Body Fluids and Circulation: Comprehensive Study Guide

Introduction to Body Fluids and Essential Transport Mechanisms

• All living cells require a continuous provision of nutrients, $O_{2}$, and other essential substances to maintain healthy functioning.

• Waste or harmful substances produced by cellular metabolism must be removed continuously from the tissues.

• Mechanisms for the movement of these substances to and from cells are essential for life.

• Simple organisms, such as sponges and coelenterates, circulate water from their surroundings through their body cavities. This facilitates the exchange of substances directly with their cells.

• Higher and more complex organisms utilize special fluids within their bodies to transport materials.

• Blood is the most commonly used body fluid for transport in higher organisms, including humans.

• Lymph, also known as tissue fluid, serves as another important body fluid that helps in the transport of specific substances.

Composition and Characteristics of Blood

• Blood is classified as a special connective tissue composed of a fluid matrix, plasma, and formed elements.

Plasma

• Plasma is a viscous, straw-coloured fluid that constitutes approximately $55\%$ of the total blood volume.

• Water makes up $90-92\%$ of plasma.

• Proteins contribute $6-8\%$ of the plasma composition. The major proteins include:

  • Fibrinogen: Essential for the clotting or coagulation of blood.

  • Globulins: Primarily involved in the body's defense mechanisms.

  • Albumins: Assist in maintaining osmotic balance.

• Mineral constituents found in plasma include $Na^{+}$, $Ca^{++}$, $Mg^{++}$, $HCO_{3}^{-}$, and $Cl^{-}$.

• Substances in transit within the body, such as glucose, amino acids, and lipids, are also present in the plasma.

• Factors required for blood coagulation are present in the plasma in an inactive form.

• Serum is defined as plasma that does not contain clotting factors.

Formed Elements

• Formed elements constitute nearly $45\%$ of the blood and include erythrocytes, leucocytes, and platelets.

Erythrocytes (Red Blood Cells - RBCs)

• These are the most abundant cells in the blood.

• A healthy adult male has an average of $5$ to $5.5$ million RBCs per $mm^{-3}$ of blood.

• In adults, RBCs are formed in the red bone marrow.

• In most mammals, RBCs are biconcave in shape and lacks a nucleus.

• They contain a red-coloured, iron-containing complex protein called haemoglobin.

• A healthy individual contains $12-16\,g$ of haemoglobin in every $100\,mL$ of blood.

• RBCs play a significant role in the transport of respiratory gases.

• The average lifespan of an RBC is $120\,\text{days}$, after which they are destroyed in the spleen, often referred to as the "graveyard of RBCs."

Leucocytes (White Blood Cells - WBCs)

• Leucocytes are colourless due to the absence of haemoglobin and are nucleated.

• Their number averages between $6000-8000\,mm^{-3}$ of blood, and they are generally short-lived.

• They are classified into two main categories: granulocytes and agranulocytes.

• Granulocytes include:

  • Neutrophils: The most abundant WBCs ($60-65\%$). They are phagocytic cells that destroy foreign organisms.

  • Eosinophils: Constitute $2-3\%$ of WBCs. They resist infections and are associated with allergic reactions.

  • Basophils: The least abundant ($0.5-1\%$). They secrete histamine, serotonin, and heparin and are involved in inflammatory reactions.

• Agranulocytes include:

  • Monocytes: Constitute $6-8\%$ of WBCs. Like neutrophils, these are phagocytic cells.

  • Lymphocytes: Constitute $20-25\%$ of WBCs. They exist in two major forms, 'B' and 'T' forms, both responsible for the body's immune responses.

Platelets (Thrombocytes)

• Platelets are cell fragments produced from megakaryocytes, which are special cells in the bone marrow.

• Normal blood contains between $150,000-350,000$ platelets per $mm^{-3}$.

• They release substances involved in the coagulation or clotting of blood.

• A reduction in platelet count can lead to clotting disorders and excessive blood loss.

ABO and Rh Blood Grouping Systems

ABO Grouping

• This system is based on the presence or absence of two surface antigens on the RBCs: Antigen A and Antigen B. Antigens are chemicals that can induce an immune response.

• Plasma contains natural antibodies, which are proteins produced in response to antigens. These are anti-A and anti-B.

• Blood Group Details:

  • Group A: Has Antigen A on RBCs and anti-B antibodies in plasma. Can receive blood from A and O.

  • Group B: Has Antigen B on RBCs and anti-A antibodies in plasma. Can receive blood from B and O.

  • Group AB: Has both A and B antigens on RBCs and no antibodies in plasma. Can receive blood from AB, A, B, and O. These are "universal recipients."

  • Group O: Has no antigens on RBCs and both anti-A and anti-B antibodies in plasma. Can donate to any group. These are "universal donors."

• Blood transfusion requires careful matching to avoid "clumping" or the destruction of RBCs.

Rh Grouping

• The Rh antigen is similar to one found in Rhesus monkeys and is observed on the surface of RBCs in nearly $80\%$ of humans.

• Individuals with the antigen are Rh positive ($Rh+ve$); those without it are Rh negative ($Rh-ve$).

• An $Rh-ve$ person exposed to $Rh+ve$ blood will develop specific antibodies against Rh antigens.

• Erythroblastosis Foetalis:

  • A specific case of incompatibility between an $Rh-ve$ pregnant mother and an $Rh+ve$ foetus.

  • During the first pregnancy, the blood of the mother and foetus are separated by the placenta.

  • During the delivery of the first child, maternal blood may be exposed to small amounts of $Rh+ve$ foetal blood, causing the mother to prepare Rh antibodies.

  • In subsequent pregnancies, these antibodies can leak into the foetal blood, destroying foetal RBCs.

  • This can cause severe anaemia, jaundice, or death for the foetus.

  • Prevention: Administering anti-Rh antibodies to the mother immediately after the first delivery.

The Mechanism of Blood Coagulation

• Blood coagulation is a mechanism to prevent excessive blood loss following injury or trauma.

• A clot (coagulum) is a dark reddish-brown scum formed of a network of threads called fibrins.

• Dead and damaged formed elements are trapped in this fibrin network.

• The Coagulation Cascade:

  • Inactive fibrinogens in plasma are converted to fibrins by the enzyme thrombin.

  • Thrombin is formed from the inactive substance prothrombin.

  • The enzyme complex required for this conversion is thrombokinase.

  • Thrombokinase is formed via a series of linked enzymic reactions known as the cascade process involving various plasma factors.

• Trauma stimulates platelets and tissues to release factors that initiate this process.

• Calcium ions ($Ca^{++}$) play a vital role in the clotting mechanism.

Lymph (Tissue Fluid) and the Lymphatic System

• As blood passes through tissue capillaries, water and small water-soluble substances move into the spaces between cells, leaving larger proteins and formed elements behind.

• This fluid is called interstitial fluid or tissue fluid. It has a mineral distribution identical to plasma.

• Nutrients and gases are exchanged between blood and cells through this fluid.

• The lymphatic system is an elaborate network of vessels that collects this fluid and drains it back into major veins.

• Lymph is a colourless fluid within this system containing specialized lymphocytes for immune responses.

• Lymph acts as a carrier for nutrients and hormones. Fats are absorbed through lymph in specialized structures called lacteals located in the intestinal villi.

Comparative Anatomy of Circulatory Pathways

Types of Circulatory Systems

• Open Circulatory System: Found in arthropods and molluscs. Blood is pumped by the heart into large vessels that open into body cavities called sinuses.

• Closed Circulatory System: Found in annelids and chordates. Blood is circulated through a closed network of vessels, allowing for more precise regulation of fluid flow.

Vertebrate Heart Structures

• Fishes: Have a 2-chambered heart ($1$ atrium, $1$ ventricle). It performs single circulation where the heart pumps deoxygenated blood to the gills for oxygenation before it travels to the body.

• Amphibians and Reptiles (except crocodiles): Have a 3-chambered heart ($2$ atria, $1$ ventricle). Oxygenated blood from the respiratory organs and deoxygenated blood from the body mix in the single ventricle (incomplete double circulation).

• Crocodiles, Birds, and Mammals: Have a 4-chambered heart ($2$ atria, $2$ ventricles). Oxygenated and deoxygenated blood remain separate in two distinct pathways (double circulation).

Human Circulatory System Anatomy

• The human circulatory system (blood vascular system) consists of a muscular chambered heart, closed branching blood vessels, and blood.

• The heart is a mesodermally derived organ located in the thoracic cavity between the lungs, slightly tilted to the left.

• It is roughly the size of a clenched fist and protected by a double-walled membranous bag called the pericardium, which contains pericardial fluid.

Internal Chambers and Septa

• Atria: Two relatively small upper chambers.

• Ventricles: Two larger lower chambers.

• Inter-atrial Septum: A thin muscular wall separating the right and left atria.

• Inter-ventricular Septum: A thick wall separating the left and right ventricles.

• Atrio-ventricular Septum: Thick fibrous tissue separating the atrium and ventricle on the same side, containing openings for blood flow.

Heart Valves

• Tricuspid Valve: Consists of three muscular flaps/cusps; guards the opening between the right atrium and right ventricle.

• Bicuspid (Mitral) Valve: Guards the opening between the left atrium and left ventricle.

• Semilunar Valves: Guard the openings of the right ventricle into the pulmonary artery and the left ventricle into the aorta.

• Function: Valves ensure unidirectional blood flow (atria to ventricles, ventricles to arteries) and prevent backflow.

Cardiac Musculature and Nodal Tissue Activity

• The heart is composed entirely of cardiac muscles. Ventricular walls are much thicker than atrial walls.

• Nodal Tissue: Specialized cardiac musculature that is autoexcitable (generates action potentials without external stimuli).

• Sino-atrial Node (SAN): Located in the right upper corner of the right atrium. It generates the maximum number of action potentials ($70-75\,min^{-1}$) and is the pacemaker of the heart.

• Atrio-ventricular Node (AVN): Located in the lower left corner of the right atrium near the atrio-ventricular septum.

• AV Bundle and Purkinje Fibres: The AV bundle continues from the AVN, passes through the interventricular septum, and divides into right and left branches. These give rise to Purkinje fibres throughout the ventricular musculature.

• Average Heart Rate: $72\,\text{beats}\,min^{-1}$.

The Cardiac Cycle and Heart Sounds

• The cardiac cycle is the sequential event in the heart that is cyclically repeated, consisting of systole (contraction) and diastole (relaxation).

• Joint Diastole: All four chambers are relaxed. Tricuspid and bicuspid valves are open. Blood flows from pulmonary veins/vena cava into the ventricles. Semilunar valves are closed.

• Atrial Systole: The SAN generates an action potential, causing both atria to contract. This increases ventricular filling by about $30\%$.

• Ventricular Systole: The action potential reaches the ventricles via the AVN and Bundle of His. Ventricles contract while atria relax. This closes the tricuspid and bicuspid valves.

• Ejection: Increased ventricular pressure forces open the semilunar valves, and blood flows into the pulmonary artery and aorta.

• Ventricular Diastole: Ventricles relax, and pressure falls, closing the semilunar valves to prevent backflow. Pressure from blood in the atria eventually forces the tricuspid and bicuspid valves open again.

Cardiac Output and Volume

• Cycle Duration: Calculation based on $72\,\text{beats}/min$ results in $0.8\,\text{seconds}$ per cycle.

• Stroke Volume: Approximately $70\,mL$ of blood pumped by each ventricle per beat.

• Cardiac Output: The volume of blood pumped by each ventricle per minute.

• Formula: $\text{Cardiac Output} = \text{Stroke Volume} \times \text{Heart Rate}$.

• Average Cardiac Output: Approximately $5000\,mL$ or $5\,litres$ per minute.

• Athletes typically have higher cardiac outputs than ordinary individuals.

Heart Sounds

• Lub: The first sound, associated with the closure of tricuspid and bicuspid valves.

• Dub: The second sound, associated with the closure of semilunar valves.

Principles of Electrocardiography (ECG)

• An electrocardiograph is a machine used to obtain an electrocardiogram (ECG), which is a graphical representation of the heart's electrical activity.

• Standard ECG Lead Placement: Three electrical leads (one for each wrist and one for the left ankle).

• ECG Waves:

  • P-wave: Represents the electrical excitation (depolarisation) of the atria, leading to atrial contraction.

  • QRS Complex: Represents the depolarisation of the ventricles, initiating ventricular contraction (start of systole).

  • T-wave: Represents the return of the ventricles to the normal state (repolarisation); the end of the T-wave marks the end of systole.

• Clinical Significance: Deviations in the shape of the ECG indicate potential abnormalities or disease.

Double Circulation and Vascular Pathways

• Arteries and veins have three layers:

  • Tunica Intima: Inner lining of squamous endothelium.

  • Tunica Media: Middle layer of smooth muscle and elastic fibres (thinner in veins).

  • Tunica Externa: External layer of fibrous connective tissue with collagen.

Circulation Pathways

• Pulmonary Circulation: Deoxygenated blood from the right ventricle $\rightarrow$ pulmonary artery $\rightarrow$ lungs $\rightarrow$ pulmonary veins $\rightarrow$ left atrium.

• Systemic Circulation: Oxygenated blood from the left ventricle $\rightarrow$ aorta $\rightarrow$ arteries/capillaries $\rightarrow$ tissues $\rightarrow$ veins/vena cava $\rightarrow$ right atrium.

• Hepatic Portal System: A unique connection where the hepatic portal vein carries blood from the intestine to the liver before it enters systemic circulation.

• Coronary System: A dedicated system of vessels for circulating blood specifically to and from the cardiac musculature.

Regulation of Cardiac Activity

• The heart is myogenic, meaning its normal activities are regulated intrinsically by the nodal tissue.

• Neural Regulation: Located in the medulla oblongata, the autonomic nervous system (ANS) moderates function.

  • Sympathetic Nerves: Increase heart rate, strength of ventricular contraction, and cardiac output.

  • Parasympathetic Nerves: Decrease heart rate, conduction speed, and cardiac output.

• Hormonal Regulation: Adrenal medullary hormones can increase cardiac output.

Disorders of the Circulatory System

• Hypertension (High Blood Pressure): Blood pressure higher than the normal $120/80\,mmHg$ ($120$ is systolic, $80$ is diastolic). A reading of $140/90\,mmHg$ or higher indicates hypertension. It can damage the brain and kidneys.

• Coronary Artery Disease (CAD): Also called atherosclerosis. Caused by deposits of calcium, fat, cholesterol, and fibrous tissues that narrow the lumen of the arteries supplying the heart muscle.

• Angina: Also called angina pectoris. Characterized by acute chest pain when insufficient oxygen reaches the heart muscle. It is more common in middle-aged and elderly individuals.

• Heart Failure: The state where the heart does not pump blood enough to meet the body's needs. Often called congestive heart failure due to lung congestion. It differs from cardiac arrest (heart stops) and heart attack (muscle damage).