Circulatory Systems & Exchange

Overview: Trading Places

• Every organism exchanges materials with its environment
• Exchanges occur at the cellular level by crossing the plasma membrane
• Unicellular organisms exchange materials directly with the environment

Concept 42.1: Circulatory systems link exchange surfaces with cells throughout the body

• Direct exchange with the environment is not possible for most cells in multicellular organisms
• Gills are a specialized exchange system in animals
• O₂ diffuses from the water into blood vessels, while CO₂ diffuses from blood into the water
• Internal transport and gas exchange are functionally related in most animals
• Diffusion time is proportional to the square of the distance
• Diffusion is only efficient over small distances
• Small and/or thin animals' cells exchange materials directly with the surrounding medium
• Most animals' cells exchange materials with the environment via a fluid-filled circulatory system

General Properties of Circulatory Systems

• A circulatory system has:
  • A circulatory fluid
  • A set of interconnecting vessels
  • A muscular pump, the heart
• The circulatory system connects the fluid surrounding cells with the organs that exchange gases, absorb nutrients, and dispose of wastes
• Circulatory systems can be open or closed, and vary in the number of circuits in the body

Open and Closed Circulatory Systems

• In insects, other arthropods, and most molluscs, blood bathes the organs directly in an open circulatory system
• In an open circulatory system, there is no distinction between blood and interstitial fluid, and this general body fluid is called hemolymph
• In a closed circulatory system, blood is confined to vessels and is distinct from the interstitial fluid
• Closed systems are more efficient at transporting circulatory fluids to tissues and cells
• Annelids, cephalopods, and vertebrates have closed circulatory systems

Organization of Vertebrate Circulatory Systems

• Humans and other vertebrates have a closed circulatory system called the cardiovascular system
• The three main types of blood vessels are arteries, veins, and capillaries
• Blood flow is one way in these vessels
• Arteries branch into arterioles and carry blood away from the heart to capillaries
• Networks of capillaries called capillary beds are the sites of chemical exchange between the blood and interstitial fluid
• Venules converge into veins and return blood from capillaries to the heart
• Arteries and veins are distinguished by the direction of blood flow, not by O₂ content
• Vertebrate hearts contain two or more chambers
• Blood enters through an atrium and is pumped out through a ventricle

Single Circulation

• Bony fishes, rays, and sharks have single circulation with a two-chambered heart
• In single circulation, blood leaving the heart passes through two capillary beds before returning

Double Circulation

• Amphibian, reptiles, and mammals have double circulation
• Oxygen-poor and oxygen-rich blood are pumped separately from the right and left sides of the heart
• In reptiles and mammals, oxygen-poor blood flows through the pulmonary circuit to pick up oxygen through the lungs
• In amphibians, oxygen-poor blood flows through a pulmocutaneous circuit to pick up oxygen through the lungs and skin
• Oxygen-rich blood delivers oxygen through the systemic circuit
• Double circulation maintains higher blood pressure in the organs than does single circulation

Mammals and Birds

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

Concept 42.2: Coordinated cycles of heart contraction drive double circulation in mammals

• The mammalian cardiovascular system meets the body's continuous demand for O₂

Mammalian Circulation

• Blood begins its flow with the right ventricle pumping blood to the lungs
• In the lungs, the blood loads O₂ and unloads CO₂
• Oxygen-rich blood from the lungs enters the heart at the left atrium and is pumped through the aorta to the body tissues by the left ventricle
• The aorta provides blood to the heart through the coronary arteries
• Blood returns to the heart through the superior vena cava (blood from head, neck, and forelimbs) and inferior vena cava (blood from trunk and hind limbs)
• The superior vena cava and inferior vena cava flow into the right atrium

The Mammalian Heart: A Closer Look

• A closer look at the mammalian heart provides a better understanding of double circulation
• The heart contracts and relaxes in a rhythmic cycle called the cardiac cycle
• The contraction, or pumping, phase is called systole
• The relaxation, or filling, phase is called diastole
• The heart rate, also called the pulse, is the number of beats per minute
• The stroke volume is the amount of blood pumped in a single contraction
• The cardiac output is the volume of blood pumped into the systemic circulation per minute and depends on both the heart rate and stroke volume
• 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 heart beat is caused by the recoil of blood against the AV valves (lub) then against the semilunar (dup) valves
• Backflow of blood through a defective valve causes a heart murmur

Maintaining the Heart's Rhythmic Beat

• Some cardiac muscle cells are self-excitable, meaning they contract without any signal from the nervous system
• The sinoatrial (SA) node, or pacemaker, sets the rate and timing at which cardiac muscle cells contract
• Impulses that travel during the cardiac cycle can be recorded as an electrocardiogram (ECG or EKG)
• Impulses from the SA node travel to the atrioventricular (AV) node
• At the AV node, the impulses are delayed and then travel to the Purkinje fibers that make the ventricles contract
• The pacemaker is regulated by two portions of the nervous system: the sympathetic and parasympathetic divisions
• The sympathetic division speeds up the pacemaker
• The parasympathetic division slows down the pacemaker
• The pacemaker is also regulated by hormones and temperature

Concept 42.3: Patterns of blood pressure and flow reflect the structure and arrangement of blood vessels

• The physical principles that govern movement of water in plumbing systems also influence the functioning of animal circulatory systems

Blood Vessel Structure and Function

• A vessel's cavity is called the central lumen
• The epithelial layer that lines blood vessels is called the endothelium
• The endothelium is smooth and minimizes resistance
• Capillaries have thin walls, the endothelium plus its basal lamina, to facilitate the exchange of materials
• Arteries and veins have an endothelium, smooth muscle, and connective tissue
• Arteries have thicker walls than veins to accommodate the high pressure of blood pumped from the heart
• In the thinner-walled veins, blood flows back to the heart mainly as a result of muscle action

Blood Flow Velocity

• Physical laws governing movement of fluids through pipes affect blood flow and blood pressure
• Velocity of blood flow is slowest in the capillary beds, as a result of the high resistance and large total cross-sectional area
• Blood flow in capillaries is necessarily slow for exchange of materials

Blood Pressure

• Blood flows from areas of higher pressure to areas of lower pressure
• Blood pressure is the pressure that blood exerts against the wall of a vessel
• In rigid vessels blood pressure is maintained; less rigid vessels deform and blood pressure is lost

Changes in Blood Pressure During the Cardiac Cycle

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

Regulation of Blood Pressure

• Blood pressure is determined by cardiac output and peripheral resistance due to constriction of arterioles
• Vasoconstriction is the contraction of smooth muscle in arteriole walls; it increases blood pressure
• Vasodilation is the relaxation of smooth muscles in the arterioles; it causes blood pressure to fall
• Vasoconstriction and vasodilation help maintain adequate blood flow as the body's demands change
• Nitric oxide is a major inducer of vasodilation
• The peptide endothelin is an important inducer of vasoconstriction

Blood Pressure and Gravity

• Blood pressure is generally measured for an artery in the arm at the same height as the heart
• Blood pressure for a healthy 20 year old at rest is 120 mm Hg at systole and 70 mm Hg at diastole
• Fainting is caused by inadequate blood flow to the head
• Animals with longer necks require a higher systolic pressure to pump blood a greater distance against gravity
• Blood is moved through veins by smooth muscle contraction, skeletal muscle contraction, and expansion of the vena cava with inhalation
• One-way valves in veins prevent backflow of blood

Capillary Function

• Blood flows through only 5-10% of the body's capillaries at a time
• Capillaries in major organs are usually filled to capacity
• Blood supply varies in many other sites
• Two mechanisms regulate distribution of blood in capillary beds:
  • Contraction of the smooth muscle layer in the wall of an arteriole constricts the vessel
  • Precapillary sphincters control flow of blood between arterioles and venules
• Blood flow is regulated by nerve impulses, hormones, and other chemicals
• The exchange of substances between the blood and interstitial fluid takes place across the thin endothelial walls of the capillaries
• The difference between blood pressure and osmotic pressure drives fluids out of capillaries at the arteriole end and into capillaries at the venule end
• Most blood proteins and all blood cells are too large to pass through the endothelium

Fluid Return by the Lymphatic System

• The lymphatic system returns fluid that leaks out from the capillary beds
• Fluid, called lymph, reenters the circulation directly at the venous end of the capillary bed and indirectly through the lymphatic system
• The lymphatic system drains into veins in the neck
• Valves in lymph vessels prevent the backflow of fluid
• Lymph nodes are organs that filter lymph and play an important role in the body's defense
• Edema is swelling caused by disruptions in the flow of lymph

Concept 42.4: Blood components contribute to exchange, transport, and defense

• With open circulation, the fluid that is pumped comes into direct contact with all cells
• The closed circulatory systems of vertebrates contain blood, a specialized connective tissue

Blood Composition and Function

• Blood consists of several kinds of cells suspended in a liquid matrix called plasma
• The cellular elements occupy about 45% of the volume of blood

Plasma

• Blood plasma is about 90% water
• Among its solutes are inorganic salts in the form of dissolved ions, sometimes called electrolytes
• Another important class of solutes is the plasma proteins, which influence blood pH, osmotic pressure, and viscosity
• Various plasma proteins function in lipid transport, immunity, and blood clotting

Cellular Elements

• Suspended in blood plasma are two types of cells:
  • Red blood cells (erythrocytes) transport oxygen O₂
  • White blood cells (leukocytes) function in defense
• Platelets, a third cellular element, are fragments of cells that are involved in clotting

Erythrocytes

• Red blood cells, or erythrocytes, are by far the most numerous blood cells
• They contain hemoglobin, the iron-containing protein that transports O₂
• Each molecule of hemoglobin binds up to four molecules of O₂
• In mammals, mature erythrocytes lack nuclei and mitochondria
• Sickle-cell disease is caused by abnormal hemoglobin proteins that form aggregates
• The aggregates can deform an erythrocyte into a sickle shape
• Sickled cells can rupture or block blood vessels

Leukocytes

• There are five major types of white blood cells, or leukocytes:
  • Monocytes
  • Neutrophils
  • Basophils
  • Eosinophils
  • Lymphocytes

Platelets

• Platelets are fragments of cells and function in blood clotting

Blood Clotting

• Coagulation is the formation of a solid clot from liquid blood
• A cascade of complex reactions converts inactive fibrinogen to fibrin, forming a clot
• A blood clot formed within a blood vessel is called a thrombus and can block blood flow

Stem Cells and the Replacement of Cellular Elements

• The cellular elements of blood wear out and are being replaced constantly
• Erythrocytes, leukocytes, and platelets all develop from a common source of stem cells in the red marrow of bones, especially ribs, vertebrae, sternum, and pelvis
• The hormone erythropoietin (EPO) stimulates erythrocyte production when O₂ delivery is low

Cardiovascular Disease

• Cardiovascular diseases are disorders of the heart and the blood vessels
• Cardiovascular diseases account for more than half the deaths in the United States
• Cholesterol, a steroid, helps maintain membrane fluidity

Atherosclerosis, Heart Attacks, and Stroke

• One type of cardiovascular disease, atherosclerosis, is caused by the buildup of plaque deposits within arteries
• A heart attack, or myocardial infarction, is the death of cardiac muscle tissue resulting from blockage of one or more coronary arteries
• Coronary arteries supply oxygen-rich blood to the heart muscle
• A stroke is the death of nervous tissue in the brain, usually resulting from rupture or blockage of arteries in the head
• Angina pectoris is caused by partial blockage of the coronary arteries and results in chest pains
• Low-density lipoprotein (LDL) delivers cholesterol to cells for membrane production
• High-density lipoprotein (HDL) scavenges cholesterol for return to the liver
• Risk for heart disease increases with a high LDL to HDL ratio
• Inflammation is also a factor in cardiovascular disease

Risk Factors and Treatment of Cardiovascular Disease

• A high LDL to HDL ratio increases the risk of cardiovascular disease
• The proportion of LDL relative to HDL can be decreased by exercise, not smoking, and avoiding foods with trans fats
• Drugs called statins reduce LDL levels and risk of heart attacks
• Inflammation plays a role in atherosclerosis and thrombus formation
• Aspirin inhibits inflammation and reduces the risk of heart attacks and stroke
• Hypertension, or high blood pressure, promotes atherosclerosis and increases the risk of heart attack and stroke
• Hypertension can be reduced by dietary changes, exercise, and/or medication

Concept 42.5: Gas exchange occurs across specialized respiratory surfaces

• Gas exchange supplies O₂ for cellular respiration and disposes of CO₂
• Animals require large, moist respiratory surfaces for exchange of gases between their cells and the respiratory medium, either air or water
• Gas exchange across respiratory surfaces takes place by diffusion
• Respiratory surfaces vary by animal and can include the outer surface, skin, gills, tracheae, and lungs

Partial Pressure Gradients in Gas Exchange

• A gas diffuses from a region of higher partial pressure to a region of lower partial pressure
• Partial pressure is the pressure exerted by a particular gas in a mixture of gases
• Gases diffuse down pressure gradients in the lungs and other organs as a result of differences in partial pressure

Respiratory Media

• Animals can use air or water as a source of O₂, or respiratory medium
• In a given volume, there is less O₂ available in water than in air
• Obtaining O₂ from water requires greater efficiency than air breathing

Lungs

• Lungs are an infolding of the body surface
• The circulatory system (open or closed) transports gases between the lungs and the rest of the body
• The size and complexity of lungs correlate with an animal's metabolic rate

Mammalian Respiratory Systems: A Closer Look

• A system of branching ducts conveys air to the lungs
• Air inhaled through the nostrils is warmed, humidified, and sampled for odors
• The pharynx directs air to the lungs and food to the stomach
• Swallowing tips the epiglottis over the glottis in the pharynx to prevent food from entering the trachea
• Air passes through the pharynx, larynx, trachea, bronchi, and bronchioles to the alveoli, where gas exchange occurs
• Exhaled air passes over the vocal cords in the larynx to create sounds
• Cilia and mucus line the epithelium of the air ducts and move particles up to the pharynx
• This "mucus escalator" cleans the respiratory system and allows particles to be swallowed into the esophagus
• Gas exchange takes place in alveoli, air sacs at the tips of bronchioles
• Oxygen diffuses through the moist film of the epithelium and into capillaries
• Carbon dioxide diffuses from the capillaries across the epithelium and into the air space
• Alveoli lack cilia and are susceptible to contamination
• Secretions called surfactants coat the surface of the alveoli
• Preterm babies lack surfactant and are vulnerable to respiratory distress syndrome; treatment is provided by artificial surfactants

Concept 42.6: Breathing ventilates the lungs

• The process that ventilates the lungs is breathing, the alternate inhalation and exhalation of air

How a Mammal Breathes

• Mammals ventilate their lungs by negative pressure breathing, which pulls air into the lungs
• Lung volume increases as the rib muscles and diaphragm contract
• The tidal volume is the volume of air inhaled with each breath
• The maximum tidal volume is the vital capacity
• After exhalation, a residual volume of air remains in the lungs

Control of Breathing in Humans

• In humans, the main breathing control centers are in two regions of the brain, the medulla oblongata and the pons
• The medulla regulates the rate and depth of breathing in response to pH changes in the cerebrospinal fluid
• The medulla adjusts breathing rate and depth to match metabolic demands
• The pons regulates the tempo
• Sensors in the aorta and carotid arteries monitor O₂ and CO₂ concentrations in the blood
• These sensors exert secondary control over breathing

Concept 42.7: Adaptations for gas exchange include pigments that bind and transport gases

• The metabolic demands of many organisms require that the blood transport large quantities of O₂ and CO₂

Coordination of Circulation and Gas Exchange

• Blood arriving in the lungs has a low partial pressure of O₂ and a high partial pressure of CO₂ relative to air in the alveoli
• In the alveoli, O₂ diffuses into the blood and CO₂ diffuses into the air
• In tissue capillaries, partial pressure gradients favor diffusion of O₂ into the interstitial fluids and CO₂ into the blood

Respiratory Pigments

• Respiratory pigments, proteins that transport oxygen, greatly increase the amount of oxygen that blood can carry
• Arthropods and many molluscs have hemocyanin with copper as the oxygen-binding component
• Most vertebrates and some invertebrates use hemoglobin
• In vertebrates, hemoglobin is contained within erythrocytes

Hemoglobin

• A single hemoglobin molecule can carry four molecules of O₂, one molecule for each iron containing heme group
• The hemoglobin dissociation curve shows that a small change in the partial pressure of oxygen can result in a large change in delivery of O₂
• CO₂ produced during cellular respiration lowers blood pH and decreases the affinity of hemoglobin for O₂; this is called the Bohr shift

Carbon Dioxide Transport

• Hemoglobin also helps transport CO₂ and assists in buffering the blood
• CO₂ from respiring cells diffuses into the blood and is transported either in blood plasma, bound to hemoglobin, or as bicarbonate ions (HCO₃-)