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CONTROL OF BREATHING The part of the brain which controls ventilation rate is the respiratory centre situated in the medulla oblogata part of the brain. The respiratory centre is divided into an inspiratory area and an expiratory area. The respiratory centre is connected to the thoracic area by the nervous system; Phrenic. nerve carries impulses from the respiratory centre to the diaphragm Intercostal nerve carries impulses from the respiratory centre to the intercostal muscles. RESTING VENTILATION The inspiratory centre sue to an intrinsic excitability discharges impulses which pass down the phrenic and intercostal nerves resulting in the diaphragm and external intercostal muscles contracting so increasing the volume of the thoracic cavity. When inspiration occurs stretch receptors in the walls of the bronchi and bronchioles are stimulated. This causes nerve impulses to be sent to the expiratory centre. The expiratory centre sends impulses to the inspiratory centre which is then inhibited and stops discharging impulses. Expiration results from passive recoil of the lungs as the diaphragm and external intercostal muscles relax. Summary Diagram 1 VENTILATION AND EXERCISE When you exercise the ventilation rate must increase in order to supply muscles with more oxygen and to remove excess carbon dioxide. The control of high ventilation rate is dependent of chemoreceptors, receptors that are sensitive to chemical changes in the blood, particularly changes in carbon dioxide and pH. The chemoreceptors are found in three parts of the body; The medulla oblongata of the brain central receptor The aorta peripheral receptors The carotid arteries The medulla oblongata During exercise, there is an increased rate of respiration in the muscles. This leads to an increase in the concentration of carbon dioxide and hydrogen ions. The membranes surrounding the medulla are permeable to carbon dioxide but not to hydrogen ions. Carbon dioxide diffuses into the medulla the following reaction occurs Therefore hydrogen ions form inside the medulla when there is an increase in the concentration of carbon dioxide. The central chemoreceptor detects this increase in hydrogen ions and sends impulses to the inspiratory centre and a group of cells called the ventral group. These both send impulses to the diaphragm and intercostal muscles which increase the strength and rate of contraction, increasing the rate and depth of ventilation. This lowers the concentration of carbon dioxide in the alveoli and so increases the concentration gradient for carbon dioxide, therefore lowering the concentration of carbon dioxide in the blood. When the carbon dioxide level falls the level of hydrogen ions in the medulla oblongata fall and the central receptors stop sending impulses. The ventilation rate returns to normal. 2 Peripheral receptors Some of the peripheral receptors send impulses to the inspiratory centre if they detect very low concentrations of oxygen in the blood. Others are sensitive to low pH which is a result of high levels of carbon dioxide in the blood. Impulses from the peripheral receptors have the same effect on the inspiratory centre as impulses from the central receptors. The rate and depth of ventilation are increased until the pH or oxygen levels return to normal. 3 CONTROL OF HEARTB EAT The heart has an internal pacemaker called the sino atrial node. This is a specialised group of cells in the wall of the right atrium that produces electrical impulses at regular intervals. The specialised muscle tissue of the heart carries these impulses across the whole heart. When nerve impulses reach the muscle cell it contracts. The wave of contraction started by the SAN travels across the atria causing them to contract (atrial systole) There is a layer of connective tissue which separates the atria and the ventricles. This stops the electrical impulses travelling directly to the ventricles. A group of cells called the atrio ventricular node is found between the atria and the ventricles and when impulses from the contracting atria reach the AVN it starts impulses in the purkyne fibres. These are specialised conductive muscle fibres which are grouped together to from the bundle of His which passes between the two ventricles. At the base of the ventricle the bundle of His divides into two branches. Fibres fan out from these and as impulses reach them the ventricles contract from the base upward (ventricular systole) SUMMARY 1. 2. 3. 4. 5. 6. 4 MODIFYING THE HEART RATE The amount of blood flowing from the heart is called the This is dependent on the volume of blood expelled at each heart beat ( ) and the heart rate. Starlings Law shows the relationship between these three One way of controlling cardiac output is by varying the heart rate. The rate at which the heart beats can be modified considerably. Resting heart rate During exercise During sleep The heart rate is changed by nerve impulses from two areas of the brain Both are located in the cardiovascular centre in the medulla oblongata of the brain. Nerve fibres pass from these two areas to the ……………………………………………….. and the ……………………………………………. CARDIOACCELERATO RY CENTRE CARDIOINHIBITORY CENTRE BRANCH OF NERVOUS SYSTEM NEUROTRANSMITTE R EFFECT ON SAN & AVN EFFECT ON HEART RATE 5 Stretch receptors and heart rate Stretch receptors are found in They are connected to the cardioinhibitory centre and have different effects on heart rate when stimulated STRETCH RECEPTOR EFFECT ON HEART RATE As the amount of blood passing through these vessels increases the stretch receptors are stimulated further so the number of impulses to the cardioinhibitory centre also increases. For example when exercising body muscles contract strongly and this increases the rate at which venous blood returns to the heart. The vena cava is stretched by the large quantities of blood returning to the heart so the stretch receptors in the vena cava are stimulated, therefore increasing the heart rate. The increased stroke volume causes the stretch receptors in the and to become stimulated which causes the heart rate to decrease. This is an automatic fail-safe mechanism to prevent the heart from beating too fast. 6 There are a number of stimuli which act directly on the cardiac muscle or the SAN. STIMULUS LOW pH HIGH pH LOW TEMERATURE HIGH TEMPERATURE HIGH OXYGEN LOW OXYGEN EFFECT ON HEART RATE Changes of this nature are usually accompanied by and increase in ventilation rate. Chemoreceptors in the aorta and carotid arteries are sensitive to changes in oxygen and carbon dioxide concentration, but these chemoreceptors are not linked to the cardiovascular centre directly. Many activities affect the cardiovascular centre, for example emotions such as anger and blushing. In such cases impulses are transmitted to the brain where they pass to the cardiovascular centre. The activity of the cardiovascular centre also fluctuates according to the health and age of the individual. 7