THE HEART Cardiovascular System

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

External Heart: Anterior View

Figure 18.4b

Heart 1. Muscular pump that is about the size of a fist; The heart weights 1/200th (0.5%) of total body

weight, yet receives 1/20th or 5% of all of the blood. Average heart weighs 0.67 lbs 2. the resting heart beats about 110,000x/day = 70 beats/min (60 min/hour)(24 hrs/day). The

heart beats around (35 million beats/yr as it moves blood through about 60,000 miles of blood vessels. The resting heart beats around 2.5 billion times across a 70-year life span. The resting

heart pumps about 2000 gallons of blood each day and 700,000 gallons each year. Average resting heart pumps 1.3 gal/min (5L/min)

3. the heart receives about 5% of the circulating blood each minute to meet its metabolic needs despite that it is only 0.5% of the body’s total weight

4. An interruption in the blood flow to the heart can cause tissue death within minutes 5. An injection of stem cells into the heart results in some of the stem cells producing new cardiac

muscle cells. There is some mitotic activity in the heart throughout life. Main Function of the Heart: receive low pressure blood and pump it out at high pressure Location: behind the lower sternum in the thoracic cavity; the apex of the heart rests on top of

the diaphragm and the major blood vessels come out of its base at the top of the heart. Coverings of the Heart 1. Collectively called the pericardium (peri = around). The pericardium is a double-walled sac that

encloses the heart 2. as the heart beats, it does not rub up against other organs 3. serous pericardium – serous pericardium consists of 2 layers; a visceral layer lies on top of the

heart and is also called the epicardium; a parietal layer forms the inner lining of the fibrous pericardium. The pericardial cavity is between the 2 layers of the serous pericardium

4. fibrous pericardium – outer layer; tough sac of dense regular connective tissue; lined with the parietal layer of the serous pericardium

5. pericardial cavity – fluid-filled cavity between the two membranes; the pericardial fluid is exuded by the serous pericardium

Wall of the Heart (3 distinct layers) 1. epicardium – same as the visceral pericardium (inner part of serous); contains blood vessels

(e.g., coronary arteries) and fat 2. myocardium – thickest layer made up mostly of a highly vascular cardiac muscle; also contains

connective tissue, blood supply, and nerves 3. endocardium – inner lining of the heart a. endothelial lining that extends from the inner lining of blood vessels (simple squamous) to

line the cavities and cover the valves; the endocardium is a continuation of the endothelial lining of blood vessels

b. endocarditis – infection of the endocardium; can occur as a result of scarlet fever (Little Women by Louisa May Alcott – one of the 5 sisters died of the complications of scarlet fever that damaged her heart valves and left her tired and bed-ridden)

Blood Vessels (General pattern of blood flow) 1. Heart → arteries → arterioles → capillaries → venules → veins → heart 2. Arteries and arterioles carry blood away from the heart towards capillaries 3. Veins and venules carry blood from capillaries to the heart 4. Most arteries, arterioles, capillaries, venules and veins are inside the organs of the body.

2

Heart Chambers and Associated Major Vessels 1. Atria (upper chambers, sing. Atrium) a. receiving chambers: receive blood returning to the heart within large veins 1. venous return from pulmonary circuit by way of 2 prs of pulmonary veins into LA 2. venous return from the systemic circuit by way of 2 vena cavae into RA b. auricle – expandable pouches on the sides of the atria c. interatrial septum – wall of cardiac muscle between the 2 atria 2. Ventricles (lower chambers) a. muscular pumps, thick-walled chambers b. RV – pulmonary pump c. LV – systemic pump (3x thicker than the RV) d. interventricular septum – muscular wall between the 2 ventricles

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Pathway of Blood Through the Heart and Lungs

Figure 18.5

Systemic and Pulmonary Circuits of Blood 1. two circuits of blood in the body, each with their own separate system of blood vessels; the

term circuit only refers to the blood vessels 2. Systemic Circuit a. delivers blood to systemic capillaries in almost every organ of the body to include the lungs

(the lens, cornea and cartilage are avascular) b. Why? So that tissue cells get a supply of O2 and nutrients and provides a way to send the

CO2 they make back to the lungs to be exhaled

3

c. the systemic circuit is a system of blood vessels that begins with the aorta and ends with the vena cavae

d. Left Ventricle (systemic pump) pumps blood into → aorta → arterial branches of the aorta → arteries into the organs of the body (pancreas, skeletal muscle, bones, brain, intestine, liver, etc) → smaller and smaller arterioles → capillary beds that go by each cell in the body (nutrient and gas exchange) → venules → major veins coming out of the organs of the body → vena cavae -> back to the RA of the heart

e. the bronchial arteries are systemic arteries that branch off the aorta and go into the lungs 3. Pulmonary Circuit a. delivers blood to the pulmonary capillaries that cover the air sacs (alveoli) within the

lungs. b. Delivers blood from the heart exclusively to the lungs c. Why? Within the lung, blood picks up O2 and gives off CO2 d. the pulmonary circuit is a system of blood vessels that begins with the pulmonary trunk

and ends with the pulmonary veins e. Right Ventricle (pulmonary pump) pumps blood to → pulmonary trunk which splits into 2

pulmonary arteries to the lung → arteries within the lung lead to arterioles → smaller and smaller arterioles → capillary beds that go across the 300 mil air sacs or alveoli within the lungs (gas exchange) → venules → major veins coming out of the lungs → back to the LA of the heart

Heart Vessels 1. Coronary arteries (heart receives 1/20th (5%) of the blood pumped into the aorta) a. arise from the base of the aorta as it leaves the heart b. branch into an interconnected network of blood vessels that supply the myocardium; as a

result the heart still functions okay if one of the coronary arteries is badly occluded (up to 80% blockage and still okay circulation)

c. Coronary bypass uses the saphenous vein in the leg (or another blood vessel like the internal mammary artery) to connect the descending aorta to the coronary artery at a place beyond the blocked area

2. Cardiac veins – drain myocardial capillaries; cardiac veins carry blood directly to the heart via the coronary sinuses that open into the RA

3. Coronary sinuses can be seen on the surface of the heart from a posterior view

4

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Gross Anatomy of Heart: Frontal Section

Figure 18.4e

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Heart Valves

Figure 18.8c, d

5

Heart Valves 1. 1-way valves that regulate the flow of blood through the heart 2. Atrioventricular Valves (AV Valves) – found between the atria and the ventricles a. Tricuspid valve (“r” for right) – between the RA and RV; has 3 cusps or flaps b. Bicuspid or mitral valve – between the LA and LV; has two flaps c. chordae tendineae (“heart strings”, s. chorda tendinea) – dense regular connective tissue

(80% collagen) 1. string like cords of dense CT that are similar to tendons; they connect the edges of the

flaps to papillary muscles (resemble the shroud lines attached to the edges of a parachute)

2. papillary muscles are columns of cardiac muscle that come off of the sides of the ventricles (papilla means little bump)

3. papillary muscles contract before other ventricular muscle to take up the slack in chordae tendineae before full force of ventricular contraction hurls blood against the AV flaps

d. abnormal valve conditions 1. stenosis – narrow valve that offers more resistance to the flow of blood; this can

overwork the heart over time; valve closes, but does not open fully; this makes it difficult for blood to flow from the atria to the ventricles

2. prolapse (e.g., mitral valve prolapse) a. irregular-shaped valve that does not close properly; valve is leaky and blood

“murmurs” as it flows through it b. most are asymptomatic and a person with a prolapsed valve may be unaware it

exists; most live normal life 3. murmur a. blood flowing through a leaky valve makes a murmuring sound b. most murmurs are not serious enough to cause problems or warrant surgery 3. Semilunar Valves (SL valves) – found between the ventricles and arterial outlets of the heart a. pulmonary SLV – between the RV and pulmonary trunk b. aortic SLV – between the LV and the aorta c. cusps – each SLV consists of 3 cup-shaped cusps or flaps (similar to a shirt pocket) 1. the pocket-like cup portion faces away from the ventricles 2. Fcn: prevent arterial blood leaving the ventricles to flow back into the ventricles 3. as the ventricles relax they create suction (negative pressure space) that tends to draw

blood in the arteries back into them. As blood flows back towards the heart, it fills up the cups of the valve. As they fill they enlarge toward the center and the opening closes off

Heart Skeleton 1. Band of dense connective tissue meshwork of collagen and elastic fibers that separate the

atria from the ventricles (they are not made of bone) and extend up into the septa 2. Heart skeleton forms rings around the openings of the heart valves. Valves are extensions of

the heart skeleton. Valve flaps are anchored to the heart skeleton 3. Function a. provide rigid support for the heart b. cardiac muscle contracts against the heart skeleton

6

c. because it is made of non-conducting CT, the cardiac impulse does not travel through it; hence, the cardiac impulse must pass from the atria to the ventricles across the AV node

Fetal Circulation Bypasses the Lung 1. circulation in the fetus occurs in such a way as to mostly bypass the pulmonary circuit since

the fetal lungs are filled with amniotic fluid and non-functional 2. ductus arteriosus a. arterial duct that shunts blood from the pulmonary trunk to the aorta thus away from the

pulmonary circuit b. this duct closes a few hours after birth; eventually fibrous CT forms within the duct and it

transforms into the ligamentum arteriosum in the newborn c. patent ductus arteriosus (patent means “open”) – congenital defect that occurs when the

duct fails to close properly; can be surgically corrected 3. Foramen ovale (“hole in heart”) a. opening in the interatrial septum that shunts blood from the RA to the LA thus routing

blood away from the pulmonary circuit b. within 48 hours of birth, the opening seals off and becomes the fossa ovalis c. interatrial septal defect – congenital defect that occurs if the foramen ovale fails to close;

it can be surgically corrected with a small patch; if the hole is small, it is simply left as is Cardiac Duty Cycle 1. describes the mechanical operation of the heart as it pumps blood (i.e., beats) a. the resting heart beats about 70-75 x/min b. each beat is a complete cardiac duty cycle lasting about 0.8 seconds (0.5 – diastole, 0.3 –

systole) 2. systole (0.3 sec) – contraction phase of a heart chamber (atrium or ventricle); as a chamber

contracts it empties (atria empty to the ventricles, ventricles empty to the arterial outlets) 3. diastole (0.5 sec) – relaxation phase; as a chamber relaxes it fills with blood 4. by convention, these terms when used alone refer to the ventricles only 5. in an upright position, about 70-80% of atrial blood normally flows into the ventricles as a

result of gravity; the contraction of the atria only adds about 20% of the blood to the ventricles. Thus, the atria are not essential to the filling of the ventricles unless a person is lying flat, upside down, or exercising

7

Resting Blood Pressure (BP) 1. reflects the pressure changes in the blood as the ventricles of the heart contract and relax 2. it is usually taken with a cuff placed over the brachial artery in the upper arm with a listening

device placed neat the antecubital fossa (elbow pit) to monitor the sounds of Korotkoff 3. Resting Systemic Arterial Pressure for healthy person 20-30 years of age (blood pressure in

the major arteries is pulsatile) Resting Systemic Arterial Pressure = Systolic P (LV contracts) = 120 mmHg Diastolic P (LV relaxes) 75 mmHg

Normal Resting BP (mmHg) = (100 to 140)/(60 to 90) 4. Diastolic pressure is usually of greater concern since this is the least amount of pressure that

develops against the arterial wall at all times in a resting individual. The higher this value, then the greater the overall stress on the blood vessel walls and the greater chance of them becoming damaged over time.

5. if resting BP >= 140/90 (160/100, 150/110), then indicates hypertension (higher than normal resting BP)

8

a. about 30% of those with hypertension can manage it by changes in lifestyle (eat better, lose weight, exercise, and reduce stress)

b. Drugs: diuretics, Beta-blockers to slow the heart, ACE inhibitors, Ca2+-channel blockers c. Problems 1. hypertension can weaken the walls of small arteries and cause aneurysms. High BP can

cause tears in the lining of large arteries that act as focal points for development of atherosclerotic plaque. It also promotes atherosclerosis which can lead to heart problems

2. heart attack and stroke 3. kidney failure 4. blindness as retinal cell degenerate 6. Resting BP in the pulmonary circuit is much lower than in the systemic circuit a. Resting BP (RV contracts/relaxes) = 25/10 b. there are fewer capillary beds in the lungs as compared to the rest of the body and the

heart doesn’t have to work as hard to counteract the effect of gravity when pumping the blood, thus the pressures in the pulmonary circuit are less. Overall, there is less resistance to flow of blood in the pulmonary circuit as compared to the systemic circuit

c. the RV is not as strong a pump as the LV. The LV wall is 3x thicker than the RV wall 7. Renin-Angiotensin System a. angiotensinogen is an inactive protein made by the liver that circulates in the blood (i.e., it

is a plasma protein). Other plasma proteins made by the liver include prothrombin and fibrinogen (clotting), and albumin (osmotic balance).

b. when the BP drops, kidney cells release an enzyme called renin into the bloodstream c. Renin is an enzyme that converts angiotensinogen to angiotensin I (Ang I) d. Angiotensin-converting enzyme (ACE) occurs on the luminal surfaces of the plasma

membranes of endothelial cells lining capillary walls (mostly in the lung). ACE converts Ang I to Ang II

e. Angiotensin II (Ang II) 1. is an extremely potent vasoconstrictor (stimulate contraction) of smooth muscle in

arteriole walls (40x more potent than Epi) that decreases diameter of arterioles and that acts to increase BP.

2. increases release of ADH and aldosterone to increase blood volume f. ACE inhibitors are often taken by individuals to control BP

Angiotensinogen is a plasma protein made by the liver that circulates at all times. Renin (kidney) and ACE (lungs) are enzymes

Normal BP -> Drop in BP within kidney →renin

Renin ACE

Angiotensinogen → Ang I → Ang II → vasoconstriction to increase BP Vasoconstriction occurs when the smooth muscle in the walls of mostly systemic arterioles contracts. This decreases their diameter which increases the PR to increase BP

9

Hypertension (“the silent killer”) – hypertension is a higher than normal resting BP > 140/90 1. Hypertension is the most common cardiovascular disease affecting about 30% of Americans

over the age of 50 and 50% by age 75. 2. it is said to be a silent killer since it can bring about its destructive effects for 10 to 20 years

before they are first noticed 3. hypertension is the major cause of heart failure, stroke, and kidney failure 4. it damages or weakens the heart because it makes the heart work harder to pump out the

blood. The myocardium enlarges and as it does it stretches and becomes less efficient at pumping blood

5. hypertension strains the blood vessels and tears the endothelial lining, thereby creating lesions (sites of injury) that become focal points for atherosclerosis

6. hypertension damages blood vessels within the kidneys that causes them to thicken. This results in reduced blood flow to the kidneys.

7. the risk factors for hypertension include a. obesity – each pound of fat results in the growth of additional blood vessels to serve it. The

extra blood vessels increase the overall peripheral resistance which acts over time to raise the blood pressure. Even a small weight loss can significantly lower the blood pressure

Defining Adult Overweight and Obesity Weight that is higher than what is considered as a healthy weight for a given height is described as overweight or obese. Body Mass Index, or BMI, is used as a screening tool for overweight or obesity. Adult Body Mass Index (BMI) Body Mass Index (BMI) is a person's weight in kilograms divided by the square of height in meters. A high BMI can be an indicator of high body fatness.

• If your BMI is less than 18.5, it falls within the underweight range. • If your BMI is 18.5 to <25, it falls within the normal. • If your BMI is 25.0 to <30, it falls within the overweight range. • If your BMI is 30.0 or higher, it falls within the obese range.

See the following table for an example.

Height Weight Range BMI Considered

5' 9" 124 lbs or less Below 18.5 Underweight

125 lbs to 168 lbs 18.5 to 24.9 Healthy weight

169 lbs to 202 lbs 25.0 to 29.9 Overweight

203 lbs or more 30 or higher Obese

271 lbs or more 40 or higher Class 3 Obese

b. lack of exercise – aerobic exercise helps to reduce hypertension by controlling weight,

reducing emotional stress, and stimulating vasodilation c. dietary factors – diets high in cholesterol and saturated fat contribute to atherosclerosis.

10

d. cigarette smoking – nicotine stimulates vasoconstriction of heart vessels and causes the heart to beat faster and harder; CO in smoke can enter circulation and damage blood vessel walls. Nicotine is a vasoconstrictor throughout the body and can raise BP

e. chronic psychological stress over relationships and work-related issues can stimulate the sympathetic nervous system which can lead to hypertension

f. Age – usually appears in individuals over the age of 40 8. treatment for hypertension typically involves a. weight loss, diet changes, exercise, and certain drugs if necessary. b. Diuretics lower blood volume by promoting urination. ACE inhibitors block the formation of

the vasoconstrictor angiotensin II. Beta blockers such as propanolol block the vasoconstrictive effects of epi and norepi. Calcium channel blockers inhibit the flow of calcium into cardiac and smooth muscle cells, thus inhibiting their ability to contract, promoting vasodilation and reducing the work of the heart

Heart Sounds 1. lub-dup that is heard as the heart beats one time (can hear with a stethoscope or just by

putting ear to another’s chest) 2. sounds occur as heart valves snap shut and shake the heart to create sound 3. lub (S1) – 1st heart sound; occurs as the ventricles contract and the AVV’s close (vibrations

within the heart and turbulent flow create sound); occurs during QRS complex 4. dup (S2) – 2nd heart sound; occurs as the SLV’s close as the ventricles relax (occurs at end of T)

Cardiac Muscle Properties 1. Myogenic a. myogenic means that the heart beat originates in the heart; the cardiac impulse that

stimulates the heart to beat begins in the heart. Cardiac muscle cells can generate the cardiac impulse without any stimulation from neurons. Skeletal muscle must be stimulated by somatic motor neurons before it will contract.

b. the heart self-stimulates as a function of the SA node. The SA node generates the cardiac impulse which is a series of self-propagating action potentials (same as nerve impulse)

c. can remove the heart and place it in a warm saline solution with nutrients; the heart will continue to beat for another hour

2. Long refractory period a. refractory – resistant to stimulation or non-responsive b. refractory period is the amount of time that cardiac muscle cells must “rest” before they

can contract again 1. refractory period of skeletal muscle cells is very short (2 msec) so they can tetanize 2. refractory period of cardiac muscle is very long (250 msec) so they cannot tetanize c. after the heart contracts, it must “rest” for a short period of time (1/4th of a second) before

it will contract again d. refractory period lasts about 250 msec (1/4 sec); max HR is about 230-250 beats per min e. consequence: heart muscle does not tetanize like skeletal muscle (a tetanized heart would

not pump blood) 3. high density of mitochondria a. 25% of the mass of a cardiac muscle cell is made up of mitochondria (compared to 2% for

skeletal muscle)

11

b. high rate of oxygen-requiring cellular respiration by cardiac muscle cells to meet their ATP demands for contraction; at rest cardiac muscle cells get most of their ATP from the catabolism of fatty acids (60%), glucose (35%), and other organic compounds such as amino acids

c. cardiac muscle cells can use almost any fuel substrate: glucose, fatty acids, amino acids, glycerol

d. all highly active cells have a lot of mitochondria (heart, neurons, liver, kidney, muscle) 4. Intercalated Discs a. a single stimulus to the heart causes the entire heart to contract b. once initiated, the cardiac impulse spreads across the heart c. cardiac muscle cells are joined by intercalated discs (junctions between cardiac muscle

cells); intercalated disks contain small tubes that are collectively called gap junctions. The tubes connect the cytoplasm of adjoining cardiac muscle cells

d. cardiac muscle impulses involve ion fluxes that spread across intercalated discs 1. anchoring desmosomes – hold cells together 2. gap junctions – allow ions to pass between the cytoplasm of 2 adjacent cells. This is a

type of electrical coupling e. cardiac muscle branching causes the impulse to spread quickly because of a multiplier

effect (i.e., 1 cell stimulates 2 cells; 2 cells stimulate 4 cells, etc.)

Conduction System (spread of cardiac impulse) 1. the cardiac conduction system is made up of specialized cardiac muscle cells that do not

contract 2. Sinoatrial node (SA node) – “pacemaker” of the heart (capable of spontaneously depolarizing

and generating a cardiac impulse. SAN firing rate = Heart Rate

a. SAN generates the cardiac impulse that causes the heart to beat. b. every time the SAN discharges, the heart beats one time

12

c. thus, the SAN firing rate determines the pace at which the heart beats d. the SAN consists of a mass of special myocardial cells embedded in the wall of the right

atrium just beneath the opening of the superior vena cava e. mechanism by which cardiac muscle impulse arises 1. the cells of the SA node are “leaky” to Na+ thus they do not have a stable resting

membrane potential; they continually depolarize towards threshold potential at regular intervals throughout life

2. there is a slow Na+ influx or current inward that eventually depolarizes the cells and generates action potentials that self-propagate as a cardiac impulse from the SA node to other myocardial cells of the heart

a. when the potential reaches threshold at about -40 mV the voltage-regulated calcium channels in the membrane open leading to a depolarization as calcium ions enter the cytoplasm from the ECF. Ca2+ entering cell slowly from IF causes SR to release Ca2+ into the cytoplasm to trigger sliding of myofilaments and contraction (Ca2+ in cytoplasm for contraction: 20% IF, 80% SR)

b. the calcium influx triggers self-propagating action potentials that lead to a cardiac impulse. It is the explosive influx of Ca2+ from IF (rather than Na+) that produces the rising phase of the AP and reverses membrane polarity. Repolarization, however, is due to an increase in K+ permeability that results in a K+ efflux.

3. unlike skeletal muscle cells and neurons, SA nodal cells do not have a stable resting membrane potential

f. Sinus Firing Rate 1. SAN firing rate without vagus nerve input = 100 beats per minute (Sinus rhythm) 2. Tonic vagal stimulation with parasympathetic release of acetylcholine drops the rate

to 75. 3. Atrioventricular Node (AV Node) a. An electrical “bridge” or coupling that connects the atria with the ventricles b. location: within the interatrial septum near the tricuspid valve c. the cardiac impulse cannot pass through the non-conducting dense CT of the heart

skeleton; instead it goes from the atria to the ventricles by passing through the AV node d. Function: delay the spread of the cardiac impulse to the ventricles long enough (about 0.1

sec or 100 msec) for the atria to empty and the ventricles to fill 4. AV Bundle, Bundle Branches and Purkinje Fibers - once the cardiac impulse crosses the AV

node it spreads rapidly through the ventricles by way of the AV Bundle (also called Bundle of His) and its branches in the interventricular septum and then by way of specialized myocardial cells called Purkinje fibers. Purkinje fibers rapidly spread cardiac impulse across the ventricles of the heart

5. all cells of the cardiac conduction system are autorhythmic and generate cardiac impulses if allowed

a. SAN – 75x/min b. AVN – 50x/min c. AV bundle and purkinje fibers – 30x/min

13

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Electrocardiography

Figure 18.16

Electrocardiogram (EKG or ECG) – graphical record of cardiac impulse spreading across the heart.

Electrocardiograph is a device used to detect the electric currents across the heart and generate an EKG

1. EKG measures the electrical activity associated with the spread of the cardiac impulse across the heart

2. Path of Cardiac Impulse: SA node (pacemaker) → atrial myocardium (atrial systole) → AV node → AV Bundle and branches → Purkinje fibers of the ventricles → ventricular myocardium (ventricular systole)

3. The spread of the cardiac impulse can be tracked by placing electrodes on the skin of an individual

4. cardiac impulse is similar to an electrical current a. depolarization is the Na+ current moving into the cardiac muscle cells. It generates 4 EKG

waves, the P and the QRS complex b. repolarization is the K+ current which occurs about 250 msec later. It generates 1 EKG

wave, the T 5. EKG Deflection Waves (P, QRS complex, T) a. P wave (atrial depolarization) 1. begins as the SA node “fires” and continues as the cardiac impulse spreads across the

atria 2. the P wave is the stimulus that causes the atria to contract (both atria contract at the

same time). The atria begin contracting about 25 msec after the start of the P wave

14

b. QRS complex (ventricular depolarization) 1. series of 3 waves: Q, R, and S 2. occurs as the cardiac impulse leaves the AV node and spreads across the ventricles

(both ventricles contract at the same time) 3. Ventricles start contracting just after peak of the R wave c. T wave (ventricular repolarization) 1. occurs as the resting potential is restored by K+ efflux 2. repolarization occurs about 250 msec after depolarization (i.e., start of Q wave) 3. the ventricles stay in a contracted state from the beginning of the QRS until start of the

T wave when they begin to relax. The ventricles are fully relaxed by the end of T wave d. repolarization of the atria occurs and creates a wave that is not seen on a typical EKG since

it is obscured by the much larger QRS complex 6. one set of waves occurs every 0.5 to 0.6 seconds, then 0.4 to 0.5 sec delay before the next set

the time between a set of waves is at least 0.25 sec (250 msec) as a result of the mandatory refractory period.

PR Interval: measured in milliseconds, that extends from the beginning of the P wave (the onset of atrial depolarization) until the beginning of the QRS complex (the onset of ventricular depolarization); it is normally between 120 and 200ms in duration. The PR interval is sometimes termed the PQ interval. PR segment – time from end of P wave to beginning of QRS complex. Corresponds to AV node delay. QT interval is a measure of the time between the start of the Q wave and the end of the T wave in the heart's electrical cycle. The QT interval represents electrical depolarization and repolarization of the ventricles. Time for ventricles to contract and fully relax

15

Cellular Respiration C6H12O6 + 6O2 → 6CO2 + 6H2O + make ATP

1. Inhale O2-rich air that moves from lungs to blood to tissue cells 2. Cells expel CO2 that moves into blood, then to lungs so can exhale CO2-rich air 3. Why ATP? Many reactions and activities that occur within cells are ATP-dependent. A cell must

make millions of ATP molecules each minute in order to survive. Failure to make ATP may lead to the death of the cell within minutes. Cellular respiration involving oxygen is the way in which cells meet their ATP demands

4. Processes that require ATP: mitosis and meiosis, muscle contraction, processing sensory information by the brain, urine production, thinking, protein synthesis within cells

16