Comprehensive Study Notes on Body Fluids and Circulation

Essential Mechanisms of Substance Transport

• All living cells require a continuous supply of nutrients, $O_2$, and other essential substances to function correctly.

• Waste or harmful substances produced by cellular metabolism must be removed continuously to ensure the healthy functioning of tissues.

• Transport mechanisms vary across the animal kingdom:

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

  • Complex organisms utilize specialized internal fluids for transport.

  • Blood is the most common body fluid used by higher organisms, including humans, for this purpose.

  • Lymph (tissue fluid) is another fluid that assists in the transport of specific substances.

Composition and Properties of Blood

Blood is a specialized connective tissue consisting of a fluid matrix called plasma and various formed elements.

Plasma

• Plasma is a straw-colored, viscous fluid that makes up nearly $55\%$ of the total blood volume.

• It is composed of $90-92\%$ water and $6-8\%$ proteins.

• Major plasma proteins include:

  • Fibrinogens: Formed in the liver and essential for the clotting or coagulation of blood.

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

  • Albumins: Assist in maintaining the osmotic balance of the blood.

• Plasma contains small amounts of minerals such as $Na^+$, $Ca^{++}$, $Mg^{++}$, $HCO_3^-$, and $Cl^-$.

• Nutrients like glucose, amino acids, and lipids are present in plasma as they are constantly in transit throughout the body.

• Coagulation factors are present in 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)

• RBCs are the most abundant cells in the blood.

• A healthy adult male averages $5 \text{ millions to } 5.5 \text{ millions}$ of RBCs per $mm^{-3}$ of blood.

• They are produced in the red bone marrow in adults.

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

• They contain a red-colored, iron-containing complex protein known as haemoglobin.

• A healthy individual typically has $12-16\,g$ of haemoglobin in every $100\,ml$ of blood.

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

• The average life span 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 colorless due to the absence of haemoglobin. They are nucleated and fewer in number, averaging $6000-8000\,mm^{-3}$ of blood.

• They are generally short-lived and categorized into two main groups:

  • Granulocytes: Includes Neutrophils, Eosinophils, and Basophils.

  • Agranulocytes: Includes Lymphocytes and Monocytes.

• Distribution and functionality of leucocytes:

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

  • Monocytes: Phagocytic cells making up $6-8\%$ of WBCs.

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

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

  • Lymphocytes: Comprise $20-25\%$ of WBCs. They exist in two 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 $1,500,00 \text{ and } 3,500,00$ platelets per $mm^{-3}$.

• They release substances involved in blood coagulation. A reduction in platelet count can lead to clotting disorders and excessive blood loss.

Blood Grouping Systems

ABO Grouping

• This system is based on the presence or absence of two surface antigens on RBCs: Antigen A and Antigen B.

• Plasma contains natural antibodies: anti-A and anti-B.

• Compatibility and distribution:

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

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

  • Group AB: Has both Antigens A and B on RBCs and no antibodies in plasma. Known as the Universal Recipient, as they can receive blood from AB, A, B, and O.

  • Group O: Has neither A nor B antigens on RBCs and both anti-A and anti-B antibodies in plasma. Known as the Universal Donor.

• Matching donor and recipient blood is critical during transfusion to avoid RBC destruction (clumping).

Rh Grouping

• Roughly $80\%$ of humans possess the Rh antigen on the surface of their RBCs, similar to that found in Rhesus monkeys. These individuals are Rh positive ($Rh^{+ve}$); those lacking it are Rh negative ($Rh^{-ve}$).

• Exposure of an $Rh^{-ve}$ person to $Rh^{+ve}$ blood induces the formation of specific antibodies against Rh antigens.

• Erythroblastosis Foetalis:

  • Occurs when an $Rh^{-ve}$ mother carries an $Rh^{+ve}$ foetus.

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

  • In subsequent pregnancies, these antibodies can leak across the placenta and destroy the RBCs of an $Rh^{+ve}$ foetus.

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

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

Coagulation of Blood

• Blood clotting is a protective mechanism against excessive blood loss following injury.

• A clot (coagulum) is a network of fibrin threads trapping dead and damaged formed elements.

• The Clotting Cascade:

  1. Inactive Prothrombin in plasma is converted to active Thrombin by the enzyme complex thrombokinase.

  2. Inactive Fibrinogen in plasma is converted to active Fibrin by Thrombin.

  3. Thrombokinase is formed via a series of linked enzymic reactions known as the cascade process, involving factors present in the plasma.

• Injury stimulates platelets and tissues to release coagulation-initiating factors.

• Calcium ions ($Ca^{++}$) are essential for the clotting process.

Lymph (Tissue Fluid)

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

• This fluid is called interstitial fluid or tissue fluid. It shares the same mineral distribution as plasma.

• It serves as the medium for the exchange of nutrients and gases between blood and cells.

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

• Lymph is a colorless fluid containing specialized lymphocytes and acts as a carrier for nutrients and hormones.

• Fats are absorbed via lymph in the lacteals within the intestinal villi.

Circulatory Pathways

Open vs. Closed Systems

• Open Circulatory System: Found in arthropods and molluscs. The heart pumps blood through large vessels 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 flow.

Vertebrate Heart Evolution

• Fishes: Two-chambered heart ($1\text{ atrium, } 1\text{ ventricle}$). Single circulation: Heart pumps deoxygenated blood to gills for oxygenation, which then goes to the body and returns as deoxygenated blood.

• Amphibians and Reptiles (except crocodiles): Three-chambered heart ($2\text{ atria, } 1\text{ ventricle}$). Incomplete double circulation: Oxygenated and deoxygenated blood mix in the single ventricle.

• Crocodiles, Birds, and Mammals: Four-chambered heart ($2\text{ atria, } 2\text{ ventricles}$). Double circulation: Oxygenated and deoxygenated blood remain completely separate.

The Human Circulatory System

Heart Anatomy

• The heart is a mesodermally derived organ located in the thoracic cavity between the lungs, tilted slightly to the left. It is roughly the size of a clenched fist.

• Protection: Enclosed in a double-walled membranous bag called the pericardium, which contains pericardial fluid.

• Chambers and Septa:

  • Two upper atria and two lower ventricles.

  • Inter-atrial septum: Thin muscular wall separating the right and left atria.

  • Inter-ventricular septum: Thick-walled structure separating the left and right ventricles.

  • Atrio-ventricular septum: Thick fibrous tissue separating the atrium and ventricle of the same side, containing an opening.

• Valves:

  • Tricuspid valve: Three muscular flaps guarding the opening between the right atrium and right ventricle.

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

  • Semilunar valves: Found at the openings of the right ventricle into the pulmonary artery and the left ventricle into the aorta.

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

Cardiac Musculature and Nodal Tissue

• The heart is composed of cardiac muscles; ventricular walls are significantly thicker than atrial walls.

• Nodal Tissue (Specialized Musculature):

  • Sino-atrial node (SAN): Located in the right upper corner of the right atrium. Known as the pacemaker because it generates the maximum frequency of action potentials ($70-75\,min^{-1}$), maintaining the heart's rhythmic activity.

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

  • AV Bundle: Continues from the AVN, passes through the atrio-ventricular septa, and divides into right and left bundles at the inter-ventricular septum.

  • Purkinje fibres: Minute fibres arising from the bundle branches throughout the ventricular musculature.

• The heart is myogenic (autoexcitable), meaning it generates action potentials without external stimuli. The average heart rate is $72\text{ beats } min^{-1}$.

The Cardiac Cycle

• The cardiac cycle is a sequential, cyclical event consisting of systole (contraction) and diastole (relaxation).

• Phases of the cycle:

  1. Joint Diastole: All four chambers are relaxed. Tricuspid and bicuspid valves are open; semilunar valves are closed. Blood flows from veins into ventricles.

  2. Atrial Systole: SAN generates an action potential. Both atria contract, increasing ventricular blood flow by about $30\%$.

  3. Ventricular Systole: Action potential reaches the ventricles via the AVN and Bundle of His. Ventricles contract; atria relax. Increased pressure closes tricuspid and bicuspid valves (producing the first heart sound, "lub").

  4. Ejection: Semilunar valves open, and blood is pumped into the pulmonary artery and aorta.

  5. Ventricular Diastole: Ventricles relax, and pressure falls. Semilunar valves close to prevent backflow (producing the second heart sound, "dub").

• Duration: Based on $72\text{ beats per minute}$, one cycle lasts $0.8\text{ seconds}$.

• Stroke Volume: Each ventricle pumps out approximately $70\,ml$ of blood per cycle.

• Cardiac Output: Total blood pumped by each ventricle per minute. $\text{Cardiac Output} = \text{Stroke Volume} \times \text{Heart Rate} \approx 5000\,ml \text{ or } 5\,litres$.

Electrocardiogram (ECG)

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

• Standardization: Three electrical leads are attached (one to each wrist and one to the left ankle).

• Waves of the ECG:

  • P-wave: Represents the depolarization (excitation) of the atria.

  • QRS complex: Represents the depolarization of the ventricles, initiating ventricular contraction.

  • T-wave: Represents the repolarization (return to normal) of the ventricles. The end of the T-wave marks the end of systole.

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

Double Circulation and Blood Vessels

• Blood vessels consist of 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 fibres.

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

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

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

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

Regulation of Cardiac Activity

• Cardiac function is primarily auto-regulated (myogenic) by nodal tissues.

• The medulla oblongata modulates heart function via the Autonomic Nervous System (ANS):

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

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

• Adrenal medullary hormones also increase cardiac output.

Disorders of the Circulatory System

• Hypertension (High Blood Pressure): Defined as blood pressure consistently at or above $140/90\,mm\,Hg$. Normal is $120/80\,mm\,Hg$ ($120$ is systolic; $80$ is diastolic). It can damage the brain and kidneys.

• Coronary Artery Disease (CAD/Atherosclerosis): Deposits of calcium, fat, cholesterol, and fibrous tissue make the lumen of coronary arteries narrower.

• Angina (Angina Pectoris): Acute chest pain resulting from insufficient oxygen reaching the heart muscle. More common in middle-aged and elderly individuals.

• Heart Failure: The heart cannot pump blood effectively to meet bodily needs. Sometimes called congestive heart failure due to lung congestion.

  • Distinctions: Heart failure is different from cardiac arrest (heart stops beating) or heart attack (muscle damage due to inadequate blood supply).