heart physiology

what does the cardiovascular system consist of

a pump (heart) that is connected to a series of tubes (blood vessels) filled with fluid (blood)

how does blood move through the body

• pressure generated in the heart propels blood through the body

• blood always flows down a pressure gradient

what are capillaries

microscopic vessels where blood exchanges materials with the interstitial fluid

what is the transition of fluid in filtration

plasma-like fluid moves to the interstitial fluid

what is the transition of fluid in absorption

plasma-like fluid within interstitial fluid goes into the blood

what materials does blood transport

• oxygen

• nutrients

• carbon dioxide

• cellular metabolic waste

• stored nutrients like glucose and fatty acids

where does blood pick up oxygen from

picks up from the lungs

where does blood pick up nutrients from

picks up from the small intestine

where does blood deliver carbon dioxide to

delivers to the lungs

where does blood deliver cellular metabolic waste to

delivers to the kidneys for excretion

where does blood pick up glucose and fatty acids from

glucose is picked up from the liver and fatty acids are picked up from adipose tissue

where does blood deliver glucose and fatty acids to

delivers to metabolically active cells like skeletal muscle and brain

what are the materials that enter the body (and into the blood)

• oxygen

• nutrients and water

what are the materials that are moved from cell to cell

• wastes

• immune cells, antibodies, clotting proteins

• hormones

• stored nutrients

what are the materials that leave the body (through the blood)

• metabolic wastes

• heat

• carbon dioxide

oxygen - where it comes from and where it is transported to

• from the lungs

• to all cells

nutrients and water - where it comes from and where it is transported to

• from intestinal tract

• to all cells

wastes - where it comes from and where it is transported to

• from some cells

• to the liver for processing

immune cells, antibodies, clotting proteins - where it comes from and where it is transported to

• present in the blood continuously

• available for any target cell that needs them

hormones - where it comes from and where it is transported to

• from endocrine cells

• to target cells

stored nutrients - where it comes from and where it is transported to

• from liver and adipose tissue

• to all cells

metabolic wastes - where it comes from and where it is transported to

• from all cells

• goes to the kidneys

heat - where it comes from and where it is transported to

• from all cells

• goes to skin

carbon dioxide - where it comes from and where it is transported to

• comes from all cells

• goes to lungs

why can’t neurons in the brain use anaerobic pathways

• neurons in the brain have a high rate of oxygen consumption

• neurons cannot meet their metabolic needs for ATP using anaerobic pathways

what happens to someone after blood flow to the brain is stopped

• 5-10 seconds after blood flow to the brain is stopped, person loses consciousness

• if oxygen delivery is interrupted for 5-10 minutes, permanent brain damage results

what does the brain do to try and maintain oxygen levels when blood flow is restricted

given the sensitivity of the brain to hypoxia, homeostatic controls do everything possible to maintain cerebral blood flow, even if it means depriving other cells of oxygen

what are the different functions of the transportation of materials in the blood

• long distance cell-to-cell communication

• defense

• heat dissipation

which substance in the blood is carried and where is it carried to in long distance cell-to-cell communication

• neurohormones secreted by neurons are carried in blood to target cells

• hormones secreted by endocrine glands are carried in the blood to target cells

which substance in the blood is carried and where is it carried to in defense

leukocytes and plasma proteins (antibodies) patrols the circulation to intercept foreign invaders

which substance in the blood is carried and where is it carried to in heat dissipation

heat circulates via the blood, moving from body core to the surface where it dissipates

where is pressure in the cardiovascular system created

high pressure originates in the chambers of the heart when cardiac muscle contracts

how does pressure change once blood moves out of the heart

• pressure is lost due to friction between the fluid and blood vessel walls

• pressure falls continuously as blood moves further from the heart

where is the highest pressure in the heart

in the left ventricle

where is the highest vessel pressure

in the aorta, followed by other downstream systemic arteries

where is the lowest vessel pressure

in the superior and inferior vena cava, just before they connect to the right atrium

what is the definition of pressure

it is the force exerted by a fluid on the container holding the fluid

what is the definition of blood pressure

it is the lateral force that blood exerts on the sides of the heart chamber walls and on blood vessel walls

what unit is used for pressure measurements in the heart and blood vessels

millimeters of mercury (mmHg)

what is it called when fluid is not moving

• pressure is called hydrostatic pressure (potential energy)

• force is exerted equally in all directions on the walls of the system

what are the two components of pressure exerted by moving fluids

• dynamic flowing component (kinetic energy)

• lateral or hydrostatic pressure (potential energy) exerted on the walls of the system

how does pressure related to distance in a system where fluid is flowing

• pressure decreases over distance because energy is lost due to friction

• inverse relationship

what happens to the pressure exerted on the blood in the heart chambers when the atria and ventricles contract

it increases

what is driving pressure

• created by the ventricles

• force that drives blood through the blood vessels

what happens to the pressure of blood inside the heart when the muscular walls of the right and left ventricles relax

it decreases

how does pressure gradient in the circulatory system compare to the absolute pressure in the system

• they are not the same thing

• two identical blood vessels can have different absolute pressure but the same blood flow (pressure gradient)

• if the pressure difference in two vessels is identical, the blood flow through the tubes is equal (i.e. - one blood vessel has 25 mmHg on the right and 5 mmHg on the left while another blood vessel has 50 mmHg on the right and 30 mmHg on the left)

how does the flow of blood in the circulatory system relate to the pressure gradient

• it is directly proportional to the pressure gradient

• the higher the pressure gradient, the greater the blood flow

how does the flow of blood in the circulatory system relate to the resistance of the system

• it is inversely proportional to the resistance of the system

• the higher the resistance is to blood flow (vasoconstriction), the lesser the blood flow

what part of the heart is made up of cardiocytes

• most of heart is myocardium and is composed of cardiocytes

• they are all called myocardial contractile cells or cardiac muscle cells

how much of the heart is comprised of autorhythmic cells

approximately one percent of the myocardial cells

what are autorhythmic cells specialized for and where are they located to do this

• specialized to generate action potentials spontaneously

• clustered in the sino-atrial node in the right atrium

what are the cell junctions of cardiocytes called

intercalated disks

what do intercalated disks consist of

interdigitated membranes tightly linked by desmosomes that tie adjacent cells together

how are cardiocytes electrically connected to each other

gap junctions in the intercalated disks

what is the anatomy of a cardiocyte cell

it is a striated muscle cell with myofibrils containing actin and myosin organized into sarcomeres

how do cardiocytes compare with skeletal muscle FIBERS

• they are smaller than skeletal muscle fibers

• contain a single nucleus per fiber

how do cardiocytes compare with skeletal muscle CELLS

• T-tubules are larger than skeletal muscle and branch inside of the cell

• sarcoplasmic reticulum is smaller than skeletal muscle because cardiocytes depend partly on interstitial fluid calcium to initiate contraction

• mitochondria occupy one third of the cell volume

how much oxygen do cardiocytes extract from the blood delivered to it by the coronary arteries

70-80% (twice the amount removed by other cells in the body)

what is the only way to get more oxygen to cardiac muscle during exercise

• during exercise, the heart removes almost all of the oxygen brought to it by the coronary arteries

• the only way to get more oxygen to exercising cardiac muscle is to increase the flow of oxygenated blood to the heart via vasodilation of the coronary arteries

what damage occurs to the heart from atherosclerosis

decreased blood flow due to narrowing of coronary vessels can damage/kill cardiocytes

what is the sympathetic' system’s responses to exercise

• increased heart rate

• increased force of ventricle contraction

• vasodilation of coronary arteries and skeletal muscle arteries

• vasoconstriction of peripheral vessels

• release of stored glucose from the liver and adipose cells

• relaxation of smooth muscle of the gastrointestinal tract

which neurotransmitters binds to which receptors during the sympathetic system’s response to exercise to increase heart rate

norepinephrine binds to beta receptors on the heart

which neurotransmitters binds to which receptors during the sympathetic system’s response to exercise to increase the force of ventricle contraction

norepinephrine binds to beta receptors on the heart

which neurotransmitters binds to which receptors during the sympathetic system’s response to exercise to vasodilate the coronary arteries and skeletal muscle arteries

epinephrine binds to beta receptors on the smooth muscle of the arteries

which neurotransmitters binds to which receptors during the sympathetic system’s response to exercise to vasoconstrict the peripheral vessels

epinephrine binds to alpha receptors on the smooth muscle of the vessels

which neurotransmitters binds to which receptors during the sympathetic system’s response to exercise to release stored glucose from the liver and adipose cells

epinephrine binds to beta receptors on the liver and adipose tissue

which neurotransmitters binds to which receptors during the sympathetic system’s response to exercise to relax the smooth muscle of the gastrointestinal tract

epinephrine binds to alpha receptors on the smooth muscle of the gastrointestinal tract

what percentage of myocardial cells do autorhythmic cells make up

they comprise approximately 1% of the myocardial cells

what are autorhythmic cells specialized to do

• generate action potentials spontaneously

• the signal for myocardial contraction does not come directly from the nervous system but rather from these specialized myocardial cells

• also called pacemaker cells because they set the heart rate

where are the autorhythmic cells located

clustered in the sino-atrial node (SA node) in the right atrium

why can the atria and ventricles contract without a connection to other parts of the body

the signal for contraction is myogenic

what does it mean that the signal for the heart to contract is myogenic

the signal originates with the heart muscle itself

how are autorhythmic cells anatomically distinct from contractile cardiocytes

• smaller cells

• contain fewer contractile fibers

• no organized sarcomeres

• do not contribute to the contractile force of the heart

what is the difference between cardiocyte muscle contraction and skeletal muscle contraction

• cardiocytes are able to execute graded contractions meaning that individual cells can vary the amount of force they generate

• skeletal muscle fibers contract only in an all or nothing fashion

what type of contraction do cardiocytes generate at rest

they develop enough cross bridges to cause a mild/moderate ventricle contraction

what type of contraction do cardiocytes generate during exercise

the sympathetic system increases the force of ventricle contraction which then increases cross bridge formation in cells leading to a moderate to maximum ventricle contraction

what happens during Step One of cardiocyte contraction

• action potentials are generated spontaneously in autorhythmic cells and propagate to cardiocytes via gap junctions

• action potentials enter from adjacent cardiocytes and travel down the cardiocyte cell membrane

• these activities take place at electrical synapses (gap junctions)

what happens during Step Two of cardiocyte contraction

• action potentials in T-tubule cause coltage gated calcium ion channels to open

• calcium from the interstitial fluid enters the cardiocyte down its electrochemical gradient

• this is an excitation-contraction coupling event

what happens during Step Three of cardiocyte contraction

• influx of calcium from outside the cell causes the sarcoplasmic reticulum to open, allowing more calcium to empty into the cytosol

• calcium release from the sarcoplasmic reticulum is called the calcium spark

• this is an excitation-contraction coupling event

what happens during Step Four of cardiocyte contraction

• calcium from the interstitial fluid and the calcium spark from the sarcoplasmic reticulum add together to cause a calcium signal

• this is an excitation-contraction coupling event

what happens during Step Five of cardiocyte contraction

• calcium ions bind to troponin which causes tropomyosin to move away from the actin binding site

• this is an excitation-contraction coupling event

what happens during Step Six of cardiocyte contraction

cross bridge formation between actin and myosin with resulting power stroke occurs

what is the force of cardiac muscle contraction proportional to

it is proportional to the number of cross bridges that are active

what determines the number of active cross bridges during cardiac muscle contraction

determined by how much calcium is bound to troponin

what happens with calcium levels in cardiac muscle cells during rest

• cytosolic calcium concentrations are low

• some cross bridges are not activated

• force of ventricular contraction is low

what happens with calcium levels in cardiac muscle cells during exercise

• additional calcium enters the cell from the interstitial fluid

• this causes more calcium to be released from the sarcoplasmic reticulum

• additional calcium binds to troponin to increase cross bridge formation between actin and myosin

• force of ventricular contraction (tension from cross bridges) increases

what three factors is the force of cardiocyte contraction dependent on

• amount of catecholamine released

• type of catecholamine released

• cardiocyte cell length

what is the process of how cardiocytes increase the force of ventricular contraction

1. more epinephrine is released at the synapse between the post-ganglionic sympathetic neuron and the cardiocyte

2. more norepinephrine binds to beta receptors on the cardiocyte

3. more binding causes increased action potential generation at the cardiocyte

4. more action potentials cause increased diffusion of calcium from the interstitial fluid into the cardiocyte

5. increased influx of calcium from outside the cell causes increased release of calcium from the sarcoplasmic reticulum

6. more calcium binds to troponin

7. more tropomyosin moves away from the actin-myosin binding site

8. more cross bridges are formed

9. increased force of cardiocyte contraction

force of cardiocyte contraction is affected by sarcomere length during which phase

it is affected by sarcomere length at the end of filling and at the beginning of contraction

what does the stretching of cardiocytes allow the cells to do

• cardiocytes are routinely stretched and are, therefore, able to exert more force when blood fills the chambers

• when the heart is empty and the cardiocytes are not stretched, there is too much overlap of the actin and myosin so less cross bridge formation is possible

what are the characteristics of the action potentials of cardiocytes

• rapid depolarization due to sodium entry

• last longer than other action potentials due to calcium entry

• repolarization due to potassium leaving the cell

what is Step One of a cardiocyte’s action potential cycle

resting membrane potential is at -90 mV

what is Step Two of a cardiocyte’s action potential cycle

depolarizing stimulus shifts membrane potential from -90 mV to -40 mV, bringing it up to threshold

what is Step Three of a cardiocyte’s action potential cycle

• voltage gated Na+ channels open when threshold is reached

• Na+ rushes into the cell via simple diffusion down its electrochemical gradient

what is Step Four of a cardiocyte’s action potential cycle

• voltage gated Na+ channels close at +20 mV

• fast voltage gated K+ channels open

what is Step Five of a cardiocyte’s action potential cycle

• voltage gated Ca++ channels open

• Ca++ rushes into the cell via simple diffusion down its electrochemical gradient

• Ca++ influx lengthens the action potentials

• fast voltage gated K+ channels close

what is Step Six of a cardiocyte’s action potential cycle

• voltage gated Ca++ channels close

• slow voltage gated K+ channels open

what is Step Seven of a cardiocyte’s action potential cycle

• slow voltage gated K+ channels close

• resting membrane potential returns to -90 mV

how much long do cardiocyte action potentials last compared to the action potentials of other cell types

• cardiocyte action potentials last 200 milliseconds or more

• typical action potential durations for neurons and skeletal muscle fibers are about 1-5 milliseconds

what purpose do the longer action potentials in cardiocytes serve

they prevent tetanus