Chapter 49
Circulatory Systems
Circulatory Systems
Circulatory Systems: Pumps, Vessels, and Blood
Vertebrate Circulatory Systems
The Human Heart: Two Pumps in One
The Vascular System: Arteries, Capillaries, and Veins
Blood: A Fluid Tissue
Control and Regulation of Circulation
Circulatory Systems: Pumps, Vessels, and Blood
A circulatory system is composed of a pump (heart), fluid (blood), and conduits (blood vessels).
This is also called a cardiovascular system.
Circulatory Systems: Pumps, Vessels, and Blood
Some simple aquatic animals do not have circulatory systems.
In small animals, nutrients, gases, and wastes can diffuse between the cells and the environment and a circulatory system is not needed.
Some aquatic animals have a flat body shape or a highly-branched __________ cavity to provide maximum surface area for exchange.
Larger animals with many cell layers must rely on extracellular tissue fluids to carry materials to and from cells.
Circulatory Systems: Pumps, Vessels, and Blood
In open circulatory systems, the blood or circulating fluid is not kept separate from the tissue fluid.
The most simple systems squeeze tissue fluid through and around intercellular spaces.
There is usually a pump or heart to help propel the fluid, and vessels direct it, but the fluid leaves the vessels to trickle through tissues.
Arthropods, mollusks, and other invertebrates utilize this type of circulatory system.
Circulatory Systems: Pumps, Vessels, and Blood
A __________ circulatory system keeps the blood and tissue fluid separate.
One or more muscular hearts and a branching network of vessels (the vascular system) move the blood.
The earthworm has a closed circulatory system.
The large __________ and __________ vessels are connected by 5 vessels that serve as __________. Direction of blood flow is controlled by one-way valves.
Circulatory Systems: Pumps, Vessels, and Blood
Closed systems have advantages over open systems:
Blood flow, nutrient delivery and waste removal are more rapid.
Closed systems can direct the blood to specific tissues.
Cellular elements and large molecules that aid in transport can be kept within the vessels.
Closed systems generally support higher levels of metabolic activities.
Insects are an exception to this rule, but they do not rely on their circulatory systems for gas exchange.
Vertebrate Circulatory Systems
Vertebrates have closed systems and hearts with two or more chambers. __________ prevent blood backflow when the heart contracts.
A theme in vertebrate evolution is a progressive separation of the blood that goes to the lungs from blood that goes to the rest of the body.
This culminates in two circuits: a pulmonary (lung) circuit and a systemic (body) circuit.
Vertebrate Circulatory Systems
Vertebrate systems have arteries to carry blood away from the heart and veins to bring blood back to the heart.
Arteries give rise to smaller vessels called arterioles, which feed blood into the capillaries.
Capillaries are thin-walled vessels through which materials are exchanged between the blood and the tissue fluid.
Small vessels called venules connect the capillary beds with the larger veins.
Vertebrate Circulatory Systems
In fish, the heart has two chambers: an atrium to receive blood and a ventricle to pump blood.
Blood is pumped to the gills for gas exchange, then through the aorta to the rest of the body.
Blood collects in the veins after passing the capillary beds and eventually returns to the atrium.
Blood pressure is low in the aorta as the narrow gill spaces dissipate flow force. This limitation does not seem to hamper the fish.
Vertebrate Circulatory Systems
African lungfish evolved a primitive lung from an out-pocketing of the gut.
A pair of modified gill arteries direct some of the blood coming from the heart to the lung, and a new vessel carries oxygenated blood from the lung back to the heart.
The heart has a partly divided atrium; the left side receives oxygenated blood from the lung, the right side receives deoxygenated blood from the body.
The blood streams stay separate as they go through the ventricle. Oxygenated blood goes mostly to the body; deoxygenated blood goes to the lung.
Vertebrate Circulatory Systems
Amphibians have __________-chambered hearts with one ventricle and two atria.
One atrium receives deoxygenated blood from the body; the other receives oxygenated blood from the lungs.
Mixing of blood in the ventricle is minimized by a __________ separation. Deoxygenated blood is directed to the pulmonary circuit; oxygenated blood is directed to the aorta.
This partial separation of pulmonary and systemic circulation permits delivery of blood in the aorta and tissues at a higher pressure than in the fish.
Vertebrate Circulatory Systems
Reptilian hearts have two atria and a ventricle that is divided, so mixing of oxygenated and deoxygenated blood is minimized.
Reptiles have a large range of metabolic needs, from very active states to resting states with very low metabolism.
This range of demands means that they do not have to breathe continuously.
The lung circuit can be bypassed when they are not breathing.
Vertebrate Circulatory Systems
Turtles, snakes, and lizards have two aortas and a ventricle that is incompletely divided by a septum.
When the animal is not breathing, this setup allows constriction of blood vessels in the lung circuit and shunting of blood through only one aorta to the systemic circuit.
Vertebrate Circulatory Systems
Crocodiles have a true __________-chambered heart and two aortas, one from each ventricle, with a channel connecting them.
When a crocodile is breathing, higher pressure in the left ventricle and aorta is communicated to the channel; this prevents right-ventricle blood from entering the aorta and right ventricle blood flows to the pulmonary circuit.
When the animal is not breathing, pressure builds up in the right ventricle until it exceeds that of the right aorta. Blood from both ventricles flows through the two aortas and the systemic circuit, and little blood flows into the pulmonary circuit.
Vertebrate Circulatory Systems
The four-chambered hearts of birds and mammals completely separate the pulmonary and systemic circuits.
The advantages of separate circuits are:
Oxygenated and deoxygenated blood cannot mix.
Gas exchange is maximized because the lungs receive only blood with low O2 and high CO2 content.
The separate circuits can operate at different pressures.
The Human Heart: Two Pumps in One
The left and right sides of the human heart may be thought of as separate pumps.
The left pump delivers blood to the systemic circuit.
The right pump delivers blood to the pulmonary circuit.
Atrioventricular valves between the atria and ventricles prevent backflow into the atria when the ventricles contract.
The pulmonary valve and aortic valve prevent backflow into the ventricles.
The Human Heart: Two Pumps in One
The right atrium receives blood from the superior and inferior vena cavas.
From the right atrium, blood goes to the right ventricle.
The right ventricle sends blood through the pulmonary artery to the lung.
Pulmonary veins return __________ blood to the left atrium.
From the left atrium, blood goes to the left ventricle.
The left ventricle sends blood through the aorta to the body and the capillary beds.
Blood returns to the right atrium via veins.
The Human Heart: Two Pumps in One
The left ventricle is more muscular because the resistance of the systemic circuit is much greater than that of the pulmonary circuit.
In the cardiac cycle, ventricle contraction is called systole and ventricle relaxation is called diastole.
At the end of dyastole, the atria contract.
The sounds of the cardiac cycle (the lub-dub) are caused by the closure of heart valves.
Defective valves produce heart murmurs, whooshing sounds following the lub.
The cardiac cycle can also be felt in artery pulsation, the surge of blood during systole.
The Human Heart: Two Pumps in One
Blood pressure may be measured by a stethoscope and a sphygmomanometer, which compares the __________ with __________ pressure.
A normal young adult might show a systolic pressure of 120 mm of Hg and a diastolic pressure of 80 mm of Hg, or 120/80.
The Human Heart: Two Pumps in One
Cardiac muscle cells are in electrical contact with one another through gap junctions.
This permits coordinated contraction for effective blood pumping.
Some cardiac muscle cells (pacemaker cells) initiate action potentials without nervous stimulation.
The primary pacemaker of the heart is the sinoatrial node located at the juncture of the superior vena cava and the right atrium.
The resting membrane potential of these cells become more negative until they reach the threshold for initiating an __________ potential.
The Human Heart: Two Pumps in One
The autonomic nervous system controls heart rate by influencing the rate at which pacemaker cells gradually depolarize.
Acetylcholine and norepinephrine from the parasympathetic and sympathetic nerve endings slow and increase the rate, respectively.
The Human Heart: Two Pumps in One
A normal heartbeat begins with an action potential in the sinoatrial node.
The action potential spreads through the atrial cells, causing them to contract in unison.
The ventricles do not contract in unison with the atria because there are no gap junctions between the cells of the atria and ventricles.
The Human Heart: Two Pumps in One
The atrioventricular node is stimulated by depolarization of the atria; with a slight delay it generates action potentials that are conducted to the ventricles via a bundle of fibers called the bundle of His.
The bundle of His fibers spread throughout the ventricular muscle mass as Purkinje fibers.
Purkinje fibers evenly distribute the action potential throughout the ventricular muscle.
The delay in the spread of action potentials through the heart assures that the atria contract before the ventricles do.
The Human Heart: Two Pumps in One
Contractions of the ventricle muscle fibers last for about 300 milliseconds.
Ventricular muscle cell action potentials are initiated by the opening of voltage-gated sodium channels.
However, unlike neurons and skeletal muscle, ventricular muscle cells remain depolarized for a long time.
The plateau of ventricular muscle cell action potential is due to a sustained opening of voltage-gated Ca2+ channels.
As long as their Ca2+ channels remain open, the ventricular muscle cells continue to contract.
The Human Heart: Two Pumps in One
An electrocardiogram (EKG) is a record of electrical events in cardiac muscle during the cardiac cycle.
Electrodes placed on the body surface at different remote locations detect heart action potentials at different times and register a voltage difference.
The EKG is used to diagnose a variety of heart problems.
The Vascular System:
Arteries, Capillaries, and Veins
Large artery walls are elastic to withstand high pressures and to squeeze blood along their lumens by elastic rebound.
Smooth muscle cells in arteries and arterioles contract and relax, varying the vessel diameters.
As the diameter changes, resistance to flow also changes, allowing blood to be distributed to different tissues.
The smooth muscles are under neuronal and hormonal control.
Arteries and arterioles are called resistance vessels.
The Vascular System:
Arteries, Capillaries, and Veins
Capillary beds lie between arterioles and venules and exchange materials between blood and tissue fluid through their __________ walls.
Blood flows slowly here, facilitating this exchange.
The high pressure in the arteries is dissipated by the __________ number of arterioles and still greater number of capillaries.
The total __________-sectional area of the capillary beds is much greater than that of any of the other vessels; thus pressure is reduced to a very low level.
The Vascular System:
Arteries, Capillaries, and Veins
Capillary walls are a single layer of epithelial cells and have fine holes, called fenestrations.
Many small molecules leak through, but larger protein molecules are held back.
Water balance in capillary beds is a result of two opposing forces, known as Starlings forces.
Blood pressure squeezes water and small solutes out of the capillaries; osmotic pressure created by the large protein molecules in the capillary (colloidal osmotic pressure) draws water into the capillary.
The Vascular System:
Arteries, Capillaries, and Veins
The medical phenomenon called edema (tissue swelling) in certain diseases, or the inflammation accompanying injury or an allergic reaction, supports this model.
However, edema does not occur during strenuous exercise, for example, or in birds, which have higher arteriole pressure and lower osmotic pressure than mammals.
The Vascular System:
Arteries, Capillaries, and Veins
CO2 and bicarbonate ions (HCO3) are major factors that pull water back into the capillaries.
As blood flows through the capillary, CO2 diffuses into the plasma and is converted into HCO3.
The increasing bicarbonate concentration causes osmotic pressure to be higher at the arterial end, especially during exercise.
In fact, CO2 and HCO3 are the major factors that pull water back into the capillaries, not colloidal osmotic pressure.
The Vascular System:
Arteries, Capillaries, and Veins
Lipid-soluble substances and many small molecules can pass easily through capillary walls.
The capillaries of the brain do not have fenestrations.
Very few substances besides lipid-soluble molecules (e.g., alcohol) can pass through the capillaries of the brain.
This highly selective barrier is referred to as the bloodbrain barrier.
The Vascular System:
Arteries, Capillaries, and Veins
Blood tends to accumulate in veins. Veins are called capacitance vessels because of their high capacity to store blood.
Pressure in veins is very low, and blood movement back to the heart relies on gravity, vessel squeezing by skeletal muscles, breathing, and limited smooth muscle contraction.
Contraction of skeletal muscles pushes blood toward the heart because one-way valves in veins prevent backflow.
If veins become stretched, the valves no longer do their job and varicose veins develop.
The Vascular System:
Arteries, Capillaries, and Veins
During walking or running, the legs act as auxiliary vascular pumps and return blood to the heart from the veins of the lower body.
A greater volume of blood is returned to the heart, which stretches the cardiac muscle cells, and the heart contracts more forcefully. This is known as the Frank-Starling law.
Breathing also helps return venous blood by creating negative pressure (suction), which pulls blood and lymph toward the chest area.
In large veins near the heart, smooth muscles also contract with exercise, increasing venous return and cardiac output.
The Vascular System:
Arteries, Capillaries, and Veins
Tissue fluid that accumulates outside of capillaries is moved by the lymphatic system.
Lymph moves from small to larger vessels and finally empties into the thoracic ducts that empty into large veins at the base of the neck.
Lymph is moved through vessels by skeletal muscle contractions, and the vessels have one-way valves to prevent backflow.
Lymph nodes are major sites of lymphocyte production.
The nodes are also filters that have phagocytic cells to remove microbes and foreign matter.
The Vascular System:
Arteries, Capillaries, and Veins
Heart attack or stroke is often the end result of atherosclerosis (hardening of arteries).
When the smooth internal lining of arteries becomes damaged, deposits called plaque form at damaged sites.
Swelling and lipid/cholesterol deposition invite fibrous connective tissue and calcium deposits.
These make the artery wall less elastic (hardening), and the plaque narrows the lumen of the artery.
Blood platelets stick in the plaque and form blood clots (a thrombus), further blocking the artery.
The Vascular System:
Arteries, Capillaries, and Veins
If the coronary arteries are affected, blood supply to the heart decreases.
A thrombus here (coronary thrombosis) can block an artery, causing a heart attack (myocardial infarction, or MI).
If part of the thrombus breaks away (an embolism), lodges in the brain, and blocks blood flow, stroke may occur.
The best approach to reducing heart disease is prevention.
Risk factors include high-fat/high-cholesterol diets, smoking, a sedentary life style, and obesity.
Blood: A Fluid Tissue
Blood is connective tissue: it consists of living cells within an extracellular matrix.
The fluid matrix is called plasma.
The cellular components of blood are the red blood cells (erythrocytes), the white blood cells (leukocytes), and the platelets (cell fragments).
The hematocrit is a measure of the cellular portions as a percentage of the total blood volume.
Blood: A Fluid Tissue
Most of the cells in blood are __________.
At maturity they are biconcave, flexible discs packed with hemoglobin.
The hemoglobin carries O2, and the flexible shape of the cell lets them squeeze through narrow capillaries.
Red blood cells are generated by stem cells in the marrow of long bones.
This production is controlled by erythropoietin, a hormone from the kidney.
Blood: A Fluid Tissue
Bone marrow makes about 2 million red blood cells per second.
While still in the marrow, the immature cells divide many times and produce hemoglobin.
When the hemoglobin level reaches 30% of the cell volume, cell organelles including the nucleus break down, and the cell enters the circulation.
Each red blood cell lives about 120 days and then breaks down.
The spleen serves as a reservoir for old blood cells that have been squeezed and ruptured. The cell remnants are then broken down by macrophages.
Blood: A Fluid Tissue
Besides producing leukocytes and erythrocytes, stem cells in marrow produce megakaryocytes.
Megakaryocytes break off cell fragments called platelets.
The platelets contain enzymes and chemicals necessary for blood clotting.
Blood vessel damage exposes collagen fibers, which activate the platelets.
The swelling, sticky platelets release clotting factors that activate other platelets and initiate clotting.
Blood: A Fluid Tissue
Blood clotting involves many chemical steps in a cascade that activates circulating substances in the blood, many of which come from the liver.
Cell damage and platelet activation lead to conversion of an inactive enzyme in the blood, prothrombin, to its active form, thrombin.
Thrombin causes a plasma protein, fibrinogen, to polymerize, forming fibrin threads.
These threads form a meshwork to seal the damaged vessel and provide a base for scar tissue.
Blood: A Fluid Tissue
Plasma contains gases, ions, nutrients, proteins, hormones, and other chemicals.
Predominant ions are Na+ and Cl, giving blood a __________ taste.
Nutrient molecules in plasma include glucose, amino acids, lipids, lactic acid, and cholesterol.
Circulating proteins include albumin, antibodies, hormones, and carrier molecules.
Plasma is similar in composition to tissue fluid but has a higher concentration of proteins.
Control and Regulation of Circulation
Neural and hormonal mechanisms control the circulatory system at local and systemic levels.
Each tissue, however, regulates its own blood flow by autoregulatory mechanisms that constrict or dilate arterioles supplying blood.
The collective autoregulatory actions of all the capillary beds influence the pressure and composition of blood. For example, if many capillary beds dilate all at once, blood pressure drops.
The nervous and endocrine systems respond to any changes by influencing breathing, heart rate, and blood distribution.
Control and Regulation of Circulation
When arteriole blood flows into a capillary bed, smooth muscle in the arteriole may constrict or relax by means of precapillary sphincters.
Low O2 and high CO2 levels cause the smooth muscle to relax and increase flow in the capillary bed.
This brings in more O2 and takes away CO2. The response is called hyperemia (excess blood).
Any activities that increase metabolism of the tissue also increase blood flow or hyperemia in the tissue.
Control and Regulation of Circulation
Arteriole smooth muscle also responds to endocrine and neural signals.
The sympathetic division of the autonomic nervous system innervates most arteries and arterioles.
Most sympathetic neurons release norepinephrine, which causes smooth muscles to contract, thus reducing blood flow.
In skeletal muscle, however, sympathetic neurons release acetylcholine, which causes smooth muscle or arterioles to relax and vessels to dilate, thus increasing blood flow.
Hormones causing arteriole constriction include epinephrine, angiotensin, and vasopressin.
Control and Regulation of Circulation
The cardiovascular centers in the medulla of the brain stem originate heart rate and blood vessel constriction through the autonomic nervous system.
The medulla is influenced by many inputs from sympathetic and parasympathetic areas.
Stretch receptors in the aorta and carotid artery provide information about blood pressure.
When atria are receiving too much venous return they release a hormone called atrial natriuretic factor, which stimulates the kidney to excrete sodium and water, resulting in a reduced blood volume.
Control and Regulation of Circulation
Chemoreceptors in the aorta and carotid arteries also stimulate the medulla regulatory system.
The chemoreceptors signal when O2 content of the blood falls drastically.
Emotions and anticipation may cause the medulla to signal an increase in heart rate and blood pressure.
Control and Regulation of Circulation
Several adaptations permit the air-breathing seal to remain under water for long periods of time.
Seals have greater blood volume, greater blood O2 carrying capacity, and more __________ in their muscles than do humans.
The diving reflex is the most important adaptation.
During a dive, the heart rate slows, and all major blood vessels are constricted except those critical to survival under water.
Metabolic rate slows, and tissues switch to glycolytic (anaerobic) metabolism.
Control and Regulation of Circulation
During the dive, the seal accumulates lactic acid in its muscles which constitutes an oxygen debt.
The oxygen debt is paid back after the dive is over, but the hypometabolism during the dive has kept the debt small.
A similar diving reflex in humans probably serves as a protective response during the birth process and is the likely explanation for remarkable feats of human underwater survival.
Animation 49.1 The Cardiac Cycle
Video 49.1 Hemolymph circulation through a prepupal fruit fly, Drosophila melanogaster
Video 49.2 Circulation in a tadpole
Video 49.3 Human heart beating
Video 49.4 Cardiac muscle cell beating
Video 49.5 Blood vessel formation
Video 49.6 Blood flow in humans