CIRCULATORY SYSTEM Circulation involves BULK FLOW of ECF. A circulatory system requires a pumping organ (heart) to generate the needed pressure, vessels to direct the flow of fluid, and one-way valves to regulate flow. Hearts may be simply pulsating vessels (e.g. as in annelids), tube hearts (e.g. arthropods), or chambered hearts (e.g. vertebrates). The hearts of some animals (e.g. mollusks, vertebrates) are MYOGENIC. These beat spontaneously and are inhibited by ACh. Other animals (e.g. annelids, arthropods) have NEUROGENIC hearts. These are accelerated by ACh and require a neural input. Some animals (e.g. arthropods, most mollusks) have OPEN circulatory systems. These are relatively low pressure. The circulating fluid = HEMOLYMPH. Other animals (e.g. annelids, cephalopod mollusks, vertebrates) have CLOSED circulatory systems. These are relatively high pressure. Vertebrate circulatory systems have four functional parts: Most vertebrates have blood volumes between 5 and 10% of total body volume. Humans are around 8% (i.e. 5 or 6 L for an average adult). [see Figure] Fish have a two-chambered heart and a SINGLE circulation. Reptiles have a three-chambered heart and a DOUBLE circulation. Mammals have a four-chambered heart and a double circulation. [see Figure] [see Figure] A PORTAL VEIN is a separate vessel and capillary bed between an artery and a vein. All vertebrates has an HEPATIC PORTAL VEIN. All but mammals also have a RENAL PORTAL VEIN. [Fig. 12-2] PULMONARY circulation goes from the Right Heart, through the lungs, to the Left Heart. SYSTEMIC circulation goes from the Left Heart, through the body, to the Right Heart. The human heart pumps about 5 to 25 L of blood per minute. Keep in mind that total blood volume is not much more than 5 L! [see Figure] The heart is composed primarily of cardiac muscle (MYOCARDIUM), which is striated, branched, and involuntary. The VENTRICLES do most of the pumping, and the left ventricle is more muscular than the right. Four valves within the heart (two A-V and two semilunar) direct the flow of blood through the heart. [Fig. 12-6] CARDIAC MUSCLE Cardiac muscle cells share some properties with smooth muscle cells; however, they are structurally more similar to skeletal muscle cells. The cells are striated, but branched. Adjacent cells are connected end-to-end at desmosome-containing structures known as INTERCALATED DISKS. There are adjacent GAP JUNCTIONS, similar to those of single unit smooth muscle cells. These represent ELECTRICAL SYNAPSES between contiguous cells, and also allow an action potential to quickly spread from cell to cell throughout the myocardium.
A few cardiac cells are part of a CONDUCTING SYSTEM network, and are specialized for the conduction of action potentials rather than for contraction.
[Fig. 12-9]
[Fig. 12-12] The action potential of a ventricular muscle cell has a prolonged depolarization (i.e. a plateau). This results from an increase in permeability to calcium ions and a temporary decrease in permeability to potassium ions. [see Figure] [see Figure] Due to its prolonged repolarization, ventricular muscle has a very long REFRACTORY PERIOD (about as long as the contraction period). As a result, cardiac muscle CAN NOT be TETANIZED. [Fig. 12-18] [Fig. 12-10] Pacemaker calls (e.g. those within the S-A NODE) are AUTORHYTHMIC. The excitatory signal then travels to the A-V NODE, and via a specialized conducting system (BUNDLE OF HIS and branching PURKINJE FIBERS) travels to the tip of the ventricles and then throughout the ventricular myocardium. [Fig. 12-11]
[Fig. 12-13] PACEMAKER POTENTIALS The ion mechanisms responsible for pacemaker potentials differ from those of myocardial cells. There are T-type calcium channels, which are not found in myocardial cells. There is also a unique group of channels that open when the membrane potential is negative (unlike typical sodium channels). There is a progressive reduction of potassium permeability during the subthreshold phase, but otherwise these channels and the L-type calcium channels are similar to those of myocardial cells.
[Fig. 12-23] Although pacemaker cells are autorhythmic, the rate of their depolarization to threshold is influenced by the ANS. [see Figure] ELECTROCARDIOGRAM (EKG or ECG) This is a composite of the electrical activity of the entire myocardium. [see Animation]
CARDIAC CYCLE This describes the events that occur during one beat of the heart. The contraction phase = SYSTOLE The relaxation phase = DIASTOLE For a typical human heart rate of 70 bpm a single sycle will last about 0.86 s. ATRIAL VENTRICULAR SYSTOLE 0.10 s 0.37 s DIASTOLE 0.76 s 0.49 s [Fig. 12-19 (top)] Note that at the beginning of ventricular systole all valves are closed (ISOVOLUMETRIC CONTRACTION). Eventually the pressure forces open the semilunar valves and the EJECTION phase begins. [Fig. 12-19 (bottom)] When ventricular pressure drops below aortic pressure, the semilunar valves close and we have ISOVOLUMETRIC RELAXATION. When the pressure drops below atrial pressure the AV valves open, leading to VENTRICULAR FILLING. [Fig. 12-20] Note the changes in pressure in the heart chambers and the aorta and the corresponding changes in ventricular volume. Also note when the valves are open and closed. [see Animation] CARDIAC OUTPUT C. O. (L/Min) = Heart rate (beats/min) x Stroke volume (ml/beat) Example: Resting human heart rate of 72 beats/min with a stroke volume of 70 ml/beat. C. O. = approximately 5 L/min = approximately 50 million gallons in a 70 year life span. [see Figure] THE FICK PRINCIPLE The Fick Principle can be used to estimate cardiac output by measuring organismal oxygen consumption and by determining the oxygen content in samples of arterial and mixed venous blood. Example: Oxygen consumption = 250 ml/min, Arterial blood: 0.19 ml oxygen/ml Venous blood: 0.14 ml oxygen/ml Cardiac Output = 250/(0.19 - 0.14) = 5,000 ml blood/min CONTROL OF THE HEART Starling's Law of the Heart (aka Frank - Starling Law (or Mechanism)) This essentially describes the length - tension relationship for the myocardium. The tension (i.e. stroke volume) increases as the muscle is stretched by diastolic filling (i.e. end diastolic volume). [see Figure] (similar to Fig. 12-25) [Fig. 12-26] Although the vertebrate heart is myogenic, autonomic nerves can have both CHRONOTROPIC (i. E. affect the rate) and INOTROPIC (i. E. affect contractility) effects. For example, in addition to accelerating the heart, norepinephrine has a pronounced inotropic effect. [Fig. 12-28] [see Figure] FIBRILLATION Fibrillation is uncoordinated electrical activity of the heart. This can be caused by heart damage, electrocution, etc. and results in a continuous recycling of electrical waves (circus rhythms) through the myocardium. As a result, no blood is pumped. Atrial fibrillation is not fatal, but ventricular fibrillation is unless reversed (spontaneously in some species, but requires a defibrillator in humans). [see Figure]