Chapter 15: Body Fluids and Circulation

In the figure : Blood is made of plasma and three kinds of formed elements.

Rule 1: Plasma — Plasma is a straw-coloured, sticky fluid. About 90 to 92 per cent is water and 6 to 8 per cent is protein (fibrinogen for clotting, globulins for defence, albumins for osmotic balance). It also carries glucose, amino acids, lipids, minerals such as sodium and calcium, hormones and wastes. Plasma without the clotting factors is called serum.

• Easy : Water makes up most of plasma.
• Medium : Serum is plasma that lacks the clotting proteins such as fibrinogen.
• Exam-type : Q: Which plasma protein helps in clotting? A: Fibrinogen.

Rule 2: Red blood cells and platelets — RBCs (erythrocytes) are the most abundant: about 5 million per mm3 in a healthy adult. They are formed in the red bone marrow, lack a nucleus in mammals, contain haemoglobin (12 to 16 g per 100 mL of blood) and live about 120 days; the spleen is called the graveyard of RBCs. Platelets (thrombocytes) are cell fragments, 1.5 to 3.5 lakh per mm3, that help in blood clotting.

• Easy : A drop of blood has millions of RBCs, which carry oxygen.
• Medium : A person with a platelet count far below normal may bleed for a long time.
• Exam-type : Q: What is the lifespan of an RBC? A: About 120 days.

Rule 3: White blood cells — WBCs (leucocytes) are colourless and fewer: 6000 to 8000 per mm3. The granulocytes are neutrophils (60 to 65 per cent, the commonest, phagocytic), eosinophils (2 to 3 per cent, resist infection and allergy), and basophils (0.5 to 1 per cent, least in number, release histamine). The agranulocytes are lymphocytes (20 to 25 per cent; B and T cells for immunity) and monocytes (6 to 8 per cent, phagocytic).

• Easy : Neutrophils engulf bacteria at an infected cut.
• Medium : Lymphocytes are involved in specific immune responses.
• Exam-type : Q: Which WBC is the most abundant? A: Neutrophil.

Spot the difference

In the figure : Steps of blood clotting after injury.

Rule 1: ABO groups — Group A has antigen A and antibody anti-B. Group B has antigen B and anti-A. Group AB has both antigens and no antibodies. Group O has no antigens and both anti-A and anti-B. A donor's red cells must not carry an antigen against which the recipient has an antibody. So O is the universal donor and AB the universal recipient.

• Easy : Group A blood cannot be given to a person of group B.
• Medium : A person of group AB can receive from A, B, AB and O.
• Exam-type : Q: Which blood group is the universal donor? A: O.

Rule 2: Rh factor — About 80 per cent of people have the Rh antigen (Rh+ve) and the rest do not (Rh-ve). If an Rh-ve mother carries an Rh+ve foetus, the mother's blood may form anti-Rh antibodies at the first delivery, which can cross the placenta in the next pregnancy and damage the foetal red cells (erythroblastosis foetalis). An anti-Rh injection to the mother after the first delivery prevents this.

• Easy : An Rh+ve person can receive Rh+ve or Rh-ve blood.
• Medium : A first Rh+ve baby of an Rh-ve mother is usually safe; the second may be at risk.
• Exam-type : Q: What is erythroblastosis foetalis? A: Destruction of foetal RBCs by the mother's anti-Rh antibodies.

Rule 3: Blood clotting — At an injury, platelets release substances (thromboplastin) that start a cascade. Inactive prothrombin changes to thrombin in the presence of Ca2+. Thrombin changes soluble fibrinogen into insoluble fibrin threads. The threads trap dead and damaged cells and form a clot. Calcium ions are required.

• Easy : A small cut stops bleeding in a few minutes because a clot forms.
• Medium : Without calcium ions the cascade would fail.
• Exam-type : Q: Which enzyme changes fibrinogen to fibrin? A: Thrombin.

Spot the difference

In the figure : Fluid leaves capillaries, becomes lymph and returns to the blood.

Rule 1: What lymph is — As blood passes through capillaries, water and small solutes leak into the spaces between cells as tissue fluid, which has the same mineral content as plasma but less protein. Most of it is returned to the veins, and the rest drains into lymphatic vessels as lymph. Lymph nodes along the vessels filter it and contain many lymphocytes.

• Easy : A swollen gland in the neck during a cold is a lymph node fighting infection.
• Medium : Lymph has the same mineral composition as plasma, but less protein.
• Exam-type : Q: Which cells does lymph carry? A: Lymphocytes.

Rule 2: Functions — Lymph carries nutrients, oxygen and hormones to cells and removes wastes. Lymphocytes in it produce antibodies and so help immune responses. Fats absorbed in the intestinal villi enter lymph vessels (lacteals) and are carried to the blood. Lymph is thus a second circulatory system that returns fluid to the blood.

• Easy : Digested fat is absorbed into the lacteals of villi.
• Medium : Lymph drains into large veins near the heart.
• Exam-type : Q: How are digested fats transported from the intestine? A: Through lymph in the lacteals.

Spot the difference

In the figure : The four chambers of the human heart.

Rule 1: Structure and valves — The heart is in the thoracic cavity between the lungs, tilted slightly left, covered by a double-walled pericardium. The right atrium opens into the right ventricle through the tricuspid valve; the left atrium opens into the left ventricle through the bicuspid (mitral) valve. At the exits of the ventricles are semilunar valves at the pulmonary artery and aorta. The left ventricle wall is thickest as it pumps blood to the whole body.

• Easy : Valves act like one-way doors.
• Medium : The left ventricle is thicker than the right because it supplies the whole body.
• Exam-type : Q: Which valve lies between the left atrium and ventricle? A: Bicuspid (mitral) valve.

Rule 2: Nodal tissue and conduction — The sino-atrial node (SAN) in the right atrium generates impulses at the fastest rate (70 to 75 per minute) and is the pacemaker. The impulse spreads through both atria, reaches the atrio-ventricular node (AVN), then the AV bundle (bundle of His), its branches and the Purkinje fibres in the ventricle walls, causing ventricular contraction. The heart is myogenic: it can beat on its own.

• Easy : The SAN starts every heartbeat.
• Medium : The delay at the AVN lets the atria empty completely before the ventricles contract.
• Exam-type : Q: Why is the SAN called the pacemaker? A: It generates action potentials at the highest rate and starts the heartbeat.

Rule 3: Cardiac cycle and output — The heart beats about 72 times a minute, so one cycle lasts about 0.8 second: atrial systole 0.1 s, ventricular systole 0.3 s and joint diastole 0.4 s. Stroke volume is about 70 mL per beat. Cardiac output = stroke volume x heart rate = 70 x 72 = about 5000 mL (5 litres) per minute. The first heart sound 'lub' is due to closing of the tricuspid and bicuspid valves and the second 'dub' is due to closing of the semilunar valves.

• Easy : At 72 beats a minute, one beat takes 60 / 72 = about 0.8 second.
• Medium : At rest, an athlete with a stroke volume of 90 mL and 60 beats per minute has a cardiac output of 5400 mL.
• Exam-type : Q: What is the cardiac output of a person with stroke volume 70 mL and rate 72 per minute? A: About 5040 mL, about 5 litres per minute.

Spot the difference

In the figure : Double circulation: lungs first, then body, before returning to the right atrium.

Rule 1: ECG — Electrodes are fixed on the chest, arms and legs and connected to an ECG machine. A normal ECG has a P wave (depolarisation of the atria, leading to atrial contraction), a QRS complex (depolarisation of the ventricles, beginning of ventricular contraction) and a T wave (return of the ventricles to normal state, repolarisation). Counting the QRS complexes in a given time gives the heart rate. Any irregularity in the shape signals abnormal function.

• Easy : A flat line with no QRS complex means the heart is not contracting.
• Medium : 10 QRS complexes in 8 seconds give 75 beats per minute.
• Exam-type : Q: Which wave shows ventricular contraction beginning? A: The QRS complex.

Rule 2: Double circulation — Deoxygenated blood from the body enters the right atrium, goes to the right ventricle and then through the pulmonary artery to the lungs (pulmonary circulation). Oxygenated blood returns by pulmonary veins to the left atrium, passes to the left ventricle and is pumped through the aorta to the body (systemic circulation). Oxygenated and deoxygenated blood never mix in the heart. There is also a hepatic portal system (gut to liver) and a coronary circulation (supplying the heart muscle).

• Easy : Blood passes through the heart twice per complete circuit.
• Medium : A drop of blood leaving the left ventricle reaches the right atrium after supplying the body.
• Exam-type : Q: Which vessel carries oxygenated blood from the lungs to the heart? A: The pulmonary vein.

Spot the difference

In the figure : Nervous and hormonal control of the heart.

Rule 1: Control of the heart — A special neural centre in the medulla moderates cardiac function. The sympathetic nerves increase the rate of heartbeat, the strength of ventricular contraction and the cardiac output; the parasympathetic nerves decrease the rate of the SAN and so reduce cardiac output. Adrenal medullary hormones (adrenaline) also raise cardiac output.

• Easy : When you run, sympathetic signals raise the heart rate.
• Medium : At rest, parasympathetic action lowers the heart rate.
• Exam-type : Q: Which part of the brain regulates cardiac activity? A: Medulla oblongata.

Rule 2: Disorders — Hypertension: blood pressure normally is 120/80 mm Hg (systolic/diastolic); repeated readings of 140/90 or higher indicate high blood pressure, which can damage the heart, brain and kidney. Coronary artery disease (CAD)/atherosclerosis: deposits of calcium, fat and cholesterol narrow the arteries supplying the heart muscle. Angina pectoris: chest pain when the heart muscle gets too little oxygen. Heart failure: the heart cannot pump enough blood; it is different from cardiac arrest, when the heart stops beating.

• Easy : Reading of 150/95 shows hypertension.
• Medium : A narrowed coronary artery causes angina during exertion.
• Exam-type : Q: What blood pressure value is considered normal? A: 120/80 mm Hg.

Spot the difference