CHAPTER 15: BODY FLUIDS AND CIRCULATION

• All living cells require nutrients, oxygen, and other essential substances, as well as the continuous removal of waste or harmful substances for healthy tissue functioning.
• Different groups of animals have evolved various methods of transport for these substances.
• Simple organisms like sponges and coelenterates circulate water from their surroundings through their body cavities to facilitate substance exchange with cells.
• More complex organisms, including humans, use special fluids within their bodies for transport. Blood is the most commonly used body fluid for this purpose.
• Blood is a vital fluid that circulates throughout the body, delivering oxygen, nutrients, and hormones, and removing waste products.
• Blood is composed of plasma, red blood cells, white blood cells, and platelets.
• Plasma is the liquid component of blood, consisting of water, proteins, hormones, electrolytes, and other substances.
• Red blood cells (erythrocytes) carry oxygen from the lungs to the body tissues and transport carbon dioxide, a waste product, from the tissues to the lungs for exhalation.
• White blood cells (leukocytes) are involved in the immune response, defending the body against infections and diseases.
• Platelets play a crucial role in blood clotting, preventing excessive bleeding when blood vessels are damaged.
• The circulatory system, comprising the heart, blood vessels, and blood, ensures the circulation of blood throughout the body.
• The heart acts as a pump, pumping oxygenated blood from the lungs to the rest of the body and receiving deoxygenated blood back from the body to be reoxygenated in the lungs.
• Arteries carry oxygenated blood away from the heart to various tissues, while veins return deoxygenated blood back to the heart.
• Capillaries are tiny blood vessels that connect arteries and veins, allowing for the exchange of substances between the blood and the surrounding tissues.
• Lymph is another body fluid that aids in the transport of certain substances. It is derived from tissue fluid and plays a role in immune response and the removal of waste products from tissues.
• Lymphatic vessels carry lymph throughout the body, eventually returning it to the bloodstream.
• The lymphatic system also includes lymph nodes, which act as filters, removing harmful substances and producing immune cells.
• The circulatory system, consisting of blood and lymph, plays a vital role in the transport of nutrients, oxygen, hormones, waste products, and immune cells throughout the body, ensuring the proper functioning of tissues and organs.

15.1 BLOOD

Plasma:

• Plasma is a straw-colored, viscous fluid that makes up about 55% of blood.
• Approximately 90-92% of plasma is water, while proteins contribute 6-8%.
• The major proteins found in plasma are fibrinogen, globulins, and albumins.
• Fibrinogen is necessary for blood clotting, globulins are involved in the body's defense mechanisms, and albumins help maintain osmotic balance.
• Plasma also contains small amounts of minerals like sodium (Na+), calcium (Ca++), magnesium (Mg++), bicarbonate (HCO3-), chloride (Cl-), etc.
• Other substances present in plasma include glucose, amino acids, lipids, and coagulation factors (in an inactive form).
• Plasma without clotting factors is referred to as serum.

Formed Elements:

• Erythrocytes (Red Blood Cells or RBCs): RBCs are the most abundant cells in the blood, constituting about 45% of the total blood volume.
• They lack a nucleus in most mammals and have a biconcave shape.
• The red color of RBCs comes from the iron-containing protein called hemoglobin, which plays a crucial role in transporting respiratory gases.
• An average adult man has about 5 million to 5.5 million RBCs per mm³ of blood.
• RBCs are produced in the red bone marrow and have a lifespan of around 120 days before being destroyed in the spleen.
• Leucocytes (White Blood Cells or WBCs): WBCs are nucleated and colorless due to the absence of hemoglobin.
• They are relatively fewer in number, averaging 6000-8000 per mm³ of blood.
• WBCs are categorized into two main types: granulocytes and agranulocytes.
• Granulocytes include neutrophils, eosinophils, and basophils, while agranulocytes include lymphocytes and monocytes.
• Neutrophils (60-65% of total WBCs) and monocytes (6-8% of total WBCs) are phagocytic cells responsible for destroying foreign organisms.
• Basophils (0.5-1% of total WBCs) secrete substances like histamine, serotonin, and heparin and are involved in inflammatory reactions.
• Eosinophils (2-3% of total WBCs) provide resistance against infections and are associated with allergic reactions.
• Lymphocytes (20-25% of total WBCs) are further divided into B and T lymphocytes, playing key roles in immune responses.
• Platelets (Thrombocytes): Platelets are cell fragments produced from megakaryocytes in the bone marrow.
• Blood normally contains 1,500,000-3,500,000 platelets per mm³.
• Platelets release various substances involved in the coagulation or clotting of blood.
• A decrease in platelet count can lead to clotting disorders and excessive blood loss.

Blood Groups

ABO Grouping

• ABO grouping is based on the presence or absence of two surface antigens, A and B, on red blood cells (RBCs).
• Individuals can have blood types A, B, AB, or O, depending on the combination of antigens on their RBCs.
• The plasma of different individuals contains natural antibodies that correspond to the antigens they lack on their RBCs.
• Blood type A has antigen A on RBCs and anti-B antibodies in plasma.
• Blood type B has antigen B on RBCs and anti-A antibodies in plasma.
• Blood type AB has both antigens A and B on RBCs and no corresponding antibodies in plasma.
• Blood type O has neither antigen A nor B on RBCs but has both anti-A and anti-B antibodies in plasma.
• During blood transfusion, compatibility must be ensured between the donor and recipient to avoid clumping (agglutination) and destruction of RBCs.
• Type O blood is considered the universal donor as it can be donated to individuals of any blood type.
• Type AB blood is considered the universal recipient as individuals with this blood type can receive blood from any other blood type.

Rh Grouping

• Rh grouping is based on the presence or absence of the Rh antigen on the surface of RBCs.
• Approximately 80% of humans have the Rh antigen and are Rh positive (Rh+ve), while those lacking the antigen are Rh negative (Rh-ve).
• Rh-ve individuals, when exposed to Rh+ve blood, can develop specific antibodies against the Rh antigens.
• Rh group compatibility should also be considered during blood transfusions.
• Rh incompatibility can occur when an Rh-ve pregnant mother is carrying an Rh+ve fetus.
• During the first pregnancy, the Rh antigens of the fetus do not usually come into contact with the Rh-ve blood of the mother.
• However, during delivery, there is a possibility of exposure of the maternal blood to small amounts of Rh+ve blood from the fetus.
• In subsequent pregnancies, if the mother's Rh antibodies (Rh-ve) cross the placenta and enter the bloodstream of an Rh+ve fetus, they can destroy the fetal RBCs.
• This condition is known as erythroblastosis fetalis and can be life-threatening or cause severe anemia and jaundice in the baby.
• Administration of anti-Rh antibodies to the mother immediately after the delivery of the first child can prevent Rh sensitization and erythroblastosis fetalis.

Coagulation of Blood

• Blood exhibits coagulation or clotting in response to an injury or trauma to prevent excessive loss of blood.
• Coagulation is the process of forming a clot or coagulum at the site of injury.
• The clot is primarily composed of a network of threads called fibrins, in which dead and damaged formed elements of blood are trapped.
• Coagulation is initiated by the conversion of inactive fibrinogens in the plasma to fibrins by the enzyme thrombin.
• Thrombin is formed from another inactive substance called prothrombin, which is present in the plasma.
• The conversion of prothrombin to thrombin requires the presence of an enzyme complex called thrombokinase.
• Thrombokinase is formed through a cascade process involving a series of linked enzymatic reactions.
• Various factors present in the plasma are involved in this cascade process and are initially in an inactive state.
• Platelets in the blood are stimulated by the injury to release certain factors that activate the coagulation mechanism.
• Factors released by the tissues at the site of injury can also initiate coagulation.
• Calcium ions play a crucial role in the clotting process.
• The formation of a clot helps to seal the wound and prevent further bleeding, allowing the healing process to begin.

15.2 LYMPH (TISSUE FLUID)

• As blood passes through capillaries in tissues, some water and small water-soluble substances move out into the spaces between cells, leaving larger proteins and most formed elements in the blood vessels.
• This fluid released into the spaces between cells is called interstitial fluid or tissue fluid.
• Interstitial fluid has the same mineral distribution as plasma and serves as a medium for the exchange of nutrients, gases, and other substances between the blood and cells.
• The lymphatic system is an elaborate network of vessels that collects the interstitial fluid and drains it back into the major veins.
• The fluid present in the lymphatic system is called lymph.
• Lymph is a colorless fluid that contains specialized lymphocytes, which play a crucial role in the immune responses of the body.
• Lymph serves as an important carrier for nutrients, hormones, and other substances.
• Fats are absorbed through lymph in specialized vessels called lacteals present in the intestinal villi.

15.3 CIRCULATORY PATHWAYS

• Circulatory patterns can be classified into open or closed systems.
• Arthropods and mollusks have an open circulatory system, where the blood pumped by the heart flows into open spaces or sinuses in the body.
• Annelids and chordates, including humans, have a closed circulatory system, where the blood is circulated through a closed network of blood vessels.
• Closed circulatory systems offer more precise regulation of fluid flow.
• Vertebrates, including humans, have a muscular chambered heart.
• Fish have a 2-chambered heart with an atrium and a ventricle, and they exhibit single circulation, where deoxygenated blood is pumped to the gills for oxygenation and then supplied to the body before returning to the heart.
• Amphibians and reptiles (except crocodiles) have a 3-chambered heart with two atria and a single ventricle. They exhibit incomplete double circulation, where oxygenated and deoxygenated blood partially mix in the ventricle before being pumped out to the body.
• Crocodiles, birds, and mammals have a 4-chambered heart with two atria and two ventricles. They exhibit complete double circulation, where oxygenated and deoxygenated blood are kept separate, and two separate circulatory pathways exist.
• Humans have a double circulation system, with oxygenated blood pumped by the left ventricle to the body and deoxygenated blood returning to the right atrium before being pumped to the lungs for oxygenation.
• The human circulatory system consists of the heart, blood vessels (arteries, veins, and capillaries), and blood.

Human Circulatory System

• The human circulatory system consists of a muscular chambered heart, a network of closed branching blood vessels, and blood.
• The heart is located in the thoracic cavity, between the two lungs, slightly tilted to the left. It is protected by a double-walled membranous bag called the pericardium.
• The human heart has four chambers: two small upper chambers called atria and two larger lower chambers called ventricles.
• The right and left atria are separated by a thin, muscular wall called the interatrial septum, while the left and right ventricles are separated by a thick-walled interventricular septum.
• The atrium and ventricle of the same side are also separated by a thick fibrous tissue called the atrioventricular septum, which has openings that connect the chambers.
• Valves in the heart ensure the one-way flow of blood. The tricuspid valve guards the opening between the right atrium and right ventricle, while the bicuspid or mitral valve guards the opening between the left atrium and left ventricle.
• The openings of the right and left ventricles into the pulmonary artery and the aorta, respectively, are provided with semilunar valves.
• The entire heart is made of cardiac muscles, with the ventricular walls being thicker than the atrial walls.
• Specialized nodal tissue is present in the heart, including the sinoatrial node (SAN) located in the right atrium and the atrioventricular node (AVN) located near the atrioventricular septum in the lower left corner of the right atrium.
• The nodal tissue, along with the atrioventricular bundle (AV bundle) and Purkinje fibers, is responsible for generating action potentials and coordinating the rhythmic contractions of the heart.
• The SAN, also known as the pacemaker, initiates and maintains a regular heartbeat. The normal heart rate in humans is around 70-75 beats per minute.

Cardiac Cycle

• The cardiac cycle consists of systole (contraction) and diastole (relaxation) of both the atria and ventricles.
• At the beginning of the cycle, all four chambers of the heart are in a relaxed state (joint diastole), and the tricuspid and bicuspid valves are open, allowing blood to flow from the atria into the ventricles.
• The sinoatrial node (SAN) generates an action potential, initiating the contraction of both atria (atrial systole), which increases the flow of blood into the ventricles.
• The action potential is conducted to the ventricles through the atrioventricular node (AVN) and AV bundle, causing the ventricular muscles to contract (ventricular systole), while the atria relax (atrial diastole).
• Ventricular systole increases the ventricular pressure, leading to the closure of the tricuspid and bicuspid valves to prevent the backflow of blood into the atria.
• As the ventricular pressure further increases, the semilunar valves guarding the pulmonary artery (right side) and the aorta (left side) are forced open, allowing blood to be ejected from the ventricles into the circulatory pathways.
• Following ventricular systole, the ventricles relax (ventricular diastole), causing the ventricular pressure to fall and the semilunar valves to close, preventing backflow of blood into the ventricles.
• As the ventricular pressure continues to decline, the tricuspid and bicuspid valves open again due to the pressure in the atria, allowing blood to flow into the ventricles, and the cycle repeats.
• Each ventricle pumps out approximately 70 mL of blood during a cardiac cycle, known as the stroke volume.
• The heart rate (number of beats per minute) multiplied by the stroke volume gives the cardiac output, which is the volume of blood pumped out by each ventricle per minute. In a healthy individual, the average cardiac output is around 5 liters.
• The stroke volume, heart rate, and cardiac output can be altered by the body based on its needs. Athletes, for example, may have a higher cardiac output than non-athletes.
• Two prominent sounds, the first heart sound (lub) and the second heart sound (dub), are produced during each cardiac cycle and can be heard through a stethoscope. The lub sound is associated with the closure of the tricuspid and bicuspid valves, while the dub sound is associated with the closure of the semilunar valves.

Electrocardiograph (ECG)

• An ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle.
• ECGs are obtained using an electrocardiograph, a machine that monitors and records voltage traces from the heart.
• A standard ECG is obtained by connecting three electrical leads to the patient (one to each wrist and to the left ankle) to monitor heart activity. Additional leads can be attached to the chest region for a more detailed evaluation.
• Each peak in the ECG is associated with a specific electrical activity of the heart and is represented by a letter from P to T.
• The P-wave corresponds to the electrical excitation (depolarization) of the atria, which leads to their contraction.
• The QRS complex represents the depolarization of the ventricles, initiating ventricular contraction (systole).
• The T-wave represents the repolarization of the ventricles, indicating their return to a normal state and marking the end of systole.
• By counting the number of QRS complexes that occur in a given time period, the heart rate of an individual can be determined.
• ECGs from different individuals with the same lead configuration typically have similar shapes, so any deviation from this shape can indicate a potential abnormality or disease.
• ECGs are of great clinical significance as they provide valuable information about the electrical activity and function of the heart, aiding in the diagnosis and monitoring of various cardiac conditions.

15.4 DOUBLE CIRCULATION

• Arteries and veins are the two main types of blood vessels in the body. They consist of three layers: tunica intima (inner lining), tunica media (middle layer of smooth muscle and elastic fibers), and tunica externa (external layer of fibrous connective tissue).
• Arteries carry oxygenated blood away from the heart, while veins carry deoxygenated blood back to the heart.
• Pulmonary circulation: Deoxygenated blood from the right ventricle is pumped into the pulmonary artery and sent to the lungs for oxygenation. Oxygenated blood returns to the left atrium through the pulmonary veins.
• Systemic circulation: Oxygenated blood from the left ventricle is pumped into the aorta, which branches into a network of arteries, arterioles, and capillaries that supply oxygen and nutrients to the body's tissues. Deoxygenated blood is collected by venules, veins, and the vena cava, and returned to the right atrium.
• Systemic circulation provides nutrients, oxygen, and other essential substances to the tissues and removes carbon dioxide and other waste products.
• The hepatic portal system is a unique vascular connection between the digestive tract and the liver. The hepatic portal vein carries blood from the intestine to the liver before it is distributed to the systemic circulation.
• The coronary system is a specialized network of blood vessels that supply and drain blood exclusively to and from the cardiac muscle.
• The structure and function of blood vessels are essential for maintaining proper circulation, delivering oxygen and nutrients to tissues, and removing waste products.

15.5 REGULATION OF CARDIAC ACTIVITY

• The normal activities of the heart are primarily regulated intrinsically, meaning they are self-regulated by specialized muscles known as nodal tissue. This property makes the heart myogenic.
• The autonomic nervous system (ANS), specifically a neural center in the medulla oblongata, can modulate cardiac function.
• The sympathetic nerves, which are part of the ANS, can increase the rate of heartbeats, the strength of ventricular contraction, and consequently, the cardiac output. This response is often associated with the "fight-or-flight" response, preparing the body for increased physical activity or stress.
• On the other hand, parasympathetic neural signals, another component of the ANS, decrease the heart rate and slow down the conduction of action potentials, resulting in a decrease in cardiac output. This response is often associated with relaxation and a "rest-and-digest" state.
• Hormones released from the adrenal medulla, such as epinephrine (adrenaline) and norepinephrine (noradrenaline), can also increase the cardiac output. These hormones have a similar effect to sympathetic neural signals, increasing heart rate and enhancing ventricular contraction strength.
• The combined effect of intrinsic regulation by nodal tissue, neural signals from the autonomic nervous system, and hormonal influence from the adrenal medulla collectively regulate the heart's activity, ensuring proper function and adaptation to various physiological demands.

15.6 DISORDERS OF THE CIRCULATORY SYSTEM

Hypertension: Hypertension refers to high blood pressure, which is defined as a blood pressure reading consistently at or above 140/90 mm Hg. It is characterized by elevated systolic (pumping) pressure and diastolic (resting) pressure. Hypertension increases the risk of heart disease and can affect vital organs such as the brain and kidneys.

Coronary Artery Disease (CAD): CAD, also known as atherosclerosis, involves the narrowing of the blood vessels that supply blood to the heart muscle. Deposits of calcium, fat, cholesterol, and fibrous tissues accumulate in the arteries, reducing the lumen's diameter. This narrowing restricts blood flow to the heart and can lead to chest pain and other symptoms.

Angina: Angina, also called angina pectoris, manifests as acute chest pain when the heart muscle doesn't receive sufficient oxygen. It can occur in individuals of any age but is more common among middle-aged and elderly individuals. Angina is a symptom of underlying conditions that affect blood flow to the heart.

Heart Failure: Heart failure refers to a condition in which the heart is unable to effectively pump blood to meet the body's needs. It is sometimes referred to as congestive heart failure due to the congestion of the lungs, which is a characteristic symptom. Heart failure should not be confused with cardiac arrest (heart stops beating) or a heart attack (sudden damage to the heart muscle due to inadequate blood supply).