Lecture 18

Heart, cont.

• Heart beat is half of the cardiac output
  
• Stroke Volume (the other half of cardiac output) – depends on 2 variables:
  
  • 1.) End Diastolic Volume – how much blood is going into the ventricles; how the ventricle chambers will fill during Diastoli
    
  • Factors the increase End Diastolic Volume:
    
    • Stronger Atrial contractions
      
    • Increased total blood volume
      
    • Increased venous tone (blood is also stored in the veins) – contracting smooth muscle of the veins increases the amount of blood going back to the heart (thus reducing the available storage area)
      
    • Increase Pumping of Limb Veins – the veins in your arms and legs have semi-lunar valves in them, and when you are running you are increasing amount of blood going back to the heart.
      
    • A drop in Interthoracic Pressure – allowing ventricles to expand a bit; a drop means more negative, you drop the Interthoracic Pressure every time you inhale.
      
    • Increased Peripheral Resistance – it is harder to push the blood out, then more of the blood will remain in the ventricle and get a bigger beat the next beat; there is greater pressure in aorta, and more blood is in the heart because you cannot get as much blood out.
      
  • Factors that decrease End Diastolic Volume:
    
    • Standing up rapidly – get dizzy and feel faint – you get dizzy very quickly b/c you decreased the End Diastolic Volume, and it reacts quickly on your brain, so you get a change in blood pressure that caused the brain to get this dizzy message
      
    • Decreased Ventricular Compliance (Elasticity – the more elastic, the harder it pulls back)(Compliance – is how far something stretches – if we lose compliance we lose the ability to stretch).
      
      • Lose of compliance: When you have a heart attack, a section of your heart dies (it is like dead skin, it can easily burst open), but the damaged area that dies is replaced with Scar Tissue.  Scar Tissue is stronger than the original cardiac tissue, but it does not beat, contract, or expand, therefore you lose compliance.
        
    • Weaker Atrial contraction
      
    • Decreased total blood volume
      
    • Decreased venous tone
      
    • Decrease Pumping of Limb Veins
      
    • An increase in Interthoracic Pressure
      
    • Decreased Peripheral Resistance
      

    2.) Contractility – how much blood is coming out of the ventricle.  Contractility is essentially the Ejection Fraction.
    

    • It is best reflected/measured by the ejection fraction – how much is coming out
      
    • Refer to Chart from Notes – it reflects changes in contractility.
      
    • Graphs:
      
      • Stroke Volume vs. End Diastolic Volume
        
      • When you are under parasympathetic control the curve you get is a log-like graph
        
      • When you get sympathetic stimulation and we are working out heart the curve you get is a higher log-like graph
        
      • This difference is due to a change in contractility and we get a greater ejection fraction.
        

 
 

• Circulation: (we have the blood out of the heart, now where is it going?)
  
  • Blood exhibits Laminar flow – refer to diagram – the blood moves faster in the center of the vessel, and as we go to the outside edges, the blood moves slower.  This keeps on going until we get to a theoretical layer of blood that isn't moving at all (it is theoretical b/c it is infinitely thin layer of blood (which is impossible) and it goes against the wall where it is not moving at all – it is theoretical because it is not there). The most important thing is that blood flow is slower towards the outside edges.
    
    • The RBC's bend to get into the arterial end of the capillary and it stays bent even at the larger venous end of the capillary because of Laminar Flow (even though the RBC diameter is 8.6um and the venous end is 10um). This shape is also ideal for Gas Exchange.
      
    • The capillary is 5um at the arteriole end and 10um at the venous end
      
    • The RBC's tend to go towards the faster flow
      
    • The cells start to get more concentrated as you get towards the center.
      
    • The hematocrit at the edge will be lower the hematorcrit at the center.
      
      • If we split the capillary, then you will get an equal Hematocrit going in both directions
        
      • If we have a capillary coming off of this at a right angle, then that capillary will get a lower Hematocirt b/c it is taking from the blood where there are fewer cells – this  is due to laminar flow and is known as Plasma Skimming.
        
      • The Capillary Hematocrit is 25% lower then the rest of the body
        
    • Laminar flow is smooth (soundless) and exists up to a certain Critical Velocity. Then the flow becomes Turbulent – Turbulent Flow is noisy, swirling, and is going no place. Laminar Flow is silent and efficient.
      
      • Turbulence is used to help take blood pressure, you can hear the sounds. You are listening to the turbulence created when you cut off the circulation of the brachial artery.
        
        • There is a unitless number that we measure that tells us the chances/probability of Turbulence called a Reynolds Number – the higher the number, the higher the probability of having turbulent flow.
          
        • Turbulence usually exists in the human body only under pathological conditions.
          
        • You hear turbulence when you are listening to a person's blood pressure, you are going to squeeze the brachial artery completely and then we reduce the pressure. Initially you are going to heart beats (caused by turbulence) – and those are called Korotkow Sounds.
          
        • Exception - ascending aorta at peak systole
          
          • Usually means accluded artery
            
• Flow is going to equal ∆pressure / resistance
  
  • ∆pressure = mean arterial pressure – mean venous pressure.
    
  • Resistance = (8 x fluid viscosity x tube length) / (π x radius of tube^4).
    
    • The biggest input is the radius – the changes in the radius are the variable that is going into the resistance.
      
    • Blood viscosity is mainly hematocrit
      
    • 19% increase in radius doubles flow
      

     
     

• Blood Pressure
  
  • Look at Pressure vs. Volume charts for arteries and veins.
    
    • For Arteries – we see a positive straight increasing linear graph
      
    • For Veins – we see a low pressure first, then suddenly we get a quick rise.
      
  • Arteries are thought of as Resistance vessels (pressure regulators)
    
  • Veins are thought of as Capacitance Vessels (a storage area for blood).
    
  • The pressure in aorta = 120/80mmHg (Systolic over Diastolic) – normal blood pressure
    
    • The blood pressure will drop when it moves into arteries and then into arterioles
      
    • The Pulse Pressure in the Aorta is 40mmHg
      
      • If you subtract the numbers (120-80) you get a Pulse Pressure
        
      • By the time you get to the end of the arterioles, the Pulse Pressure is 5mmHg.
        
      • In the Capillaries the Pulse Pressure is 0mmHg (non-existent)
        
      • Further away from heart = lower pulse pressure
        
  • If we look at a recording of a blood pressure going up and down, and we are possibly recording it on a polygraph, we will see a little notch, and that is called the Dicrotic Notch – this little notch is due to the closure of the Aortic Semi-lunar Valve
    
  • The blood pressure in Capillaries at the Arteriole end it is approximately 32mmHg.
    
  • The blood pressure in Capillaries at the Venous end it is approximately 15mmHg.
    
    • In the venules (when it gets into the venules), it will be between 12 to 18mmHg.
      
    • When it gets back to the large veins (outside of the thorax) it will be approximately 5.5mmHg.
      
    • At the entrance to Vena Cava (at the entrance of the Right Atrium) it is approximately 4.6mmHg
      
      • This 4.6mmHg will fluctuate due to 2 things: (it will go to 2mmHg and then it will go to 6mmHg cycling back and forth)
        
        • 1.) Part of this is due to the action of the heart contracting
          
        • 2.) It is mainly due to breathing
          
          • During Inspiration it drops to 2mmHg.
            
          • During Expiration it rises to 6mmHg.
            
  • (We are still looking at the capillary picture) The filtration pressure of a capillary is determined by Starling Forces.
    
  • Starling Forces – are:
    
    • Hydrostatic pressure – the measurable pressure (can be measured with a gauge); that is what we are looking at here, the 15mmHg and the 32mmHg
      
    • Osmotic Gradient – osmosis is the flow of water from a greater concentration to a lesser concentration. It is a gradient b/c we will stated in terms of mmHg
      
      • The membrane will only allow water to cross (salt cannot get though). Think of a two chamber system with a permeable membrane between them.
        
        • If one side is diluted with salt, water will flow to that side.
          
        • The level of water stops because the weight of the water equals the draw of the gradient (this is how we determine where the gradient is) – it is how much pressure is needed to offset them
          
        • The Osmotic gradient at the Arteriole end is 25mmHg
          
        • The osmotic gradient at the Venule end is approximate the same at 25mmHg
          
    • At the Arterial end the Hydrostatic Pressure in the capillary is 32mmHg – pushing out, and the Osmotic Gradient will be about 25mmHg pushing in (trying to draw the fluid towards it/inward).
      
    • At the Venule end Hydrostatic Pressure of 15mmHg out, but Osmotic Gradient is still about 25mmHg still inward.
      
    • The area outside the capillary but not inside a cell is called the Interstitium.
      
      • In the Interstitum, we consider the pressure to be about 0mmHg. 
        
      • There is 3mmHg Osmotic Gradient – it is drawing fluid out.
        
    • Calculations:
      
      • Arterial end (we have 32 out, 3 out, and 25 in) = 32 + 3 – 25 = 10mmHG outward – this is the Filtration Pressure
        
      • Venule end (we have 15 out, 3 out, and 25 in) = 15 + 3 – 25 = 7mmHg in (the Filtration Pressure is never a negative values – it just changes inward or outward flow)
        
    • The Filtration pressure is outward at the arterial end and inward at the venous end. The Filtration pressure will change from a 10mmHg out to a 7mmHg get inward (there is a change in direction)
      
    • We will be taking less fluid in then we will be putting out.  This is due to both Filtration Pressure and Starling Forces
      
    • THERE WILL BE FORCES LIKE THIS ON THE TEST (WE WILL HAVE TO CALCULATE A FILTRATINO PRESSURE ON THE TEST) HE WILL GIVE US BLANKS TO FILL IN DURING THE SHORT ANSWER SECTION
      
  • The design of the capillary is a very good explanation of the Starling Forces.
    
  • You never get negative numbers when adding the Starling Forces, you just reverse the flow. It is possible to get zero.  There was a point where zero was reached.
    
• More fluid will be leaving the capillary then will be retrieved by the capillary.
  
• 85% of the fluid/filtrate returns through the Capillaries and Pericytic Venules.
  
• 15% of the fluid/filtrate will return as Lymph (in the Lymphatic System)
  
• Starling Forces will vary from tissue to tissue (ex. The Kidney – filtration and reabsorption)
  
  • This picture that we drew is the drawing that Starling drew when he first talked about Filtration Pressure, but it didn't give a good idea of how the system was working.
    
  • In the Renal Glomerulus, the flow is outward for the entire length of the capillary – this is in humans. In rats and other experimental mammals, there filtration pressure will drop to zero but it never reverses and comes back in. It is all outward flow.
    
  • Intestinal villi is inward flow almost always over its entire length.
    
  • Peritubular capillaries in the kidney – 100% inward flow.
    
• We have an Arteriole coming down and then we have a Through Channel, and they come down the other side, and we have a Venule there
  
• Coming off of this is a Capillary, and that will go into a Capillary Bed
  
• Capillary Beds – consist of a Through Channel with an arteriole end and a venule end.
  
  • This can drain back directly into the Venule or it can go on and connect back on into the Through Cannel.
    
  • Sitting around this is a Capillary Sphincter, it is not innervated by anything, but it is just regulated by local prostaglandins in the tissue
    
• Skeletal muscle doesn't have capillary sphincters
  
  • The Capillary sphincter will wink open and close (they are not innervated) – there is an outward and inward flow. When they are open the fluid is basically going out into the tissue, and when they are closed the fluid is going back into the capillary bed.  There are a series of capillary beds functioning like this, and there is even another set of capillaries coming off too.
    
    • Capillary sphincters aren't opening or closing at the same time, so we are getting a steady flow of blood in and out of the tissue at the same time.
      
  • If we drop the pressure inside the blood vessel, then the Hydrostatic pressure of the Interstiutium/outside will be enough to collapse the capillary, stopping the blood flow through – this is called the Critical Closing Pressure.
    
    • Once it collapses, there is nothing going back into it, it is not absorbing anything.
      
    • There are some other ways that the capillaries get things out (besides Starling Forces and Hydrostatic Pressure) such as through little Vacuoles. 
      
  • Capillaries mainly in the CNS will use Starling Forces and Transcytosis.
    
  • Transcytosis (CSF, aqueous humor, endolymph of ear) – are all leaving in this direction, all being returned to the venous system. This is the main system for the CNS.
    
    • How Transcytosis works is that a cell that has Vacuoles will open up, which pick up fluid at one end, then travel to the other end and let the fluid out. The vacuole will then travel back to the beginning to where it can pick up more fluid.
      
      • Sometimes the vacuoles are stacked together (taking stuff in or letting stuff out).  Sometimes a vacuole comes between them and makes a channel between them.
        
    • We see Transcytosis associated with the Central Nervous System.  Some examples are:
      
      • When the Cerebral Spinal Fluid is being returned to the blood stream (if we increase the Hydrostatic Pressure behind the CSF it will increase the flow of the CSF in the Dura Sinus)
        
      • When we are returning Aqueous Humor in our eye into the Canal of Schlemm.