Key Words

It is important that you know the meanings of all the key scientific words as this will make it easier for you to understand what quesitons are asking and you may be tested on the meanings of some of these.

Look at the words below. Do you know a meaning for each? Click/Tap on the word to check that you know the correct definition.



A painful condition where the flow of blood to cardiac muscle is reduced because of a partial blockage of a coronary artery.


The major artery of the body which carries oxygenated blood away from the left ventricle of the heart to the rest of the body (systemic circulation).


These carry blood away from the heart under high pressure due to their small lumen. As with all vessels these contain an endothelium which surrounds the lumen. They have a thin layer of elastic tissue surrounded by thick walls made out of thick layers of smooth muscle and elastic tissue to withstand the high pressure that the blood is under. They have an outer layer of collagen.


These are smaller versions of arteries which also carry blood away from the heart. As with arteries they contain an endothelium with a thin layer of elastic tissue surrounded by thick layers of smooth muscle and elastic tissue and an outer layer of collagen. These vessels experience the greatest pressure change from the high pressure of the blood in the arteries down to the low pressure in the capillaries. Constriction of these vessels control the direction of blood flow.


These are thin walled chambers found at the top of the heart.

Atrial Systole

The contraction of the atria walls which causes any blood in the atria to be pushed into ventricles.

Atrio-ventricular node

A node at the base of the right atrium which picks up the electrical impulse initiated by the sino-atrial node and spread across the atria. It delays the impulse for a short period of time to allow all of the blood to move out of the atria before sending the impulse down the bundle of His to the bottom of the heart.

Atrio-ventricular valve

This is a pocket valve found between the atrium and the ventricle to ensure that blood flows in the correct direction around the heart. These valves are supported by the chordae tendineae (tendons) which ensure that the valves are not able to invert.


The fluid that is found in the blood vessels in a closed circulatory system. It is made out of a liquid called plasma and blood cells such as erythrocytes, leucocytes and platelets.

Blood vessels

These carry the blood around the body. In closed system these vessels are arteries, arterioles, capillaries, venules and veins.

Bohr effect

The effect that carbon dioxide has on the uptake of oxygen by haemoglobin causing the dissociation curve to shift to the right.


The term to describe a slow heart rate.


These are vessels which consist of just a single layer of endothelial tissue and allows some of the contents of the blood to leave the vessels and bathe the cells as tissue fluid.


The protein formed when carbon dioxide binds with haemoglobin for transportation. This accounts for how around 10% of the carbon dioxide is transported.

Carbonic Acid

The acid formed by the reaction of carbon dioxide with water catalysed by the enzyme carbonic anhydrase. The carbonic acid dissociates to form hydrogen carbonate ions and hydrogen ions.

Carbonic Anhydrase

The enzyme that catalyses the reaction between water and carbon dioxide to form carbonic acid which dissociates to form a hydrogen ion and hydrogen carbonate ion.

Cardiac cycle

The sequence of atrial systole, ventricular systole and diastole which takes place in each heartbeat.

Cardiac muscle

Sometimes called myocytes, this is the muscular tissue which makes up the majority of the structure of the heart. These are made up of muscle fibres called myofibrils between which are lots of mitochondria to supply energy. Each cardiac muscle cell is connected to others by intercalated discs and divided into contractile sections called sarcomeres.

Chloride shift

The movement of chloride ions into the erythrocyte to balance the positive charge of the heamoglobinic acid in the erythrocyte.

Closed Circulatory system

A system where the blood is kept within vessels and does not mix with the tissues. Fluid that escapes from the blood forms tissue fluid which bathes the tissues.

Conformational change

The change in the shape of a haemoglobin molecule when in the presence of carbon dioxide.


The term that describes when the smooth muscle in a blood vessel contracts to narrow the size of the lumen which restricts the flow of blood or raise the pressure.

Coronary arteries

The arteries which supplies blood to the cardiac muscles. It is a blockage of these arteries which can cause angina or myocardial infarction (heart attack).


The relaxation of the muscle in the heart tissue and due to the elastic tissue in the heart. The ventricle and atria return to their original size. The decrease in pressure in the ventricles forces the semi-lunar valves closed and the pressure of blood already in the atria forces the atrio-ventricular valves open so that blood can enter the ventricles.


The term that describes the relaxation of smooth muscle in a blood vessel and the recoil of elastic tissue to increase the size of the lumen and increase the flow of blood in the vessel and reduce the pressure.


The process of oxygen being released from oxyhaemoglobin to form haemoglobin.


A graph curve which shows the saturation of haemoglobin with oxygen for different partial pressures.

Double circulatory system

A system where blood goes through the heart twice for each circuit of the body. This keeps oxygenated and deoxygenated blood separated. Eg in mammals where blood goes from the heart to the lungs (pulmonary circulation) back to the heart and then to the body tissue (systemic circulation) and back to the heart.

Ectopic heartbeat

The term of an irregular heartbeat.

Elastic tissue

Tissue which can be stretched and recoil to help withstand the high pressure that blood is under in the vessels.

Electrocardiograms (ECG)

A graphical representation of the electrical activity that takes place in the heart. It is made up of 5 peaks – a P wave which shows atrial systole, a QRS complex which shows ventricular systole and then the T wave which shows diastole.


A smooth layer of cells which is found in all types of vessels.


This is a red blood cell which has a biconcave disc shape and is between 6 and 10 µm in diameter. The cell is packed with haemoglobin and has no organelles present to maximise the oxygen carrying capacity.

Fetal haemoglobin

The haemoglobin which is found in a fetus. This has a higher affinity for oxygen than adult haemoglobin which means that it is able to take up oxygen in low oxygen partial pressures when adult haemoglobin cannot hold onto it (such as in the placenta).


A process where the cardiac muscle cells are contracting and relaxing at their own rate causing no overall pumping of blood. Atrial fibrillation is much less serious than ventricular fibrillation which is life threatening and needs an electric shock to get the heart back into rhythm.


The protein that makes up the erythrocyte. It is made up of four subunits each one consisting of a polypeptide chain and a haem group. Each haem contains a Fe (II) ion which has a high attraction for oxygen and can hold one oxygen molecule.

Heamogloibinic acid (+HHb)

The protein that is formed when haemoglobin reacts with a hydrogen ion to buffer the acidic effect of carbonic acid being formed.

Hydrogencarbonate ions

The way that 85% of carbon dioxide is transported in the blood. The reaction to form the hydrogencarbonate ions is catalysed by carbonic anhydrase.

Hydrostatic Pressure

The pressure that a fluid exerts on the side of a container or vessel. It is this pressure that forces the some of the plasma out of the capillary to form tissue fluid. The pressure is always expressed as a positive value.

Intercalating plates

The connections between one cardiac muscle cell and the next. These plates are very good at conducting nerve impulses and ensuring the contraction of both the atria and ventricles happen smoothly.


This is a white blood cell which is involved in the immune response of the body.


The name given to the hole through which the blood flows in a vessel.


A fluid which is found in the lymphatic system. It is formed from the excess tissue fluid which does not return to the capillary. It also contains white blood cells particularly lymphocytes.

Lymph nodes

The structures in the body that create lymphocytes and play an important part in the immune response.

Myocardial Infarction

Also known as a heart attack. This usually happens as a result of a coronary artery becoming blocked and restricting the flow of blood to part of the cardiac tissue which then dies.


The muscle fibres which are found in cardiac muscle cells.


A property of cardiac muscle cells which means that they will contract and relax on their own.

Oncotic pressure

The pressure created by the hormones and plasma proteins that cause water to move back in to the capillary at the venule end by osmosis. This pressure is always expressed as a negative value.

Open Circulatory System

A system where the blood is not always kept within the vessels and is free to bathe the tissue cells.


Small openings that blood can re-join the circulatory system in small insects.

Oxygen partial pressure

The relative pressure of oxygen in a mixture of gases. This is also known as oxygen tension.


The chemical formed when haemoglobin becomes associated with oxygen.


The pumping movement made by smooth muscles to move fluid along a tube. For example peristalsis is used to move the blood in small insects.


The liquid part of the blood. It is made from water which has dissolved substances such as sugars (eg glucose), amino acids, lypoproteins, ions (eg sodium, potassium and chloride), oxygen, carbon dioxide and urea as well as non-dissolved substances such as plasma proteins and hormones.


These are fragments of cells which are involved in the clotting process.

Pulmonary artery

This is the artery which carries deoxygenated blood from the right ventricle to the lungs to be oxygenated. This is part of the pulmonary circulation.

Pulmonary circulation

The part of the circulatory system which delivers deoxygenated blood to the lungs for oxygenation.

Pulmonary vein

The vein which carries oxygenated blood from the lungs where it has been oxygenated back to the left atrium of the heart. This is part of the pulmonary circulation.

Purkyne tissue

A conductive tissue found in the ventricles which spreads the impulse from the bottom of the ventricles upwards.


One contractive section of a cardiac muscle cell.

Semi-lunar valves

These are pocket valves which look like half-moons and are found in the pulmonary artery, aorta and most of the veins in the body. They ensure that blood flows in the correct direction.


A centre of the heart which divides the left (oxygenated) side of the heart from the right (deoxygenated) side. The bundle of His runs through this.

Single circulatory system

A system where the blood goes through the heart once for each circuit of the body. Eg in a fish where blood goes from the heart to the gills to the body tissue and then back to the heart.

Sino-atrial node

A small patch of tissue in the right atrium which generates the electrical impulse that is spread over both atria and causes them to contract.

Sinus Rhythm

The normal rate at which the heart beats at.

Smooth muscle

A circular type of muscle found in blood vessels which can constrict to narrow the flow of blood in the vessel.

Systemic circulation

The part of the circulatory system which delivers oxygenated blood to the tissues of the body.


The term for a fast heart rate.

Tissue fluid

This is the fluid from the blood which leaks out of the capillaries and bathes the tissue cells. It is made up of the dissolved parts of plasma (all of plasma apart from the plasma proteins and the hormones). It is from this fluid that gases and nutrients are exchanged with cells.


The main chamber of the heart which is involved in creating high pressure to pump the blood out of the heart. These usually have thick walls with the wall on the left being significant thicker than the wall on the right.


These carry blood back to the heart. They have a large lumen to carry the blood under low pressure. They consist of endothelium surrounded by a thin layer of elastic tissue surrounded by a thin layer of smooth muscle and a second layer of elastic tissue and finally an outer layer of collagen. These contain valves to ensure that the blood flows in the correct direction.

Vena Cava

The main vein of the body which delivers deoxygenated blood from around the body to the right atrium. It is made of a superior branch which delivers blood from above the heart and an inferior branch which delivers blood from below the heart.


These join capillaries to veins and are small versions of veins. They contain a larger lumen than arterioles carrying blood at lower pressure. They contain an endothelial layer surrounded by a thin layer of elastic fibres, smooth muscle, a second layer of elastic tissue and finally a layer of collagen.

Ventricular Systole

The contraction of the ventricles which causes the atrio-ventricular valves to close and the semi-lunar valves to open. The ventricles contract from the bottom of the heart upwards pushing the blood out of the heart and into the aorta/pulmonary artery.

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