Lecture 19

Vascular Regulation: (what regulates blood flow)

• To a certain extent, a tissue can regulate its own blood flow – this is called Autoregulation. (Autoregulation is found in the kidney too) Autoregulation has several mechanisms:
  
  • Myogenic Mechanism – (has to do with the blood vessels) the greater the blood pressure out on the arteriole smooth muscle, the more it stretches. The smooth muscle will contract to resist the stretch. This keeps blood flow more constant.
    
  • Metabolic Mechanism – decreases the oxygen level (decreases the partial pressure of oxygen (P_O2)) in a tissue.  This will cause Vasodilation of the blood vessel and will allow more blood to go to the tissues with low amount of oxygen.
    
  • Things that cause Vasodilation:
    
    • Decrease the Oxygen Level (decreasing the partial pressure of oxygen)
      
    • Increase the Carbon Dioxide Level (increasing in Partial pressure of carbon dioxide)
      
    • Decrease the pH.
      
    • Increase the Temperature (more active tissue will generate more head – so if we increase the temperature we will increase the blood going to that tissue)
      
    • In Skeletal muscle – if we increase the Potassium level or if we increase the Lactate level (lactate builds up when we are doing a great deal of anaerobic activity of the muscle) it will increase the amount of blood going to the muscle.
      
    • In Cardiac muscle – increase in Adenosine
      
    • The more active the tissue, the more blood it will receive.
      
• Endothelial Factors:
  
  • Endothelin – is a vasoconstrictor that is created by endothelial tissue.  It is found in arterioles and arteries. Endothelin is secreted in response to stretching of blood vessels, causing smooth muscle to contract, and it will be secreted with other stimuli
    
  • Nitroic Oxide (NO) – is a Vasodialator released in response to the stretching of blood vessels plus other stimuli (it is released the same way Endothelin was released)
    
    • We don't find NO in every blood vessel.  If we push on the walls of the endothelial cells, this will constrict to fight that, but if we have NO in the system, the more you push on it the bigger it gets.  It will dilate and the pressure drops in the vessel itself.
      
    • The Arcuate Artery and Interlobar Artery (are the last 2 arteries in the kidney (that go to the cortex of the kidney) prior to the blood going to the afferent arterioles going to the nephrons) will stretch with the presence of NO.  The afferent arterioles and the arteries prior to the Arcuate artery do not have this ability to stretch in the presence of NO.
      
      • It helps to be part of the Myogenic system of the kidney, as you increase the blood pressure, it will expand, and the blood pressure will drop. It maintains the blood pressure to the kidney. 
        
  • The roles vary from tissue to tissue.
    
  • Sympathetic effects can change the circulation
    
• Sympathetic effects can change the Circulation, such as:
  
  • 1.) Vasomotor Tone
    
  • 2.) Fight or Flight
    
  • 3.) Baroreceptors – are stretch receptors in heart and surrounding vessels, which are monitoring blood pressure. Baroreceptors tell the brain lots of information.  The higher the pressure, the higher the firing rate will be.  There are 2 types of Baroreceptors:
    
    • 1.) High Pressure Baroreceptors –they are located in the Aortic arch (how much pressure is going out to the body), and are in the Carotid Sinus (how much pressure is going out to the brain) (located at the base of the internal carotid artery – regulating pressure to brain, at split of common carotid to the internal and external).  These High Pressure Receptors are monitoring blood pressure.  They monitor Atrial blood pressure, which is the pressure going out into the brain and out of the heart. These receptors increase their firing rates when the blood pressure increases.
      
    • 2.) Low Pressure Baroreceptors – they increase their firing rate when blood pressure goes up. They are located in the walls of both atria of the heart.
      
      • In right atrium they are very near the openings for the Superior vena cava and Inferior vena cava – in the walls of the atrium.
        
      • In left atrium they are near the Pulmonary veins – in the walls of the atrium.
        
      • There are 2 Types of Low Pressure Baroreceptors:
        
        • Type A – they discharge their electrical activity during Atrial Systole – how high pressure goes during contraction
          
        • Type B – they discharge their electrical activity late in Diastole – how high and low it goes during the Relaxation stage.
          
      • Because of this discharge system, the brain can tell pressure differences due to contraction, and due to over filling.
        
      • With these 2 types of firing mechanisms, the brain can monitor the blood volume in the venous and arteriole systems (it know the volume of the blood in the body), and it also does this with the pulmonary system (how much blood is going in)
        
        • They will lower their firing rate in response to lower blood volume even though artial pressure stays high – we are not quite sure how this works, but we think veins give input.
          
• Chemoreceptors (Chemical Receptors) – are called Bodies. They are actually made for regulating breathing, but as a secondary function they are used for blood input changes.
  
  • Carotid Bodies (sit on the back of the carotid sinus, and monitor things going specifically what is going to the brain) and Aortic Bodies (monitor things going to the rest of the body) – the bodies are much smaller
    
  • These chemical receptors respond to:
    
    • 1.) Changes in the amount of Oxygen
      
    • 2.) Changes in the amount of Carbon Dioxide
      
    • 3.) Change in the pH levels
      
    • These receptors are mainly involved in regulating breathing
      
    • The chemical receptors can also affect blood pressure.
      
    • The following increase heart rate and increase blood pressure:
      
      • A decrease in oxygen levels increase heart rate and blood pressure.
        
      • An increase in carbon dioxide levels increase heart rate and blood pressure.
        
      • A decrease the pH increase the heart rate and blood pressure.
        
    • Anything that regulates fluid volume can directly or indirectly effect heart rate or blood pressure:
      
      • ADH (Anti-diuretic hormone) will increase blood pressure
        
      • Angiotension II will increase blood pressure.
        
      • ANP - Atrial Natrauretic Protein/Factor will decrease the blood pressure (it is the hormone produced by the atria of the heart in response to high blood pressure in the atria)
        
      • VIP – Vasoactive Intestinal Peptide will decrease the blood pressure.  It throws electrolytes into GI tract lumen, causing water to come out, and you lose water/fluid this way.
        
      • ETC.