49.0 Introduction

1. Blood and Blood Vessels Comprise the Circulatory System fig 49.1
1. Blood Is Analogous to Trucks Carrying Food to Market
2. Blood Vessels Are Analogous to Highways

49.1 The circulatory systems of animals may be open or closed

1. Open and Closed Circulatory Systems
1. All Organisms Capture Nutrients and Gases from the Environment
1. Simple organisms transport materials across membrane of each cell fig 49.2a
2. Interior of large organisms cannot communicate with environment
1. Fluids within body cavity facilitate movement of materials
2. Circulation: Transport of materials through an internal fluid

2. Types of Circulatory Systems
1. Open system: No distinction between circulating fluid and body fluid
1. Molluscs and arthropods have open system fig 49.2b
2. Fluid called hemolymph
3. Muscular tube in body cavity pumps fluid through network of channels
4. Fluid drains back into central cavity

2. Closed system: Blood enclosed within vessels
1. Blood transported via actions of a pump, the heart
2. Annelids have a closed system fig 49.2c
1. Dorsal artery contracts rhythmically, functions as pump
2. Five connecting arteries also function as pumps
3. Ventral artery pumps blood posteriorly
4. Blood eventually reenters dorsal artery
5. Smaller vessels branch from arteries to supply all tissues fig 49.2c

3. Blood vessels form tubular network
1. Arteries: Direct blood away from heart
2. Veins: Carry blood back to heart
3. Capillaries: Transition from arteries to veins

4. Pressure of blood forces some blood plasma out of capillaries
1. Interstitial fluid passes into surrounding tissues
2. Some fluid returns directly to capillaries
3. Some fluid enters lymph vessels in connective tissues around blood vessels
1. Fluid called lymph
2. Returned to venous blood at certain points

3. The Functions of Vertebrate Circulatory Systems
1. Transportation
1. Respiratory
1. Erythrocytes transport oxygen to tissue cells
2. Oxygen diffuses into capillaries in lungs or gills
3. Oxygen attaches to hemoglobin of red blood cells
4. Transported to cells for aerobic respiration
5. Carbon dioxide, a metabolic product, is released by cells into blood
6. Carbon dioxide carried back to gills or lungs and released

2. Nutritive
1. Digestive system breaks down food
2. Nutrients enter blood through wall of intestine
3. Carried to liver and to all body cells

3. Excretory
1. Metabolic wastes, water, ions carried to kidney for removal
2. Filtered through capillaries
3. Excreted in urine

2. Regulation
1. Hormone transport
1. Body activities coordinated by hormones produced in endocrine glands
2. Hormones transported to target tissues throughout body

2. Temperature regulation
1. Warm-blooded vertebrates are homeotherms
2. Maintain constant body temperature
3. Heat distributed by circulating blood
4. Temperature adjusted by directing flow to interior or extremities
1. Decrease body temperature by dissipating heat to environment fig 49.3
2. Retain heat by directing blood from extremities to interior

5. Some animals use counter current heat exchange system fig 49.4
1. Vessels lie adjacent to one another
2. One carries warm blood from interior
3. Other carries cold blood from body surface
4. Warm blood heats cold blood, so it isn't cold when it reaches interior

3. Protection
1. Blood clotting
1. Protects against blood loss when vessels are damaged
2. Involves proteins in plasma and platelets

2. Immune defense
1. Leukocytes, white blood cells, provide immunity against disease agents
2. Are phagocytic, produce antibodies or have other actions

49.2 A network of vessels transports blood through the body

1. The Blood Plasma
1. Composition of Blood fig 49.5
1. Fluid plasma
2. Several kinds of cells
1. Platelets are not complete cells
2. Are fragments of cells found in bone marrow

3. Blood plasma contains three primary solutes
1. Metabolites, wastes and hormones
1. Dissolved within are glucose, amino acids, vitamins
2. Also includes wastes, nitrogenous compounds, carbon dioxide
3. And hormones that regulate cell activities

2. Ions
1. Plasma is a dilute salt solution
2. Primarily sodium, chloride and bicarbonate
3. Trace amounts of calcium, magnesium and metallic ions
4. Similar to sea water with slightly lower total ion concentration

3. Proteins
1. Liver produces most plasma proteins, including albumin
2. Alpha and beta globin proteins are carriers of lipids and steroid hormones
3. Fibrinogen associated with blood clotting
4. Serum is blood fluid minus the fibrinogen

2. The Blood Cells
1. Erythrocytes and Oxygen Transport
1. Each milliliter of blood contains 5 million erythrocytes or red blood cells
2. Hematocrit: Volume of blood composed of red blood cells, 45% of blood volume
3. Structure of a red cell
1. Flat disk with a central depression
2. Contain hemoglobin, function in oxygen transport
3. Plasma lacks hemoglobin, opposite situation in many invertebrates

4. Mature mammal cells lack nuclei and protein-synthesis machinery
1. All other vertebrates have nucleated red cells
2. Removed by phagocytic cells of spleen, bone marrow, liver
3. Cells produced in bone marrow during erythropoiesis
4. Process stimulated by erythropoietin hormone
1. Produced in kidneys
2. In response to decrease in plasma oxygen concentration

2. Leukocytes Defend the Body
1. Less than 1% of total blood cells
2. Larger than red cells, possess nuclei
3. Circulate in blood, present in interstitial fluid
4. Function to defend body against microbes and foreign substances
1. Colorless, lack hemoglobin, difficult to see without staining
2. Granular leukocytes include neutrophils, basophils, eosinophils
3. Nongranular leukocytes include monocytes and lymphocytes
4. Most neutrophils, lymphocytes, monocytes, eosinophils, basophils !’ least

5. Role in inflammatory response
1. Neutrophils leave capillaries, accumulate at site of injury
2. Joined by monocytes which are converted into macrophages
3. Neutrophil and macrophages entrap microorganisms and foreign particles
4. Lymphocytes play key role in specific immune defense
5. Eosinophils help defend against parasitic infections, play role in allergic reaction

3. Platelets Help Blood to Clot
1. Platelets are cell fragments that pinch off from megakaryocytes, no nuclei
2. Play important role in blood clotting
1. Ruptured vessel constricts due to contraction of smooth muscle in wall
2. Platelets accumulate and form plug with tissues
3. Fibrin protein reinforces plug fig 49.6

3. Characteristics of Blood Vessels
1. Kinds of Blood Vessels
1. Arteries: Direct blood away from heart
2. Arterioles: Large network of microscopic vessels of arterial tree
3. Capillaries: Fine network of thin-walled tubes
4. Venules: Small vessels that collect blood from capillaries
5. Veins: Large vessels carry blood back to heart

2. Anatomy of a Blood Vessel
1. Similar structures found in arteries, arterioles, veins and venules fig 49.7
2. Walls are composed of four layers of tissue
1. Innermost endothelium: Epithelial sheet of cells
2. Thin layer of elastic fibers
3. Layer of smooth muscle
4. Encased in connective tissue

3. Walls too thick to permit exchange of materials
4. Exchange occurs in capillaries, have only endothelium
1. Molecules and ions leave blood plasma by filtration
2. Travel through pores in capillary walls
3. Transported through endothelial cells

3. Arteries and Arterioles
1. Elastic fibers allow large artery to expand and recoil when receiving blood from heart
2. Smaller arteries and arterioles are less elastic, but have thicker smooth muscle
3. Network of small vessels provides frictional resistance to flow
1. Inversely proportional to radius of the tube to the fourth power
2. Small diameter arteries and arterioles cause greatest resistance to blood flow
3. Contraction of smooth muscle causes vasoconstriction
1. Increases resistance
2. Decreases flow

4. Relaxation of smooth muscle causes vasodilation
1. Decreases resistance
2. Increases flow

5. Blood around some organs regulated by precapillary sphincters fig 49.8
1. Rings of smooth muscle around arterioles where they empty into capillaries
2. Close off specific capillary beds to all blood flow
3. Close beds in skin to limit heat loss in cold environments

4. Exchange in the Capillaries
1. Heart provides sufficient pressure to pump against resistance of arterial tree and into capillaries
2. Every cell within 100 æm of a capillary
3. Average capillary 1 mm long, 8 æm wide, just larger than red blood cell fig 49.9
4. Capillaries have greatest cross-sectional area of all types of vessels
1. Blood velocity decreases in capillary beds
2. Provides greater time for exchange of materials with extracellular fluid
3. Blood releases oxygen and nutrients, picks up carbon dioxide and wastes
4. Blood pressure greatly reduced when blood enters veins

5. Venules and Veins
1. Blood flows from venules to larger vessels to heart
2. Veins and venules have thinner layer of smooth muscle than arteries fig 49.7c
3. Pressure one-tenth that of arteries
1. Most blood in body held in veins
2. Can expand to hold greater quantities

4. Venous pressure not sufficient to return blood to heart from feet and legs
1. Aided by contraction of skeletal muscles
2. One-way venous valves direct flow toward heart
3. Varicose veins produced when valves don't work, blood pools in veins

4. The Lymphatic System
1. The Lymphatic System Recovers Lost Fluid
1. Circulatory system open to diffusion through capillary walls
1. Filtration driven by pressure of blood, supplies cells with oxygen and nutrients
2. Most fluid returned by osmosis due to concentration of protein in blood
3. Difference in protein concentration called osmotic pressure fig 49.10

2. High capillary blood pressure causes production of too much interstitial fluid
1. Commonly occurs in pregnant women
1. Fetus compresses veins, increases blood pressure in mother's lower limbs
2. Causes swelling, edema, in tissues of feet

2. Edema also results when plasma protein concentration is too low
1. May be caused by liver disease, liver produces most plasma protein
2. May be caused by protein malnutrition

3. Open lymphatic system collects rest of fluid and returns it to blood
1. Composed of lymphatic capillaries, lymphatic vessels, lymph nodes and lymphatic organs like spleen and thymus
2. Fluid in tissues drains into blind-ended lymph capillaries
3. Lymph passes into progressively larger vessels
4. Lymphatic vessels contain vein-like one-way valves fig 49.11
5. Major lymphatic ducts drain into veins on sides of neck

4. Lymph fluid movement assisted by movement of muscles
1. Some lymph vessels contract rhythmically
2. Many fish, all amphibians and reptiles, some birds have lymph hearts

5. Lymph modified by phagocytic cells in nodes and lymphatic organs
1. Contain germinal centers for production of lymphocytes
2. Thymus plays central role in immune system

49.3 The vertebrate heart has undergone progressive evolutionary change

1. The Fish Heart
1. The Early Chordates Had Simple Tubular Hearts
1. Peristaltic contractions of muscular wall of ventral artery
2. Pumps in direction of wave
1. Uncontracted portion of vessel has larger diameter
2. Provides less resistance to blood flow

2. Fish Developed a True Chamber-Pump Heart
1. Development of gills required more efficient pump
2. Four consecutive chambers fig 49.12a
1. Two collection chambers: Sinus venosus and atrium
2. Two pumping chambers: Ventricle and conus arteriosus

3. Heartbeat sequence: Sinus venosus, atrium, ventricle, conus arteriosus
1. Pattern maintained in all vertebrates
2. Impulse initiated in sinus venosus (or its equivalent)

4. Blood delivered to body tissues is fully oxygenated
5. Flow: Heart gills tissues heart fig 49.12b
6. Circulation to body is sluggish due to resistance in gill capillaries

2. Amphibian and Reptile Circulation
1. Amphibian and Reptile Hearts Reflect the Evolution of Pulmonary Circulation
1. Blood pumped from heart to pulmonary arteries to lungs
1. Blood does not go directly to body tissues
2. Returns to heart via pulmonary veins

2. Creation of two circulations
1. Pulmonary circulation: Heart to lungs and back to heart
2. Systemic circulation: Heart to body and back
3. Without change oxygenated blood mixed with unoxygenated blood

3. Structure of the amphibian heart reduces mixing fig 49.13
1. Atrium divided into right and left chambers
1. Right atrium gets deoxygenated blood from systemic circulation
2. Left atrium gets oxygenated blood from lungs

2. Conus arteriosus partially separated by a septum
3. Imperfect separation of blood flow into pulmonary and systemic circulations
4. Deficiency partly compensated for by cutaneous respiration

4. Structure of the reptile heart reduces mixing even better
1. Have two separate atria
2. Ventricle partially divided by a septum
3. Greater separation of aerated/nonaerated blood, greater efficiency
4. Complete separation into two ventricles in crocodiles
5. Conus arteriosus absent, fully subdivided into arteries leaving heart

3. Mammal and Bird Hearts
1. Mammals and Birds Have Four-Chambered Hearts
1. Two separate atria, two separate ventricles fig 49.14
2. Advent of a double circulatory system
1. Right atrium gets deoxygenated blood from body
2. Delivers it to right ventricle, pumped to lungs
3. Left atrium gets oxygenated blood from lungs
4. Delivers it to left ventricle, pumped to body

3. Produces a two-cycle pump
1. Both atria fill and contract simultaneously
2. Ventricles contract at same time

4. Evolution of double pump related to development of endothermy
1. Requires more efficient circulation
2. Needed to support high metabolic rate

5. Same volume of blood moves through each circuit
1. Left ventricle pumps blood through higher resistance pathway than right
2. Left ventricle is more muscular and generates more pressure than right one

2. The Pacemaker Is a Remnant of the Sinus Venosus
1. Sinus venosus served as collection chamber and pacemaker in early vertebrates
2. Remaining tissue is site of origin of the heartbeat in mammals and birds
1. Located in wall of right atrium
2. Called sinoatrial node (SA node)

49.4 The cardiac cycle drives the cardiovascular system

1. The Cardiac Cycle
1. Double Pump System Operates within a Single Organ
1. Right side sends blood to lungs
2. Left side sends blood to rest of body

2. Circulation through the Heart fig 49.21
1. Heart has two pairs of valves
1. Atrioventricular (AV) valves lie between atria and ventricles
1. AV valve on right side is called the tricuspid valve
2. AV valve on left side is the bicuspid or mitral valve

2. Semilunar valves lie between ventricles and main arteries
1. Pulmonary valve is at exit of right ventricle
2. Aortic valve is at exit of left ventricle

2. Cardiac cycle: Complete journey of blood through body and heart
3. Blood returns to resting heart, into left and right atria
1. Atria fill, increase pressure, AV valves open
2. Blood flows from atrium to opening in left ventricle
3. Ventricle about 80% full
4. Contraction of right atrium produces final 20 % of blood volume to ventricle
5. Occurs while ventricles are relaxing, period called ventricular diastole

4. Ventricles contracts, called ventricular systole
1. Blood forced out of ventricles
2. AV valves close, prevents backflow into atria
3. Blood flows through arterial system

5. Pulmonary arteries deliver oxygen-depleted blood to lungs
1. Return blood to heart via pulmonary veins

6. Systemic arteries include aorta and its branches fig 49.15
1. Carry oxygen-rich blood from left ventricle to body
2. Coronary arteries branch off aorta first, supply heart muscle with blood
3. Blood returns to heart via systemic veins, has less oxygen

7. Systemic veins empty into two major veins
1. Superior vena cava drains upper body
2. Inferior vena cava drains lower body
3. Vena cava empty into right atrium

2. Electrical Excitation and Contraction of the Heart
1. How the Heart Is Stimulated to Contract fig 49.16
1. Caused by membrane depolarization, reversal of electrical polarity
2. Contraction triggered by sinoatrial (SA) node, not nervous system
1. Derived from sinus venosus
2. SA node is pacemaker
3. Membrane of cells depolarize spontaneously with regular rhythm

3. Depolarization passes from one cardiac muscle cell to another
4. Spreads because cardiac cells are electrically coupled by gap junctions
5. Ventricular wave of depolarization delayed by nearly 0.1 second
1. Atria and ventricles separated by connective tissue
2. Connective tissue cannot propagate depolarization
3. Wave passes via atrioventricular (AV) node
4. Delay permits atria to completely empty before ventricles contract

6. Depolarization conducted over both ventricles at same time
1. Carried over network of fibers called atrioventricular bundle
2. Also called bundle of His

7. Transmitted by Purkinje fibers that stimulate ventricle myocardial cells
8. Right and left ventricles contract almost simultaneously

2. Monitoring the Heart's Performance
1. Depolarization in heart generates electrical signals that spread throughout body
2. Monitor electrocardiogram (ECG/EKG) that records waves of depolarization
1. Depolarization causes contraction of heart
2. Repolarization causes relaxation

3. Meaning of ECG tracing
1. First peak: Depolarization associated with atrial contraction, atrial systole
2. Second peak: Depolarization of ventricles, ventricular systole
3. Last peak: Ventricular repolarization, ventricles begin diastole

3. Cardiac Output
1. Output is the volume pumped by each ventricle per minute
2. Calculated by: Rate of heart beat x stroke volume (volume of blood ejected)
3. Cardiac output is increased with exercise
1. Heart rate and stroke volume increase
2. Skeletal muscles squeeze on veins,returning blood to heart more rapidly
3. Increases rate at which heart fills and ejects blood
4. Ventricles contract more strongly, empty more completely

3. Blood Flow and Blood Pressure
1. Cardiac Output Increases with Exercise
1. Not all organs receive same increase in blood flow
2. Arterioles in some organs constrict, those to other organs dilate
1. Decrease flow to digestive system
2. Increases blow flow to heart and skeletal muscles

2. Blood Pressure and the Baroreceptor Reflex
1. Arterial blood pressure depends on two factors
1. Cardiac output, how much blood ventricles pump
2. Resistance to flow

2. Increased blood pressure caused by
1. Increased heart rate or blood volume (both increase cardiac output)
2. Vasoconstriction (increases resistance to flow)

3. Blood pressure will fall if
1. Heart rate slows
2. Blood volume reduced, by dehydration or hemorrhage

4. Baroreceptors respond to changes in systemic arterial blood pressure
1. Located in walls of aortic arch and carotic arteries
2. Connected to cardiovascular control center in medulla

5. When baroreceptors detect decrease in blood pressure
1. Stimulates sympathetic activity, inhibits parasympathetic activity
2. Results in increased heart rate
3. Also stimulate sympathetic neurons to blood vessels in skin and viscera to constrict
4. Further raises blood pressure
5. Restores normal pressure and cardiac output

6. Baroreceptors act to maintain blood flow to brain with rapid standing
1. Changes venous pressure in lower body, reduces pressure above heart
2. Increases volume of blood in lower body
3. Pressure in veins at right side of heart decreased
4. Decreases cardiac output and blood flow to brain = fainting
5. Reflex rapidly increases heart rate, constricts arterioles
6. Maintains normal blood pressure values

3. Blood Volume Reflexes
1. Blood pressure depends partly on blood volume
2. Lower blood volume means lower blood pressure
3. Volume regulation via three hormones
1. Antidiuretic hormone (ADH)
2. Aldosterone system
3. Atrial natriuretic hormone (ANH)

4. Antidiuretic hormone (ADH) system (also called vasopressin)
1. Secreted by posterior pituitary with increased osmotic concentration of blood
2. Example: Dehydration decreases volume, increases plasma concentration
3. Stimulates osmoreceptors in brain hypothalamus
4. Promote thirst and stimulate ADH secretion
5. ADH stimulates kidneys to reduce amount of water lost in urine
6. Individual drinks more and urinates less, increasing blood volume

5. Aldosterone system
1. When blood flow through kidney is decreased, cells secrete angiotensin II
2. Angiotensin II has two effects
1. Promotes vasoconstriction (raises blood pressure)
2. Stimulates production of aldosterone by adrenal cortex

3. Aldosterone increases total body Na⁺, reduces water loss
4. Animal without aldosterone dies, blood volume lost in urine

6. Atrial natriuretic hormone (ANH) system
1. Body responds to need to excrete Na⁺ and lower blood volume
2. Inhibits aldosterone secretion
3. Also secretes ANH
1. Secreted by endocrine cells in atrial walls
2. Occurs when atrium is stretched by high blood volume

4. More Na⁺ excreted in urine, water follows, blood volume lowered