Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
178 Cards in this Set
- Front
- Back
What is the function of the Circulatory System |
to transport blood and nutrients to all parts of the body; to continually perfuse oxygenate, and nourish all the vital organs in the body; to remove wastes and CO2 |
|
Describe the location of the Heart |
Lies in the mediastinum, behind the sternum, between the points of attachment of ribs two through 6 Posteriorly: T5-T8 |
|
Function of arteries |
Carry blood away from heart (except pulmonary artery) |
|
Three types of arteries |
Muscular (distributing), Arterioles (resistance vessels), Metarterioles |
|
Function of capillaries |
Primary gas and nutrient exchange vessels; carry blood from arterioles to venules |
|
What are three types of capillaries |
Continuous, fenestrated and sinusoid |
|
Function of veins |
carry blood toward the heart, act as collectors and reservoir vessels(they have the capacity to take on large volumes of blood without rupture) |
|
What are the three main layers of vessel structure |
Tunica externa(arteries and veins) Tunica media(arteries and veins) Tunica intima(all blood vessels)(only layer of capillaries) |
|
Describe collagen fibers |
woven appearance, formed from potein molecules that aggregate into fibers, limited ability to stretch(2-3%) |
|
Describe elastic fibers |
composed of insoluble protein called elastin, form highly elastic networks, can stretch more than 100%, play important role in creating passive tension to help regulate blood pressure |
|
Describe smooth muscle fibers |
most numerous in elastic and muscular arteries, exert active tension in vessels when contracting, present in all segments of vascular system except capillaries, very little in veins, ANS connected |
|
Systematic Circulation |
blood flows from the left ventricle of the heart through blood vessels to all parts of the body (except gas exchange tissues of lungs) and back to the right atrium |
|
Pulmonary Circulation |
venous blood moves from right atrium to right ventricle to pulmonary artery to lung arterioles and capillaries, where gases are exchanged; oxygenated blood returns to left atrium by pulmonary veins; from left atrium, to the left ventricle |
|
Arterial anastomoses |
arteries that open into other branches of the same or other arteries; incidence of arterial anastomoses increases as distance from the heart increases. Allows for collateral circulation |
|
When do arterio-venous anastomoses (shunts) occur? |
When blood flows from an artery directly into a vein |
|
dural sinuses |
large veins of the cranial cavity |
|
hepatic portal vein |
veins from the spleen, stomach, pancreas, gallbladder, and intestines send blood to the liver by the hepatic portal vein |
|
What are the 6 vessels and shunts necessary to allow a fetus access to maternal oxygen and nutrition |
Two umbilical arteries, placenta, umbilical vein, ductus venosus, foramen ovale, ductus arteriosus |
|
Function of the two umbilical arteries in fetal circulation |
extensions of the internal iliac arteries that carry fetal blood to the placenta |
|
Function of the placenta in fetal circulation |
where exchange of oxygen and other substances between the separate maternal and fetal blood occurs; attached to uterine wall |
|
Function of the umbilical vein in fetal circulation |
returns oxygenated blood from the placenta to the fetus; enters body through the umbilicus and goes to the undersurface of the liver, where it gives off two or three branches and then continues as the ductus venosus |
|
Function of the ductus venosus in fetal circulation |
continuation of the umbilical vein; drains into inferior vena cava |
|
Function of the foramen ovale in fetal circulation |
opening in septum between the right and left atria--> allows blood circulation to by-pass the lungs |
|
Function of the ductus arteriosus in fetal circulation |
small vessel connecting the pulmonary artery with the descending thoracic aorta |
|
What happens to the umbilical vein after birth |
within the baby's body becomes the round ligament of the liver |
|
What happens to the ductus venosus after birth |
becomes the ligamentum venosum of the liver |
|
What happens to the foramen ovale after birth |
usually functionally closed shortly after a newborn's first breath and pulmonary circulation is established; structural closure takes approximately 9 months |
|
What happens to the ductus arteriosus after birth |
contracts with establishment of respiration; becomes ligamentum arteriosum |
|
About how much blood does a young adult male have |
5L |
|
What are the three formed elements in blood |
red blood cells(erythrocytes), platelets(thrombocytes) and white blood cells(leukocytes) |
|
Facts about red blood cells |
-No nucleus -Does not contain ribosomes, mitochondria, or other organelles typical of most body cells -Primary component is hemoglobin -Most numerous of the formed elements |
|
Function of red blood cells |
Critical role in the transport of oxygen and carbon dioxide depends on hemoglobin |
|
Explain erythropoiesis |
This is the formation of RBCs and it begins in the red bone marrow as hematopoietic stem cells and goes through several stages of development to become erythrocytes; entire maturation process requires approximately 4 days |
|
How many RBCs are created and destroyed per minute? |
100 million |
|
What is the average lifespan of a circulating RBC |
105 to 120 days |
|
Describe hemoglobin |
-Within 1 RBC there are 200-300 million molecules of hemoglobin -Composed of 4 globin chains, each attached to a heme group -Able to unite with 4 oxygen moleucles |
|
General definition of anemia |
a decrease in number or volume of functional RBCs in a given unit of whole blood |
|
5 types of white blood cells (most prevalent to least) |
Neutrophiles Lymphocytes Monocytes Eosinophils Basophils |
|
Why do WBC have clinical significance? |
because they change with certain abnormal conditions; infection, etc. |
|
Explain the formation of WBC |
-granular and agranular leukocytes mature from the undifferentiated hematopoietic stem cell -neutrophils, eosinophils, basophils, and a few lymphocytes and monocytes originate in red bone marrow; most lymphocytes and monocytes develop from hematopoietic stem cells in lymphatic tissue |
|
Functions of platelets |
-Important role in hemostasis and blood coagulation(clots); secondary role in defending against bacterial attacks |
|
Describe platelet plug formation |
-1-5 seconds after injury to vessel wall, platelets adhere to damaged endothelial lining and to each other, forming a temporary platelet plug -normal platelets(positive charge) adhere to damaged capillary wall and underlying collagen fibers, both of which have a negative charge -"sticky platelets" form physical plug and secrete several chemicals involved in the coagulation process. Fibrin accumulates |
|
Formation of platelets |
formed in red bone marrow, lungs, and spleen by fragmentation of megakaryocytes |
|
Lifespan of platelets |
7 to 10 days |
|
What are the 4 ABO blood types |
Type A, Type B, Type AB, Type O |
|
What is the universal donor |
Type o |
|
What is the universal recipient |
Type AB |
|
What is plasma composed of? |
90% water and 10% solutes |
|
What percent of the plasma solutes are proteins and what are the three main compounds |
6-8% Albumins, Globulins, Fibrinogen |
|
Albumins |
help maintain osmotic balance of the blood |
|
Globulins |
essential component of the immunity mechanism |
|
Fibrinogen |
key role in blood clotting |
|
What are 4 components critical to coagulation |
prothrombin, thrombin, fibrinogen, fibrin |
|
What are the 3 stages of coagulation |
Stage 1: Production of thromboplastin activator b either of the following: -chemicals released from damaged tissues(extrinsic pathway) -chemicals present in the blood(intrinsic pathway) Stage 2: Conversion of prothrombin to thrombin Stage 3: Conversion of fibrinogen to fibrin and production of fibrin clot |
|
fibrinolysis |
physiological mechanism that dissolves clots |
|
Point of Maximal Impact (PMI) |
the apical beat created by the repeated impact of the beating heart on the inner chest wall |
|
pericardium |
the heart is covered with a loose-fitting membranous sac |
|
Average size and weight of human heart |
size of an adult fist, weighs 250-300 grams |
|
Pericardium |
double walled sac surrounding the heart |
|
Fibrous pericardium |
tough, loose-fitting inextensible(mostly) sac; anchored to great vessels |
|
Serous pericardium |
2 layers: -Parietal layer lies inside the fibrous pericardium -Visceral layer(epicardium) integral part of heart wall |
|
Pericardial space |
with 10-15 mL pericardial fluid separates the two layers. Provides protection from friction |
|
What are the 3 layers of the heart wall? |
-Epicardium: outer layer of heart wall -Myocardium: thickest, contractile middle layer of heart wall; compresses the heart cavities -Endocardium: delicate inner layer of endothelial tissue |
|
How many chambers are in the heart? |
4 |
|
Describe the Atria |
-Two superior chambers known as receiving chambers because they receive blood from veins -Alternately contract and relax to receive blood and then push it into ventricles -Myocardial wall of each atrium is not very thick because little pressure is needed to move blood such a small distance |
|
What is an auricle? |
earlike flap protruding from each atrium(little effect) |
|
Describe the ventricles |
-Two lower chambers known as pumping chambers -Ventricular myocardium is thicker than the myocardium of the atria -Myocardium of left ventricle is thicker than the right due to high afterload pressures in the aorta |
|
What are all the valves of the heart? |
Semilunar valve Left atrioventricular (mitral) valve (bicuspid) Right atrioventricular (tricuspid) valve Aortic SL valve Pulmonary SL valve |
|
Describe what happens to the valves of the heart during atrial contraction |
Semilunar valve closed Left atrioventricular (mitral) valve open Right atrioventricular (tricuspid) valve open Aortic SL valve closed Pulmonary SL valve closed |
|
Describe what happens to the valves of the heart during ventricular contraction |
Semilunar valve open Left atrioventricular (mitral) valve closed Right atrioventricular (tricuspid) valve closed Aortic SL valve open Pulmonary SL valve open |
|
Function of Atrioventricular (AV) valves |
prevent blood from flowing back into the atria |
|
Tricuspid valve(Right AV) valve |
guards the right atrioventricular orifice; free edges of three flaps of endocardium are attached to papillary muscles by chordae tendinae |
|
Mitral (Left AV) (Bicuspid) Valve |
similar in structure to tricuspid valve except has only two flaps. same function |
|
semilunar valves |
-half-moon shaped flaps growing out from the lining of the pulmonary artery and aorta -prevent blood from flowing back into the ventricles from the aorta and pulmonary artery |
|
pulmonary valve |
valve at entrance of the pulmonary artery |
|
aortic valve |
valve at entrance of the aorta |
|
intertrial septum |
separates the two atria and prevents mixing of oxygenated and deoxygenated blood |
|
interventricular septum |
separates the right and left ventricles, preventing mixing of oxygenated and unoxygenated blood |
|
papillary muscles |
each ventricle contains these cone-shaped pillars from which thin, strong connective tissue strings (chordae tindineae) attach to tricuspid and mitral valve cusps |
|
prolapse |
if the size disproportion of mitral and tricuspid rings is too great, the valve cusps balloon upward into the atrium during ventricular contraction |
|
Describe coronary arteries |
-Myocardial cells receive blood from the right and left coronary arteries, the first branches to come from the aorta -Ventricles receive blood from both right and left coronary arteries |
|
myocardial infarction (MI) |
myocardial tissue death (can occur when the heart muscles supplied by the occluded artery becomes hypoxic(ischemic)) |
|
angina pectoris |
occurs when ischemia produced by coronary artery occlusion stimulates myocardial nociceptive (pain-eliciting) fibers. This pain is often felt beneath the sternum, in the left arm, and in the neck. |
|
What happens at the coronary sinus? |
after going through cardiac veins, blood enters the coronary sinus to drain into the right atrium |
|
sinoatrial (SA) node (pacemaker) |
-initiates each heartbeat and sets its pace -embedded in the right atrial muscle near the superior vena cava, is responsible for initiating the electrical impulses that produce sequential atrial and ventricular contraction |
|
atrioventricular (AV) node |
the impulses of the SA node travel over specialized internodal pathways to the AV node. The AV node and AV bundle slow the impulse velocity considerably before transmitting it into the ventricles. |
|
What are the time differences in SA node impulse and AV node impulse? |
It takes the SA node impulse about 0.03 seconds to reach the AV node, which along with the AV bundle delays the impulse another 0.13 seconds before conducting it into the ventricles. |
|
Why is there a transmission delay from the SA node impulse to time the ventricles receive the signal? |
this transmission delay prevents impulses from arriving at the ventricles too rapidly in succession; in this way, the ventricles have enough time to fill between contractions |
|
cardiac plexuses |
located near the arch of the aorta; composed of sympathetic and parasympathetic fibers that accompany the right and left coronary arteries to enter the heart |
|
What is another name for the AV bundle? What is its function? |
Bundle of His: -conducts impulses to the right and left bundle branches in the forward direction only (toward the ventricles). This one-way transmission prevents impulses from traveling backward from ventricles to atria. |
|
Purkinje fibers |
The bundle branches travel downward through the ventricular septum and subdivid many times to form these fibers. They spread out diffusely from the apex throughout the ventricles, transmitting impulses at a velocity up to 4 m/s, which is about six times faster than the impulses can travel over regular ventricular muscle. |
|
What is the Frank-Starling Mechanism? |
The inherent ability of the heart to increase its force of contraction as increasin amounts of blood flow into it. |
|
What is important about the Frank-Starling Mechanism? |
***The greater the diastolic volume of the heart, the greater the force of contraction -The force of myocardial contraction is proportional to the rate of cross-bridge formation between actin and myosin filaments, which is influenced by the sarcomere's length(length-dependent activation) |
|
What is the systolic sound caused by? |
This is the first sound; believed to be caused primarily by the contraction of the ventricles and vibrations of the closing AV valves |
|
What is the diastolic sound caused by? |
The second, short, sharp sound; thought to be caused by vibrations of the closing of SL valves |
|
Preload |
-The amount of blood in the heart at the end of diastole -As soon as the ventricles relax, the pressure that accumulated in the atria during ventricular contraction pushes the AV valves open, and blood rushes into the ventricles, creating a rapid and slow filling phase |
|
Afterload |
The ejection period occurs immediately after aortic and pulmonary semilunar valves open; the resistance to this ejection is called afterload |
|
Why does blood circulate from the left ventricle to the right atrium of the heart? |
because pressure gradient: -Aorta mean 90 -Atrium mean 1 |
|
perfusion pressure |
pressure gradient needed to maintain blood flow through a local tissue |
|
Cardiac output |
volume of blood pumped out of the heart per unit of time (ml/min or L/min) |
|
stroke volume (SV) |
volume pumped per heartbeat |
|
How can CO be calculated? |
Fick's Formula: CO=VO2/Ca-Cv |
|
What is contractility? |
strength of contraction |
|
What are cardiac pressor reflexes? what do they do? |
-aortic and carotid baroreceptors(located in the aorta and carotid sinus) -affect the autonomic cardiac control center, and therefore parasympathetic and sympathetic outflow, to help control blood pressure |
|
Carotid sinus reflex |
-sensory fibers from carotid sinus baroreceptors run through the carotid sinus nerve and the glossopharyngeal nerve to the cardiac control center -parasympathetic impulses leave the cardiac control center, travel through the vagus nerve to reach the SA node |
|
Aortic reflex |
-sensory fibers extend from baroreceptors located in the wall of the arch of the aorta through the aortic nerve and through the vagus nerve to terminate in the cardiac control center |
|
What is peripheral vascular resistance? |
resistance to blood flow imposed by the force of friction between blood and the walls of its vessels |
|
hematocrit |
percentage of the blood volume composed of cells-- affects vascular resistance |
|
What are 3 factors affecting peripheral vascular resistance? |
-Blood viscosity -Diameter of arterioles -Vasomotor mechanism |
|
Vasomotor pressor reflexes |
-sudden INCREASE in arterial blood pressure stimulates aortic and carotid baroreceptors; results in arterioles and venules of the blood reservoirs dilating therefore reducing blood pressure -DECREASE in arterial blood pressure results in stimulation of vasoconstrictor centers, causing vascular smooth muscle to constrict therefore increasing blood pressure |
|
vasomotor chemoreflexes |
chemoreceptors located in aortic and carotid bodies are sensitive to hypercapnia, hypoxia and decreased arterial blood pH |
|
medullary ischemic reflex |
acts during emergency situation when blood flow to medulla is decreased; causes marked arteriole and venous constriction theoretically to force more blood to the head |
|
venous return |
the amount of blood returned to the heart by the veins |
|
Name and describe two variables of venous return |
1) Stress-relaxation effect: occurs when a change in blood pressure causes a change in vessel diameter(because of elasticity) and thus adapts to the new pressure to keep blood flowing(works only within certain limits) 2) Gravity: the pull of gravity on venous blood while sitting or standing tends to cause a decrease in venous return (orthostatic effect) |
|
venous pumps |
blood-pumping action of respirations and skeletal muscle contractions facilitate venous return by increasing pressure gradient between peripheral veins and venae cava |
|
How does the antidiuretic hormone mechanism affect changes in the total blood volume? |
-decreases the amount of water lost by the body by increasing the amount of water that kidneys resorb from urine before it is excreted from the body -triggered by input from baroreceptors and osmoreceptors(hypothalamus) |
|
How does renin affect changes in total blood volume? |
renin is released when blood pressure in the kidney is low; leads to increased secretion of aldosterone, which stimulates retention of sodium, causing increased rentention of water and an increase in blood volume |
|
How does Angiotensin II affect changes in total blood volume? |
This is an intermediate compound that causes vasoconstriction, which complements the volume-increasing effects of renin and promotes an increase in overall blood flow |
|
How does the atrial natriuretic peptide mechanism affect changes in total blood volume? |
Adjusts venous return from an abnormally high level by promoting the loss of water from plasma, causing a decrease in blood volume. Also increases urine sodium loss, which causes water to follow osmotically |
|
Systolic blood pressure |
force of the blood pushing against the artery walls while ventricles are contracting |
|
Diastolic blood pressure |
force of the blood pushing against the artery walls when ventricles are relaxed |
|
Pulse pressure |
the difference between systolic and diastolic blood pressure |
|
Describe what a pulse is and what it indicates |
-alternate expansion and recoil of an artery -can indicate adequacy of perfusion and blood volume as well as heart function |
|
What is a pulse wave |
-each pulse starts with ventricular contraction and proceeds as a wave of expansion throughout the arteries. It gradually dissipates as it travels, disappearing in the capillaries |
|
What is an electrocardiogram (ECG) |
graphic record of the heart's electrical activity, and its conduction of impulses |
|
How does an ECG work? |
Electrodes of an electrocardiograph are attached to the subject. Changes in voltage are recorded that represent changes in the heart's electrical activity- the cardiac cycle |
|
What does the P wave represent? |
represents depolarization of the atria |
|
What does the QRS complex? |
Represents depolarization of the ventricles and repolarization of the atria |
|
What does the T wave represent? |
represents repolarization of the ventricles |
|
Describe the atrial systole |
Contraction of the atria -causes the pumping of blood out of the atria into the ventricles -AV valves are open -SL valves are closed -Ventricles are relaxed and fill with blood -This cycle begins with the P wave of the ECG |
|
Describe the isovolumetric ventricular contraction |
-Occurs between the start of ventricular systole and the opening of the SL valves -Ventricular volume remains constant as the pressure increases rapidly -Onset of ventricular systole coincides with the R wave of the ECG and the appearance of the first heart sound |
|
What is ejection? |
-When the SL valves open and blood is ejected from the heart when the pressure gradient in the ventricles exceeds the pressure in the pulmonary artery and aorta |
|
What is rapid ejection? |
the initial short phase characterized by a marked increase in aortic pressure and in aortic blood flow |
|
What is reduced ejection? |
characterized by a less abrupt decrease in ventricular volume; coincides with the T wave of the ECG |
|
Describe isovolumetric ventricular relaxation |
-Ventricular diastole begins with this phase -Occurs between closure of the SL valves and opening of the AV valves -Dramatic fall in intraventricular pressure but no change in volume -The closure of the aortic and pulmonary valves is caused by the pressures in the aorta and pulmonary artery being higher than the ventricles |
|
Describe passive ventricular filling |
-Returning venous blood increases intra-atrial pressure until the AV valves are forced open and blood rushes into the relaxing ventricles -Influx lasts approximately 0.1 seconds and results in a dramatic increase in ventricular volume |
|
Diastasis |
later, longer period of slower ventricular filling at the end of ventricular diastole lasting approximately 0.2 seconds; characterized by a gradual increase in ventricular pressure and volume |
|
If a P wave is missing what's happening? |
SA node not firing---> pathology |
|
What does measuring P, QRS, and T waves provide us? |
Provides us information about the rate and direction of conduction of an action potential through the heart |
|
membrane potential |
the difference in ion concentrations creates an electrical charge across the membrane of the fiber |
|
resting membrane potential |
the electrical charge difference between two sides of a polarized myocardial cell membrane |
|
action potential |
an electrical event ultimately leading to the mechanical event of muscle fiber contraction |
|
threshold potential |
when the membrane potential reaches a critical level all activation gates open instantaneously |
|
Cardiac tissue has properties of 4 things: |
1) Automaticity- the ability to depolarize spontaneously 2) Rhythmicity-the ability to depolarize spontaneously in a repetitive manner 3) Excitability- the inclination to depolarize spontaneously 4) Contractility- the ability to shorten muscle fibers |
|
sinus rhythm |
the heart rate pattern established by the SA node |
|
Stokes-Adams syndrom |
delayed ventricular takeover of the heartbeat |
|
ventricular escape |
ventricles escape the dominance of the SA node |
|
vulnerable period |
part of the heart can respond to an additional stimulus and part of it cannot |
|
normal sinus rhythm(NSR) |
occurs when the sinus node initiates each depolarizing impulse at a rate of 60-100bpm, and each impulse is conducted normally through the atria and ventricles |
|
Premature atrial contractions (PACs) |
occur when an ectopic focus fires. An ectopic focus fires before the SA node would normally fire, causing an early atrial contraction. |
|
atrial flutter |
type of supraventricular tachycardia caused by a single ectopic pacemaker located above the AV node firing at a rate of 200-350 bpm |
|
atrial fibrillation |
is caused by uncoordinated, chaotic electrical discharge from numerous ectopic foci in the atria |
|
ventricular fibrillation (VF) |
the most lethal of all cardiac arrhythmias and is equivalent to full cardiac arrest |
|
thromboemboli |
blood clots: sick-cells clumping with other sickle-cells |
|
oxyhemoglobin (HbO2) |
when a hemoglobin molecule combines with O2 |
|
deoxyhemoglobin (Hb) |
unoxygenated hemoglobin |
|
arterial oxygen saturation (SaO2) |
refers to the amount of oxygenated haemoglobin in the blood |
|
mixed venous oxygen saturation (SvO2) |
corresponds to a partial pressure of oxygen in mixed venous blood |
|
anemia |
a condition that develops when your blood lacks enough healthy red blood cells or hemoglobin |
|
HbO2 curve (dissociation curve) |
shows the relationship between plasma PO2 and the percentage of hemoglobin saturated with oxygen (SO2) |
|
Bohr effect |
the decreased affinity of hemoglobin for O2, or the rightward curve shift caused by PCO2 |
|
carbaminohemoglobin |
carbon dioxide also directly combines with hemoglobin |
|
hypothermia |
the condition of having an abnormally low body temperature--> sometimes deliberately reduced to decrease tissue O2 requirements, they require less O2 because they have reduced metabolic rate |
|
C(a-v)O2 |
arterial-venous oxygen content difference |
|
cyanosis |
a long-recognized clinical sign associated with severe hypoxia. When the hemoglobin releases O2 and becomes desaturated, it changes shape and turns deep purple. Severely hypoxic patients may have enough desaturated Hb in their blood that the skin, nail beds, lips, and mucous membranes appear blue/grey |
|
ischemia |
tissue hypoxia |
|
methemoglobin (metHb) |
formed when the ferrous ion (Fe++) of normal HbA is oxidized to the ferric form (Fe+++) |
|
carbonic acid (H2CO3) |
the presence of CO2 in the blood creates a special problem because CO2 reacts with water to form this acid |
|
CO2 hydration reaction: |
H2O + CO2 --> H2CO3 --> HCO3- + H+ |
|
chemical equilibrium |
exists when the reaction velocities to the left and right are equal |
|
law of mass action |
the rate of a chemical reaction is directly proportional to the product of the activities or concentrations of the reactants |
|
volatile acid |
H2CO3 because it is in equilibrium with CO2 gas. It can be excreted by the body by ventilation |
|
carbonic anhydrase |
an intracellular enzyme that makes the reaction between CO2 and H2O occur 13000x faster in the erythrocyte than in the plasma |
|
chloride shift |
the outward movement of HCO3- creates an electropositive environment inside the erythrocyte: to maintain |
|
Haldane effect |
the affinity of hemoglobin for CO2 is greater when it is not combined with oxygen |
|
How many mL of oxygen can each gram of hemoglobin bind? |
1.34 mL |
|
Can a hemoglobin bind 2 or 3 molecules of oxygen? |
NO. fully loaded with 4 molecules of oxygen or unloaded, there is no in between |
|
Explain cooperative binding |
the binding of an O2 molecule to one heme group induces a structural change in the shape of the hemoglobin molecule, which increases the affinity of the next heme subunit for O2 |