Circulatory Systems: Open vs. Closed

Open and Closed Circulatory Systems

• A circulatory system needs a circulatory fluid, interconnecting vessels, and a muscular pump (heart).
• The circulatory system uses fluid to connect cells with organs that exchange gases, absorb nutrients and dispose of waste.
• Circulatory systems are either open or closed.
• Hemolymph, a circulatory fluid, bathes organs directly in an open circulatory system in insects, some mollusks, and other arthropods.
• Blood is confined to vessels and is distinct from interstitial fluid in a closed circulatory system.
• Annelids, cephalopods, and vertebrates have closed circulatory systems.

Organization of Vertebrate Circulatory Systems

• Humans and other vertebrates possess a closed cardiovascular system.
• Arteries, veins, and capillaries are the three main types of blood vessels.
• Blood flows in one direction within the vessels.
• Arteries branch into arterioles, which carry blood away from the heart to capillaries.
• Capillary beds are networks of capillaries where the exchange of chemicals occurs between blood and interstitial fluid.
• Venules converge into veins, which return blood from capillaries back to the heart.
• Arteries and veins are distinguished by the of blood flow, not by oxygen levels.
• The atria are where blood enters and ventricles are where blood is pumped out in hearts with two or more chambers.

Single Circulation

• Bony fishes, rays, and sharks have a two-chambered heart and single circulation.
• Blood leaving the heart passes through two capillary beds before returning in single circulation.

Double Circulation

• Amphibians, reptiles, and mammals have double circulation.
• Oxygen-rich and oxygen-poor blood are pumped independently from the right and left sides of the heart.
• Oxygen-poor blood flows through the pulmonary circuit to get oxygen via the lungs in reptiles and mammals.
• Oxygen-poor blood flows through a pulmocutaneous circuit to get oxygen via the lungs and skin in amphibians.
• Oxygen-rich blood moves oxygen via the systemic circuit.
• Double circulation keeps pressure in the organs higher than in single circulation.

Evolutionary Variation in Double Circulation

• Some vertebrates with double circulation are intermittent breathers.
• Amphibians and many reptiles can go long times without gas exchange, instead relying on gas exchange from skin.
• A three-chambered heart is present in frogs and other amphibians, two atria and one ventricle.
• The ventricle contains a ridge that diverts oxygen-rich blood into systemic and oxygen to the pulmocutaneous circuit.
• Blood flow to the lungs is nearly shut off when underwater.
• Turtles, snakes, and lizards have a three-chambered heart with two atria and one ventricle, partially divided by an incomplete septum
• Alligators, caimans, and other crocodilians have a septum dividing the ventricles, but pulmonary and systemic circuits connect where arteries exit the heart.

Mammals and Birds

• Mammals and birds have a four-chambered heart with two atria and two ventricles.
• The left side of the heart receives/pumps oxygen-rich blood, while the right side receives/pumps oxygen-poor.
• Mammals and birds are endotherms needing more O2 than ectotherms.

Coordinated Cycles of Heart Contraction in Mammals

• The mammalian cardiovascular system needs to meet body's continuous need for O2.

Mammalian Circulation

• The right ventricle contracts and pumps blood to the lungs via the pulmonary arteries.
• Oxygen is loaded, and CO2 is unloaded once blood passes through capillary beds in the lungs.
• Oxygen-rich blood from the lungs then travels through the pulmonary veins to the left atrium.
• The oxygen-rich blood moves into the left ventricle, then is pumped out via the systemic circuit to body tissues.
• Blood exits the left ventricle through the aorta, which transports blood to arteries throughout the body.
• Coronary arteries, which supply the heart muscle, are the starting branches.
• Branches then go to capillary beds in the head and forelimbs.
• Additional branches then lead to capillary beds in the abdominal organs and hind limbs.
• Blood delivers O2 and picks up CO2 in tissues.
• Capillaries then link up to form venules, moving blood to veins.
• Oxygen-poor blood from the head, neck, and forelimbs is channeled into the superior vena cava.
• The inferior vena cava drains blood from the trunk and hind limbs.
• The two venae cavae brings blood into the right atrium where oxygen-poor blood flows into the right ventricle.

Mammalian Heart

• A human heart is about the size of a clenched fist and mostly cardiac muscle.
• The walls of the two atria are thin, serving as blood collectors when returning to heart.
• Ventricles walls are thicker than atria and contracts more forcefully.
• The heart contracts and relaxes in a cardiac cycle rhythm.
• The contraction, or pumping, phase is called systole.
• The relaxation, or filling, phase is called diastole.
• The cardiac output is the volume of blood pumped into systemic circulation per minute depending on heart rate/stroke volume (72 beats x 70 ml = 5 liters).
• The heart rate is the number of beats per minute (72 beats per minute).
• Stroke volume is the amount of blood pumped in a contraction (70 ml).
• Four valves prevent backflow of blood in the heart.
• The atrioventricular (AV) valves separate each atrium and ventricle.
• The semilunar valves control blood flow to the aorta and the pulmonary artery.
• The "lub-dup" sound of a heartbeat is from the recoil of blood against the AV valves (lub) then against the semilunar valves (dup).
• The backflow of blood through a broken valve creates a heart murmur.

Maintaining the Heart’s Rhythmic Beat

• Some cardiac muscle cells are autorhythmic; they contract without a signal.
• The sinoatrial (SA) node/pacemaker decides timing and rate for cardiac muscle cells.
• It is located near where the superior vena cava joins the right atrium.
• Muscle cells are electrically connected through gap junctions.
• Impulses travel from the SA node to the atrioventricular (AV) node in the wall between the right and left atria.
• The impulses are delayed (0.1 second) and then travel to the Purkinje fibers, causing ventricles to contract.
• An electrocardiogram, ECG or EKG records impulses during the cardiac cycle.
• Neural and hormonal control regulates the pacemaker.
• The nervous system regulated the pacemaker through parasympathetic divisions divisions.
• The sympathetic division speeds up the pacemaker, while parasympathetic slows it down.

Blood Pressure and Blood Vessels

• Vertebrate circulatory uses vessels that exhibit a close match of structure and function.
• All blood vessels have a central lumen with an epithelial layer lining blood.
• The endothelium is smooth, lessening resistance.
• Capillaries are only slightly wider than a red blood cell.
• Capillaries' thin walls plus the endothelium and basal lamina facilitate material exchange.
• Arteries and veins possess connective tissue, smooth muscle, and endothelium.
• Arteries have thick elastic walls that accommodate pressure of pumped blood.
• Arteries bulge when blood pressure increases and recoils when the heart relaxes.
• Blood flow is mainly from muscle action in the thinner-walled veins.
• Veins contain valves to maintain unidirectional blood flow.

Blood Flow Velocity

• Physical laws governing movement of fluids through pipes affect blood flow and blood pressure.
• Water cannot be pressurized under pressure.
• Due to high resistence and large area, the blood flows more slowly in the capillary beds (500 times slower).
• Blood flow in capillaries must be slow in order be exchange materials.
• Blood flows from areas of higher to lower pressure.
• Blood pressure is a force exerted in all directions, including against blood vessel walls.
• Force exerted sideways stretches the artery walls.
• Recoil of elastic arterial walls helps maintain blood pressure.
• Blood flow resistance in tiny capillary/arteriole diameters dissipates much of pressure.

Blood Pressure and Cardiac Cycle

• Systolic pressure is the pressure in the arteries during ventricular systole, and it's the highest pressure in the arteries.
• A pulse is the rhythmic bulging of artery walls with each heartbeat.
• Diastolic pressure is the pressure in the arteries during diastole; it is lower than systolic pressure.

Blood Pressure Regulation

• Homeostatic mechanisms control blood arterial pressure by altering arteriole diameter.
• Constriction of smooth muscle in arteriole walls results in vasoconstriction, increasing blood pressure.
• Relaxation of smooth muscles in the arterioles leads to vasodilation, causing blood pressure to fall.
• Nitric oxide (NO) induces vasodilation.
• The peptide endothelin induces vasoconstriction.
• Vasoconstriction and vasodilation are often coupled to changes in cardiac output that affect blood pressure.
• Cardiac output can increases and arterioles dilate during exercise
• Blood pressure is usually determined for an artery in the arm close to the heart the heart.
• Blood pressure for a healthy 20-year-old human at rest is around 120 mm Hg at systole and 70 mm Hg at diastole.
• Gravity can affect blood pressure.
• Being upright will lower blood pressure in the brain is about 27 mm Hg less.
• Fainting can happen from inadequate blood flow to the head.
• High systolic pressure is required for animals with long necks such as giraffes to pump blood (systolic= 250 mm Hg).
• Blood pressure is low in veins, so valves stop backflow of blood.
• Smooth muscle in the walls of venules and skeletal must contract muscles to return blood.