Lecture 11: circulation 2

Where does the heartbeat originate in vertebrates

The heart

What are autorhythmic cells

Some cardiac muscle cells are autorhythmic: they contract and relax repeatedly without any signal from the nervous system

What is the SA node

Also known as the pacemaker, the sa node is a group of autorhythmic cells in the right atrium which set the rate and timing at which the cardiac muscle cells contract. The heartbeat originates here.

What type of signal does the SA node produce

Electrical impulses

What happens to the electrical impulse produced in the SA node

Rapidly spread within the heart tissue via internodal tracts

What is measured in an ECG

Currents which are generated from the electrical impulses produced in the SA node.

How are cardiac muscles electrically coupled

Through gap junctions

Heart conduction system stages

1. Impulse originates in the SA node and spreads through both atria causing them to simultaneously contract

2. Signal is deliberately delayed by the AV node for around 0.1 seconds

3. Signal spreads to the apex of the heart and up either side of the ventricles

4. Signal travels up purkinje fibres and spreads through ventricles causing them to simultaneously contract

AV node

A group of autorhythmic cells located in the wall between the left and right atria which acts as a relay point

Why does the AV node delay the signal

To ensure the atria empty completely before the ventricles contract

ECG wave

1. P wave: atrial systole (atria contract)

2. QRS complex: ventricle systole (ventricles contract)

3. T wave: repolarisation of SA node and chamber diastole

What can alter heart rate by affecting the rate of depolarisation of the SA node

• Nerve impulses

• Hormones ( eg. Epinephrine secreted by adrenal glands speeds up SA node )

• Body temperature ( a 1º C increase raises heartbeat by 10 beats per minute)

What is responsible for regulating heart rate

The autonomic nervous system

What parts of the ans regulate heart rate

Sympathetic and parasympathetic

Role of sympathetic nerves

• Originate from the TI → T4 (thoracic vertebrae 1 -4 ) levels of the spinal cord and increases heart rate and the force of contraction

What hormone do sympathetic nerves release

Norepinephrine or noradrenaline which act on the entire heart

Role of parasympathetic nerves

Originate from the vagus nerves and decrease heart rate & force of contraction

Cardiac cycle stages

1. AV valves open and SL valves closed

2. 0.4 seconds: Atrial and ventricular diastole- both chambers are relaxed and filling up

3. The SA node depolarises

4. 0.1 seconds: Atrial systole and ventricular diastole- the atria contract and blood pushed to the ventricles

5. AV valves close

6. 0.3 seconds: Ventricular systole and atrial diastole- the ventricles load up pressure and contract pushing open the SL valves

7. Pressure in ventricles drop opening AV valves and closing SL valves

When are the lub dub sounds produced

• Lub: when av valves close

• Dub: when SL valves close

Cardiac output

Volume of blood leaving the heart and entering the aorta to be pumped into the systemic circulation per minute

Stroke volume X heart rate = cardiac output

Heart rate

Beats per minute

Stroke volume

Amount of blood pumped out of the ventricles in one contraction

Blood pressure

Determined by cardiac output and peripheral resistance met by blood as it passes from the arteries into the capillaries

What does the vertebrate circulatory system rely on

Blood vessels that are anatomically suited to their function

What is the central lumen

All blood vessels have a central space called the lumen which is lined with an epithelial layer

• this endothelium is smooth and minismises resistance

Structure of arteries

• small lumen/ endothelium

• Thick muscular walls

• Thick elastic walls

• Thick connective tissue

Why do arteries have a thick elastic wall

To allow them to stretch to accommodate higher blood pressure

Structure of veins

• Wider lamen/endothelium

• Thinner muscular wall

• Thinner connective tissue

• Contain valves

• Low blood pressure

What blood vessels contain valves

Veins

Structure of capillaries

• Have the smallest diameter

• Very thin wall

• No smooth muscle or connectivetissue

• Only have endothelium

Why do capillaries not have multiple layers

To allow the diffusion the nutrients, waste etc in and out of tissues

Function of arteries

• Transports blood away from the heart & towards the capillaries

• Act as a pressure reservoir for forcing blood into small diameter capillaries as the thick arterial walls withstand pressure

• Dampen oscillations in pressure & flow generated in the heart which produces a more even blood flow to the capillaries

Role of arterioles

Control flow to the capillary beds

Function of veins

• Transport blood towards the heart from the capillary beds

• Act as a storage reservoir for blood (50 - 60 % of blood in veins)

How is blood returned to the heart if blood pressure in veins is low

• Valves

• Skeletal muscle contraction

How do skeletal muscles improve blood flow to the heart

Skeletal muscles press on the veins which pushes blood back to the heart

What is atherosclerosis

A build of plaque which is mainly composed of fatty substances in the arterial wall

The plaque can break off and become stuck elsewhere

What conditions can arise from atherosclerosis

• Sequela

• Myocardial infarction- if coronary arteries get blocked

• Stroke - if plaque goes to a blood vessel in the brain

What is deep vein thrombosis

A pooling of blood in the deep veins which leads to a blood clot usually due to a malfunctioning valve

Causes of a DVT

• Prolonged inactivity after an operation

• Illness

• Long journeys

Complication of a DVT

The blood clot (thrombus) can break off and go to the lungs causing a pulmonary embolism

What blood vessels have the biggest area

Capillaries → arterials & venules → arteries & veins → aorta & vena cava

What blood vessels have the highest velocity

aorta → arteries → vena cava → veins → Arteriole's → venules → capillaries

What blood vessels have the highest blood pressure

Aorta → arteries → arterioles → capillaries → venules → veins → vena cava

Why is blood flow in the capillaries slow

To allow the exchange of materials

What happens to the velocity of a blood vessel as the blood pressure increases

Decreases

What artery is used to measure blood pressure

Brachial artery

Systolic pressure

Pressure in arteries during ventricular systole highest pressure in arteries

Pulse

The rhythmic bulging of artery walls with each heartbeat

Diastolic pressure

Pressure in arteries during diastole (lower than systolic)

Healthy blood pressure for an average 20 y/o

120 / 70 mmHg

How do homeostatic mechanisms regulate blood pressure

Altering the diameter of arteries

Vasoconstriction

Contraction of the smooth muscle in the arterial walls which decreases the width of the lumen and increases blood pressure

vasodilation

Relaxation of the smooth muscles in the arterial walls which increases the width of the lumen and decreases blood pressure

What are vasodilation and vasoconstriction often coupled to

Changes in cardiac output that affect blood pressure

Where do cusps of valves lie

Flat against the blood vessel wall

How do cusps control blood movement

They fill from the top to the bottom to prevent backflow of blood and when they fill, they close off the blood vessel

How do AV valves work

Cusps lie flat against the ventricular wall and when the atria contract, blood is pushed into the ventricle. The ventricle cntraction causes the blood to push the cusps of the valves to close the atrioventricular opening.

role of the chordae tendinae

held by the papillary muscles which hold the cusps of valves across the opening and prevent them from opening into the atria