Circulatory system

Blood Vessels -Humans, and all vertebrates, have a closed circulatory system composed

of blood vessels that carry blood from the heart to and from the tissues. As the system is closed, the pressure within the system can be maintained

by the contractions of the heart and the properties of the blood vessels

Like all fluids, blood will follow a hydrostatic

pressure gradient - it flows from a high-

pressure area to a region with lower pressure.

Blood flows in the same direction as the

decreasing pressure gradient: arteries to

capillaries to veins.

Arteries

 Arteries are large, thick-walled

vessels that carry blood away

from the heart under relatively

high pressure

 Arteries are strong and elastic,

allowing them to withstand the high

pressure of blood being pumped from

the heart

 They are also able to recoil in

between heart beats

 They transport blood to the arterioles,

the smaller vessels that carry blood to

the capillaries

Blood Pressure

 Arterial blood is transported around a body at very high pressure

(80 – 120 mmHg)

 Blood flows through arteries in repeated and rhythmic surges called pulses

 Fibres in arterial walls assist in maintaining blood pressure between

pump cycles

 Elastic fibres can stretch and contract, while muscle fibres prevent rupturing

Veins

 Veins are blood vessels that return blood to the heart from the tissues

and organs under relatively lower pressure

 Venules collect blood from the capillaries and transport it to the veins

 Veins are made of the same three layers as arteries, but the vessel walls are

thinner, and the lumen of the vessel has a larger diameter

Veins

 The pressure in the veins is much lower after flowing through the

capillaries.

 Valves prevent backflow in blood returning to the heart.

 Veins are less elastic than arteries but are somewhat capable of

adapting to changes in blood pressure and volume.

Valves

 Valves ensure the unidirectional flow of blood

and prevent the pooling of blood in lower

extremities

 One-way valves exist in both veins and the heart

 Veins can be compressed by contractions of

skeletal muscles, which helps promote blood flow

against gravity

 Veins often run parallel to arteries and can also be

compressed by arterial bulges created by a pulse

Capillaries

 Capillaries are very small blood vessels that connect arterial and venous

circulation and enable direct exchange of nutrients and wastes between

the blood and the cells/tissues

 Capillaries are composed of a single layer of endothelium

 The endothelium is composed of squamous (flat) epithelial cells

 Capillaries form networks and are abundant where metabolic rates are high

Exchanges in the Capillaries

 Blood flows through capillaries slowly and at low pressure to maximise

exchange

 Materials that exit the blood include nutrients and oxygen (for cell respiration)

 Materials that enter the blood include carbon dioxide and urea (waste products)

Capillary Adaptations

 Capillaries are adapted for exchange:

 Their small size, high number, and numerous

branches greatly increases surface area

 No cell is more than 25 μm away from a capillary

 Capillary walls are thin enough to allow all the

exchanges of materials between tissue and blood

cells to take place

 Some have larger openings (fenestrations) that

allow the quick exchange of substances

 This type of capillary is found in the kidneys, small

intestine and endocrine glands

Composition of Blood

 Blood is the fluid medium in which materials are

transported around the body via blood vessels

 The liquid plasma (~55% of blood) is responsible

for transporting dissolved and suspended materials,

such as blood cells, electrolytes and proteins

 Plasma contains three types of blood cells:

 Red blood cells/erythrocytes (~45% of blood)

transport oxygen

 White blood cells/leukocytes (<1% of blood) fight

infections

 Platelets/thrombocytes (<1% of blood) are involved in

clotting

Composition of Blood

 As well as the blood cells, various materials are also transported in the

plasma:

 Nutrients (glucose, amino acids)

 Antibodies (immunoglobulins)

 Carbon dioxide (respiratory waste)

 Hormones (chemical messengers)

 Oxygen (respiratory requirement)

 Urea (nitrogenous waste product)

 Heat (important for thermoregulation)

Extracellular Fluid

 Extracellular fluid is the body fluid

outside the cells

 The primary types are:

 Blood plasma – fluid component of

blood

 Tissue fluid – fluid that surrounds

cells in the organs and tissues

 Also referred to as interstitial fluid

 Lymph – fluid found in lymph

vessels

Tissue Fluid

 Tissue fluid forms due to the differences in pressure at the beginning of

the capillary network and the end

 The arterial blood pressure is high enough to force fluid out of the capillary

(pressure filtration)

 The lower pressure at the venous end of the capillary allows tissue fluid to drain

back into the capillary

Tissue Fluid

 The composition of plasma and tissue/interstitial fluid is very similar,

although tissue fluid contains fewer proteins and cells

 Many blood proteins, as well as the red blood cells and platelets, are too large to

pass through the capillary walls so are not found in the tissue fluid

 Nutrients and minerals are pushed out from the capillaries into the tissue fluid, and

later taken up by the cells in the tissues

 Because the cells need a constant supply of nutrients and oxygen, these are

generally found at lower concentrations in the tissue fluid compared to the blood

plasma as they are taken up by the cells from the tissue fluid

Lymph

 The lymphatic system is a subsystem of the

circulatory system

 It is a series of vessels, nodes, and organs

 It helps maintain fluid balance in the body

by collecting excess tissue fluid and

particulate matter from tissues and returning

much of it back to the bloodstream

 It also helps defend the body against

infection by supplying disease-fighting cells

called lymphocytes

Lymph

 Excess tissue fluid drains into the lymph vessels

 These vessels have thin walls with gaps between the

cells, which allows movement of the fluid in and out

 There is no central pump, so the movement of fluid

occurs due to peristalsis, valves, and compression

from muscle contraction and arterial pulsation

 Lymph nodes filter out unwanted materials such as

bacteria and damaged cells

 Lymph fluid drains into the subclavian vein (near

the collar bone) to return the fluid back to the

heart for re-circulation

The Heart

 The structure of the human heart includes a number of key components:

 It contains four chambers: two atria (reservoirs) and two ventricles (pumps)

 Every chamber possesses a heart valve to prevent the backflow of blood

 Chambers are connected to blood vessels (veins ⟶ atria ; ventricles ⟶ arteries)

 The heart can functionally be divided into a left side and a right side:

 The right side transports deoxygenated blood to lungs (pulmonary circulation)

 The left side transports oxygenated blood to the body (systemic circulation)

The Heart

The Heart

The Heart

Asymmetry of the Heart

 The asymmetry is related to the necessary pressure differences between

the pulmonary and systemic circulations, not the distance the blood must

travel

Blood pressure during

contraction (systole)

Blood pressure

during relaxation

(diastole)

The greatest fall in pressure occurs when the

blood moves into the capillaries, even though the

distance through the capillaries represents only

a tiny proportion of the total distance traveled.

 Pulmonary circuit - lower

pressure to prevent fluid

accumulating in the lungs

 Systemic circuit - enough

pressure to enable

increased blood flow to the

muscles and maintain

kidney filtration without

decreasing the blood

supply to the brain

Heart Contractions

 A heart beat comprises a period of contraction (systole) and relaxation

(diastole)

 The contraction of the heart increases pressure in the atria and ventricles

 Blood will flow from areas of higher pressure to areas of lower pressure

 Systole: When ventricles contract, AV valves close (first heart sound)

 This prevents backflow into atria and forces blood into the arteries

 Diastole: When ventricles relax, semilunar valves close (second heart

sound)

 This prevents backflow into ventricles, so blood must flow through the arteries

Heart Contractions

Control of the Heartbeat

 When removed from the body, the heart continues to beat for a short period

as the contraction of a heart is myogenic – it is initiated by signals within the

heart itself

 Electrical signals arise from a pacemaker called the sinoatrial node (SA node)

within the myocardium of the right atrium

 The SA node causes atria to contract and also triggers the atrioventricular

node (AV node), which is responsible for the subsequent contraction of the

ventricles

 The AV node is located in the septum of the heart (between atria and ventricles)

 The delay in signalling between the two nodes allows time for ventricles to fill

following atrial contraction – this serves to maximise blood flow from the heart

Control of the Heartbeat

 The electrical conduction of a heart

beat occurs according to the following

events:

 The sinoatrial node sends out an electrical

impulse that stimulates contraction of the

atria

 The atrioventricular node briefly delays the

signal then sends it down the septum via a

nerve bundle (Bundle of His)

 The Bundle of His innervates nerve fibres in

the ventricular wall, causing ventricular

contraction

Control of the Heartbeat

 The sequence of events ensures regular

and continuous beating of the hear, and

results in two heart sounds (valves

opening and closing)

 The interference of the pacemakers will lead

to the irregular and uncoordinated

contraction of the heart muscle (fibrillation)

 When fibrillation occurs, normal sinus rhythm

may be re-established with a controlled

electrical current (defibrillation)

Nervous Control

 The SA node is under autonomic (involuntary) control from the medulla

oblongata (brainstem)

 The sympathetic nerve releases a neurotransmitter called noradrenaline

(also called norepinephrine) to increase heart rate

 The vagus nerve (parasympathetic) releases a neurotransmitter called

acetylcholine to decrease heart rate

Hormonal Control

 Heart rate can also be moderated by chemicals in the bloodstream

(hormones) that are slower-acting compared to nerves, but signals can be

sustained

 Adrenaline (also called epinephrine) is released in preparation for

vigorous or sustained physical activity

 Adrenaline functions to increase the heart rate for more extended

durations (same chemical pathway as noradrenaline)

Changing the Heart Rate

 Changes in a number of factors can influence heart rate in order to

speed it up or slow it down:

 Blood pressure changes

 Blood pH changes

 Oxygen content changes

 Physical activity levels and arousal

 Sympathetic nervous system (fight or flight = speed up)

 Parasympathetic nervous system (rest and digest = slow down)

Cardiac Cycle

 The cardiac cycle describes the series of events that take place in the

heart over the duration of a single heart beat

 It is comprised of a period of contraction (systole) and relaxation

(diastole)