Body Fluids and Circulation

Learning Outcomes:

1. Understand the composition and function of blood and lymph.
2. Identify the role of the heart and blood vessels in circulation.
3. Recognize the significance of blood groups and Rh factor.
4. Comprehend the cardiac cycle and the role of pacemakers.
5. Examine disorders related to the circulatory system.

All living cells must receive nutrients, oxygen, and other essential substances, while waste products must be removed for optimal tissue function. Various organisms have developed mechanisms for the efficient transport of these substances. In simpler organisms like sponges and coelenterates, water from their surroundings circulates through body cavities, enabling cell exchanges. More complex organisms rely on specialized body fluids for transport, such as blood and lymph. This chapter explores the composition and functions of blood, lymph, and the mechanisms that circulate these fluids.

Blood

Blood is a specialized connective tissue composed of a fluid matrix called plasma and formed elements. It plays a crucial role in transporting gases, nutrients, and waste products.

Plasma

Plasma is a straw-colored, viscous fluid that makes up about 55% of blood volume. It consists of:

1. Water: Constituting 90-92% of plasma, water serves as the solvent for various substances.
2. Proteins: Plasma contains 6-8% proteins, primarily:

• Fibrinogen: Necessary for blood clotting.
• Globulins: Participate in defense mechanisms.
• Albumins: Help maintain osmotic balance.

1. Minerals: Includes ions like Na+, Ca++, Mg++, HCO3–, and Cl–.
2. Glucose, amino acids, and lipids: Always in transit within the body.
3. Clotting factors: Present in plasma in an inactive form. Plasma without these clotting factors is termed serum.

Formed Elements

The formed elements include erythrocytes, leucocytes, and platelets, collectively making up 45% of the blood.

1. Erythrocytes (RBCs): The most numerous blood cells, ranging between 5 million to 5.5 million cells per mm³ in a healthy adult male. Formed in the red bone marrow, these cells are biconcave and lack nuclei in mammals. Hemoglobin, an iron-containing protein, gives RBCs their red color and helps in oxygen transport. RBCs have a lifespan of 120 days before being destroyed in the spleen.
2. Leucocytes (WBCs): Colorless cells due to the absence of hemoglobin, with a count of 6000-8000 per mm³. They are further classified into:

• Granulocytes: Include neutrophils, eosinophils, and basophils.
  • Neutrophils: Make up 60-65% of WBCs and are phagocytic.
  • Eosinophils: Constitute 2-3% and are linked to allergic reactions.
  • Basophils: The least abundant, at 0.5-1%, and release histamine and heparin, playing a role in inflammation.
• Agranulocytes: Comprise lymphocytes and monocytes.
  • Lymphocytes: Include B-cells and T-cells, essential for immune responses.
  • Monocytes: 6-8% of WBCs, involved in phagocytosis.

1. Platelets (thrombocytes): Fragments derived from megakaryocytes, with a count of 1.5-3.5 lakh per mm³. Platelets release substances that aid in blood clotting. A reduction in platelet count leads to clotting disorders and excessive bleeding.

Blood Groups

Blood differs between individuals, necessitating the identification of specific blood groups. The two most common systems are ABO and Rh.

ABO Grouping

ABO grouping is based on the presence or absence of surface antigens A and B on RBCs. Corresponding antibodies exist in plasma, and blood transfusion compatibility is crucial to avoid clumping. The four blood types are A, B, AB, and O.

Important Note: People with O blood are called universal donors as their blood can be donated to anyone, while those with AB blood are universal recipients because they can receive blood from any group.

Rh Grouping

The Rh antigen is present in about 80% of humans. Individuals with the antigen are Rh-positive, while those without it are Rh-negative. Rh compatibility is essential during blood transfusions and pregnancy. Erythroblastosis foetalis can occur if an Rh-negative mother has an Rh-positive fetus, but it can be prevented by administering anti-Rh antibodies.

Coagulation of Blood

Blood coagulation or clotting is a critical process to prevent blood loss following an injury. It involves the transformation of fibrinogen to fibrin, forming a clot. This process is initiated by thrombin, which activates fibrinogen, and thrombokinase, an enzyme complex, facilitates this conversion. Calcium ions also play a pivotal role in clotting.

Lymph (Tissue Fluid)

As blood flows through capillaries, some water and small molecules pass into the interstitial fluid. This fluid, similar in composition to plasma, allows for the exchange of nutrients and gases between blood and cells. The lymphatic system collects and drains this fluid back into the venous system. Lymph, which contains specialized lymphocytes, is crucial for immune responses and acts as a carrier of nutrients and fats.

Important Note: Fats are absorbed through lymph vessels called lacteals in the intestinal villi.

Circulatory Pathways

Circulation in organisms can follow either an open or closed system.

1. Open Circulatory System: Found in arthropods and molluscs, where blood flows through open spaces called sinuses.
2. Closed Circulatory System: Present in annelids and chordates, where blood flows through a network of vessels. This system is advantageous as it allows for better regulation of blood flow.

All vertebrates have a chambered heart. Fishes possess a two-chambered heart with an atrium and a ventricle, resulting in single circulation. Amphibians and reptiles (except crocodiles) have a three-chambered heart, where blood partially mixes. Crocodiles, birds, and mammals have a four-chambered heart, ensuring double circulation—complete separation of oxygenated and deoxygenated blood.

Human Circulatory System

The human circulatory system consists of a heart, a network of blood vessels, and blood. The heart is a mesodermal organ located in the thoracic cavity and has four chambers: two atria and two ventricles.

1. Heart Chambers: The atria are separated by the inter-atrial septum, while the ventricles are divided by the inter-ventricular septum. Valves, such as the tricuspid and bicuspid (mitral) valves, prevent the backflow of blood.
2. Cardiac Musculature: The heart contains specialized tissue called the nodal tissue, which is auto-excitable. The sinoatrial node (SAN) generates action potentials to initiate the cardiac cycle, earning it the title pacemaker.
3. Cardiac Cycle: The cycle includes the systole and diastole phases of the heart, ensuring the rhythmic contraction and relaxation of atria and ventricles. Each cycle lasts 0.8 seconds, and the heart pumps approximately 70 mL of blood per cycle, known as stroke volume. The cardiac output is the product of stroke volume and heart rate, averaging 5 liters per minute.

Electrocardiograph (ECG)

An electrocardiogram (ECG) records the electrical activity of the heart. The P-wave indicates atrial depolarization, the QRS complex represents ventricular depolarization, and the T-wave signifies ventricular repolarization. Deviations in the ECG can indicate cardiac abnormalities.

Important Note: The ECG is an essential tool for diagnosing heart conditions.

Double Circulation

Humans have double circulation, involving pulmonary and systemic circulation. Pulmonary circulation carries deoxygenated

blood from the right ventricle to the lungs for oxygenation. The systemic circulation distributes oxygenated blood from the left ventricle to the body tissues, where deoxygenated blood is collected and returned to the heart.

Regulation of Cardiac Activity

The heart is myogenic, meaning its activity is regulated by nodal tissues. However, the autonomic nervous system (ANS) can modulate the heart rate:

1. Sympathetic Nervous System: Increases heart rate and cardiac output.
2. Parasympathetic Nervous System: Decreases heart rate and cardiac output.

Disorders of the Circulatory System

Several conditions affect the circulatory system:

1. Hypertension (High Blood Pressure): A systolic pressure above 140 mm Hg and a diastolic pressure over 90 mm Hg indicates hypertension, leading to heart disease and kidney damage.
2. Coronary Artery Disease (CAD): Also known as atherosclerosis, this condition narrows arteries due to deposits of calcium, fat, and cholesterol.
3. Angina Pectoris: A condition causing chest pain due to reduced oxygen supply to the heart muscles.
4. Heart Failure: The heart’s inability to pump blood efficiently, resulting in congestive heart failure.

MCQ: What is the primary function of the sinoatrial node (SAN)?
Answer: It acts as the pacemaker, initiating the heart’s rhythmic contractions.