All living cells require a constant supply of nutrients and oxygen and the continuous removal of waste products. In complex multicellular animals, this exchange is carried out by the circulatory system, in which a fluid medium transports substances between the cells and the external environment. In humans and other vertebrates, the body fluids are of two main types: the intracellular fluid (within the cells) and the extracellular fluid (outside the cells). The extracellular fluid includes blood plasma, tissue fluid (interstitial fluid) and lymph.
Blood is the most important body fluid, a connective tissue composed of a fluid matrix called plasma and formed elements (red blood cells, white blood cells and platelets). The circulatory system comprises the blood, the blood vessels (arteries, veins and capillaries) and the heart, which acts as the pump. This chapter describes the composition and functions of blood, the structure of the human heart, the cardiac cycle, the different types of circulation, and the regulation of cardiac activity, including the disorders of the circulatory system.
Blood is a specialised connective tissue consisting of a fluid matrix, the plasma, and formed elements. In a healthy adult, blood constitutes about 7-8 percent of the total body weight, with a volume of about 5 to 6 litres.
Plasma is the straw-coloured, viscous fluid of the blood, constituting about 55 percent of the blood volume. Plasma contains about 90 to 92 percent water and about 6 to 8 percent proteins. The plasma proteins are of three types: - Fibrinogen: Involved in blood clotting. - Globulins: Involved in defence mechanisms (antibodies are globulins) and lipid transport. - Albumins: Help in the osmotic balance of the blood. Plasma also contains glucose, amino acids, lipids, hormones, enzymes and waste products, as well as inorganic ions like sodium, potassium, chloride and calcium. Serum is plasma without the clotting factors (fibrinogen).
The formed elements include erythrocytes (red blood cells, RBCs), leucocytes (white blood cells, WBCs) and thrombocytes (platelets).
The ABO blood group system is based on the presence or absence of antigens on the surface of the RBCs and the corresponding antibodies in the plasma. Karl Landsteiner discovered the ABO blood groups. Blood group A has antigen A and antibody anti-B; group B has antigen B and antibody anti-A; group AB has both antigens A and B but no antibodies (universal recipient); group O has no antigens but both antibodies anti-A and anti-B (universal donor). The compatibility of blood transfusion depends on the avoidance of antigen-antibody reactions.
The Rh (Rhesus) factor is another antigen present on the surface of RBCs. Individuals with the Rh antigen are Rh positive (Rh+), and those without it are Rh negative (Rh-). An Rh negative mother carrying an Rh positive foetus may produce antibodies against the Rh antigen if foetal RBCs leak into the maternal circulation, causing erythroblastosis foetalis in a subsequent Rh positive pregnancy, a condition that can be prevented by the administration of anti-Rh antibodies (RhoGAM) to the mother.
Blood coagulation, or clotting, is the process by which blood forms a solid mass to prevent excessive loss of blood from a wound. The process involves the enzyme thrombin, which converts the soluble fibrinogen into insoluble fibrin, forming a clot. This conversion is catalysed by the enzyme thrombokinase (thrombin activator), which is released from damaged tissues and platelets, along with calcium ions and vitamin K. The clot consists of fibrin threads entangling blood cells and platelets. Vitamin K is essential for the synthesis of clotting factors in the liver.
The human circulatory system consists of the heart, blood vessels and blood. It is a closed, double circulation system.
The human heart is a muscular organ about the size of a clenched fist, located in the thoracic cavity between the lungs, slightly tilted towards the left. It is enclosed in a double-walled membrane called the pericardium, and the pericardial fluid between the layers reduces friction. The heart has four chambers: two upper atria (auricles) and two lower ventricles. The right atrium receives deoxygenated blood from the body through the superior and inferior vena cavae. The right ventricle pumps this blood to the lungs through the pulmonary artery. The left atrium receives oxygenated blood from the lungs through the pulmonary veins. The left ventricle pumps oxygenated blood to the whole body through the aorta. The right and left atria are separated by the interatrial septum, and the right and left ventricles by the interventricular septum.
The heart valves prevent the backflow of blood: the atrioventricular (AV) valves are the bicuspid (mitral) valve between the left atrium and ventricle, and the tricuspid valve between the right atrium and ventricle. The semilunar valves are present at the openings of the pulmonary artery and the aorta. The heart wall is made of cardiac muscle (myocardium), lined by the endocardium.
The cardiac cycle is the sequence of events in one heartbeat, consisting of systole (contraction) and diastole (relaxation) of the atria and ventricles. The duration of the cardiac cycle is about 0.8 seconds. The events are: 1. Atrial systole (0.1 s): The atria contract, forcing blood into the ventricles (which are in diastole). 2. Ventricular systole (0.3 s): The ventricles contract, the AV valves close (producing the first heart sound, lub), and blood is pumped into the aorta and pulmonary artery. The semilunar valves open. 3. Ventricular diastole (0.4 s): The ventricles relax, the semilunar valves close (producing the second heart sound, dup), and the atria and ventricles refill. The heart beats about 72 times per minute at rest. The volume of blood pumped out by each ventricle per minute is called the cardiac output, about 5 litres per minute (stroke volume times heart rate).
The lub (first) heart sound is produced by the closure of the AV valves at the start of ventricular systole, and the dup (second) heart sound is produced by the closure of the semilunar valves at the start of ventricular diastole.
Since blood passes through the heart twice in one complete circuit of the body, the circulation in humans is called double circulation.
The human heart is myogenic, meaning the impulse for its rhythmic contraction originates in the heart itself, in a specialised patch of tissue called the sinoatrial (SA) node, located in the right atrium. The SA node is the pacemaker of the heart. The impulse spreads to the atrioventricular (AV) node, then through the bundle of His and Purkinje fibres to the ventricular muscles. The contraction of the atria is followed by the contraction of the ventricles. The cardiac activity is also regulated by the autonomic nervous system: sympathetic nerves increase the rate of heart beat, and parasympathetic nerves decrease it. Adrenaline and thyroid hormones also increase the heart rate.
| Cell | Number | Feature | Function |
|---|---|---|---|
| RBC (Erythrocyte) | 5 million/mm3 | Biconcave, enucleated, haemoglobin | Oxygen transport |
| WBC (Leucocyte) | 6000-8000/mm3 | Nucleated, colourless | Defence |
| Platelets | 1.5-3.5 lakh/mm3 | Cell fragments | Blood clotting |
| Blood Group | Antigen on RBC | Antibody in Plasma | Donates to |
|---|---|---|---|
| A | A | anti-B | A, AB |
| B | B | anti-A | B, AB |
| AB | A and B | None | AB only |
| O | None | anti-A and anti-B | All (universal donor) |
| Event | Duration | Key Change |
|---|---|---|
| Atrial systole | 0.1 s | Blood pushed into ventricles |
| Ventricular systole | 0.3 s | AV valves close (lub), blood pumped out |
| Ventricular diastole | 0.4 s | Semilunar valves close (dup), refilling |
Body fluids and circulation constitute the transport system that sustains every cell of the human body. Blood, with its plasma and formed elements, carries oxygen, nutrients, hormones and waste products, while the ABO and Rh blood groups ensure the safe matching of blood for transfusion. The heart, a remarkable muscular pump with four chambers and a precise system of valves, drives the double circulation of blood between the lungs and the body. The cardiac cycle, regulated by the myogenic SA node and modulated by the autonomic nervous system, maintains a steady output that matches the body's needs. Understanding the composition of blood, the mechanism of clotting and the common circulatory disorders provides the essential knowledge for appreciating how the body delivers oxygen and nutrients, preparing the ground for the study of excretion in the next chapter.