Heart and Circulation:

Heart and Circulation:

  • In small unicellular organisms, substances move around slowly by diffusion.
  • Diffusion is too slow to move substances round the larger bodies of multicellular organisms.
  • Thus has a circulatory system: substances are carried in blood pumped by a heart.
  • There are 3 types of circulatory system:
  1. Closed circulatory system (e.g. in vertebrates) blood is enclosed in narrow blood vessels. This increases efficiency: blood travels faster as a higher pressure is generated.
  2. Single circulation: (e.g. fish) heart pumps blood to gills for gas exchange, then to tissues and back to the heart.
  3. Double circulation: (e.g. birds and mammals) right ventricle pumps blood to lungs. Blood returns to the left atrium and then the left ventricle pumps it to the rest of the body. Blood travels round the body faster, delivering nutrients faster, so the animals have a higher metabolic rate.
  • Valves ensure blood flows in one direction.



  • The heart:




  • The cardiac cycle:


Phase Detail
Atrial systole §  Pressure in the atria increases as they fill with blood returning from the veins.

§  Increased pressure opens the atrioventricular valves allowing blood to enter the ventricles.

§  The atria contract to force remaining blood into ventricles.

Ventricular systole §  Ventricles contract from the base up, increasing the pressure and closing the atrioventricular valves.

§  The semilunar valves open and blood is forced into the arteries.

Diastole §  As the atria and ventricles relax, pressure falls.

§  In the ventricle, this causes closure of the semilunar valves.

§  In the atria, blood is drawn into the heart from the veins.



  • The initiation and conduction pathways of the heartbeat:
  • Cardiac muscle contracts without being stimulated by a nerve impulse.
  • The electrical charge in the heart muscle cells changes – depolarisation. This spreads from cell to cell (like a wave) causing them to contract.
  • Depolarisation starts in the Sino atrial node or SAN (pacemaker) in the right atrium and spreads across the left and right atria causing them to contract.
  • The atria are electrically insulated from the ventricles so the wave of depolarisation converges on the atrioventricular node (AVN).
  • It then travels down the Bundle of His in the septum and into the Purkyne fibres, which then make the ventricles contract from the bottom upwards pushing blood into the aorta and pulmonary artery.
  • When the cells are depolarised, there is a small electrical current detectable on the skin.
  • This is measured in an electrocardiogram or ECG, which can be used to diagnose cardiovascular disease, problems with the conducting system or irregular heartbeat rhythms (arrhythmias).






  • Blood Vessels:
  Image Structure & Function

Carry high-pressure blood away from the heart.

  – Thick muscle layer to withstand high-pressure blood.

– Elastic tissue allows artery to stretch when blood is forced into it. The elastic layer recoils during diastole, so there is a continuous blood flow.

– They have a protective collagen layer.

– They have a round shape.

– They have a relatively small lumen.

– They have no valves.


Carry low-pressure blood towards the heart.

  – Thin muscle layer (low pressure blood).

– They have valves to stop backflow.

– Have a protective collagen layer.

– Not a round shape (wall not thick enough to hold shape).

– Large lumen (decreases effect of friction).


Adapted for exchange of substances.

  – Walls are one cell thick (cells are called endothelial cells).

– Lumen is the same width as one RBC (therefore more of RBC in contact with wall, therefore smaller diffusion distance).

– No muscle or elastic tissue.

– They are very small.



  • Atherosclerosis:
  • A disease where fatty deposits block an artery or increase its chances of being blocked by a blood clot (thrombosis).
  • It occurs…

  • After atherosclerosis has developed there is a chance that a blood clot might form in the damaged area.



  • Clot Formation:

It occurs when…

  • The damaged tissue in the blood vessel wall releases thromboplastin.
  • Blood contain platelets. When they contact the damaged tissue, they are activated. They become sticky and release calcium ions.
  • Blood plasma contains prothrombin. Thromboplastin causes prothrombin to change to an enzyme – thrombin.
  • Blood plasma also contains fibrinogen. In the presence of calcium ions, thrombin causes fibrinogen to change to fibrin.
  • Fibrin precipitates to form long fibres. Platelets and red blood cells get tangled in the fibres.


  • Risk Factors of Cardiovascular Disease:


Genetic Risk is increased if your parents have CVD.


– Some vitamins act as antioxidants, reducing the damaging effects of free radicals.

– High salt levels cause the kidneys to retain water, increasing blood pressure.

Age More likely as you get older.
Gender Incidence is much higher for men than women.
High Blood Pressure  


– Carbon monoxide prevents haemoglobin from carrying sufficient O2 – heart rate increases.

– Nicotine stimulates adrenaline release, increasing heart rate and blood pressure.

– Chemicals damage endothelium, triggering atherosclerosis.

– Decreased levels of HDLs.



– Most common risk factor.

– Exercise can halve the risk of developing CHD.

– Reduces blood pressure.

Stress Leads to increased blood pressure, poor diet and increased alcohol consumption.


– Heavy drinkers have an increased risk of CHD as alcohol raises blood pressure, contributes to obesity, and causes irregular heartbeat.

– It also increases levels of LDLs.

– Moderate amounts of alcohol may increase HDL levels.


  • Treatments for Cardiovascular Disease:
Treatment What it does? Benefits Risks
Antihypertensive – Reduce blood pressure by affecting the nervous system.

– Stop the muscles in blood vessel walls to contract.

– Blood pressure is reduced. – If dosage is not correct, blood pressure may become too low.
Plant statins – Inhibit an enzyme in the liver that catalyzes a reaction involved in producing cholesterol. – Levels of cholesterol are lowered. – Liver failure
Anticoagulants – Inhibit blood clotting. – Reduced risk of forming a blood clot in the circulatory system. – Blood clots slowly or not when it should.
Platelet inhibitory – Reduce the activation of platelets (become less sticky). – Reduced risk of forming a blood clot in the circulatory system. – Aspirin affects the stomach wall, thus the risk of stomach ulcers developing increases (for people with a tendency to this condition).





  • Cholesterol:
  • Insoluble cholesterol is transported combined with proteins to form soluble lipoproteins.


High-density lipoproteins or HDLs Contain more protein and transport unsaturated fats to the liver where they are broken down. – Reduce blood cholesterol deposition



Low-density lipoproteins or LDLs

(The main blood cholesterol carriers)

Associated with saturated fats.


– Overload membrane receptors and reduce cholesterol absorption from the blood.

– Associated with the formation of atherosclerotic plaques.


  • Saturated fats also reduce the activity of LDL membrane receptors and therefore increase blood cholesterol levels.
  • Eating both monounsaturated and polyunsaturated fats reduces the level of LDLs in the blood.




  • Determining Risks:
  • Risk is the probability of occurrence of some unwanted event or outcome.
  • A time period is always quoted.
  • Not all individuals are at risk to the same degree.
  • Risk factors increase the chance of the harmful outcome.
  • Factors that contribute to health risks include:
  • Heredity
  • Physical environment
  • Social environment
  • Lifestyle and behaviour choices
  • Two factors are positively correlated if an increase in one is accompanied by an increase in the other.
  • A positive correlation does not necessarily mean that the two are causally linked!


  • People’s behaviour is affected by the perception of risk.
  • They overestimate the risk of something happening if the risk is not under their control, unnatural, unfamiliar, dreaded, unfair, or very small.
  • There is a tendency to underestimate the risk if it has an effect in the long-term future.



  • Body Mass Index:

BMI = Mass / Height2

  • Your energy budget balances the number of calories you require with those that you consume. They should be the same.
  • Energy consumed > energy expended -> mass gain.
  • Energy consumed < energy expended -> mass lost.