The Circulatory System

ICSE Class 10 Biology • Chapter 07 • Detailed Master Notes

Almost all organisms, including humans, have fluids circulating in their bodies. Such fluids constitute a distributing system (to supply substances) as well as a collecting system (to pick up substances) to and from various parts of the body including the remotest cell.

7.1 Need for Transport

All living cells need continuous supply of nutrients and oxygen, and a continuous removal of waste products and carbon dioxide. In simple organisms (like Amoeba), this occurs by simple diffusion. In multicellular, complex organisms like humans, the cells are located deep inside the body, hence a specialized Circulatory System is required.

7.2 Fluids in our Body

There are three principal fluids in our body:

1. Blood: Contained in the heart and in the blood vessels (arteries, veins, and capillaries) of the circulatory system.
2. Tissue Fluid: Occupies the spaces between cells in the organs.
3. Lymph: Contained within lymph vessels and lymphatic organs (like the spleen and tonsils).

7.3 Blood

Blood is the most important circulating fluid. It is a slightly thick, opaque liquid. It is never stationary and is always in motion from the heart to the arteries and back through the veins.

• Colour: Bright red (when oxygenated in arteries) and dark red (when deoxygenated in veins).
• Taste: Slightly salty.
• pH: Slightly alkaline (pH = 7.3 to 7.4).
• Volume: An average adult human has about 5 to 6 litres of blood (about 8% of body weight).

7.3.1 Functions of Blood

The functions of blood fall under two main categories: Transport and Protection.

A. Transport

• 1. Transport of Digested Food: Absorbed in the alimentary canal (glucose, amino acids) and supplied to all tissues.
• 2. Transport of Oxygen: From the lungs to tissues by combining with Haemoglobin to form an unstable compound, Oxyhaemoglobin.
• 3. Transport of Carbon dioxide: From tissues to lungs, partly dissolved in plasma and partly combined with haemoglobin as Carbaminohaemoglobin.
• 4. Transport of Excretory Material: Urea and uric acid are transported from tissues to the liver and kidneys for elimination.
• 5. Distribution of Hormones: Secreted by endocrine glands directly into the blood and transported to target organs.
• 6. Distribution of Heat: Helps in regulating body temperature to a constant $37^\circ C$ by distributing heat uniformly.

B. Protection

• 1. Clotting: Blood forms a clot at the site of injury to prevent further loss of blood and entry of disease-causing germs.
• 2. Engulfing Bacteria (Phagocytosis): White Blood Cells (WBCs) protect the body by engulfing and destroying bacteria.
• 3. Antibodies: Blood produces antibodies and antitoxins which neutralize poisonous substances produced by germs.

7.4 Composition of Blood

Blood consists of two main parts:

1. Plasma: The fluid part (about 55%).
2. Cellular Elements: Red Blood Cells, White Blood Cells, and Platelets (about 45%).

1. Plasma

Plasma is a light yellow, alkaline liquid containing about 90-92% water and 8% proteins and inorganic salts. It contains fibrinogen, a protein necessary for blood clotting.

2. Red Blood Cells (RBCs or Erythrocytes)

These are the most numerous cells in the blood (about 5 million per cubic mm in adult males and slightly less, about 4.5 million, in adult females).

• Shape: Biconcave discs (indented on both sides). They are very small (about 7 microns in diameter).
• Site of Production: In adults, produced in the marrow of long bones (especially ribs, breast bone, and ilium). In embryos, produced in the liver and spleen. In children, produced in the bone marrow of all bones until 5 years of age.
• Small size advantage: Helps them squeeze through the narrowest blood capillaries in a single file, maximizing the surface area exposed for oxygen exchange.
• Haemoglobin: An iron-containing respiratory pigment present in RBCs that gives blood its red colour and has a high affinity for oxygen.

Life Span and Destruction: The average life span of an RBC is 120 days. Old and weak RBCs are destroyed primarily in the Spleen, liver, and bone marrow. The iron part is retained, while the rest is excreted as bile pigments (bilirubin).

3. White Blood Cells (WBCs or Leukocytes)

WBCs are far fewer in number (about 4000 to 8000 per cubic mm) and lack haemoglobin.

• Shape: They are mostly amoeboid (can change shape) and possess a distinct nucleus.

Categories of WBCs

4. Blood Platelets (Thrombocytes)

These are very minute, oval, non-nucleated fragments of giant cells (megakaryocytes) formed in the red bone marrow. They number about 200,000 to 400,000 per cubic mm. They are essential for blood clotting.

Note: If the number of platelets falls to an abnormally low count, coagulation occurs very slowly and often leads to haemorrhage. This occurs in viral dengue fever.

7.5 Coagulation of Blood (Blood Clotting)

When a blood vessel is cut, blood escapes, but soon a clot is formed preventing further loss. This cascade mechanism involves several steps:

Haemophilia: A genetic disorder where the blood does not clot normally due to a lack of certain clotting factors.

7.6 Blood Groups and Transfusion

Blood is not entirely identical in all humans. It is classified under the ABO system based on the presence or absence of specific proteins on the RBC surface called Antigens (A and B), and complementary proteins in the plasma called Antibodies (anti-A and anti-B).

The Rh Factor

Besides ABO antigens, another antigen called the Rh factor (Rhesus factor) may be present. People with this antigen are Rh positive (Rh+), and those without it are Rh negative (Rh-).
Pregnancy Complication: If an Rh- mother carries an Rh+ fetus, the mother's blood may develop antibodies against the Rh+ cells. While the first child is usually safe, subsequent Rh+ pregnancies can result in the destruction of fetal RBCs, a severe condition called erythroblastosis fetalis.

7.7 The Blood Vessels

Blood flows through three main types of vessels in a closed circuit.

7.8 The Heart

The heart is a muscular pumping organ located in the centre of the chest cavity, slightly tilted to the left. It is about the size of one's closed fist.

• Protective Covering: The heart is enclosed by a double-walled membranous sac called the Pericardium. The space between the two membranes is filled with pericardial fluid, which reduces friction and protects the heart from mechanical shocks.

Chambers of the Heart

The mammalian heart has four chambers:

• Two Auricles (Atria): The upper chambers. They have thin walls because their major function is only to receive blood and pump it into the ventricles directly below them.
• Two Ventricles: The lower chambers. They have thick, muscular walls because they have to pump blood over long distances.
  Note: The wall of the Left Ventricle is much thicker than the right ventricle because it has to pump blood to the farthest extremities of the entire body, whereas the right ventricle only pumps blood a short distance to the lungs.

Valves of the Heart

Valves are flap-like structures that allow blood to flow in only one direction.

1. Right Atrioventricular Valve (Tricuspid Valve): Located between the right auricle and right ventricle. It has three flaps (cusps).
2. Left Atrioventricular Valve (Bicuspid or Mitral Valve): Located between the left auricle and left ventricle. It has two flaps.
3. Semilunar Valves: Pocket-shaped valves located at the origin of the Pulmonary Artery and the Aorta to prevent backflow into the ventricles.

7.9 Circulation of Blood in the Heart (Double Circulation)

Blood flows twice through the heart to complete one full cycle. This is called Double Circulation.

1. Pulmonary Circulation: The flow of deoxygenated blood from the right ventricle to the lungs (via pulmonary artery) for oxygenation, and the return of oxygenated blood to the left auricle (via pulmonary veins).
2. Systemic Circulation: The flow of oxygenated blood from the left ventricle to all body parts (via aorta), and the return of deoxygenated blood to the right auricle (via vena cavae).

7.10 Hepatic Portal System

The Hepatic Portal Vein collects nutrient-rich blood from the stomach and intestines and enters the Liver, where it breaks up into capillaries.
Importance: The liver acts as a checkpoint. It extracts excess glucose and stores it as glycogen, detoxifies harmful substances, and processes amino acids before the blood enters the general circulation.

7.11 Heart Beat and Cardiac Cycle

A complete heartbeat consists of the contraction (Systole) and relaxation (Diastole) of the heart chambers.

• Sino-atrial Node (SAN) or Pacemaker: A specialized patch of tissue in the wall of the right auricle that originates the electrical impulse for the heartbeat.
• The impulse spreads to the Atrio-ventricular Node (AVN), then travels down the Bundle of His, and finally into the ventricle walls via the Purkinje fibres, causing the ventricles to contract.

Heart Sounds

You can hear two distinct sounds through a stethoscope during a heartbeat:

1. LUBB (First sound): Produced during ventricular systole when the tricuspid and bicuspid (AV) valves close sharply to prevent backflow.
2. DUP (Second sound): Produced at the beginning of ventricular diastole when the semilunar valves at the roots of the aorta and pulmonary artery close.

7.12 Blood Pressure and Pulse

• Pulse: The alternate expansion and elastic recoil of the wall of the artery during ventricular systole. The arteries distend due to their elastic muscular walls. This distension can be easily felt by pressing gently over a superficial artery, such as the radial artery of the wrist. Counting the pulse is indirectly counting the heart beat. The pulse rate increases due to physical exercise or certain emotions.
• Blood Pressure: The pressure which the blood flowing through the arteries exerts on their walls. It has two limits:
  • Systolic Pressure (Upper limit): Recorded when fresh blood is pushing through the artery as a result of ventricular contraction. Normal range: 100 - 140 mm Hg.
  • Diastolic Pressure (Lower limit): Recorded when the wave has passed over (ventricular relaxation). Normal range: 60 - 80 mm Hg.
  Blood pressure is measured using a Sphygmomanometer. A rise in blood pressure above 140/90 is known as Hypertension (high blood pressure).

7.13 Tissue Fluid and Lymphatic System

As blood flows under high pressure in the capillaries, the liquid part (plasma minus large proteins) and some WBCs (by diapedesis) leak out through the walls and bathe the cells. This fluid is called Tissue Fluid. It helps in the actual exchange of materials directly with the cells.

Most of this fluid re-enters the blood capillaries, but some enters another set of minute channels named lymph vessels, where it is called Lymph. The lymph flows in these vessels due to the contraction of surrounding muscles (a beneficial effect of physical exercise) and ultimately pours back into major anterior veins close to the right auricle, returning to circulation.

Composition of Lymph

• Cellular part: Only leukocytes (mostly lymphocytes). No blood platelets.
• Non-cellular part: Water (94%) and Solids (6% - proteins, fats, carbohydrates, enzymes, antibodies, etc.).

Functions of Lymph

1. Nutritive: Supplies nutrition and oxygen to those parts where blood cannot reach.
2. Drainage: Drains away excess tissue fluid and metabolites, returning proteins to the blood from tissue spaces.
3. Absorption: Fats from the intestine are absorbed through lymphatics called lacteals located in the intestinal villi.
4. Defence: Lymphocytes and monocytes of the lymph function to defend the body. Lymph nodes filter out and destroy bacteria.

The Spleen

A large lymphatic organ situated in the abdomen behind the stomach. Functions:

1. Acts as a blood reservoir.
2. Produces lymphocytes.
3. Destroys old and worn-out RBCs (known as the "Graveyard of RBCs").
4. In embryos, it produces RBCs.

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