Cardiovascular System: Function & Structure

Cardiovascular System Functions

• The cardiovascular system is a transport system where blood is pumped by the heart within a closed circuit of vessels
• Blood circulation delivers substances and removes waste
• All living cells receive necessary materials from the blood like O2 and nutrients

General Function

• It delivers necessary materials, such as O2 and nutrients, to living cells
• It removes metabolic waste like CO2
• Interstitial fluid facilitates the indirect passage of substances into and out of the bloodstream

Heart Characteristics

• The heart is a muscular organ in the thorax, shaped like an inverted cone
• An adult heart is about the size of a man's fist and weighs around 300 grams
• The heart pumps blood directly into arteries

Heart Structure

• The heart has four chambers: two atria (right and left) and two ventricles (right and left)
• The right atrium and right ventricle receive deoxygenated blood from systemic veins and pump it to the lungs
• The left atrium and left ventricle receive oxygenated blood from the lungs and pump it to systemic vessels throughout the body
• Interatrial and interventricular septa separate the left and right sides of the heart
• These septa are continuous with each other

Heart Valves

• Blood flows from the atria into the ventricles via atrioventricular orifices, which open and close periodically
• Heart valves separate atria and ventricles, as well as ventricles from great vessels
• Heart valves have two or three leaflets (cusps)
• Cusps open to allow blood flow in one direction and close to prevent backflow
• Chordae tendineae prevent backward prolapse of cusps

Types of Heart Valves

• Atrioventricular valves prevent backflow from the ventricles to the atria
• The right atrioventricular valve is also known as the tricuspid valve and is between the right atrium and right ventricle
• The left atrioventricular valve is also known as the mitral valve, and is between the left atrium and ventricle.
• Semilunar valves prevent backflow from the great vessels to the ventricles
• The pulmonary semilunar valve is between the right ventricle and the opening of the pulmonary artery, and has three semilunar cusps
• The aortic semilunar valve is between the left ventricle and the opening of the aorta, and has also three semilunar cusps

Arteries

• Arteries deliver oxygenated blood to the body.
• The pulmonary and umbilical arteries are exceptions, that carry deoxygenated blood
• Blood flows from the arteries to arterioles, then to capillaries for gaseous exchange

Types of Arteries

• Elastic arteries are the largest and have low resistance for blood flow; they can expand and recoil to handle blood volume changes
• Muscular arteries are smaller and regulate local blood flow to individual organs, with more smooth muscle and less elastin
• Arterioles are the smallest arteries that comprise a single smooth muscle layer overlying endothelial cells, and regulate blood flow to the capillaries

Capillaries

• Capillaries connect arterioles and venules and allow for the exchange of nutrients, gases, and waste
• Blood flow into capillaries is determined by the diameter of the arterioles
• Capillary blood flow can be increased through arteriolar vasodilation

Veins

• Veins transport blood back to the heart and contain valves to prevent backflow
• Veins are thin and elastic, and act as a reservoir of blood
• Veins do not need much elastin and smooth muscle because they transport low-pressure blood
• Veins have a large lumen and valves to maintain one-way blood flow

Cardiac Cycle

• The heart pumps oxygenated blood from the left ventricle into the aorta for systemic circulation
• Blood supplies cells, then returns deoxygenated to the right atrium
• The deoxygenated blood flows from the right atrium to the right ventricle
• The heart pumps blood from the right ventricle into the pulmonary arteries for pulmonary circulation
• Blood moves to the lungs, exchanges carbon dioxide for oxygen, then returns to the left atrium
• The blood is then pumped to the left ventricle to begin systemic circulation again

The Circulatory System

• The human circulatory system includes the Systemic and Pulmonary circulations
• Systemic circulation pumps blood to the left ventricle (85% of the circulation) utilizing high intravascular pressure
• Pulmonary circulation pumps blood to the right ventricle (10% of the circulation) utilizing low intravascular pressure and low intravascular pressure
• The remaining 5% is in the heart
• The pulmonary circulation allows for oxygenation of the blood
• The systemic circulation provides for systemic blood

Cardiac Muscle Cells

• Two main types of cardiac muscle cells exist: ordinary myocardial contractile fibers and specialized myocardial conducting fibers

Cardiac Muscle Fibers

• Ordinary fibers make up 99% of atrial and ventricular cells, responsible for contractions to pump blood
• Specialized fibers, comprising 1% of cells, form the conduction system, similar to neurons, initiating and propagating action potentials to trigger contractions

Properties of Cardiac Muscle

• Rhythmicity (automaticity) is the ability to depolarize spontaneously without external electrical stimulation
• Conductivity is the ability to conduct cardiac impulses initiated in the SA node (the pacemaker)
• Excitability is the ability to respond to adequate stimuli by generating an action potential
• Contractility is the ability to convert electrical energy into mechanical work, demonstrated by myocardial fibers having 'functional syncytium'
• Cardiac rhythmicity/automaticity is the ability of cardiac muscle to initiate its own electrical impulse, triggering mechanical contraction

Primary Pacemaker

• The sinoatrial node (SA node), located on the right atrium wall near the superior vena cava entrance, is the primary pacemaker
• SA node cells spontaneously depolarize, generating action potentials approximately 100 times per minute
• The native rate is modified by sympathetic and parasympathetic nerve fibers
• The average resting cardiac rate in adults is about 70 beats per minute

Secondary and Tertiary Pacemakers

• Secondary pacemaker: Atrioventricular node (AV), in the lower right posterior interatrial septum, will act as the pacemaker if the SA node does not function, discharging at 40-60 beats per minute
• Tertiary pacemaker: The bundle of His and Purkinje fibers produce action potentials at 30-40 bpm if the SA and AV nodes do not function
• It controls the whole heart because of the most rapid rate of discharge

Cardiac Conductivity and Conduction

• The electrical conduction system allows synchronous contraction of the heart's sides in a sequential order
• The SA Node is the physiological pacemaker, generating electrical activity spontaneously, propagated throughout the right atrium, through Bachmann’s bundle to the left atrium and to the Internodal Tracts to get to the AV node.
• Internodal Tracts comprise three bundles Bachmann, Wenkebach, Thorel
• The AV Node functions as a critical delay for the conduction system to allow blood to flow from atria to ventricles

Cardiac Conduction Components and Velocities

• The distal portion of the AV node is known as the bundle of His. The bundle of His splits into two branches in the interventricular septum
• The left bundle branch and the right bundle branch branch
• Purkinje Fibers then stimulate individual groups of myocardial cells to contract
• SA node velocity is 0.05 m/sec
• AV node velocity is 0.01 m/sec (slowest)
• Bundle of His velocity is 1.00 m/sec
• Purkinje fibers velocity is 4.00 m/sec (fastest)
• Atrial and ventricular muscles velocity is 0.3 to 0.4 m/sec
• The AV node has the slowest velocity to allow blood to flow from the Atria to the Ventricles
• Purkinje fibers have the fastest velocity, contracting the ventricles at the same time

Cardiac Excitability

• Ventricular muscle has a resting membrane potential of -90 mV
• Cardiac muscle fibers can generate a transmembranous action potential, that goes to +20 mV
• Transmembranous action potential of ventricular muscle is characterized by the presence of 5 phases with a plateau
• ARP: the excitability of cardiac muscle is completely lost during the whole period of systole
• The heart cannot be tetanized, as its ARP occupies the whole contraction phase
• RRP: The excitability and time for diastole that requires normal and stronger stimuli than normal to excite the muscle

Cardiac Contraction

• The strength of myocardial contraction determines the heart pumping power
• The mechanism depends on the contractile filaments, which contain the protein molecules
• Extracellular Ca2+ diffuses down gradient into cell through T- tubules and SR to finally releases SR to bind to troponin promoting [SYSTOLE]
• During repolarization, Ca2+ in sarcoplasm is actively transported and pumped out of the cell [DIASTOLE] until new action potential occurs
• A single sub-minimal stimulus does not evoke any response
• Threshold stimuli lead to maximal cardiac contraction, and further increase in stimulus strength does not increase the contraction
• Cardiac muscle cannot be stimulated while it is contracted

Cardiac Output (CO)

• CO: the volume of blood pumped by each ventricle per minute
• Stroke Volume (SV): the volume of blood pumped by each ventricle per beat
• Heart Rate (HR): The number of times the heart beats per minute

CO Equation

• Cardiac output = Stroke volume X Heart Rate
• Typically, it's 70ml(SV) X 72(HR) = about 5 liters/minute in a resting supine man

Stroke Volume

• Normal SV is about 70 ml in a resting man of average size in the supine position
• Increasing SV would increase the cardiac output, provided HR is unchanged

Factors Affecting SV

• Venous return, several mechanisms that help return blood to the RA increase the SV and CO
• Pressure: a pressure gradient in the right atrium directly affects venous return, depending on Blood volume and venous compliance
• Muscle pump: skeletal muscles exert a muscle tone compressing veins, enhanced with muscular exercise
• The diameter of Arterioles: dilation increases return while constriction decreases it
• The capillary tone: only 10% are open, maintaining return
• Valves stop the blood from returning to the heart, not present in big/small veins or brain
• Inspiratory pump: inspiration increases negativity increasing the return
• Sympathetic stimulation: this constricts the veins for blood circulation

Cardiac Contractility and Blood Pressure Effects

• increased SV with force of contraction, and is affected by sympathetic stimulation. Epinephrine/norepinephrine are also a factor
• Warming of the heart in addition to Thyroxine increases contractility
• decreased SV is affected by arterial blood pressure and heart rate
• Cooling, Parasympathetic Stimulation and Acetylcholine all reduce contractility

Heart Rate

• Normal is 60–100 bpm
• Tachycardia occurs if it's >100 bpm
• Bradycardia occurs if it's <60 bpm
• Increased HR within limits increases the CO
• The HR has a direct effect on the CO
• Shortening the diastolic decreases the CO
• factors affecting HR and coronary activity is 0.5, decreasing output

Factors Affecting heart rate

• Faster through, Sympathetic activity and hormones
• Slower through, Parasympathetic activity and Hormones
• Body temperature, Higher heat increases electrical activity and vice versa
• Hormones, Thyroxine and adrenaline which, increase heat
• Hypoxia levels, and various Ions control activity

Arterial Blood Pressure

• Definition: The pressure exerted by the blood in the large arteries
• Systolic pressure: The pressures is at its highest point while your heart beats
• The level is about 120 mmHg
• Diastolic pressure: The pressure is at its lowest point between the hearts beats
• This pressure is typically 80 mmHg

Pulse Pressure and Arterial Pressure

• Pulse Pressure is the difference between Systolic and Diastolic
• Mean arterial pressure = diastolic pressure + 1/3 pulse pressure

Blood Pressure Controls

• These can be short term, intermediate, regulatory etc
• Medullary sympathetic center: increases the body output, increasing pressure by causing the blood streams to constrict
• Medullary parasympathetic center: This controls the Cardio and its fibers to the hear, decreasing heart rate output.
• Stimulation of this center will have the opposite effect of blood pressure
• Baroreceptors, cardio, Chemo: Is affected by mean arterial blood pressure, and can be adjusted at 60 -160

Body Functions Effect on BP

• At rest: decreases heart rate. In response to a drop: heart rate increases
• Opposite responses, the arterial will dilate from pressure, which is controlled via multiple mechanisms
• High is an effort to cause dialation so the body is balanced

Effects on Pressure Receptors and Muscles

• Heart is affected by the bodies chambers, with the atrium and the heart having venous effects
• With an increase in the inspiration or expirations
• Brain functions through the cortex and hypothalamus, rising the blood flow
• Lowering function in the brain will constrict and reduce levels
• With short term fluid, there is a shift, and during stress its at its lowest

Long Term Regulations

• In the blood stream long term has a huge roll, and the Renin in the body helps to control this function
• The salt levels needs to balance, and the bodies receptors needs blood for water, and increase the tubules volume
• Renin stimulates the kidney to increase water volume, for proper flow
  • The angiotensin levels help with production, filtration, control of salt, and reasortion
  • Helping the body with fluid for blood control, by water consumption

Shock in Circulation

• Is a life threatening condition, that has an inadequate amount needed to help with respiration
• The are reduced blood flow, as well as tissue/substrate
• Blood P, Tachycardia will be present
• Reduced organ function

Causes of Shock

• Hypovolemic Shock, is the typical type involving reduction via hemorrhage or other factors
• Cardiogenic is when heart has defects. Obstructive is an object that impedes flow
• Distrubtive is caused by the distribution, from sepsis or even allergic like effects

Treatment for Shock

• Position patient in a comfortable position, with legs lifted
• Remove, and manage source effects
• Make sure to maintain proper blood transfusions
• Give fluids if needed, keep sedative in the patient

Edema Problems

• This is caused by filtration, and capillary drainage issues
• Unbalanced pressure, and vascular issue, that can cause hydrostatic effects
• Disturbance to the Lymphatic system and vascular system

Types of Edema

• Pitting refers to fluid caused by tissue function
• Bone pressure may form
• Unrelated caused maybe systemic, or localized
• Non pitting means is not related, and comes in three types
• Unrelated causes for Lymp, Myxedema, Lipedema maybe systemic and vein located systems