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FULL NAME OF STUDENT: MAWONEI LINAH MASAWI PIN: P1721110B
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COURSE NAME: PHYSIOLOGY FOR HEALTH SCIENCES II COURSE CODE: BSHN III
ASSIGNMENT NO. 1 DUE DATE 10/03 /2018
ASSIGNMENT TITLE: PHYSIOLOGY FOR HEALTH SCIENCES 11
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OVERALL MARK: _____________ MARKER’S NAME: ________________________
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Issue Date: 3 October 2013 Revision
DISCUSS HOW THE BRAIN’S BLOOD FLOW IS CONTROLLED UNDER NORMAL CIRCUMSTANCES
It is of utmost importance that blood flow to the brain is never interrupted as a few minutes (4-5minutes) interruption would result in damage to that part of the brain. This is because the brain cells depend mostly on oxidative metabolism of glucose for energy. Without oxygen, the brain cannot produce sufficient energy for its upkeep. The brain cannot use other food substrates than glucose and therefore need a constant supply of oxygen and glucose through the blood for its survival.
The circulation of blood to the brain has several distinctive features that are important in preserving a constant for the nerve cells.
The cerebral blood vessels have a highly engineered ‘sprinkler system’ type of arrangement called the circle of Willis by which pressure can be maintained equally around the many units which make up the brain. The carotid arteries have baroreceptors which monitor the main cerebral supply of blood but the brain vessels are independent of the baroreceptor reflex and even when the blood pressure is low in the rest of the system, cerebral vessels do not constrict. The brain has overall control over blood pressure and can maintain its own supply at the expense of all other tissues except the heart. The flow rate is very high and the high capillary density provides rapid and efficient transfer of materials.
The blood flow to the brain remains constant because there are mechanisms that prevent the blood supply from either rising too high or from falling too low. These mechanisms are inherent within the brain itself, and so the blood is said to be autoregulated.
Metabolic factors such as adenosine, potassium ions, hydrogen ion concentration of brain tissue, excess carbon dioxide and low oxygen content of brain tissue all are involved in metabolic autoregulation of brain blood flow. When blood supply to the brain decreases. All the above metabolic factors cause dilatation of the brain blood vessels. Such dilatation improves blood flow to the brain. When blood supply to the brain increases, metabolic products like adenosine, hydrogen and potassium ions are carried away from the brain causing vasoconstriction of the brain blood vessels.
When oxygen supply decreases(hypoxia), the carbon dioxide and hydrogen ion content of the brain tissue increases causing vasodilation of the brain blood vessels and therefore increased blood supply to the brain.
When there is increased oxygen supply to the brain, carbon dioxide and hydrogen ion content in the blood decreases causing vasoconstriction of the brain blood vessels leading to a person feeling dizzy. This happens in hyperventilation i.e. breathing in and out forcefully. Hyperventilation washes out carbon dioxide from the body and hence the fall of carbon dioxide levels. Low carbon dioxide content of the tissues, including the brain would lead to low hydrogen ion concentration of the body fluids. Hyperventilation also supplies more oxygen to the lungs, hence to all body tissues including the brain.
DESCRIBE THE RELATIONSHIP BETWEEN THE ECG AND THE CARDIAC CYCLE.
The heart is a muscle that works continuously like a pump. Each beat of the heart is set in motion by the electrical signal from within the heart muscle.
The electrical activity can be recorded by an electrocardiogram (ECG). It is used to measure the rate and regularity of heartbeats as well as the size and position of the chambers, the presence of damage to the heart. The ECG device detects and amplifies the tiny electrical changes on the skin that are caused when the heart muscle depolarizes during each heartbeat, and then translates the electrical pulses of the heart into a graphic representation.
A typical ECG tracing of the cardiac cycle (heartbeat) consists of the P wave (atrial depolarization), a QRS complex (ventricular depolarization), and a T wave (ventricular repolarization). An additional wave, the U wave (Purkinje repolarization), is often visible but not always.
Each beat of the heart begins with an electrical signal from the Sino Atrial node (SA node), the heart’s natural pacemaker. The SA node is located in the heart’s right atrium. When the heart’s right atrium is full of blood, the electrical signal spreads across the cells of the right and left atrium, causing the atria to contract or squeeze, pumping blood through the open valves from the atria into both ventricles. The P wave on the ECG marks the contraction of the heart’s atria (atrial depolarization).
When the signal arrives at the Atrioventricular node (AV node), near the ventricles, it is slowed for an instant to allow the heart’s right and left ventricle to fill up with blood. on an ECG this interval is represented by the line segment between the P and the Q wave.
The action potential is released and moves next to the Bundle of His located in the interventricular septum and the signal fibres divide into the right and left bundle branches which run through the heart’s septum. On the ECG, this is represented by the Q wave.
The action potential travels from the right and the left bundle branches to the Purkinje fibres that connect directly to the walls of the heart’s ventricles. The signal spreads quickly across the heart’s ventricles causing them to contract but not exactly at the same moment. The left ventricle contracts an instant before the right ventricle. The R wave on an ECG marks the contraction of the heart’s left ventricle. The S wave marks the contraction of the heart’s right ventricle. Q, R and S waves (ventricular depolarisation)
The contraction of the heart’s right ventricle pushes blood through the pulmonary valve to the lungs while the contraction of the heart’s left ventricle pushes blood through the aortic valve to the rest of the body.
As the signal passes, the walls of the heart’s ventricles relax and await the next action potential. On the ECG, the T wave marks the point at which the heart’s ventricles are relaxing. This process continues over and over with the heart’s pacemaker, the SA node firing at a normal rate of 60 -100beats per minute while the AV node slows the impulses from the SA node firing at a normal rate of 40-60beats per minute.
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https://www.ncbi.nlm.nih.gov Glial and neuronal control of brain blood flow -NCBI NIH