Feedback control, C3.1.14, C3.1.15

• What are SAN?


  The pacemaker cells, also known as the sinoatrial node (SAN) in the heart, are a group of cardiac muscle cells in the wall of the right atrium, that receives signals from the cardiovascular centre (in the medulla oblongata) to regulate heart rate.

  The baseline heart rate and stroke volume can be modulated by hormones and nervous system input.
• What is heart rate?

  Heart rate- the pace at which the pacemaker cells trigger the heart to contract .
• What is stroke volume?

  Stroke volume- the volume of blood pumped out of the left ventricle, determined by the force of the contraction.
• What are factors that increase heart rate?

  Exercise

  Anxiety

  Lack of sleep

  Alcohol

  Treat

  Increase in body temperature

  Nicotine

  Caffeine

  Altitude

  Infections
• What are factors that decrease heart rate?

  Being fit

  Relaxation

  Decrease in body temperature

  Sleep
• How does adrenaline affect the heart?

  When experiencing a threat, the amygdala sends distress signals to the hypothalamus, that sends signals to the cells in the adrenal gland that simulate the release of adrenaline into the blood. The adrenaline binds to receptors on the SAN.

  Then, a signal transduction pathway is activated, which leads to the response of the cells initiating electrical impulses at a faster pace, which increases the heart rate.
• What is the medulla oblongata?

  The medulla oblongata, located at the base of the brainstem, controls vital involuntary bodily functions like breathing, heart rate, blood pressure, swallowing and digestion.

  It receives input from sensory neurons that are located in the walls of the aorta and carotid arteries, coordinating a response and sends electrical impulses to the heart to change stroke volume and rate.

  The aortic and carotid artery sensory neurons have receptors that monitor blood pressure, blood pH and blood oxygen. These are known as baroreceptors and chemoreceptors.
• What are baroreceptors?


  Baroreceptors stretch or relax when blood pressure increases or decreases, stimulating a neural signal to the medulla oblongata.
• What are chemoreceptors?


  Chemoreceptors are stimulated when blood oxygen level decreases, or increases (identified via a drop in pH), to activate a neural signal to the medulla oblongata.
• What are the two major nerves that innervate the heart?

  The sympathetic cardiac nerve- causes the heart to increase frequency and force of the contraction

  The vagus nerve- causes the heart to decrease frequency and force of the heart contraction
• What is the negative feedback loop for when blood oxygen levels decreases, and for when blood pressure decreases?

  1- A stimulus (change in physiological variable that triggers the feedback system, such as exercise) decreases blood O2 levels

  2- A receptor (specialised cells that detect the change in the stimulus and send information to the CNS), specifically chemoreceptors on sensory neurons in the aorta and carotid arteries sense decreasing blood O2 levels and send messages to the CNS.

  3- A control centre (the brain) is the part of the system that processes information from the sensor and determines the appropriate response. The medulla oblongata is the part of the brain that modifies heart rate and stroke volume, and is the region of the brain that receives the signal from the chemoreceptors on the aorta and carotid arteries.

  4- An effector (cell tissue or organ that carries out the response) makes adjustments according to the decrease in blood O2 levels. The sympathetic and vagus nerves output from the brain to communicate to the heart, which is the effector.

  5- A response to counteract the original physiological stimulus is carried out, which in this case, is an increase of heart rate and stroke volume, as this increases blood that enters the lungs and therefore increases blood O2 levels.

  The EXACT SAME feedback loop is used to counteract a drop in blood pressure, apart from the receptor- baroreceptors detect the drop in blood pressure, instead of chemoreceptors.
• What is ventilation?


  Ventilation is the process of moving air in and out of the lungs. It is essential for maintaining a concentration gradient of gasses for gas exchange, so that diffusion will transfer oxygen from the lungs to the bloodstream, and carbon dioxide from the bloodstream into the lungs.

  It occurs through inspiration and expiration.

  The ventilation rate is the number of inspiration-expiration cycles per minute. The adult average is between 12-20 breaths per minute.

  The brain, as the central information integration organ, controls the ventilation rate.
• How is ventilation rate controlled?

  Ventilation rate must be modified in response to changes in metabolic demand. For example, during physical activity, metabolically active tissues require more oxygen and produce more carbon dioxide, so the ventilation rate will increase to meet this demand.

  Blood pH influences ventilation rate too. The pH of the blood fluctuates between 7.35 and 7.45, and acidosis (a decrease beyond this) can result from the overproduction of acid (such as lactic acid) or inadequate removal of acids.

  Ventilation rate increases when blood pH drops, as it increases the amount of carbon dioxide exhaled, aiding return of blood pH to a normal ventilation range.

  The rate of ventilation is controlled by the medulla oblongata- it receives input from sensory neurons and coordinates a response to the intercostal muscles and diaphragm.
• What is the negative feedback loop for when blood ph decreases?

  1- A stimulus (change in physiological variable that triggers the feedback system, such as exercise) decrease in blood ph.

  2- A receptor (specialised cells that detect the change in the stimulus and send information to the CNS), specifically chemoreceptors on sensory neurons in the aorta and carotid arteries sense decreasing blood pH levels and send messages to the CNS.

  3- A control centre (the brain) is the part of the system that processes information from the sensor and determines the appropriate response. The medulla oblongata is the part of the brain that regulates ventilation rate, and is the region of the brain that receives the signal from the chemoreceptors on the aorta and carotid arteries.

  4- An effector (cell tissue or organ that carries out the response) makes adjustments according to the decrease in blood pH, carrying the signal to the diaphragm and the intercostals.

  5- A response to counteract the original physiological stimulus is carried out, which in this case, is an increase of ventilation rate that allows secretion of excess CO2 to increase blood pH levels.