial plaque or occlusion allows anastomoses between vessels to become functional.

CORONARY VEINS

• Left ventricular venous drainage occurs primarily through the coronary sinus. The Thebesian veins and coronary-luminal vessels (connections between the coronary vessels and the lumen of the heart) also return small amounts of blood to the left ventricle.

• Right ventricular venous drainage occurs through the multiple anterior coronary veins. The coronary-luminal connections carry a larger proportion of the flow in the right ventricle than in the left ventricle.

CORONARY FLOW PATTERN• A continuous FLOW OF BLOOD to the heart is

essential to maintain an adequate supply of O2 and nutrients.

• NORMAL CORONARY BLOOD FLOW represents approximately 5% of the resting cardiac output (250 ml/min), or approximately 60-80 ml blood/100 g tissue/min.

• During ventricular systole, myocardial wall tension increases and COMPRESSES THE PENETRATING VESSELS, thus increasing the resistance to flow.

• This extravascular compression produces a complex CORONARY FLOW PATTERN. Maximal flow in the left coronary vessels occurs during isovolumic relaxation while the arterial pressure is still relatively high and the myocardium is relaxed.

FACTORS INFLUENCING CORONARY FLOW

Coronary blood flow is characterized by a supply-demand relationship. The heart metabolizes all substrates in approximate proportion to their vascular concentration.

FACTORS INFLUENCING CORONARY FLOW

• Myocardial O2 consumption averages 6-8 ml O2 /100 g of tissue/min. Normally, hemoglobin releases approximately 50% of its arterial O2 content to the myocardium (hemoglobin releases about 25% of its O2 content for the body as a whole).

• The coronary venous PO2 is approximately 25-30 mm Hg under resting conditions and decreases during exercise or stress (the cardiac O2 consumption may increase five- to sixfold in well-trained athletes).

FACTORS INFLUENCING CORONARY FLOW

ADENOSINE is a major factor in production of coronary vasodilation during hypoxic states. Adenosine is derived from the degradation of adenosine monophosphate (by 5'-nucleotidase). Release of adenosine into the myocardium produces an extremely strong vasodilator response.

FACTORS INFLUENCING CORONARY FLOW

• SYMPATHETIC STIMULATION increases the cardiac rate and contractility. The resultant increase in myocardial metabolic activity leads to coronary vasodilation.

• If vasodilation is not adequate, the breakdown of adenosine triphosphate (ATP) will lead to the release of ADENOSINE to enhance vasodilation.

FACTORS INFLUENCING CORONARY FLOW

• The coronary vessels contain both alpha- and beta-receptors.

• The alpha vasoconstrictor activity is rather weak, allowing the vasodilator “beta- response” to predominate.

MYOCARDIAL OXYGEN CONSUMPTION AND CORONARY BLOOD FLOW

• In contrast to skeletal muscle, increased myocardial oxygen requirements can be met only by increasing the supply.

• Additional oxygen extraction from the blood is limited. While the oxygen content of venous blood draining resting skeletal muscle is 15 Vol%, coronary sinus blood contains only about 5 Vol%.

• Any increase in cardiac work that increases oxygen consumption, causes an increase in coronary blood flow.

MYOCARDIAL OXYGEN CONSUMPTION AND CORONARY BLOOD FLOW

• The type of cardiac work also plays a role in determining oxygen consumption.

• Increases in cardiac output (induced by increasing preload) at constant arterial blood pressure (afterload).

• Pressure work, require considerably less oxygen than increases in cardiac work involving increased afterload with constant preload (pressure work).

• Stresses such as aortic stenosis and hypertension are much more energy-costly than equal increases in stroke work associated with exercise.

INTERRELATIONSHIP BETWEEN AORTIC PERFUSION PRESSURE

AND THE CORONARY BLOOD FLOW• Between 60 to 150 mm

Hg of BP (mean pressure) in the aorta, the coronary flow practically remains unchanged.

• This is due to autoregulation of the coronary circulation, which survives even when the nerve supply to the coronary vessels are paralysed or removed.

CLINICAL NOTE: TACHYCARDIA AND CORONARY FLOW

• During tachycardia, the diastolic period of a cardiac cycle is shortened but the systolic period is not.

• The left coronary artery is filled principally in the diastole. Conclusion is, all other things remaining constant tachycardia causes myocardial ischemia (hypoxia).

• However, the rise of cardiac metabolism which occurs during tachycardia (by virtue of O2 lack), causes coronary vasodilatation, which cancels the harmful effects of tachycardia.

• Despite this, in elderly people or people with CHD, tachycardia may be dangerous.

CLINICAL NOTE: BLOOD PRESSURE AND CORONARY FLOW

• Conditions characterized by very low diastolic pressure, (e.g. aortic incompetence) are bad for coronary filling.

• In severe cardiovascular shock, where the mean BP falls less than 60 mm Hg, a vicious cycle may be produced.

• Thus, severe myocardial infarction → severe cardiovascular shock and severe fall of BP → insufficient coronary filling due to the low BP → further infarction. Therefore the duty of the attending physician is to cut the vicious cycle off as quickly as possible.

• Vicious cycle - one vice leading to further vices and ultimately causing repetitions of the first vice and thus is an example of + ve feed back.

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