Chapter overview: body fluids and circulation
Living cells need a continual supply of nutrients, oxygen and other essentials, while wastes must be removed. Blood is the main transport fluid in higher organisms; lymph or tissue fluid also transports certain substances. The human circulatory system consists of a muscular heart, closed blood vessels and blood.
15.1 Blood: plasma and formed elements
Blood is a special connective tissue with a fluid matrix, plasma, and formed elements.
| Component | Features and functions |
|---|---|
| Plasma | Straw-coloured viscous fluid, about 55% of blood. It is 90-92% water; proteins make up 6-8%. Fibrinogen is needed for clotting, globulins mainly for defence and albumins for osmotic balance. It also carries minerals, glucose, amino acids and lipids. |
| Serum | Plasma without clotting factors. |
| Formed elements | RBCs, WBCs and platelets; together about 45% of blood. |
| Formed element | Key NCERT facts |
|---|---|
| RBCs / erythrocytes | Most abundant; 5-5.5 million mm-3 in healthy adult male. Formed in red bone marrow; biconcave and non-nucleated in most mammals; haemoglobin transports respiratory gases. Life span about 120 days; destroyed in spleen. |
| WBCs / leucocytes | Nucleated, colourless, 6000-8000 mm-3. Granulocytes: neutrophils, eosinophils, basophils. Agranulocytes: lymphocytes and monocytes. |
| Platelets / thrombocytes | Cell fragments from megakaryocytes; 1.5-3.5 lakh mm-3. Release factors important in blood clotting. |
Leucocytes: types and roles
| WBC type | Approximate proportion / role |
|---|---|
| Neutrophils | 60-65% of WBCs; phagocytic, destroy foreign organisms. |
| Monocytes | 6-8%; phagocytic. |
| Eosinophils | 2-3%; resist infections and are associated with allergic reactions. |
| Basophils | 0.5-1%, least abundant; secrete histamine, serotonin and heparin; involved in inflammatory reactions. |
| B and T lymphocytes | 20-25% collectively; responsible for immune responses. |
Blood groups: ABO and Rh
| ABO group | Antigen on RBC | Antibody in plasma | Compatible donor groups |
|---|---|---|---|
| A | A | Anti-B | A, O |
| B | B | Anti-A | B, O |
| AB | A and B | Nil | AB, A, B, O |
| O | Nil | Anti-A and anti-B | O |
Group O individuals are called universal donors and group AB individuals are called universal recipients; blood must still be carefully matched before transfusion to prevent RBC clumping.
About 80% of people possess the Rh antigen and are Rh positive. An Rh-negative person exposed to Rh-positive blood forms anti-Rh antibodies. In an Rh-negative mother carrying an Rh-positive foetus, antibodies formed after first delivery can destroy foetal RBCs in later pregnancies, causing erythroblastosis foetalis. Anti-Rh antibodies given immediately after first delivery prevent this.
Coagulation of blood
Clotting prevents excessive blood loss after injury. A clot or coagulam is mainly a network of fibrin threads in which dead and damaged formed elements are trapped.
15.2 Lymph (tissue fluid)
At tissue capillaries, water and small water-soluble substances leave blood and enter spaces between cells, while larger proteins and most formed elements remain in vessels. This interstitial fluid or tissue fluid has mineral distribution similar to plasma. Exchange between blood and cells occurs through it.
| Feature | Blood | Lymph |
|---|---|---|
| Origin / pathway | Circulates in blood vessels through heart-driven closed system | Tissue fluid collected by lymphatic vessels and returned to major veins |
| Colour and cells | Red because of haemoglobin-containing RBCs; includes all formed elements | Colourless; contains specialised lymphocytes, but lacks most formed elements |
| Functions | Broad transport of gases, nutrients, hormones and wastes | Immune response; carries nutrients and hormones; absorbs fats through lacteals in intestinal villi |
15.3 Circulatory pathways and vertebrate hearts
| System / group | Pattern |
|---|---|
| Open circulatory system | Arthropods and molluscs: heart pumps blood through vessels into open spaces or sinuses. |
| Closed circulatory system | Annelids and chordates: blood remains within closed vessels; flow can be regulated more precisely. |
| Fishes | Two-chambered heart: one atrium and one ventricle; single circulation. |
| Amphibians and most reptiles | Three-chambered heart: two atria and one ventricle; oxygenated and deoxygenated blood mix, producing incomplete double circulation. |
| Crocodiles, birds and mammals | Four-chambered heart: two atria and two ventricles; complete double circulation with no mixing. |
Human heart: chambers, valves and nodal tissue
The heart lies in the thoracic cavity between the lungs, slightly tilted left, and is enclosed by double-walled pericardium containing pericardial fluid. It has two upper atria and two lower ventricles; ventricular walls are thicker.
| Structure | Role |
|---|---|
| Inter-atrial and inter-ventricular septa | Separate right and left atria and ventricles respectively. |
| Tricuspid valve | Three cusps; guards right atrium-right ventricle opening. |
| Bicuspid / mitral valve | Guards left atrium-left ventricle opening. |
| Semilunar valves | Guard openings of right ventricle into pulmonary artery and left ventricle into aorta. |
| SA node (SAN) | Right upper corner of right atrium; generates 70-75 action potentials min-1, initiating rhythm. It is the pacemaker. |
| AV node, AV bundle and Purkinje fibres | Conduct impulse from atria to ventricular musculature. Heart is autoexcitable and therefore myogenic. |
Cardiac cycle, cardiac output and heart sounds
At joint diastole, all four chambers are relaxed; atrioventricular valves are open and semilunar valves closed. Blood flows through atria into ventricles. SAN firing causes atrial systole, adding about 30% more blood to ventricles.
Impulse reaches ventricles via AVN and AV bundle. During ventricular systole, tricuspid and bicuspid valves close; pressure opens semilunar valves and blood enters pulmonary artery and aorta. Ventricular relaxation closes semilunar valves; a fall in pressure reopens AV valves and joint diastole returns.
| Term | Value / significance |
|---|---|
| Cardiac cycle | One sequence of systole and diastole; at 72 beats min-1, duration is about 0.8 s. |
| Stroke volume | About 70 mL pumped by each ventricle per cycle. |
| Cardiac output | Stroke volume x heart rate; volume pumped by each ventricle per minute, about 5 L in healthy individual. |
| Lub sound | Closure of tricuspid and bicuspid valves. |
| Dub sound | Closure of semilunar valves. |
Electrocardiogram (ECG)
An ECG is a graphical record of electrical activity of the heart during a cardiac cycle. A standard ECG uses three leads attached to each wrist and left ankle; multiple chest leads are used for detailed evaluation.
| ECG wave | Meaning |
|---|---|
| P wave | Atrial depolarisation; leads to contraction of both atria. |
| QRS complex | Ventricular depolarisation; initiates ventricular contraction and marks beginning of systole shortly after Q. |
| T wave | Ventricular repolarisation; its end marks the end of systole. |
Heart rate can be determined by counting QRS complexes in a given period. Deviations from normal ECG shape may indicate disease.
15.4 Double circulation
| Pathway | Route and role |
|---|---|
| Pulmonary circulation | Right ventricle -> pulmonary artery -> lungs -> pulmonary veins -> left atrium. Deoxygenated blood becomes oxygenated. |
| Systemic circulation | Left ventricle -> aorta -> arteries, arterioles and capillaries -> venules, veins and vena cava -> right atrium. Delivers O2 and nutrients, removes CO2 and wastes. |
| Hepatic portal system | Hepatic portal vein carries blood from intestine to liver before systemic circulation. |
| Coronary system | Supplies and drains the cardiac musculature. |
Arteries and veins have tunica intima (inner endothelium), tunica media (smooth muscle and elastic fibres) and tunica externa (fibrous connective tissue). Tunica media is comparatively thinner in veins.
15.5 Regulation and 15.6 disorders
Heart activity is intrinsically regulated by nodal tissue. The medulla oblongata can moderate it through the autonomic nervous system: sympathetic signals increase heart rate, ventricular contraction strength and cardiac output; parasympathetic signals decrease heart rate, impulse conduction and cardiac output. Adrenal medullary hormones also increase cardiac output.
| Disorder | NCERT description |
|---|---|
| Hypertension | Persistent blood pressure at or above 140/90 mm Hg. Normal is 120/80; 120 is systolic/pumping and 80 is diastolic/resting pressure. It damages heart, brain and kidney. |
| Coronary artery disease (CAD) | Also called atherosclerosis; calcium, fat, cholesterol and fibrous deposits narrow vessels supplying heart muscle. |
| Angina pectoris | Acute chest pain when insufficient oxygen reaches heart muscle; common in middle-aged and elderly but can occur at any age. |
| Heart failure | Heart cannot pump effectively enough for body needs; lung congestion is a main symptom. It is not cardiac arrest or heart attack. |
NCERT revision prompts
- Why is blood a connective tissue? State the functions of plasma proteins.
- Compare RBCs, WBCs and platelets.
- Explain ABO compatibility, Rh incompatibility and erythroblastosis foetalis.
- Trace the clotting cascade from injury to fibrin formation.
- Differentiate lymph from blood and open from closed circulation.
- Compare vertebrate heart chambers and circulatory patterns.
- Trace blood through the human heart and state functions of the valves.
- Explain cardiac cycle, stroke volume, cardiac output and heart sounds.
- Identify P, QRS and T waves in an ECG.
- Differentiate pulmonary and systemic circulation; define double circulation.