Comprehensive Notes on Body Fluids and Circulation
Body Fluids and Circulation Notes
Introduction
All living cells need nutrients, oxygen, and essential substances.
Waste and harmful substances must be removed for healthy tissue function.
Efficient mechanisms are required for substance transport to and from cells.
Different animals have evolved different transport methods.
Sponges and coelenterates circulate water for substance exchange.
Complex organisms use special body fluids for transport.
Blood is the most common body fluid in higher organisms, including humans.
Lymph also aids in the transport of certain substances.
This chapter covers the composition and properties of blood and lymph, as well as the mechanism of blood circulation.
Blood
Blood is a special connective tissue with a fluid matrix (plasma) and "formed elements."
Plasma
Plasma is a straw-colored, viscous fluid comprising about 55% of blood.
Composition:
90-92% water
6-8% proteins (fibrinogen, globulins, albumins)
Fibrinogen: Needed for clotting/coagulation.
Globulins: Primarily involved in defense mechanisms.
Albumins: Help maintain osmotic balance.
Plasma contains minerals like , , , , .
Glucose, amino acids, and lipids are present in transit.
Coagulation factors are present in an inactive form.
Plasma without clotting factors is called serum.
Formed Elements
Erythrocytes, leucocytes, and platelets constitute nearly 45% of the blood.
Erythrocytes (Red Blood Cells - RBCs):
Most abundant blood cells.
A healthy adult man has 5-5.5 million RBCs per of blood.
Formed in the red bone marrow in adults.
Devoid of a nucleus in most mammals; biconcave shape.
Contain hemoglobin (iron-containing complex protein) for respiratory gas transport.
A healthy individual has 12-16 gms of hemoglobin in every 100 ml of blood.
Life span of 120 days; destroyed in the spleen (graveyard of RBCs).
Leucocytes (White Blood Cells - WBCs):
Colorless due to lack of hemoglobin; nucleated.
Fewer in number: 6000-8000 per of blood.
Generally short-lived.
Two main categories: granulocytes and agranulocytes.
Granulocytes: Neutrophils, eosinophils, and basophils.
Agranulocytes: Lymphocytes and monocytes.
Neutrophils:
Most abundant WBCs (60-65%).
Phagocytic; destroy foreign organisms.
Basophils:
Least abundant WBCs (0.5-1%).
Secrete histamine, serotonin, heparin; involved in inflammatory reactions.
Eosinophils:
2-3% of WBCs.
Resist infections; associated with allergic reactions.
Lymphocytes:
20-25% of WBCs.
Two major types: B and T lymphocytes.
Responsible for immune responses.
Monocytes:
6-8% of WBCs.
Phagocytic; destroy foreign organisms.
Platelets (Thrombocytes):
Cell fragments from megakaryocytes (bone marrow).
Normal count: 150,000 - 350,000 per .
Release substances involved in coagulation/clotting.
Reduction in number leads to clotting disorders and excessive blood loss.
Blood Groups
Human blood differs in certain aspects.
Two main groupings: ABO and Rh.
ABO Grouping
Based on the presence or absence of A and B surface antigens on RBCs.
Plasma contains natural antibodies (anti-A, anti-B).
Four blood groups: A, B, AB, and O.
Blood transfusion requires matching donor and recipient blood to avoid clumping (RBC destruction).
Group O:
Universal donors; can donate to any blood group.
Group AB:
Universal recipients; can receive blood from any group.
Blood Group | Antigens on RBCs | Antibodies in Plasma | Donor's Group |
|---|---|---|---|
A | A | anti-B | A, O |
B | B | anti-A | B, O |
AB | A, B | nil | AB, A, B, O |
O | nil | anti-A, B | O |
Rh Grouping
Rh antigen (similar to Rhesus monkeys) present on RBCs of most humans (80%).
Rh positive (Rh+ve): Individuals with the antigen.
Rh negative (Rh-ve): Individuals without the antigen.
Rh-ve person exposed to Rh+ve blood forms specific antibodies.
Rh group must be matched before transfusions.
Rh Incompatibility:
A special case occurs when an Rh-ve pregnant mother carries an Rh+ve fetus.
During the first pregnancy, the mother may get exposed to Rh+ve blood from the foetus during delivery, leading to antibody production.
In subsequent pregnancies, Rh antibodies from the mother (Rh-ve) can leak into the fetus (Rh+ve) and destroy fetal RBCs.
This causes erythroblastosis fetalis: can be fatal or cause severe anemia and jaundice.
Prevention: Administer anti-Rh antibodies to the mother immediately after the first child's delivery.
Coagulation of Blood
Blood clots in response to injury or trauma to prevent excessive blood loss.
A dark reddish-brown scum (clot/coagulum) forms at the injury site.
The clot consists of a network of fibrins trapping dead and damaged formed elements.
Fibrin formation: Inactive fibrinogens are converted to fibrins by thrombin.
Thrombin formation: Formed from inactive prothrombin.
Thrombokinase (enzyme complex) is required for this reaction.
The complex forms through a series of linked enzymatic reactions (cascade process).
Platelets release factors that activate the coagulation mechanism.
Tissue factors at the injury site can also initiate coagulation.
Calcium ions play a vital role in clotting.
Lymph (Tissue Fluid)
As blood passes through capillaries, water and small water-soluble substances move into the spaces between tissue cells.
Larger proteins and most formed elements remain in the blood vessels.
The released fluid is called interstitial fluid or tissue fluid.
It has the same mineral distribution as plasma.
Nutrient and gas exchange between blood and cells occurs through this fluid.
The lymphatic system collects this fluid and drains it back to major veins.
Lymph: Fluid present in the lymphatic system.
Colorless fluid containing specialized lymphocytes (immune responses).
Important carrier for nutrients and hormones.
Fats are absorbed through lymph in the lacteals present in the intestinal villi.
Circulatory Pathways
Two types: open and closed.
Open Circulatory System:
Present in arthropods and mollusks.
Blood pumped by the heart passes through large vessels into open spaces/body cavities called sinuses.
Closed Circulatory System:
Present in annelids and chordates.
Blood pumped by the heart circulates through a closed network of blood vessels.
More advantageous: allows for more precise regulation of fluid flow.
Vertebrate Hearts:
All vertebrates have a muscular, chambered heart.
Fishes: 2-chambered heart (atrium and ventricle).
Amphibians and Reptiles (except crocodiles): 3-chambered heart (two atria, single ventricle).
Crocodiles, Birds, and Mammals: 4-chambered heart (two atria, two ventricles).
Single Circulation (Fishes):
The heart pumps out deoxygenated blood, which is oxygenated by the gills and supplied to the body.
Deoxygenated blood returns to the heart.
Incomplete Double Circulation (Amphibians and Reptiles):
The Left atrium receives oxygenated blood from gills/lungs/skin, and the right atrium receives deoxygenated blood from other body parts.
Blood mixes in the single ventricle.
Double Circulation (Birds and Mammals):
Oxygenated and deoxygenated blood received by the left and right atria, respectively.
Blood passes to the ventricles on the same sides.
Ventricles pump blood out without mixing; two separate circulatory pathways.
Human Circulatory System
Also called the blood vascular system.
Consists of:
Muscular, chambered heart
Network of closed, branching blood vessels
Blood (the fluid being circulated)
Heart:
Mesodermally derived organ.
Located in the thoracic cavity, between the two lungs, slightly tilted to the left.
Size of a clenched fist.
Protected by a double-walled membranous bag called the pericardium, enclosing the pericardial fluid.
Four chambers: two relatively small upper chambers (atria) and two larger lower chambers (ventricles).
Septa:
Interatrial septum: separates the right and left atria (thin, muscular wall).
Interventricular septum: separates the left and right ventricles (thick-walled).
Atrioventricular septum: Separates the atrium and ventricle of the same side (thick fibrous tissue).
Valves:
Openings in the septa connect the chambers on the same side.
Tricuspid valve: guards the opening between the right atrium and right ventricle (three muscular flaps/cusps).
Bicuspid/mitral valve: guards the opening between the left atrium and left ventricle.
Semilunar valves: present at the openings of the right and left ventricles into the pulmonary artery and aorta, respectively.
Valves allow blood flow in one direction: from atria to ventricles and from ventricles to pulmonary artery/aorta, preventing backflow.
Cardiac Muscle:
The entire heart is made of cardiac muscles.
Ventricle walls are much thicker than those of the atria.
Nodal Tissue:
Specialized cardiac musculature is distributed in the heart.
Sino-atrial node (SAN): a patch of tissue in the right upper corner of the right atrium.
Atrio-ventricular node (AVN): mass of tissue in the lower-left corner of the right atrium, close to the atrioventricular septum.
Atrio-ventricular bundle (AV bundle): a bundle of nodal fibers continuing from the AVN, passing through the atrioventricular septa, emerging on top of the interventricular septum, and immediately dividing into a right and left bundle.
Purkinje fibers: Minute fibers arising from these branches throughout the ventricular musculature on the respective sides.
Autoexcitable Nature:
The nodal musculature can generate action potentials without external stimuli (autoexcitable).
The number of action potentials generated per minute varies in different parts of the nodal system.
SAN generates the maximum number of action potentials (70-75 per minute) and initiates/maintains the rhythmic contractile activity of the heart; it is called the pacemaker.
Normal heart rate: 70-75 beats per minute (average 72 beats per minute).
Cardiac Cycle
Four chambers of the heart are initially in a relaxed state (joint diastole).
The tricuspid and bicuspid valves are open, allowing blood from the pulmonary veins and vena cava to flow into the left and right ventricles, respectively.
The semilunar valves are closed.
The SAN generates an action potential stimulating both atria to undergo simultaneous contraction (atrial systole).
This increases blood flow into the ventricles by about 30%.
The action potential is conducted to the ventricular side by the AVN and AV bundle, from where the bundle of His transmits it through the entire ventricular musculature.
This causes ventricular muscles to contract (ventricular systole), while the atria undergo relaxation (diastole).
Ventricular systole increases ventricular pressure, causing the closure of tricuspid and bicuspid valves due to attempted backflow of blood into the atria.
As ventricular pressure increases further, the semilunar valves guarding the pulmonary artery (right side) and the aorta (left side) are forced open.
This allows blood in the ventricles to flow through these vessels into the circulatory pathways.
The ventricles now relax (ventricular diastole), and ventricular pressure falls, causing the closure of semilunar valves to prevent backflow of blood into the ventricles.
As ventricular pressure declines further, the tricuspid and bicuspid valves are pushed open by the pressure in the atria exerted by the blood being emptied into them by the veins.
Blood now moves freely to the ventricles once again.
The ventricles and atria are now again in a relaxed (joint diastole) state, as earlier.
Soon, the SAN generates a new action potential, and the events described above are repeated in that sequence; the process continues.
This sequential event in the heart, which is cyclically repeated, is called the cardiac cycle, and it consists of systole and diastole of both the atria and ventricles.
The heart beats 72 times per minute, meaning that many cardiac cycles are performed per minute.
Duration of a cardiac cycle: 0.8 seconds.
During a cardiac cycle, each ventricle pumps out approximately 70 mL of blood (stroke volume).
Cardiac output - The stroke volume multiplied by the heart rate equals the cardiac output.
Cardiac output: volume of blood pumped out by each ventricle per minute, averaging 5000 mL or 5 liters in a healthy individual.
The body can alter the stroke volume and heart rate, thereby changing the cardiac output. For example, the cardiac output of an athlete will be much higher than that of an ordinary man.
2 prominent sounds are produced during each cardiac cycle that can be heard through a stethoscope.
First heart sound (lub): closure of the tricuspid and bicuspid valves.
Second heart sound (dub): closure of the semilunar valves.
These sounds are of clinical diagnostic significance.
Electrocardiogram (ECG)
ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle.
A patient is connected to the machine with three electrical leads (one to each wrist and to the left ankle) that continuously monitor heart activity.
For a detailed evaluation of the heart’s function, multiple leads are attached to the chest region.
Each peak in the ECG is identified with a letter from P to T that corresponds to a specific electrical activity of the heart.
P-wave: Represents the electrical excitation (or depolarization) of the atria, leading to the contraction of both the atria.
QRS complex: Represents the depolarization of the ventricles, which initiates ventricular contraction. The contraction starts shortly after Q and marks the beginning of systole.
T-wave: Represents the return of the ventricles from the excited to normal state (repolarization). The end of the T-wave marks the end of systole.
By counting the number of QRS complexes that occur in a given time period, one can determine the heart-beat rate of an individual.
Any deviation from this shape indicates a possible abnormality or disease, hence it is of great clinical significance.
Double Circulation
Blood flows strictly by a fixed route through blood vessels: arteries and veins.
Each artery and vein consists of three layers:
Tunica intima: Inner lining of squamous endothelium.
Tunica media: Middle layer of smooth muscle and elastic fibers; thinner in veins.
Tunica externa: external layer of fibrous connective tissue with collagen fibers.
Blood pumped by the right ventricle enters the pulmonary artery, whereas the left ventricle pumps blood into the aorta.
Pulmonary Circulation:
Deoxygenated blood is pumped into the pulmonary artery and passed on to the lungs.
Oxygenated blood is brought back by the pulmonary veins into the left atrium.
Systemic Circulation:
Oxygenated blood entering the aorta is carried by a network of arteries, arterioles, and capillaries to the tissues.
Deoxygenated blood is collected by a system of venules, veins, and the vena cava and emptied into the right atrium.
The systemic circulation provides nutrients, , and other essential substances to the tissues and takes and other harmful substances away for elimination.
Hepatic Portal System:
A unique vascular connection exists between the digestive tract and liver.
The hepatic portal vein carries blood from the intestine to the liver before it is delivered to the systemic circulation.
Coronary System:
A special coronary system of blood vessels is present exclusively for the circulation of blood to and from the cardiac musculature.
Regulation of Cardiac Activity
Normal activities of the heart are regulated intrinsically (auto-regulated) by specialized muscles (nodal tissue).
Thus, the heart is called myogenic.
A special neural center in the medulla oblongata can moderate cardiac function through the autonomic nervous system (ANS).
Neural signals through the sympathetic nerves (part of ANS) can increase the rate of the heartbeat, the strength of ventricular contraction, and thereby the cardiac output.
Parasympathetic neural signals (another component of ANS) decrease the heart rate, the speed of conduction of action potential, and thereby the cardiac output.
Adrenal medullary hormones can also increase cardiac output.
Disorders of Circulatory System
High Blood Pressure (Hypertension):
Higher than normal blood pressure (120/80).
120 mm Hg: systolic (pumping) pressure.
80 mm Hg: diastolic (resting) pressure.
Repeated checks of blood pressure of an individual is 140/90 or higher, it shows hypertension.
High blood pressure leads to heart diseases and also affects vital organs like the brain and kidney.
Coronary Artery Disease (CAD):
Often referred to as atherosclerosis.
Affects the vessels that supply blood to the heart muscle.
Caused by deposits of calcium, fat, cholesterol, and fibrous tissues, making the lumen of arteries narrower.
Angina:
Also called ‘angina pectoris’.
A symptom of acute chest pain appears when not enough oxygen is reaching the heart muscle.
Can occur in men and women of any age but is more common among the middle-aged and elderly.
Occurs due to conditions that affect the blood flow.
Heart Failure:
The state of heart when it is not pumping blood effectively enough to meet the needs of the body.
Sometimes called congestive heart failure because congestion of the lungs is one of the main symptoms of this disease.
Heart failure is not the same as cardiac arrest (when the heart stops beating) or a heart attack (when the heart muscle is suddenly damaged by an inadequate blood supply).