Circulation Through Special Regions 1.CORONARY CIRCULATION Prof.Z.Y.Jiang Oct 3, 2008

The right coronary artery has a greater flow in 50% of individuals The left has a greater flow in 20% The flow is equal in 30%. Most of the venous blood returns to the heart through the coronary sinus and anterior cardiac veins The two coronary arteries that supply the myocardium arise from the sinuses behind two of the cusps of the aortic valve at the root of the aorta.

Coronary flow at rest in humans is about 250 mL/min (5% of the cardiac output).

Systole. Flow occurs in the arteries supplying the subendo-cardial portion of the left ventricle only during diastole. Because no blood flow occurs during systole in subendocardial portion of the left ventricle, this region is prone to ischemic damage and is the most common site of myocardial infarction. Blood flow to the left ventricle is decreased in patients with stenotic aortic valves. Since diastole is shorter when the heart rate is high, left ventricular coronary flow is reduced during tachycardia. the coronary vessels are severely compressed during systole, particularly prone to develop symptoms of myocardial ischemia

The autoregulation Factors suspected of playing vasodilator role include: - O2 lack - increased local concentrations of CO2, H+, K+, lactate, prostaglandins, adenine nucleotides, and adenosine.

Neural Factors Activity in the noradrenergic nerves to the heart and injections of norepinephrine cause coronary vasodilation. However, NE increases the heart rate and the force of cardiac contraction, and the vasodilation is due to production of vasodilator metabolites in the myocardium secondary to the increase in its activity.

Neural Factors When the inotropic and chronotropic effects of noradrenergic discharge are blocked by a β-adrenergic blocking drug, stimulation of the noradrenergic nerves or injection of NE in unanesthetized animals elicits coronary vasoconstriction. Thus, the direct effect of noradrenergic stimulation is constriction rather than dilation of the coronary vessels. Stimulation of vagal fibers to the heart dilates the coronaries.

Coronary Artery Disease It is the leading cause of death in both men and women. angina pectoris myocardial infarction. the most common cause of myocardial infarction is rupture of an atherosclerotic plaque, or hemorrhage into it, which triggers the formation of a coronary-occluding clot at the site of the plaque.

CAD happens when the coronary artery muscle become hardened and narrowed, due to the buildup of cholesterol, called plaque, less blood can flow through the arteries, the heart muscle can't get the blood or oxygen it needs. This can lead to chest pain (angina) or a heart attack. Most heart attacks happen when a blood clot suddenly cuts off the hearts' blood supply, causing permanent heart damage.

Angina pectoris Nitroglycerin: great value in the treatment of angina act by stimulating guanylyl cyclase in the same manner as NO does. tonic release of NO is necessary to maintain normal blood pressure

2. CEREBRAL CIRCULATION two internal carotids and two vertebrals.The vertebral arteries unite to form the basilar artery, and the basilar artery and the carotids form the circle of Willis below the hypothalamus.

From this circle the ACA, the MCA, the PCA - arise and travel to all parts of the brain.

The external carotid arteries supply the face and scalp with blood. The internal carotid arteries supply blood to the anterior three-fifth of cerebrum, except for parts of the temporal and occipital lobes. The vertebro-basilar arteries supply the posterior two-fifths of the cerebrum, part of the cerebellum, and the brain stem.

Anterior Cerebral Artery extends upward and forward from the internal carotid artery. It supplies the frontal lobes, the parts of the brain that control logical thought, personality, and voluntary movement, especially the legs. Stroke in the anterior cerebral artery results in opposite leg weakness.  If both anterior cerebral territories are affected, profound mental symptoms may result(akinetic mutism).  

Middle Cerebral Artery is the largest branch Middle Cerebral Artery is the largest branch. The artery supplies a portion of the frontal lobe and the lateral surface of the temporal and parietal lobes, including the primary motor and sensory areas of the face, throat, hand and arm and in the dominant hemisphere, the areas for speech. It is the artery most often occluded in stroke.

The PCA stem in most individuals from the basilar artery but sometimes originate from the ipsilateral internal carotid artery. They supply the temporal and occipital lobes of the left cerebral hemisphere and the right hemisphere. When infarction occurs in the territory of thePCA, it is usually secondary to embolism from lower segments of the vertebral basilar system or heart.

Lenticulostriate Arteries branch from the MCA. Occlusions of these vessels or penetrating branches are referred to as lacunar strokes. About 20% of all stokes are lacunar and have a high incidence in patients with chronic hypertension and in the elderly CT scanning shows signs of infarction in only approximately half of the most of the common form of lacunar stroke (pure motor stroke), but MRI has increased the yield: the probability that CT or MRI will be positive is generally a function of the severity of the deficit .The cells distal to the occlusion die, but since these areas are very small ,Often only minor deficits are seen. When the infarction is critically located, however, more severe manifestations may develop, including paralysis and sensory loss.

Relation of fibrous astrocyte (3) to a capillary (2) and neuron (4) in the brain. The endfeet of the astrocyte processes form a discontinuous membrane around the capillary (1). Astrocyte processes also envelop the neuron. Transport across cerebral capillaries.

The blood–brain barrier: maintains the constancy of the environment of the neurons to refer to the barrier in the capillary walls and the term blood–CSF barrier to refer to the barrier in the choroid epithelium. it seems more appropriate to use the term blood–brain barrier to refer to exchange across both barriers. Passive diffusion very limited, and little vesicular transport takes place. there are numerous carrier-mediated and active transport systems in the cerebral capillaries.

Autoregulation of cerebral blood flow(CBF) during steady-state conditions. The dotted line shows the alteration produced by sympathetic stimulation during autoregulation.