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Questions 1 and 2
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Physiology for health sciences 2
1 Discuss how the brain’s blood flow is controlled under normal circumstances
Blood flow refers to the movement of auto regulatory mechanism and severely depresses brain activity.

The mechanism remains blood through a vessel, tissue or organ, and is usually expressed in terms of volume of blood per unit of time.

The brain uses 20% of available oxygen for normal function .Total blood flow to the brain average about 650-750 mills /min and is maintained at a relatively constant level. The mechanism of autoregulation in the brain are not completely understood and likely differ with increase vs decrease and pressure.

Oxygen deficit is a much less potent stimulus for autoregulation, greatly excessive carbon dioxide levels abolish
Poorly understood autoregulation is multifactorial phenomenon, of the cerebral circulation, including myogenic autonomic and metabolic mechanism.

The myogenic mechanism
It is the intrinsic property of the vascular smooth muscles to respond to changes in intravascular pressure, then myogenic activity arises from arteries and arterioles denuded of endothelium and autonomic nerve control.

Endothelium produces vasoactive mediators, nitric oxide, prostacyclin and endothelium derived polarizing factors
-744220000DDdlationship between the ECG and cardiac cycle
The brain receives blood from two sources; the internal carotid arteries which arise at the point in the neck where the common carotid arteries bifurcate, and the vertical arteries. The internal carotid arteries branch to form the two major cerebral arteries the internal and middle cerebral arteries, the right and left vertebral arteries come together at the level of the brainstem to form the midline basilar artery.

The basilar artery joins the blood supply from the internal carotids in an arterial ring at the base of the brain called the circle of Wills
The posterior cerebral arteries arise at this confluence as do two small bringing arteries the anterior and posterior communicating arteries conjoin the two major sources of cerebral vascular via the circle of Wills improving the chance of any region of the brain continuing to receive blood if one of the major arteries become occluded.

The major branches that arise from the internal carotid artery –the anterior and middle cerebral arteries form the anterior circulation that supplies the forebrain.

These arteries also originates from the circle of Wills, each gives to branches that supply the cortex and branches that penetrates the basal surface of the brain ,supplying deep structures such as the basal ganglia ,thalamus and internal capsule. Particularly prominent are the lenticulostrate arteries that branch from the middle cerebral artery .These arteries supply the basal ganglia and thalamus.

The posterior circulation of the brain supplies the posterior cortex, the mid-brain and the brainstem ,it comprises an arterial branch as arising from the posterior cerebral ,basilar and the vertebral arteries ,midline arteries supply medial structures lateral arteries supply the lateral brainstem, dorsal lateral arteries supply dorsal brainstem structures and the cerebellum.

The posterior inferior cerebellar artery and the anterior inferior cerebellar artery, supply distinct regions of the medulla and the pons, these arteries as well as the branches of the basilar artery that penetrate the brainstem from its ventral and lateral surfaces are especially common sites of occlusion and results in specific functional deficits of cranial nerve, somatic sensory and motor function cited; (Sinecure Associates Inc.; 2001).


The ECG works by detecting and amplifying tiny electrical changes on the skin and that occur during heart muscle depolarization .The output for the ECG form a graph that shows several different waves, each corresponding to a different electrical and mechanical event with the heart changes are used to identify problems.

P – Indicates atrial depolarization in which the arterial contract (atria systole), increased or decreased P wave can indicate problems with the potassium ion concentration in the body that will later the nerve activity .A missing P wave indicates arterial defibrillation a cardiac arrhythmia ,the heart beat irregularly .

QRS complex
Ventricular defibrillation and contraction (ventricular systole),
Q&S downward and R upward wave QRS represents action potentials moving front the AV node through the bundle of His and left and right brunches and Purkinje fibers into the ventricular tissues, abnormalities may indicate hypertrophy or myocardial infarction.

The T wave, ST segment
The T wave indicates ventricular depolarization, ventricles relaxes following depolarization and contraction.

ST segment, the gap (flat or slightly up curved line) between the S wave and T wave indicates the time between ventricular depolarization and repolarization.
An elevated ST indicates myocardial infarction myocardial ischemia
U wave indicates repolarization of the purkinje fibers.

The cardiac cycle
The relaxation and contraction occurs as the heart works to pump blood through the body, the heart rate describes the frequency of the cardia cycle and the heart rate is the rate is the number of contractions of the heart in one
And is expressed as ‘beats per minute’, there are three major stages; arterial systole, ventricular systole and the cardiac diastole.

Arterial systole is the contraction of the atria that causes ventricular filling.

Ventricular systole is the contraction of the ventricles in when blood is ejected into the pulmonary artery or aorta, depending on side.

Complete cardiac diastole occurs after systole .The blood chambers of the heart relaxes and fill with blood once more continuing the cycle.

Every single beat of the heart include the three major stages
However, pulse is a way of measuring heartbeat based on the arterial distention’s or pulses that occur as blood is pushed through the arteries., normal record of blood pressure is from 90/60-120/80mmhg.

Cardiac diastole
During the systole phase, the ventricles contract pumping blood into the arteries. The right ventricle sends via the pulmonary artery. The left ventricle pumps blood to the aorta.

One cardiac cycle is completed when the heart fills with blood and the blood is then pumped form the heart
C .T.Musabayane (2004) Module BSHN 111
Ross and Wilson (2011); Anatomy and Physiology
Google Sninaure Associates incl et al (2014)
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