TOPIC 4

4.4 = CIRCULATION

what is heart

The heart is an organ made mainly of cardiac muscle. Its job is to pump blood around the body.

Pump = something that creates pressure to move fluid from one place to another.

Describe heart

4 chambers:

Chamber	Side	Blood type	Main job
Right atrium	Right side	Deoxygenated	Receives blood from body
Right ventricle	Right side	Deoxygenated	Pumps blood to lungs
Left atrium	Left side	Oxygenated	Receives blood from lungs
Left ventricle	Left side	Oxygenated	Pumps blood to bodyAtrium = upper chamber of the heart. Ventricle = lower chamber of the heart.

Right = deox

left = ox

2 sides separated by SEPTUM

describe atria

Upper chamber of heart

Thinner walls - only push blood a short distance into v

describe ventricles

lower chamber of heart

thicker muscular walls

pump blood OUT of heart

what is septum

muscular wall that separates left and right side of heart

stop ox and deox blood mixing

list blood vessels in heart

vena carva

pulmonay artery

pulmonar vein

aorta

PULMONARY = relating to lungs

describe vena carva

Carries blood from body

—> right artium

deox blood

describe pulmonary artery

carries blood from R ventricle —> lungs

deox blood

describe pulmonary vein

carries blood from lungs —> left artium

ox blood

describe aorta

carries blood from L ventricle —> body

ox blood

blood flow thru heart

Body → vena cava → right atrium → tricuspid valve → right ventricle → pulmonary semilunar valve → pulmonary artery → lungs → pulmonary vein → left atrium → bicuspid/mitral valve → left ventricle → aortic semilunar valve → aorta → body

list valves in heart

AV valves: atrioventricular valves

• between atria + ventricles

• stops blood flowing back into atria

Semilunar valves :

• between ventricles and arteries

• stops blood flowing back to ventricles

AV valves

Valve	Location
Tricuspid valve RIGHT	Between right atrium and right ventricle
Bicuspid / mitral valve LEFT	Between left atrium and left ventricle

semilunar valves

Valve	Location
Pulmonary semilunar valve RIGHT	Between right ventricle and pulmonary artery
Aortic semilunar valve LEFT	Between left ventricle and aorta

valve function

ensure one way flow

prevents backflow

Describe arteries

carry blood away from heart

Feature	Why
Thick muscular wall	Withstands high pressure
Thick elastic tissue	Stretches and recoils to maintain pressure
Small/narrow lumen	Helps maintain high pressure
Smooth endothelium	Reduces friction
No valves along most arteries	Blood is already under high pressure

Lumen = the hollow space inside a blood vessel where blood flows.
Endothelium = the thin inner lining of a blood vessel.

describe veins

carry blood towards the heart.

Feature	Why
Thin muscular wall	Blood is at low pressure
Less elastic tissue	Less recoil needed
Large/wide lumen	Reduces resistance to blood flow
Valves	Prevent backflow of blood
Smooth endothelium	Reduces friction

describe capillaries

tiny blood vessels that connect arteries/arterioles to veins/venules.

to exchange - move substances between blood and body cells

Examples of substances exchanged:

• oxygen

• carbon dioxide

• glucose

• amino acids

• urea

• water

capillary structure

Feature	Why
Wall one cell thick	Short diffusion distance
Made of endothelium only	Allows substances to pass through easily
Very narrow lumen	Red blood cells pass close to cells
Many branches / networks	Large surface area for exchange
Gaps/pores between cells	Allow tissue fluid to form

Arteries vs veins vs capillaries

Feature	Artery	Vein	Capillary
Direction of blood flow	Away from heart	Towards heart	Between arteries and veins
Pressure	High	Low	Low-medium
Wall thickness	Thick	Thin	One cell thick
Muscle	Lots	Little	None
Elastic tissue	Lots	Little	None
Lumen size	Narrow	Wide	Very narrow
Valves	No, except near heart	Yes	No
Main function	Transport blood under pressure	Return blood to heart	Exchange substances

what does “circulatory system” mean?

A circulatory system is a system that transports substances around an organism.

It usually includes:

Part	Meaning
Heart	Muscular pump that generates pressure
Blood	Transport fluid
Blood vessels	Tubes that carry blood around the body

The point of a circulatory system is mass transport.

mass transport meaning

bulk movement of substances around an organism - usually fluid

Single circulatory system

Blood passes through the heart once during one complete circuit of the body.

fish:

Heart → gills → body → heart

single circulatory fish

The fish heart mainly receives and pumps deoxygenated blood.

The blood goes to the gills first to pick up oxygen. Then it goes straight from the gills to the rest of the body.

Why does pressure fall in fish?

When blood passes through the gill capillaries, pressure drops.

This means the blood reaches the body more slowly and at lower pressure than in mammals.

That is okay for fish because many fish have a lower metabolic demand than mammals, but it is less suitable for mammals.

Blood loses pressure in capillaries because:

• capillaries are very narrow

• there is resistance to blood flow

• blood slows down to allow gas exchange

• pressure is lost as blood passes through the capillary network

double circulatory system

Blood passes through the heart twice during one complete circuit of the body.

That is why it is called double circulation: the blood goes through the heart twice.

advantages of double circulatory system

what is cardiac cycle

The sequence of events that happens during one heartbeat.

events in a heart beat

• the heart filling with blood

• the atria contracting

• the ventricles contracting

• the heart relaxing again

stystole and diastole

Systole = squeeze; contract
Diastole = relax heart muscle

order of caridac cycle

1. Cardiac diastole

2. Atrial systole

3. Ventricular systole

4. Back to cardiac diastole

myogenic meaning

heart can generate its own electrical impulse without needed nerve impulse from brain

=independent

produced by muscle itself

Nerves can change the rate of the heartbeat, but the heartbeat itself is started by the heart’s own pacemaker tissue.

myogenic hstructures

Structure	Full name	Role
SAN	Sinoatrial node	Starts the heartbeat
AVN	Atrioventricular node	Delays and passes on the impulse
Bundle of His	Conducting tissue in septum	Carries impulse down the septum
Purkyne fibres	Conducting fibres in ventricle walls	Spread impulse through ventricles

myogenic process

step	Key event	detail	keyworkds
1	SAN starts electrical impulse  Causes atrial systole	SAN located in right atrium Acts as pace maker  Generates electrical excitation SAN initiates wave of excitation actress atrial walls  = causes atrial systole NOTE: impulse cant go straight thru ventricals as tissues between atria and ventricles are non conducting  Adv: atria must contract before ventricles, not at same time as blood would not flow props	Pacemaker Group of cells that set rhythm of heartbeat  Wave of excitation = electrical activity spreads thru cardiac muscle = contract
2	AVN receives + delays impulse  (Atrioventricular node)	Impulse reaches AVN Located between atria nad ventricles Receives impulse Delays impulse Passes it to bundle of his Delay: So atria can finish contracting before ventricles contract  Allows blood to move into ven from atria before ventricles contraact
3	Bundle of hiss carries impulse down septum	BOH = conducting tissue Septum separates L and R  Impulse: Down BOH → apex Apex = bottom tip of heart  So that ventricles contract from bottom upwards	BOH Apex
4	Purkyne fibres spread impulse thru ventricles	Purkyne fibres = branches of BOH PF spread impulse thru ventricle walls  = ventricle systole  Contract from apex upwards  =pushes blood upwards and out of heart  Forced blood into pulmonary artery (RV)+ aorta (LV)

Myogenic stimulation vs nervous stimulation

myogenic control = heartbeat starts in heartFor example:

Situation	Effect
Exercise	Heart rate increases
Rest	Heart rate decreases

But the brain does not normally start every heartbeat.

• san generates electical impulse

nervous control = ns changes heartbeat

What is an ECG?

ECG stands for electrocardiogram.

An ECG trace is a graph showing the electrical activity of the heart.

The electrical activity causes heart muscle to contract, so ECG changes happen just before mechanical events like atrial systole or ventricular systole.

name parts of ecg

ECG part	What it shows	What happens after it
P wave	Electrical activity spreading across atria	Atria contract
QRS complex	Electrical activity spreading through ventricles	Ventricles contract
T wave	Ventricles recovering electrically	Ventricles relax

P wave = atria stimulated

QRS complex = ventricles stimulated

T wave = ventricles relax/recover

principles of ecg

blood moves from high to low pressure

Valve rule

Situation	Valve response
Pressure behind valve is higher	Valve opens
Pressure in front of valve is higher	Valve closes

4.7

Xylem tissue

Main job of xylem

Xylem transports:

• water

• mineral ions

from the roots → stem → leaves.

This movement is mostly one-way, upwards through the plant.

xylem struc

Xylem vessels are long, hollow tubes made from dead cells that transport water and mineral ions through plants.

dead = less resistance (slow down movement)

no cytoplasm too

mature = dead

End walls break down = continuous tube

Thickened walls of lignin

• waterproof

• strenght

• no colapse under tension

pits - small lignified gaps

• sideays movement to near tissue

2. Phloem tissue

Phloem transports organic substances, mainly sucrose, around the plant.

This process is called translocation.

Translocation = movement of organic solutes, such as sucrose, through the phloem from sources to sinks.

source and sink

Source

A source is where sucrose is produced or released.- leabes

Sink

A sink is where sucrose is used or stored.

Examples:

• roots

• fruits

• growing shoots

• seeds

• storage organs

phloem struc

Phloem tissue is made mainly of:

1. sieve tube elements

2. companion cells

1. Sieve tube elements

Sieve tube elements are long living cells joined end to end to form tubes.

They transport sucrose solution through the plant.

They are alive, but they have very few organelles.

They usually have:

• no nucleus

• little cytoplasm

• few organelles

Why this helps

There is more space inside the cell for sap to flow.

Sap = liquid in the phloem containing sucrose and other dissolved substances.

---

2. Sieve plates

Between sieve tube elements are sieve plates.

A sieve plate is an end wall with lots of pores.

Pores are tiny holes.

Why this helps

The pores allow phloem sap to move from one sieve tube element into the next.

So phloem forms a continuous transport pathway.

---

3. Companion cells

Companion cells are living cells next to sieve tube elements.

They have:

• a nucleus

• dense cytoplasm

• many mitochondria

Why this helps

Companion cells control and support the sieve tube elements.

They provide energy for active transport of sucrose into and out of the phloem.

---

Key definition

Active transport = movement of substances against their concentration gradient using ATP.

ATP = energy-carrying molecule used by cells.

---

4. Many mitochondria in companion cells

Mitochondria are organelles where aerobic respiration happens.

Respiration releases ATP.

Why this helps

Phloem transport needs ATP because sucrose is loaded into phloem by active transport.

So companion cells need many mitochondria to supply ATP.

---

5. Plasmodesmata connect cells

Plasmodesmata are tiny channels between plant cells.

They connect companion cells and sieve tube elements.

Why this helps

They allow substances to move between the companion cell and sieve tube element.

This is important for loading sucrose into the sieve tube.

soil = higher wp thana cytop;as, of root hair cell

water = soil —? rhc

apoplastic

The apoplastic pathway is the movement of water through the cell walls and spaces between cells.

Important: water does not enter the cytoplasm in this pathway.

FAST:

doesnt need to cross plasma membranes

• selective permeability

BUT CAS[ARIAN STRIP

The Casparian strip is a waterproof band of suberin in the cell walls of endodermal cells.

Suberin = a waterproof substance.

The Casparian strip blocks the apoplastic pathway because water cannot pass through it.

The Casparian strip forces water to leave the cell wall pathway and enter the symplastic pathway.

That means water must cross a plasma membrane.

This is useful because the plant can control which ions enter the xylem.

So the plant can selectively absorb mineral ions rather than letting everything from the soil enter the xylem.

symplastic

The symplastic pathway is the movement of water through the cytoplasm of cells, via plasmodesmata.

Plasmodesmata are tiny cytoplasmic channels between neighbouring plant cells.

They allow substances to move directly from one cell’s cytoplasm into another.

cohesion

Cohesion is the attraction between water molecules.

Water molecules stick to each other because they form hydrogen bonds.

adhesion

Adhesion is the attraction between water molecules and the walls of the xylem.

Water sticks slightly to the xylem walls.

tension

Tension means a pulling force.

In xylem, water is under tension because it is being pulled upwards from the leaves.

cohesion tension model

Step 1: Water evaporates from mesophyll cells

Inside the leaf, water evaporates from the surface of mesophyll cells.

Mesophyll cells are photosynthetic cells inside the leaf.

Water changes from liquid water to water vapour.

---

Step 2: Water vapour diffuses out through stomata

The air spaces inside the leaf become humid because they contain lots of water vapour.

Usually, the air outside the leaf has less water vapour.

So water vapour diffuses:

leaf air spaces → outside air

through the stomata.

This is transpiration.

---

Step 3: Water loss lowers water potential in leaf cells

As water evaporates from mesophyll cell walls, those cells lose water.

This lowers their water potential.

So water moves into them from nearby cells by osmosis.

---

Step 4: Water is pulled out of the xylem

Water moves from the xylem into leaf cells to replace the water lost by evaporation.

This creates tension in the xylem.

The leaf is basically pulling water up the plant.

---

Step 5: Cohesion keeps water molecules together

Water molecules are cohesive because they form hydrogen bonds with each other.

So when water molecules at the top are pulled upwards, they pull the next water molecules with them.

This creates a continuous column of water in the xylem.

---

Step 6: The whole water column moves upwards

Because the water column is continuous, the pull is transmitted down the xylem.

So water moves:

roots → stem → leaves

This movement does not directly require ATP.

It is a passive process driven by water loss from the leaves.

---

Why xylem structure is perfect for this

Xylem vessels are:

• dead

• hollow

• lignified

• continuous tubes

• narrow

This allows water to move with little resistance, and lignin prevents the vessels collapsing under tension.

Higher light intensity increases transpiration

In bright light, photosynthesis increases.

The plant needs carbon dioxide for photosynthesis.

So stomata open to allow carbon dioxide to diffuse into the leaf.

But when stomata open, water vapour can also diffuse out.

mass flow hypothesis

Translocation

Translocation is the movement of organic solutes through the phloem from sources to sinks.

The main organic solute is sucrose.

1. sucrose loaded into phloem