Small, Thin Organisms do not Need Circulations

• Cells must move materials & heat from one part to another; organisms must move materials & heat to different parts of the organism
• Diffusion works well for delivering materials over short distances (up to ~100 microns)
• Average diffusion time needed to travel a certain distance can be calculated from this equation:
  
  
  
  

  Distance	Ave. Diffusion Time	Significance
  100 angstroms	0.0000001 sec	Cell membrane thickness
  1 micron	0.001 sec	Size of most bacteria or mitochondria
  10 microns	0.1 sec	Diameter of small eukaryotic cells
  100 microns	10 sec	Diameter of large eukaryotic cells
  250 microns	1 min	Radius of giant squid axon
  2 millimeters	1 hr	Thickness of frog sartorius muscle, half thickness of lens of eye
  5 millimeters	7 hr	Radius of mature ovarian follicle
  2 centimeters	5 days	Thickness of ventricular myocardium
  10 centimeters	120 days	Diameter of sea urchins & other small animals
  1 meter	32 yrs	Half height of human
  Data from Robert Macey. Mathematical models of membrane transport processes. In: Membrane Physiology, edited by Thomas Andreoli, Joseph Hoffman & Darrell Fanestil. NY: Plenum, 1980, p. 125-146.

  
  
• As you can see, diffusional delivery of substances across membranes and within cells will take 0.001 to 60 seconds for most single cells; this is probably fast enough
• Diffusion also seems to be adequate for large flat organisms, such as kelp fronds and flatworms such as tapeworms
  • There is no circulatory system in Porifera (sponges), Cnidaria (sea anenomes & hydra), Nematoda (roundworms) or Platyhelminthes (flatworms) and respiration is by simple diffusion in these phyla

Diffusion is not Sufficient in Larger Animals

• Over longer distances (above ~250 microns) diffusion is inefficient and pumping or convection is required to speed up the transport process
• As animals become larger and more complex circulatory and respiratory systems are required to transport food, wastes, heat, etc.
• In large animals both air and blood are moved by pumps
• Air movement usually uses a simple tidal (in/out) pump- this works because there is an unlimited external source of air
• Blood is circulated, used over and over again

Circulations Need Pumps, Tubing and Valves

• Pumps:
  • Circulatory pumps (hearts) are vessels filled with the circulatory fluid or blood
  • When muscles around the container contract they exert pressure on the blood, causing it to flow
  • Peristaltic type: ring of contraction moves along the vessel in one direction, pushing the blood ahead of it
  • Heart type: pump produces a high pressure which causes blood to flow out through arteries
  • Muscle pump: muscle contraction squeezes veins, causing blood to flow toward heart
• Tubing or pipes = arteries, capillaries, veins
  • Carry blood toward delivery site
  • Sometimes deliver within a few microns of site (capillaries in tissue)
  • Other types open into a large cavity bathing the tissues
  • Blood vessels may be elastic (which helps keep the pressure high between heartbeats)
  • Vessels may constrict and dilate (which gives control over the flow)
• Valves give direction to the flow
  • Blood vessels (including hearts & veins) have flap valves that open in only one direction
  • Example: when pressure increases in veins this opens valves toward the heart and closes those in the other direction -> blood flows toward heart

Circulations Can be Open or Closed

• Open systems:
  • In many invertebrates the circulation is an open system; when blood leaves the heart it flows into a large cavity (hemocoel) which bathes the tissues
  • There are no capillaries and there is no separate interstitual fluid around the cells
  • This type of circulation is found in Arthropods and most Molluscs (except squids & octupuses)

• Closed systems:
  • In most larger animals the blood travels entirely through tubing (arteries, capillaries, veins)
  • There is a separate interstitial fluid around the cells
  • Blood is pumped to within a few microns of where it is needed
  • This type is found in Annelids (segmented worms), Echinoderms (sea urchins & relatives) and Vertebrates (animals with backbones)

• The diagram shows the circulation in an annelid worm: the dorsal blood vessel is a peristaltic pump which brings blood forward
• It is supplemented by 5 pairs of accessory anterior hearts whic are simply contractile blood bessels
  
• Comparison of the 2 types of systems:
  • Open systems have low pressures and are not too efficient
  • "Old" and "new" blood not separated very well
  • Return of blood to heart slow
    • But, this type of system works fine for small animals- note the success of the arthropods
  • Closed systems have high pressures, deliver faster
  • Keep "old" and "new" blood well separated
    • Used in all large animals (including squids & octopuses)
• The last few microns of all delivery systems is always done by simple diffusion
  • Advantageous to have blood vessels close to the cells where delivery occurs

Vertebrates Have an Elaborate Heart with Multiple Chambers

• Circulatory systems are most highly developed in the vertebrates
• Heart has progressed from 2 chambers in the fish, to 3 chambers in the amphibian, then to 4 chambers in mammals and birds
• Chambers are paired:
  • Chamber for collecting the blood = atrium
    • Thin walls
  • Chamber for expelling the blood at high pressure = ventricle
    • Thick walls
• Fish have a 2-chamber heart
  • Output goes to gills, picks up O2, then goes to tissues
    
    
    
• Amphibia expand the chambers to 3
  • Blood pumped to lungs & skin to pick up O2
  • Back to heart to boost pressure
  • One ventricle used to pump to both lungs and rest of tissues
  • Partial separation of oxygenated and deoxygenated blood streams in ventricle due to anatomical features
    
    
    
• Mammals and birds expand chambers to 4
  • Like amphibia, but better separation of oxygenated and deoxygenated blood
  • Allows different pressures in systemic and lung circulations
    
    

The Mammalian Heart is Really 2 Hearts in Series

• Right heart:
  • Pumps to lungs: nearly 100% of the flow goes through the lungs
  • Low pressure side: 25 mm Hg systolic pressure in humans
  • Right ventricle has thin walls
• Left heart:
  • Pumps to rest of body
  • High pressure side: 120 mm Hg systolic pressure in humans
  • Left ventricle has thick walls
• Right & aleft tria:
  • Help fill the ventricles
  • Very low pressure
  • Thin walls
• Pumping of the right & lift sides occurs together
  • Must be accurately balanced, otherwise fluid may accumulate in the lungs (pulmonary edema)
• The muscle pump helps return blood to the heart
  • When muscles contract the veins passing through them are squeezed
  • This causes blood to flow toward the heart
  • Valves prevent flow away from the heart

The Heartbeat Does not Require Nerves

• The heart generates its own beat
  • If all the nerves to the heart are cut it will continue to beat
• Stimulated by series of electrical impulses generated in SA node of right atrium
• Nerves do modulate the heart beat, slowing it (vagus nerve), or speeding it up (cardioaccelerator nerve)
• Heart stimulus travels across the heart, generating the EKG trace on the body surface
  
  
  
• EKG waves:
  • P wave associated with atrial depolarization (stimulation)
  • QRS complex associated with ventricular depolarization (stimulation)
  • T wave associated with ventricular repolarization (recovery)
  • Atrial recovery wave hidden under QRS wave
• Stimulus causes atria to contract before ventricles
• Delay in spread of stimulus to ventricles allows time for ventricles to fill

The Cardiac Output is the Product of Heart Rate and Stroke Volume

• The cardiac output per minute (CO) is the product of the size of a single output, the stroke volume (SV), and the heat rate (HR) in beats/minute:
• It is normally about 5 liters/min in humans
  • Normal heart rate is ~70 beats/in; normal stroke volume is ~70 mL = 0.07 liters
  • CO = HR X SV
  • = 70 beats/min X .07 liters/beat = 5 liters/min
  • Other things being equal, doubling the HR or the SV will double the CO
• In humans heart rate can rise to ~200 beats/min in exercise
  • Faster rates are inefficient because the heart does not have time to fill completely
• Stroke volume can increase about 50% in exercise, to ~100 to 120 mL
  • The changes in HR and SV allopw the exercise cardiac output to increase to 25-30 liters/min

Cardiac Contraction Produces the Blood Pressure

• Blood pressure at the output of the left heart alternates between a high pressure (systole) and a lower pressure (diastole)
• Systole:
  • When the heart beats (systole) the pressure in the arteries leaving the heart rises to about 120 millimeters of mercury (mm Hg)
• Diastole:
  • Between beats (diastole) the arterial pressure drops to about 80 mm Hg
  • The diastolic pressure does not drop to 0 because the arterial walls are elastic
  • The 80 mm Hg diastolic pressure keeps the blood flowing between beats

Pressure Causes Valves to Open and Close in the Heart Cycle

• The heart has 2 sets of valves:
  • AV valves, between atria and ventricles
    • Flap type
    • Chorda tendinae & papillary muscles keep them from being pushed too far
    • Left heart: bicuspid or mitral (2 flaps)
    • Right heart: tricuspid: 3 flaps
  • Semilunar valves where arteries leave heart
  • Blood caught in the 3 cusps pushes them closed
    • Right heart: pulmonary semilunar
    • Left heart: aortic semilunar
  • Leaks in valves -> murmurs
  • No valves where veins enter the atria- not needed, low pressure
• When the heart contracts pressure builds up, forcing the valves to close
• Valves in the cardiac cycle:
  
  

  Event	AV Valves: Right: Tricuspid Left: Mitral	Semilunar Valves: Right: Pulmonary Left: Aortic
  Filling of ventricle	Open	Closed
  Building up pressure	Closed	Closed
  Expelling blood	Closed	Open

  
  
• Opening and closing of valves depends upon the pressures on opposite sides
  • Example: aortic valve
  • Closed during filling and building up of pressure in the left ventricle because pressure in the aorta is higher than pressure in the ventricle
  • When pressure in the left ventricle becomes higher than pressure in the aorta the aortic valve opens and blood is expelled from the heart
• Mark the chambers (RA, RV, LA, LV) and valves (mitral, bicuspid, aortic semilunar, pulmonary semilunar) on this drawing of the mammalian heart. Also label the major blood vessels entering and leaving the heart (aorta, inferior & superior vena cava, pulmonary artery, pulmonary veins). Finally draw arrows to show the direction of blood flow. To check your answers click here.

Different Blood Vessels have Different Functions

• Arteries:
  • Designed for high pressure
  • Elastic: must swell to take up blood expelled by the heart
  • Swelling stretches elastic tissue and keeps the blood pressure fairly high between heart beats
  • Small arteries (arterioles) have muscles that contol their diameters (precapillary sphincters): used to control blood flow through an organ
• Veins:
  • Low pressure
  • Expand to take up blood when animal is not active
• Capillaries:
  • This is where materials are delivered from blood to cells, and vice versa
  • Thin: one layer of flattened (squamous) cells
  • Not elastic