Body Fluids and Circulation: common doubts, answered
The questions students ask most often about Body Fluids and Circulation, each with a short answer. For the full chapter, read the Body Fluids and Circulation notes.
Blood and plasma
Read this section in the notes →What is the difference between plasma and serum?
Plasma is the straw-coloured liquid part of blood, about 55 per cent of it, containing water, proteins, minerals and clotting factors. Serum is plasma from which the clotting factors, such as fibrinogen, have been removed, as in blood that has already clotted. So serum has no clotting proteins, while plasma does.
What are the functions of albumin, globulin and fibrinogen?
Fibrinogen is needed for blood clotting, globulins are mainly involved in the body's defence mechanisms, and albumins help in maintaining the osmotic balance of the blood. These are the three major plasma proteins. Plasma is 90 to 92 per cent water, and proteins make up 6 to 8 per cent of it.
Formed elements: RBCs, WBCs and platelets
Read this section in the notes →Which is the most abundant and which the least abundant WBC?
Neutrophils are the most abundant white cells, about 60 to 65 per cent, and basophils are the least abundant, about 0.5 to 1 per cent. Lymphocytes are 20 to 25 per cent, monocytes 6 to 8 per cent and eosinophils 2 to 3 per cent. Neutrophils and monocytes are phagocytic, and lymphocytes are responsible for immune responses.
Why do mature human RBCs lack a nucleus and what is their lifespan?
Mature human red cells lack a nucleus, which leaves more room for haemoglobin to carry oxygen. They live about 120 days, after which they are destroyed in the spleen, which is why the spleen is called the graveyard of RBCs. A normal count is 5 to 5.5 million per cubic millimetre of blood.
What do platelets do and where are they made?
Platelets, or thrombocytes, help blood clotting by releasing substances that start the clotting process. They are fragments of special cells in the bone marrow called megakaryocytes. A normal count is about 1,50,000 to 3,50,000 per cubic millimetre, and a reduction in their number can lead to clotting problems.
Blood groups
Read this section in the notes →Why is O the universal donor and AB the universal recipient?
Group O red cells have neither antigen A nor antigen B, so they do not trigger a reaction in recipients, making O the universal donor. Group AB plasma has neither anti-A nor anti-B antibodies, so AB can receive blood from all groups and is the universal recipient. The antigens are on red cells and the antibodies in plasma.
What is Rh incompatibility and erythroblastosis foetalis?
It arises when an Rh negative mother carries an Rh positive foetus. The placenta keeps the two bloods apart during the first pregnancy, but at delivery some foetal blood may reach the mother, who then starts making anti-Rh antibodies. In a later Rh positive pregnancy these antibodies can pass into the foetus and destroy its red cells. Giving the mother anti-Rh antibodies right after the first delivery prevents this.
Coagulation of blood
Read this section in the notes →What is the sequence of events in blood clotting?
An injury triggers platelets and tissue to release clotting factors, and thrombokinase is formed, which converts inactive prothrombin into active thrombin with the help of calcium ions. Thrombin then converts soluble fibrinogen into insoluble fibrin threads, which trap blood cells and form the clot. Calcium ions are essential for the process.
Lymph (tissue fluid)
Read this section in the notes →What is lymph and how does it differ from blood?
Lymph is a colourless fluid formed from tissue fluid that has entered the lymphatic vessels. It contains specialised lymphocytes responsible for immunity, but unlike blood it has no red cells and fewer proteins. It carries nutrients, hormones and fat, and returns tissue fluid to the circulation.
Circulatory pathways in animals
Read this section in the notes →What is the difference between open and closed circulation?
In an open circulatory system, blood is pumped by the heart into open spaces and sinuses, as in arthropods and molluscs. In a closed system, blood stays within vessels, which allows more precise control of the flow, as in annelids and chordates. Vertebrates thus have a closed system.
How many chambers are there in the hearts of different vertebrates?
Fishes have two chambers, amphibians and most reptiles have three, and birds and mammals have four. Crocodiles are the exception among reptiles, with four chambers. In fishes the heart pumps only deoxygenated blood, which is then oxygenated at the gills and sent on to the body, so the circulation is single.
The human heart
Read this section in the notes →Where are the SA node and AV node located?
Both are in the right atrium of the human heart. The sinoatrial node lies in the upper right corner and the atrioventricular node lies in the lower left corner of the right atrium. The SA node is the pacemaker and generates about 70 to 75 impulses a minute, which sets the heart rate.
The cardiac cycle
Read this section in the notes →What causes the lub and dub sounds of the heart?
Lub is the first sound, caused by the closure of the tricuspid and bicuspid valves at the start of ventricular systole. Dub is the second sound, caused by the closure of the semilunar valves at the end of ventricular systole. These sounds are heard through a stethoscope and are useful in diagnosis.
How long is the cardiac cycle and what is cardiac output?
The cardiac cycle lasts about 0.8 seconds in a person with a heart rate of 72 beats a minute. Cardiac output is the volume of blood pumped by each ventricle in a minute, equal to stroke volume times heart rate. With a stroke volume of about 70 mL it works out to about 5 litres per minute.
Electrocardiogram (ECG)
Read this section in the notes →What do the P wave, QRS complex and T wave of an ECG represent?
The P wave shows atrial depolarisation, which leads to atrial contraction. The QRS complex shows ventricular depolarisation, which starts ventricular contraction. The T wave shows ventricular repolarisation, the return of the ventricles to the normal state, and its end marks the end of systole.
Double circulation
Read this section in the notes →What is double circulation?
Double circulation means blood passes twice through the heart in one complete circuit: once in the pulmonary circulation, between the heart and lungs, and once in the systemic circulation, between the heart and the rest of the body. This keeps oxygenated and deoxygenated blood separate in birds and mammals, supplying the tissues with oxygen-rich blood.
What is the hepatic portal system?
The hepatic portal system is a special venous link between the gut and the liver: the hepatic portal vein carries blood from the intestine to the liver before that blood reaches the systemic circulation. A portal vein begins and ends in capillaries rather than at the heart. The liver therefore receives the absorbed products of digestion first and can process them before they reach the rest of the body.
Regulation of cardiac activity and disorders
Read this section in the notes →How is the heartbeat regulated?
The human heart is myogenic, which means that its activity is initiated by the muscle itself, through the SA node. The nervous system modifies it: sympathetic nerves increase the heart rate and the force of contraction, while parasympathetic nerves reduce them, both through the medulla oblongata. Adrenal hormones also increase cardiac output.
What are normal and high blood pressure values?
Normal blood pressure is 120 mm Hg systolic over 80 mm Hg diastolic. A person with repeated readings of 140/90 or higher is considered to have hypertension. High blood pressure affects the heart and other vital organs, and is a risk factor for heart disease.
What is the difference between heart failure, cardiac arrest and heart attack?
Heart failure is a state where the heart does not pump blood effectively enough to meet the body's needs. Cardiac arrest is when the heart stops beating. A heart attack is the sudden damage to heart muscle because of an inadequate blood supply. They are different conditions and the terms are not interchangeable.
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