Introduction to the Cardiovascular Module

module leader: Dr. ali yahya al-salimeemail: aliy.alsalime@uokufa.edu.iq

How the module is structured

• First 3 sessions– introduction to cardiovascular system, the

anatomy & development of the heart• important physiology of the cardiovascular

system• final sessions more clinically focussed

• group work throughout• case studies

Formative assessments

• 2 formative assessments– week 7– week 8 ECG quiz– week 14

• Sample summative questions at end of module

Recommended text books

Cardiovascular Module

• The broad aim of this module is that you should understand the structure and function of the human cardiovascular system, how its condition is assessed, and how cardiovascular function is altered in common diseases

Why do we need a cardiovascular system?

• single-celled or tiny organisms can get O2 and nutrients directly by diffusion from environment

• larger organisms need a circulatory system• human body has around 1014 cells

– most cells far away from source of O2 and nutrients

• cells are metabolically active– use O2 and produce CO2

• a system is required to carry O2 and nutrients to cells and carry waste products away

We need a cardiovascular system because -

• Distance of cells from the source of O2 and metabolic substrates

THEREFORE• A mechanism for transporting these

substances to the cells is essential to allow diffusion to take place

Components of the cardiovascular system

• Distribution system – vessels & blood

• Pump – the heart

• Exchange mechanism - capillaries

Blood

• Transports O2, metabolic substrates, CO2 and waste products around the body

• There is exchange of these substances between the blood and cells of the body

Blood

• Exchange occurs by diffusion• Substances diffuse between the blood and

the surrounding tissue• The function of the cardiovascular system

is to provide adequate conditions for diffusion to each tissue of the body

Where does diffusion take place?

• Capillaries are the site for diffusion between blood and the tissues

• Capillaries are composed of a single layer of endothelial cells surrounded by basal lamina

Diffusion

• Some substances eg O2 and CO2 are lipophilic and diffuse through the lipid bilayer

• Others molecules such as glucose, amino acids and lactate are hydrophilic and diffuse through small pores in the capillaries

• All molecules will move down their concentration gradient

O2 CO2

lipid soluble molecules

small water soluble molecules

endothelial cell

capillary lumen

interstitial fluid

Diffusion through capillaries

small hydrophilic (lipid insoluble) substance such as glucose diffuse through aqueous pores between endothelial cells

What factors affect diffusion

• rate of diffusion depends on:– area– diffusion ‘resistance’– concentration gradient

Area

• rate of diffusion depends on the area available for exchange

Area

• rate of diffusion depends on the area available for exchange

Area

• area for exchange between capillaries and tissues is generally very large– it depends on capillary density

• a tissue which is more metabolically active will have more capillaries

Diffusion resistance

• resistance to diffusion depends on– nature of the molecule

• eg lipophilic or hydrophilic, size – nature of the barrier

• eg pore size and number of pores for hydrophilic substances

– path length• depends on capillary density• path is shortest in the most active tissues

• diffusion resistance is mostly low

Concentration Gradient• rate of diffusion is dependent on the concentration gradient

– the greater the concentration gradient the greater the rate of diffusion

– the concentration which matters is between capillary blood and tissues

• for exchange to continue the concentration gradient between the capillary blood and tissues must be maintained

• a substance which is used by the tissues will have a lower concentration in capillary blood than arterial blood

• how much lower depends on– rate tissues use the substance– rate of blood flow through the capillary bed

Concentration Gradient

arterial endvenous end

blood flowcapillary

Maintaining the concentration gradient

• at any rate of use, the lower the blood flow the lower the capillary concentration

• rate of blood flow must be high enough to maintain a sufficient concentration gradient for diffusion

• i.e. rate of blood flow determines the concentration gradient driving O2 diffusion into the cells

Supply and Demand

• blood flow must match the tissues’ metabolic needs– the higher the rate of metabolism the greater

the demand for O2 and nutrients– increases in metabolism must be met by

increases in blood flow– the rate of blood flow is known as the

perfusion rate

Blood flow to different tissues• Brain needs high, constant flow

– 0.5 ml.min-1.g-1

• Heart muscle needs high flow which increases during exercise– 0.9 to 3.6 ml.min-1.g-1

• Kidneys need high, constant flow– 1.0 ml.min-1.g-1

• Blood flow to skeletal muscle can be very high during exercise and gut blood flow is high after a meal

Blood flow to different tissues

Tissue Min. flow l.min-1 Max. flow l.min-1

brain 0.75 0.75heart 0.3 1.2kidneys 1.2 1.2gut 1.4 2.4muscle 1.0 16.0skin 0.2 2.5other 0.2 0.2TOTAL 5.0 24.5

Blood flow must adjust to meet requirements

• At rest total blood flow is about 5 l.min

• In exercise this can rise to 25 l.min

Specification for the CVS

• The cardiovascular system must supply between 5 and 25 l.min-1 of blood to the tissues whilst at all times maintaining perfusion to vital organs such as the brain, heart and kidneys

Regulating blood flow

• If a pump is just connected to a network of vessels blood will only flow to the parts that are easiest to perfuse

• The brain is harder to perfuse due to gravity• To regulate blood flow you need to add

resistance to the system– Reduce the ease with which some regions are

perfused in order to direct blood flow to the more difficult to perfuse regions

• Arterioles are the resistance vessels

The pump• Heart is two pumps in

series

• The left heart pumps blood around the systemic circulation

• The right heart pumps blood around the pulmonary circulation

Rightheart

Leftheart

A

B

C

Lungs

systemiccirculation

pulmonarycirculation

The Circulation

• Heart pumps blood to arteries

• Arteries supply arterioles

• Arterioles supply capillaries

• Capillaries drain into venules

• Venules drain into veins

• Veins return blood to the heart

heart

arteriolesvenules

arteriesveins

capillaries

Need for flexibility

• The total flow in the system has to be able to change. This requires a temporary store of blood which can be returned to the heart at a different rate.

• Veins have thin walls which can easily distend or collapse enabling them to act as a variable reservoir for blood.

• The capacitance of the veins provides the temporary store

Distribution of blood in the CVS

67%

11%5%

17%

arteries and arterioles

capillaries

heart andlungs

veins