Download Overview of the Cardiovascular System Overview of Cardiovascular

Overview of the Cardiovascular System
The Heart
Blood Vessels
Arteries
Arterioles
Capillaries
Venules
Veins
Blood
Diagrams: Germann and Stanfield,
Principles of Human Physiology
Dr. Áine Kelly [email protected]
https://medicine.tcd.ie/physiology/student/
Overview of Cardiovascular System Functions
Transport
oxygen & carbon dioxide
absorbed products of digestion
metabolic wastes delivered (to liver and kidneys)
hormones, immune cells, clotting proteins
Regulation
Hormones
Thermoregulation (skin blood flow)
Protection
Blood clotting (protects against haemorrhage)
Pathogens (immune system)
Exchange between blood and tissue takes place in capillaries
Blood gases:
Pulmonary capillaries
Blood entering lungs is deoxygenated
Oxygen diffuses from tissue to blood (CO2 from blood to tissue)
Blood leaving lungs is oxygenated
Systemic capillaries
Blood entering tissues is oxygenated
Oxygen diffuses from blood to tissue (CO2 from tissue to blood)
Blood leaving tissues is deoxygenated
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Blood
On average, 5L of blood (approx 8% body weight)
Cellular portion of blood (45% blood volume)
Erythrocytes (red blood cells): oxygen transport
Leukocytes (white blood cells): immune function
Platelets: Blood clotting
Plasma (55% blood volume)
Water
Dissolved solutes eg. ions
Plasma proteins
Other components eg. metabolites,
hormones, enzymes, antibodies.
Path of blood flow through cardiovascular system
Cardiovascular system is a closed system
Flow through systemic and pulmonary circuits
are in series
Left ventricle  systemic circuit  right atrium
 right ventricle pulmonary circuit  left
atrium  left ventricle
Flow within systemic (and pulmonary) circuit is
in parallel: allows independent regulation of
blood flow to organs
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Anatomy of the Heart
Size of fist; weighs approximately 250 – 350 grams
Location of the Heart
Located in thoracic
cavity
Diaphragm separates
abdominal cavity from
thoracic cavity
Internal anatomy of the heart
Walls of ventricles thicker
than walls of atria
Left ventricle wall thicker than
right ventricle wall
Cardiac muscle: gap junctions
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Function of Cardiac Muscle
Contraction and relaxation generates pumping action
Contraction pushes blood into vasculature
Relaxation allows heart to fill with blood
Heartbeat
Wave of contraction through cardiac muscle
Atria contract as a unit
Ventricles contract as a unit
Atrial contraction precedes ventricle contraction
Valves and Unidirectional Blood Flow
Pressure within chambers of heart vary with heartbeat cycle
Pressure difference drives blood flow: High pressure to low pressure
Normal direction of flow: Atria to ventricles, then ventricles to arteries
Valves prevent backward flow of blood
All valves open passively based on pressure gradient
Atrioventricular valves = AV valves
R: tricuspid valve; L: bicuspid (mitral)
Papillary muscles and chordae
tendinae keep AV valves from everting
Semilunar valves
Aortic Valve
Pulmonary Valve
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Electrical Activity of the Heart
Autorhythmic cells generate their own rhythm
Conduction System
Pacemaker cells: Coordinate and provide rhythm to heartbeat
The Sinoatrial (SA) node is the pacemaker of the heart
Conduction fibers: Rapidly conduct action potentials initiated by pacemaker
cells to myocardium
Atrioventricular (AV) node
Bundle of His
Purkinje fibers
Spread of Excitation (NB gap junctions)
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The Electrocardiogram
A non-invasive technique used to record the electrical activity of the heart
Tests for clinical abnormalities in conduction of electrical activity in the heart
Body is conductor: currents in body can spread to surface (ECG, EMG, EEG)
Distance & amplitude of spread depends on size of potentials and
synchronicity of potentials from other cells
Heart electrical activity is synchronized
Standard ECG Trace
P wave: atrial depolarization
QRS complex: ventricular depolarization
T wave: ventricular repolarization
Abnormal Heart Rates
“Sinus rhythm”: generated by SA node
Abnormal Heart Rates:
Tachycardia- fast
Bradycardia- slow
Ventricular Fibrillation
Loss of coordination of electrical activity.
Can be corrected by defibrillation
Atrial fibrillation - weakness
Ventricular fibrillation - death within
minutes
Damage to heart muscle
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Cardiac Cycle
Events associated with the flow of blood through the
heart during a single complete heartbeat
Mechanical Events
Systole - Ventricular contraction and blood ejection
Diastole - Ventricular relaxation and filling
Opening of Valves
Valves open passively due to pressure gradients
AV valves open when P atria > P ventricles
Semilunar valves open when P ventricles > P arteries
Cardiac Cycle
volume of blood
ejected from
heart each cycle
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Volume of blood pumped by each ventricle per minute
Cardiac Output = CO = SV x HR
Equal on both sides of the heart
Average CO = 5 litres/min at rest (70ml/beat x 70beat/min)
Can increase 5-fold during exercise
Regulation of Cardiac Output
We regulate CO by regulating heart rate and stroke volume
These can change from moment to moment
Regulation by nerves and hormones
Neural and hormonal input to the Heart
Neural:
Nerves can increase or
decrease (a) heart rate and
(b) contractility of the
myocardium (hence stroke
volume)
Hormonal:
Hormones such as adrenaline can increase (a)
heart rate and (b) contractility of the
myocardium (hence stroke volume)
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Blood Flow and Blood Pressure
Physical laws governing blood flow:
Pressure Gradients & Resistance in the Cardiovascular System
Pressure gradients: Flow occurs from high pressure to low pressure
Heart creates the pressure gradient for flow of blood
A gradient must exist throughout circulatory system to maintain blood flow
Resistance: systemic circuit is high pressure, high resistance; pulmonary
circuit is low pressure, low resistance
Flow = ΔP/R = pressure gradient/resistance
Poiseuille’s Law
R=
8ηL
r4
Flow = ΔP/R =
ΔP r4
8ηL
Factors Affecting Resistance to Flow
Length of vessel (normally doesn’t change)
Viscosity of fluid = η (normally doesn’t change)
Radius of vessel
In arterioles (and small arteries) - can regulate radius
RADIUS IS THE MOST IMPORTANT FACTOR
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Blood Vessel anatomy
Arteries
Rapid transport pathway: large diameter - little resistance
Under high pressure: walls contain elastic and fibrous tissue
Arteries are pressure reservoirs
Thick elastic arterial walls expand as
blood enters arteries during systole &
recoil during diastole
Arteries & disease
Atherosclerosis - ‘hardening of the arteries’
A plaque composed of cholesterol, calcium
and other substances builds up in an artery
Plaques reduce blood flow; they can rupture &
cause clots – heart attacks or strokes can
result
Risk factors: age; smoking; diabetes; obesity
Treatments: Angioplasty; stent implantation
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Blood pressure: Mean Arterial Pressure
MAP is the driving force for blood flow
F = ΔP/R
Regulating MAP is critical for normal function
MAP < normal
Hypotension
Inadequate blood flow to tissues
MAP > normal
Hypertension
Stress on heart and walls of blood vessels
Arterioles
Resistance vessels in microcirculation
Connect arteries to capillaries
Contain smooth muscle: regulate
radius (& thus resistance; below)
Arterioles provide greatest resistance
to blood flow
Largest pressure drop in vasculature
(90 mmHg to 40 mmHg)
Radius dependent on contraction state of smooth muscle
in arteriole wall
Vasoconstriction: increased contraction
(decreased radius)
Vasodilation: decreased contraction
(increased radius)
Functional importance
Controlling blood flow to individual capillary beds
Regulating mean arterial pressure
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Control of Blood Flow Distribution to Organs
Cardiac output increases during exercise
Distribution of blood does not increase proportionally
Dilation to skeletal muscle and heart increases blood flow
Constriction to GI tract and kidneys decreases blood flow
Dilation to skin increases heat loss to environment (thermoregulatory
response mediated by the brain in response to increased body temp during
exercise)
Capillaries
Site of exchange between blood and tissue
5-10 µm diameter - small diffusion distance
Walls : 1 cell layer (small diffusion barrier)
10-40 billion in the body. Total surface area = 600 m2
Most cells within 1 mm of a capillary
1 mm long
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Venules
Smaller than arterioles
Connect capillaries to veins
Thin walls
Little smooth muscle in walls
Some exchange of material between blood and interstitial fluid
Veins
Large diameter, but thin walls, which contain muscle and elastic
tissue
Valves allow unidirectional blood flow
Volume reservoirs: at rest, systemic veins contain 60% of total
blood volume
Return of blood to heart from veins is called venous return
Summary of cardiovascular physiology
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Respiratory Physiology
Pulmonary ventilation
(breathing)
Gas exchange between
lungs and blood
Transport of
gases in blood
Gas exchange between
blood and tissues
Anatomy of the Respiratory System
Conducting airways
(Nasal passages, pharynx,
trachea, bronchii,
bronchioles)
Inspired air is warmed and
humidified in these tubes.
Moistening of air is essential
to prevent drying out of
alveolar linings.
Photomicrograph of Tracheal Epithelium
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Defence mechanisms
Respiratory system is largest area of the body in direct
contact with the environment.
Large particles filtered out in hairs in nasal passages
Respiratory airways lined with mucus to trap foreign objects
Cilia move mucus upwards towards throat to be swallowed
Coughs and sneezes
Alveolar macrophages scavenge within the alveoli
Function of the Alveoli
Exchange of gases between air and blood by diffusion
300 million alveoli/lung
(tennis court size)
Rich blood supplycapillaries form sheet
over alveoli
Alveolar pores
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Resin cast of pulmonary
blood vessels
Scanning electron
micrograph of capillaries
around alveoli
Pulmonary Circulation is low-pressure, low-resistance
Ventilation-perfusion matching: blood flow through the pulmonary
circulation is matched to ventilation
Structures of the Thoracic Cavity
Chest wall – air tight, protects lungs
Skeleton: rib cage;sternum; thoracic vertebrae
Muscles: internal/external intercostals; diaphragm
Lungs are surrounded by pleural sac
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Role of Pressure in Pulmonary Ventilation
Air moves in and out of lungs by bulk flow
Pressure gradient drives flow (air moves from high to low pressure)
Atmospheric pressure = Patm (760mmHg at sea level)
Intra-alveolar pressure = Palv
Pressure of air in alveoli during inspiration is negative (< atmospheric)
Pressure of air in alveoli during expiration is positive (> atmospheric)
Difference between Palv and Patm drives ventilation
Mechanics of Breathing
Movement of air in and out of lungs due to pressure gradients
Mechanics of breathing describes mechanisms for creating pressure gradients
Boyle’s Law (pressure and volume are inversely related)
The lungs follow the movement of the rib cage
Respiratory Muscles
Expiratory muscles
(internal intercostal &
abdominal muscles)
decrease volume of
thoracic cavity
Expiration is
generally passive
(recoil; no muscle
contraction required)
Inspiratory muscles
(diaphragm &
external intercostals)
increase volume of
thoracic cavity
INSPIRATION
EXPIRATION
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Spirometry
A pulmonary function test (method of measuring lung volumes)
Can be used diagnostically
Volume-time curves
Flow volume loops
Dependent upon patient effort
Used to measure several lung volumes, including tidal volume
(VT) - the volume of a normal breath (approx. 500ml)
Minute Ventilation
Total volume of air entering and leaving respiratory system each minute
Minute ventilation = VT x RR
Normal respiration rate = 12 breaths/min
Normal VT = 500 mL
Normal minute ventilation =
500 mL x 12 breaths/min = 6000 mL/min
NB: Ventilation-perfusion matching: blood flow through the pulmonary
circulation is matched to ventilation
Airway Resistance
Like blood vessels, the resistance of the airways affects air flow
Airway radius affects airway resistance
Airway Resistance and disease
Asthma – caused by contractions of smooth muscle of bronchioles
Chronic obstructive pulmonary diseases – COPD
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Movement of Oxygen and Carbon Dioxide
Gas Composition of Air
Composition of Air:
79% Nitrogen; 21% O2;
trace amounts of CO2, helium, argon,
etc.; water vapour (varies with
humidity)
Diffusion of Gases
Gases diffuse down pressure
gradients
High pressure  low pressure
In gas mixtures, gases diffuse
down partial pressure gradients
High partial pressure  low
partial pressure
A particular gas diffuses down its
own partial pressure gradient presence of other gases irrelevant
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Gas Transport in the Blood
Oxygen
Carbon dioxide
Not very soluble in plasma
(1.5%)
Some transported
dissolved in plasma
Most (98.5%) transported by
haemoglobin - a protein present
in red blood cells
Some transported bound to
haemoglobin
Each haemoglobin protein can
bind 4 oxygen molecules
Most converted to
bicarbonate ions by red
blood cells, then
transported into plasma
Hb + O2  Hb.O2
Haemoglobin has greater affinity
for carbon monoxide (CO) than
for oxygen: Prevents oxygen
from binding to haemoglobin.
CO is poisonous
Summary
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