Body Fluids and Circulation

• All living cells need nutrients and waste removal for healthy tissue function
• Efficient mechanisms are essential for substance movement to and from cells
• Animals have evolved different transport methods
• Sponges and coelenterates circulate water for substance exchange
• Complex organisms use specialized fluids for transport
• Blood is the most common body fluid in higher organisms, including humans
• Lymph also aids substance transport

Blood

• Specialized connective tissue with fluid matrix (plasma) and formed elements

Plasma

• Straw-colored, viscous fluid; about 55% of blood volume
• 90-92% water
• 6-8% proteins, including fibrinogen, globulins, and albumins
• Fibrinogens are needed for clotting/coagulation
• Globulins are involved in defense mechanisms
• Albumins help in osmotic balance
• Contains small amounts of minerals (Na+, Ca++, Mg++, HCO3, Cl-)
• Glucose, amino acids, lipids are present in plasma
• Coagulation factors are present
• Serum is plasma without clotting factors

Formed elements

• Erythrocytes, leucocytes, and platelets
• Constitute about 45% of blood

Erythrocytes

• Red blood cells (RBCs)
• Most abundant blood cells
• 5-5.5 million RBCs per mm3 of blood in healthy adults
• Formed in red bone marrow in adults
• Devoid of nucleus
• Biconcave shape
• Contain hemoglobin (red-colored, iron-containing protein)
• 12-16 gms of hemoglobin/100 ml of blood
• Transports respiratory gases
• 120 day lifespan
• Destroyed in the spleen (graveyard of RBCs)

Leucocytes

• White blood cells (WBCs)
• Colorless (lack hemoglobin)
• Nucleated
• 6000-8000 per mm-3 of blood
• Short-lived
• Two categories of WBCs: granulocytes and agranulocytes
• Granulocytes: neutrophils, eosinophils, and basophils
• Agranulocytes: lymphocytes and monocytes
• Neutrophils: most abundant WBCs (60-65%)
• Basophils: least abundant (0.5-1%)
• Monocytes (6-8%): Phagocytic that destroy foreign organisms
• Basophils: secrete histamine, serotonin, heparin, involved in inflammatory reactions
• Eosinophils (2-3%): resist infections and allergic reactions
• Lymphocytes (20-25%): B and T forms, for immune responses

Platelets

• Thrombocytes
• Cell fragments from megakaryocytes in bone marrow
• 150,000-350,000 platelets per mm-3
• Release substances for coagulation/clotting
• Reduction in number leads to clotting disorders and excessive blood loss

Blood Groups

• Blood differs among humans
• ABO and Rh groupings are widely used

ABO Grouping

• Based on presence/absence of A and B surface antigens on RBCs
• Plasma contains natural antibodies against antigens
• Distribution of antigens/antibodies in A, B, AB, and O groups

Blood groups and Donor Compatibility

• Type A: A antigens, anti-B antibodies, can receive A, O
• Type B: B antigens, anti-A antibodies, can receive B, O
• Type AB: A and B antigens, no antibodies, can receive all
• Type O: no antigens, anti-A and anti-B antibodies, can receive O
• Group O: 'universal donors'
• Group AB: 'universal recipients'

Rh Grouping

• Rh antigen (similar to rhesus monkeys) on RBCs: Rh positive (Rh+ve)
• Absence of Rh antigen: Rh negative (Rh-ve)
• Rh-ve person exposed to Rh+ve blood forms antibodies
• Rh group should be matched before transfusions
• Rh incompatibility: Rh-ve mother with Rh+ve fetus.
• First pregnancy: no issues due to placental separation
• Delivery of first child: maternal blood exposure to fetal Rh+ve blood
• Subsequent pregnancies: maternal Rh antibodies attack fetal RBCs
• Causes erythroblastosis foetalis (fatal to fetus)
• Prevented by administering anti-Rh antibodies to the mother after the first delivery

Coagulation of Blood

• Blood clots to prevent excessive loss after injury or trauma
• Dark reddish brown scum (clot/coagulum) formed at injury site
• Clot: network of fibrins with dead and damaged blood elements
• Fibrins formed by conversion of fibrinogens (inactive) by thrombin
• Thrombins: formed from prothrombin (inactive)
• Thrombokinase (enzyme complex): required for the above reaction
• Cascade process: series of linked enzymic reactions
• Injury/trauma: platelets release factors to activate coagulation
• Tissue factors at injury site initiate coagulation
• Calcium ions are very important in clotting

Lymph

• As blood flows through capillaries in tissues, water and small water-soluble substances move out of spaces
• Larger proteins most of the formed elements remain in the blood vessels
• Fluid released is interstitial fluid or tissue fluid
• Mineral distribution similar to plasma
• Nutrients, gases exchanged between blood and cells through this fluid
• Lymphatic system: network of vessels that collect this fluid back to major veins
• Lymph: fluid present in the lymphatic system
• Colorless fluid with specialized lymphocytes (immune responses)
• Important carrier for hormones, and nutrients
• Fats are absorbed through lymph in lacteals of intestinal villi

Circulatory pathways

• Two types: open and closed

Open circulatory system

• Blood pumped passes through large vessels into open spaces/body cavities
• Present in arthropods and molluscs

Closed circulatory system

• Blood pumped by Heart circulated through a network of vessels
• Present in annelids and chordates
• More advantageous as a fluid flow can be regulated

Vertebrate Hearts

• Possess a muscular chambered heart
• Fishes: 2-chambered (atrium and ventricle)
• Amphibians and reptiles (except crocodiles): 3-chambered (two atria, single ventricle)
• Crocodiles, birds, and mammals: 4-chambered (two atria, two ventricles)
• Fishes: heart pumps out deoxygenated blood, oxygenated by gills, returned to heart (single circulation)
• Amphibians and reptiles: left atrium - oxygenated blood, right atrium - deoxygenated blood; mix in single ventricle (incomplete double circulation)
• Birds and mammals: oxygenated and deoxygenated blood in left and right atria respectively, ventricles pump without mixing (double circulation)

Human Circulatory System

• Consists of muscular chambered heart, blood vessels, and blood
• Heart: mesodermally derived, in thoracic cavity, between lungs, tilted left, size of fist
• Protected by pericardium (double-walled membranous bag with pericardial fluid)
• Four chambers: two upper atria, two lower ventricles
• Inter-atrial septum: thin, muscular wall separating atria
• Inter-ventricular septum: thick-walled, separates ventricles
• Atrio-ventricular septum: separates atrium and ventricle on each side

Valves

• Atrio-ventricular septum has openings between same-side chambers
• Tricuspid valve: guards opening between right atrium and right ventricle (three flaps/cusps)
• Bicuspid/mitral valve: guards opening between left atrium and left ventricle
• Pulmonary artery and aorta have semilunar valves

Heart Valves

• Allow blood flow in one direction only: atria to ventricles, ventricles to pulmonary artery/aorta
• Prevent backward flow
• Heart is made of cardiac muscles
• Ventricle walls are thicker than atria walls
• Nodal tissue: specialized cardiac musculature in the heart
• Sino-atrial node (SAN): patch in the right upper corner of the right atrium
• Atrio-ventricular node (AVN): mass in lower left corner of right atrium near the atrio-ventricular septum
• Atrio-ventricular bundle (AV bundle): nodal fibers from AVN, passes through atrio-ventricular septa, divides into right/left bundles
• Purkinje fibers: minute fibers from bundles throughout ventricular musculature
• Autorhythmic: Nodal musculature generates action potentials without external stimuli
• SAN generates maximum action potentials (70-75/min), initiates/maintains rhythmic activity
• Pacemaker: SAN
• Heart beats 70-75 times per minute (average 72 beats min-1)

Cardiac cycle

• Four chambers in relaxed state (joint diastole)
• Tricuspid and bicuspid valves are open, blood flows from pulmonary veins/vena cava into ventricles through atria
• Semilunar valves are closed
• SAN generates action potential, stimulates simultaneous atrial contraction (atrial systole)
• Increases blood flow into ventricles by ~30%
• Action potential is conducted to ventricular contraction
• Ventricles contract (ventricular systole), atria relax (diastole)
• Ventricular pressure increases, closing tricuspid/bicuspid valves
• Semilunar valves forced open as ventricular pressure increases further
• Blood flows from ventricles into pulmonary artery (right) and aorta
• Ventricles relax (ventricular diastole), ventricular pressure falls, semilunar valves close to prevent backflow
• Ventricular pressure declines, tricuspid/bicuspid valves open due to atrial pressure
• Blood flows freely into the ventricles
• Ventricles and atria are relaxed again (joint diastole)
• SAN generates new action potential and the cycle repeats

Terms

• Repeated sequential events in the heart: cardiac cycle
• Cardiac cycle consists of systole and diastole of both atria and ventricles
• Heart beats 72 times per minute (72 cardiac cycles/minute)
• Cardiac cycle duration: 0.8 seconds
• Stroke volume: 70 mL of blood pumped out by each ventricle per cardiac cycle
• Cardiac output stroke volume multiplied by heart rate (5000 mL or 5 liters in healthy individual)
• Body can alter stroke volume and heart rate to change cardiac output
• Athlete: higher cardiac output than ordinary person
• Lub (first heart sound): closure of the tricuspid and bicuspid valves.
• Dub (second heart sound): closure of the semilunar valves
• Heart sounds are clinically significant

Electrocardiograph (ECG)

• Machine to obtain an electrocardiogram (ECG)
• ECG: graphical representation of electrical activity of the heart during cardiac cycle

ECG waves and complexes

• Patient connected to machine with three electrical leads
• P wave: electrical excitation (depolarization) of the atria (contraction)
• QRS complex: depolarization of ventricles (ventricular contraction)
• Starts shortly after the Q wave, marks the systole beginning
• T wave: return of ventricles from excited to normal state (repolarization)
• End of the T wave marks end of systole
• Counting QRS complexes determines heart beat rate

Abnormality

• ECG deviations from normal shape indicates a disease condition
• Deviation has great clinical significance

Double Circulation

• Blood flows through blood vessels (arteries and veins)
• Artery and vein structure is 3 layers
• Tunica intima: inner lining of squamous endothelium
• Tunica media: middle layer of smooth muscle and elastic fibers
• Tunica externa: external layer of fibrous connective tissue with collagen
• Tunica media is thinner in veins

Pulmonary circulation: pulmonary artery carries deoxygenated blood from right ventricle to lungs, pulmonary veins carry oxygenated blood to left atrium

Systemic circulation: blood flows through the aorta

• Network of arteries, arterioles, capillaries to the tissues
• Deoxygenated blood passes through a system of venules, veins, and vena cava into right atrium
• Systemic circulation: provides O2 nutrients, eliminates harmful wastes
• Hepatic portal system: blood from intestine goes to the liver before systemic

Coronary System

• Exists for the circulation of blood to and from the cardiac musculature

Regulation of Cardiac Activity

• Activities of the heart are regulated intrinsically and are autorhythmic
• The heart is myogenic
• Neural center in the medulla oblongata moderates cardiac function through ANS
• Sympathetic nerves increase heart rate and cardiac output
• Parasympathetic nerves decrease heart rate and cardiac output
• Adrenal medullary hormones increases cardiac output

Disorders of the circulatory System

• Hypertension: high blood pressure (higher than 120/80)
• 120 mm Hg: systolic
• 80 mm Hg: diastolic
• 140/90 or higher: hypertension
• High blood pressure leads to heart disease and affects brain and kidney

Coronary artery Disease

• Atherosclerosis
• Affects vessels supplying blood to the heart muscle
• Caused by fat, cholesterol, fibrous tissues deposits
• Narrower lumen to arteries

Angina Pectoris

• Symptom of chest pain when not enough oxygen reaches the heart muscle
• Common in middle-aged and elderly
• Occurs due to conditions that impair blood flow

Heart Failure

• Heart is not pumping blood effectively to meet the needs of Body
• Congestive heart failure: lung congestion
• Heart failure is not a cardiac arrest
• Heart failure is not a heart attack