Introduction

The circulatory system encompasses the cardiovascular and lymphatic systems. The cardiovascular system consists of the heart and the vessels that transport blood, while the vascular system consists of arteries, veins and capillaries. Vertebrates have closed cardiovascular systems, for which there are two main circulation pathways, the single and double circulation pathways. Invertebrates may have open or closed circulatory systems.

Invertebrate vascular systems

Insects have an open circulatory system (below, a), with haemolymph (equivalent of blood) moving freely within the body and making direct contact with tissues. The haemolymph carries nutrients, salts, hormones, and metabolic waste throughout the insect’s body and also has a role in wound healing and defence from pathogens. the hydrostatic pressure generated by movement of muscles also helps the insect to moult. The dorsal vessel running through the body is the equivalent of the heart and it has valves that allow movement of heamolymph only from back to front of the insect (towards the head). Oxygen and carbon dioxide are not carried in the haemolymph, but are delivered directly via tubes called trachea (and tracheoles).

Earthworms have a closed circulatory system (below, b) with five main blood vessels carrying nutrients, metabolic waste and respiratory gases. The dorsal vessel, as with insects, lies at the top of the earthworm and pumps blood forward. The other four vessels carry blood back towards the rear.

Vertebrate vascular systems

Closed circulatory systems are characteristic of vertebrates; however, there are differences in the structure of the heart and the circulation of blood between the different vertebrate groups due to adaptation during evolution and associated differences in anatomy.

Fish have a single circulatory system and a two-chambered heart with a single atrium and ventricle. The atrium collects blood returning from the body and the ventricle pumps the blood to the gills where gas exchange occurs and the blood is re-oxygenated = gill circulation. The blood moves through the rest of the body before arriving back at the atrium = systemic circulation. This unidirectional flow of blood produces a gradient of oxygenated to deoxygenated blood around the fish’s systemic circuit resulting in a limited amount of oxygen reaching organs and tissues, which will reduce the metabolic capacity of fish.

Amphibians, reptiles, birds, and mammals have a double circulatory system where blood flows two circuits, one through the lungs and back to the heart, which is called pulmonary circulation, and the other throughout the rest of the body and its organs including the brain (systemic circulation).

In amphibians, gas exchange also occurs through the skin during pulmonary circulation and is referred to as pulmocutaneous circulation. Amphibians and reptiles have a three chambered heart with two atria and only one ventricle, which is divided to a greater (reptiles) or lesser (amphibians) extent to reduce mixing of oxygenated and deoxygenated blood.

In mammals and birds, the heart is fully divided into four chambers, keeping the oxygenated blood separated from the deoxygenated blood, which improves the efficiency of double circulation. Interestingly, the four-chambered heart of birds and mammals evolved independently from the three-chambered heart. This independent evolution of the same or a similar biological trait is referred to as convergent evolution.

Blood vessels

Blood from the heart is carried through the body by a network of blood vessels. Arteries carry blood from the heart, the main artery being the aorta which branches from the right ventricle of the heart into major arteries that take blood to different limbs and organs. The major arteries diverge into minor arteries, and then smaller vessels called arterioles, to reach all the muscles and organs of the body. Arterioles diverge into capillary beds that made of capillaries that branch among the cells and tissues of the body. Capillaries are one cell thick and so are the sites for the exchange of nutrients, waste, and oxygen with tissues at the cellular level. Tissue fluid (the non cell portions of the blood) from capillaries is forced into the interstitial space and here the exchange of nutrients, waste, and oxygen takes place. The capillaries converge again into venules that connect to minor veins that finally connect to major veins that take blood high in carbon dioxide back to the heart. Fluid is also brought back to the heart via the lymphatic system.

All blood vessels may be made of three layers:

Inner lining of endothelial cells
Smooth muscle
Connective tissue

Th relative amounts of layers vary depending on their function. The diagram below shows the layer make-up for each type of vessel.

Function and constituents of blood

Blood is involved in the regulation of pH, temperature, osmotic pressure, the circulation of nutrients and removal of waste, the distribution of hormones from endocrine glands, and the elimination of excess heat in the body. Blood clotting factors necessary to control blood clotting are also carried in the blood.

Blood is made of of several components:

1. Erythrocytes (red blood cells) have haemoglobin which binds to oxygen allowing the erythrocytes to deliver oxygen to cells and remove CO_2.In most mammals, erythrocytes do not have nuclei to allow more room for haemoglobin. Camels for example have nucleated erythrocytes, as do new-born babies. Haemoglobin is not the only protein capable of oxygen transportation. Haemocyanin is a copper-containing protein found in molluscs, crustaceans, and some arthropods, chlorocruorin is an iron-containing pigment found in four families of polychaete tubeworms and haemerythrin is an iron-containing protein found in some annelid worms.

Haemoglobin itself is made of four polypeptide chains, each with an iron-containing haem group that reversibly binds oxygen, so each haemoglobin can reversibly bind up to four oxygen molecules (Hb + 4O₂ = HbO₈). As each O₂ molecule binds, it alters the conformation of haemoglobin, making subsequent binding easier; this is known as co-operative binding and means where there are oxygen-rich areas in the body, such as in the lungs, the haemoglobin will have a higher affinity for oxygen, allowing more oxygen loading. In areas with lower oxygen, such as in muscles, haemoglobin will have a lower affinity for O₂ and so oxygen unloading will be promoted.

Oxygen dissociation curves show the relationship between oxygen levels (as partial pressure) and haemoglobin saturation. As the oxygen binding affinity changes, the saturation of haemoglobin is not a linear relationship.

In adults (below, a), the oxygen dissociation curve is sigmoidal (S-shaped) because of this co-operative binding. So when there is little oxygen, the haemoglobin releases oxygen (low saturation) and when the oxygen levels are high, haemoglobin binds oxygen (high saturation).

In a foetus (below, b), the haemoglobin has a slightly different molecular composition to that of adults and a higher affinity for oxygen, loading oxygen when adult haemoglobin is unloading it, shifting the curve to the left. This allows it to gain oxygen from the mother's blood (otherwise it would not be able to do so.

Skeletal muscle has myoglobin (below, c) instead of haemoglobin. Ti=his has just one site for oxygen and so cannot operate the co-operative binding that haemoglobin can. As a result of this, the curve is a different shape (logarithmic). Myoglobin has a higher affinity for oxygen that haemoglobin and can become saturated when oxygen levels are low allowing it to hang on to oxygen when muscles are working hard, and delaying the onset of anaerobic respiration.


a) Adult dissociation curve of haemoglobin	b) Dissociation curve of foetal haemoglobin	c) Myoglobin dissociation curve

BUT this doesn't take into account the effects of increased CO₂ and consequent changes in pH of the blood. As cells work harder, they produce more CO2, that will need to be carried by the red blood cells. As CO2 enter the red blood cells, it combines with water to form carbonic acid (H₂CO₃), that then dissociates to form hydrogen ions (H⁺) and bicarbonate (HCO₃^–). The bicarbonate is pumped out of the cells in exchange with chloride ions to balance the charge. This bicarbonate will then travel to the lungs. H+ in the red blood cell change the pH, making the cell less alkaline and cause the haemoglobin to release oxygen as a result of the change in pH. The haemoglobin absorbs the H⁺ ions and acts as a buffer to keep the pH inside the cell stable. When the red blood cell reaches the lungs, bicarbonate is pumped back into the cell and the entire process is reversed

The oxyhaemoglobin dissociation curve demonstrates the saturation of haemoglobin by oxygen under normal conditions, but as the pH changes the affinity of haemoglobin for oxygen, we have to take this into account. The change in affinity with pH is known as the Bohr shift, whereby decreasing pH moves the curve to the right

2. Leucocytes (white blood cells), including neutrophils, monocytes, lymphocytes, eosinophils, and basophils are transported in the blood to sites of infection. Each leucocyte has different roles and methods of working. See the Immunology section later for more details.

3. Thrombocytes(or platelets) are cell fragments that are attracted and adhere to the wound site where interact with other coagulation factors causing the blood to clot.

4. Plasma, the liquid portion of the blood that contains water, proteins, salts, lipids, and glucose, as well as coagulation factors and serum.

The human heart and cardiac cycle

The structure of the heart

The heart is divided into four chambers: two atria and two ventricles. There is one atrium and one ventricle on the right side and one atrium and one ventricle on the left side. The atria are the chambers that receive blood, and the ventricles are the chambers that pump blood. The heart muscle is asymmetrical as the right side of the heart pumps blood to the pulmonary circuit so is smaller, whereas the left side pumps blood to the whole body in the systemic circuit.

The right atrium receives deoxygenated blood from the superior vena cava. The superior vena cava receives blood from the brain and arms and from the inferior vena cava that receives blood from the lower organs and legs.

This deoxygenated blood passes to the right ventricle through the atrioventricular or tricuspid valve, that prevents backflow of blood. Once filled, the right ventricle pumps the blood through pulmonary arteries, via the semilunar valve to the lungs for re-oxygenation. The right semilunar valves close preventing the blood from flowing backwards into the right ventricle.

The left atrium receives oxygen-rich blood from the lungs via the pulmonary veins. The blood passes through the bicuspid or mitral valve to the left ventricle.

The left ventricle pumps blood out through the aorta, the major artery of the body that takes oxygenated blood to the organs and muscles of the body. The aortic semilunar valve (or aortic valve) prevent backflow of blood into the left ventricle.

The cardiac cycle

The cardiac cycle is the control of the contraction and relaxation of the relevant heart muscle to move blood around the body. In each cardiac cycle, the heart contracts (systole), followed by a relaxation phase (diastole), where the heart fills with blood. The atria contract at the same time, forcing blood through the atrioventricular valves into the ventricles. Then the ventricles contract forcing blood through the semilunar valves into the aorta and the artery transporting blood to the lungs (via the pulmonary artery).

Illustration A shows cardiac diastole. The cardiac muscle is relaxed, and blood flows into the heart atria and into the ventricles. Illustration B shows atrial systole; the atria contract, pushing blood into the ventricles, which are relaxed. Illustration C shows atrial diastole; after the atria relax, the ventricles contract, pushing blood out of the heart.

Cardiac muscle cells beat by themselves under the right conditions, and are regulated by electrical signals that co-ordinate their beats to allow effective beating of the heart. The sinoatrial (SA) node found near the wall of the right atrium spontaneous produces electrical charges causing the two atria to contract in unison. The pulse reaches the atrioventricular (AV) node, found between the right atrium and right ventricle. The pulse is paused for a moment (~ 0.1 second) and then spreads through the walls of the ventricles. From the AV node, the electrical impulse enters the bundle of His, then to the left and right bundle branches extending through the interventricular septum. Finally, the Purkinje fibres conduct the impulse from the apex of the heart up the ventricular myocardium, and then the ventricles contract. This pause allows the atria to empty completely into the ventricles before the ventricles pump out the blood. The electrical impulses in the heart produce electrical currents that flow through the body and can be measured on the skin using electrodes and observed as an electrocardiogram (ECG).