Cardiovascular System and Diseases: Comprehensive Notes
Cardiovascular Disease
Cardiovascular diseases (CVDs) are diseases of the heart and circulation.
CVDs are the main cause of death in the UK, accounting for almost 180,000 deaths a year.
Over 46,000 of these deaths are premature (under the age of 75).
Around one in three people in the UK die from CVDs.
The main forms of CVDs are coronary heart disease (CHD) and stroke.
Almost half of all deaths from CVDs are from CHD (45%), and over a quarter are from stroke (28%).
CHD is the most common cause of death in the UK.
About one in five men and one in ten women die from CHD.
Premature Deaths by Cause in the UK (2010)
Females:
Coronary heart disease: 8%
Stroke: 5%
Other CVD: 6%
Lung cancer: 10%
Colo-rectal cancer: 4%
Other cancers: 24%
Diabetes: 1%
Respiratory disease: 10%
Injuries and poisoning: 5%
All other causes: 19%
Males:
Coronary heart disease: 17%
Stroke: 4%
Other CVD: 7%
Lung cancer: 9%
Colo-rectal cancer: 4%
Other cancers: 23%
Diabetes: 1%
Respiratory disease: 9%
Injuries and poisoning: 9%
All other causes: 18%
One person dies of a heart attack in the UK every 7 minutes.
Key Biological Principle: Why Have a Heart and Circulation?
The primary purpose of the heart and circulation is to move substances around the body.
In small organisms, substances move by diffusion.
Diffusion: The movement of molecules or ions from a region of high concentration to a region of low concentration by relatively slow random movement of molecules.
Complex multicellular organisms rely on a mass transport system to move substances efficiently over long distances by mass flow.
Mass flow: All the particles in a liquid move in one direction through tubes due to a difference in pressure.
Animals have blood to carry vital substances and a heart to pump it.
Some animals have more than one heart (e.g., earthworms have five).
Open Circulatory Systems
In insects and some other animal groups, blood circulates in large open spaces.
A simple heart pumps blood out into cavities surrounding the animal’s organs.
Substances can diffuse between the blood and cells.
When the heart muscle relaxes, blood is drawn from the cavity back into the heart through small, valved openings.
Closed Circulatory Systems
Many animals, including all vertebrates, have a closed circulatory system.
Blood is enclosed within tubes (blood vessels).
This generates higher blood pressures, forcing blood along narrow channels.
Blood travels faster, making the system more efficient at delivering substances.
The blood leaves the heart under pressure and flows along arteries and then arterioles to capillaries.
There are extremely large numbers of capillaries in close contact with most cells, where substances are exchanged.
After passing along the capillaries, the blood returns to the heart by means of venules and then veins.
Valves ensure blood flows only in one direction.
Animals with closed circulatory systems are generally larger and more active than those with open systems.
Single Circulatory Systems
Animals with a closed circulatory system have either single circulation or double circulation.
Fish have single circulation:
The heart pumps deoxygenated blood to the gills.
Gaseous exchange takes place in the gills.
Carbon dioxide diffuses from the blood into the water, and oxygen diffuses from the water into the blood.
The blood leaving the gills then flows round the rest of the body before returning to the heart.
Blood flows through the heart once for each complete circuit of the body.
Double Circulatory Systems
Birds and mammals have double circulation:
The right ventricle pumps deoxygenated blood to the lungs where it receives oxygen.
The oxygenated blood then returns to the heart to be pumped a second time (by the left ventricle) out to the rest of the body.
Blood flows through the heart twice for each complete circuit of the body.
The heart gives the blood returning from the lungs an extra boost, reducing the time it takes for the blood to circulate.
This allows birds and mammals to have a high metabolic rate, as oxygen and food substances can be delivered more rapidly.
How Does the Circulation Work?
In the circulatory system, a liquid and all the particles it contains are transported in one direction due to a difference in pressure in a process known as mass flow.
In animals, the transport medium is usually called blood.
The fluid, plasma, is mainly water and contains dissolved substances such as digested food molecules (e.g., glucose), oxygen, and carbon dioxide.
Proteins, amino acids, salts, enzymes, hormones, antibodies, and urea are also transported in the plasma.
Cells are also carried in the blood: red blood cells, white blood cells, and platelets.
Blood is not only important in the transport of dissolved substances and cells but also plays a vital role in the regulation of body temperature, transferring energy around the body.
Key Biological Principle: Properties of Water That Make It an Ideal Transport Medium
Water (H_2O) is a liquid at room temperature, unlike most other small molecules.
Water is a polar molecule with an unevenly distributed electrical charge.
The hydrogen end is slightly positive, and the oxygen end is slightly negative (dipole).
This polarity accounts for many of its biologically important properties.
Hydrogen bonding holds water molecules together.
Solvent Properties
Many chemicals dissolve easily in water due to their dipole nature, allowing vital biochemical reactions to occur in the cytoplasm of cells.
Ionic substances, such as sodium chloride (NaCl), dissolve easily in water, with ions becoming hydrated.
Polar molecules also dissolve easily in water (hydrophilic).
Non-polar, hydrophobic substances, such as lipids, do not dissolve in water; they combine with proteins to form lipoproteins for transport in blood.
Thermal Properties
The specific heat capacity of water is very high because a large amount of energy is required to break the hydrogen bonds.
This helps organisms avoid rapid changes in their internal temperature and maintain a steady temperature.
Bodies of water do not change temperature rapidly.
Water also has a high boiling point because there are so many hydrogen bonds.
The Structure of the Heart
The heart is a double pump made of cardiac muscle.
The right side receives deoxygenated blood from the body and pumps it to the lungs.
The left side receives oxygenated blood from the lungs and pumps it to the body.
Arteries carry blood away from the heart, and veins return blood to the heart.
Key structures include:
Superior vena cava
Inferior vena cava
Right atrium
Right ventricle
Pulmonary artery
Pulmonary veins
Left atrium
Left ventricle
Aorta
Coronary arteries and veins
Atrioventricular valves
Semilunar valves
The Structure of Blood Vessels
Arteries and veins contain collagen (tough fibrous protein), elastic fibers, and smooth muscle cells.
Arteries
Narrow lumen
Thicker walls
More collagen, smooth muscle, and elastic fibers
No valves
Veins
Wide lumen
Thinner walls
Less collagen and smooth muscle, fewer elastic fibers
Valves
Capillaries
Very narrow (about 10 µm in diameter)
Walls are only one cell thick.
How Blood Moves Through Vessels
During systole (heart contraction), blood is forced into arteries, and their elastic walls stretch.
During diastole (heart relaxation), the elasticity of the artery walls causes them to recoil, helping to push the blood forward and smoothing blood flow.
The pulsing flow of blood through the arteries can be felt anywhere an artery passes over a bone close to the skin.
Blood flows more slowly in the capillaries due to their narrow lumens, allowing for exchange between the blood and the surrounding cells.
Blood flows steadily and without pulses in veins where it is under relatively low pressure.
In the veins, blood flow is assisted by the contraction of skeletal muscles and breathing.
Low pressure developed in the thorax (chest cavity) when breathing in also helps draw blood back into the heart from the veins.
Backflow is prevented by semilunar valves within the veins.
Coronary Circulation
The heart muscle is supplied with blood through its own coronary circulation.
Coronary arteries, a network of capillaries, and coronary veins.
How the Heart Works
The chambers of the heart alternately contract (systole) and relax (diastole) in a rhythmic cycle.
One complete sequence of filling and pumping blood is called a cardiac cycle or heartbeat.
During systole, cardiac muscle contracts, and the heart pumps blood out through the aorta and pulmonary arteries.
During diastole, cardiac muscle relaxes, and the heart fills with blood.
The cardiac cycle includes three phases: atrial systole, ventricular systole, and diastole.
Phase 1: Atrial Systole
Blood returns to the heart due to the action of skeletal muscles involved in breathing.
Blood under low pressure flows into the left and right atria from the pulmonary veins and vena cava.
The atria contract, forcing more blood into the ventricles.
Phase 2: Ventricular Systole
The ventricles contract from the base of the heart upwards, increasing the pressure in the ventricles.
The pressure forces open the semilunar valves and pushes blood up and out through the pulmonary arteries and aorta.
The pressure of blood against the atrioventricular valves closes them and prevents blood flowing backwards into the atria.
Phase 3: Cardiac Diastole
The atria and ventricles then relax during cardiac diastole.
Elastic recoil of the relaxing heart walls lowers pressure in the atria and ventricles.
Blood under higher pressure in the pulmonary arteries and aorta is drawn back towards the ventricles, closing the semilunar valves and preventing further backflow into the ventricles.
The coronary arteries fill during diastole.
Low pressure in the atria helps draw blood into the heart from the veins.
Pressure Changes and Valves Determine Flow
At each stage in the cycle, blood moves from high pressure to low pressure.
The closing of the valves causes the sounds that we recognize as a heartbeat.
The first sound (‘lub’) is caused by the closing of the atrioventricular valves, and the second (‘dub’) by the closing of the semilunar valves.
What is Atherosclerosis?
Atherosclerosis is the disease process that leads to coronary heart disease and strokes.
Fatty deposits can either block an artery directly or increase its chance of being blocked by a blood clot (thrombosis).
If the blood supply is blocked completely and not restored very quickly, the affected cells are permanently damaged.
In the coronary arteries, this results in a heart attack (myocardial infarction).
In the arteries supplying the brain, it results in a stroke.
Narrowing of arteries to the legs can result in tissue death and gangrene (decay).
An artery can burst where blood builds up behind an artery that has been narrowed.
What Happens in Atherosclerosis?
The endothelium, a delicate layer of cells that lines the inside of an artery and separates the blood from the muscular wall, becomes damaged and dysfunctional.
Once the inner lining of the artery is breached, there is an inflammatory response. White blood cells leave the blood vessel and move into the artery wall. These cells accumulate chemicals from the blood, particularly cholesterol. A fatty deposit builds up, called an atheroma.
Calcium salts and fibrous tissue also build up at the site, resulting in a hard swelling called a plaque on the inner wall of the artery. The buildup of fibrous tissue means that the artery wall loses some of its elasticity; in other words, it hardens, giving the word ‘atherosclerosis’.
Plaques cause the lumen of the artery to become narrower. This makes it more difficult for the heart to pump blood around the body and can lead to a rise in blood pressure.
Why Does the Blood Clot in Arteries?
Rapid blood clotting is vital when a blood vessel is damaged.
The blood clot seals the break in the blood vessel, limits blood loss, and prevents entry of pathogens through any open wounds.
When platelets come into contact with the damaged vessel wall, they change from flattened discs to spheres with long thin projections.
Their cell surfaces change, causing them to stick to the exposed collagen in the wall and to each other to form a temporary platelet plug.
They also release substances that activate more platelets.
The direct contact of blood with collagen within the damaged blood vessel wall also triggers a complex series of chemical changes in the blood.
The Clotting Cascade
Platelets and damaged tissue release a protein called thromboplastin.
Thromboplastin activates an enzyme that catalyses the conversion of the protein prothrombin into an enzyme called thrombin. A number of other protein factors, vitamin K, and calcium ions must be present in the blood plasma for this conversion to happen.
Thrombin then catalyses the conversion of the soluble plasma protein, fibrinogen, into the insoluble protein fibrin.
A mesh of fibrin forms that traps more platelets and red blood cells to form a clot.
Consequences of Atherosclerosis
Coronary Heart Disease
Narrowing of the coronary arteries limits the amount of oxygen-rich blood reaching the heart muscle.
The result may be a chest pain called angina.
Angina is usually experienced during exertion when the cardiac muscle is working harder and needs to respire more.
Because the heart muscle lacks oxygen, it is forced to respire anaerobically. It is thought that this results in chemical changes which trigger pain, but the detailed mechanism is still not known.
If a fatty plaque in the coronary arteries ruptures, collagen is exposed, which leads to rapid clot formation.
The blood supply to the heart may be blocked completely.
If the affected muscle cells are starved of oxygen for long, they will be permanently damaged. This is what we call a heart attack or myocardial infarction.
Stroke
If the supply of blood to the brain is only briefly interrupted, then a mini-stroke may occur.
If a blood clot blocks one of the arteries leading to the brain, a full stroke will result.
If brain cells are starved of oxygen for more than a few minutes, they will be permanently damaged, and it can be fatal.
Symptoms of Cardiovascular Disease
Coronary Heart Disease
Shortness of breath and angina are often the first signs of coronary heart disease.
The main symptom of angina is intense pain, ache, or a feeling of constriction and discomfort in the chest, or in the left arm and shoulder.
Other symptoms are unfortunately very similar to those of severe indigestion and include a feeling of heaviness, tightness, pain, burning, and pressure – usually behind the breastbone, but sometimes in the jaw, arm, or neck.
Women may not have chest pain but experience unusual fatigue, shortness of breath, and indigestion-like symptoms.
Sometimes coronary heart disease causes the heart to beat irregularly. This is known as arrhythmia and can itself lead to heart failure.
Stroke
The effects of a stroke will vary depending on the type of stroke, where in the brain the problem has occurred, and the extent of the damage.
Symptoms normally appear very suddenly and include:
Numbness
Dizziness
Confusion
Slurred speech
Blurred or lost vision, often only in one eye.
Paralysis on one side of the body with a drooping arm, leg, or eyelid, or a dribbling mouth.
Aneurysms
If part of an artery has narrowed and become less flexible, blood can build up behind it.
The artery bulges as it fills with blood, and an aneurysm forms.
Aortic aneurysms are likely to rupture when they reach about 6–7 cm in diameter.
The resulting blood loss and shock can be fatal.
Earlier signs of pain may prompt a visit to the doctor, and the bulge can often be felt or seen with ultrasound examination.
It may be possible to surgically replace the damaged artery with a section of artificial artery.
Congenital Heart Disease
Congenital heart disease refers to a heart defect or condition that is present at birth.
There are many different types of congenital heart disease, with some being minor and easily treated, whereas others are more serious.
Some conditions are inherited, and researchers are working to understand the causes.
Variants of the NR2F2 gene are responsible for the development of severe forms of congenital heart disease.
Approximately one percent of all babies are born with congenital heart disease.
Because the damage to the heart is structural, most babies will need surgery to correct the problem.
Genetic variants that completely deactivate the NR2F2 gene tended to cause damage to the left side of the heart.
Genetic variants that alter activity of the gene but do not deactivate it more commonly caused a specific sub-type of holes in the hearts of patients.