Blood and Circulation

Structure of the heart :

·                The heart has two pumps side by side.

·                Each pump has two chambers, so there are 4 chambers altogether.

·                The upper chambers are called atrium and they receive blood.

·                The lower chambers are called ventricles and they pump blood to organs by contraction.

·                Deoxygenated blood flows into the right atrium via the vena cava.

·                It is pumped out to the lungs via the pulmonary artery, where the red blood cells are oxygenated.

·                Oxygenated blood returns to the left atrium via pulmonary vein.

·                Left ventricle has more cardiac muscle to generate high pressure by contracting more forcefully to pump blood all around the body at a long distance.

·                Coronary arteries supply blood, containing oxygen and glucose to cardiac muscle wall.

·                More cardiac muscle is present in the left ventricle than in the right ventricle.

·                Chordae tendinae are cords / tendons that prevent inversion of the atrioventricular valves.

Note : There is more cardiac muscle in the left ventricle to generate high blood pressure and push blood all around the body. Right ventricle pumps blood at lower pressure. Hence, it has less cardiac muscle. Right ventricle pumps blood to the lungs only at low blood pressure as this prevents damage to the lungs.

Valves :

·                There are parachute-like cuspid valves between the atria and the ventricles to prevent backflow of blood to the atria when ventricle contracts.

·                There are semi lunar valves at the base of the aorta and the pulmonary artery that prevent backflow of blood into the ventricles when ventricles relax.

·                Valves have flaps that fill up with blood as pressure in the atria / ventricles rises, pushing the valve shut.

·                They prevent backflow by closing the passageway to blood.

·                Chordae tendinae are cords / tendons that prevent inversion of the atrioventricular valves.

Note : If the semilunar valves did not work properly, some blood from pulmonary artery would return to the heart, causing the heart to exert even more pressure.

Circulation:

·                Blood enters the heart from vena cava via right atrium. Blood in vena cava has raised carbon dioxide levels and lower oxygen levels as carbon dioxide is produced by respiring body cells. Oxygen is removed from blood as it passes through tissues. Blood is at lower pressure in vena cava.

·                When atrium wall contracts, blood is pushed to ventricles via cuspid valves.

·                When ventricle wall contracts, blood is forced to pulmonary artery via semi lunar valves. Blood pressure is raised as the heart pumps.

·                Alveolar air has high concentration of oxygen. So, oxygen diffuses into blood capillaries down concentration gradient. Red blood cells are oxygenated in the lungs as oxygen combines with haemoglobin to form oxyhaemoglobin. Blood passes through lung capillaries. Carbon dioxide concentration is low in alveolar air. Carbon dioxide diffuses out of blood down concentration gradient. Glucose level drops due to respiration in alveoli.

·                Blood returns to the left atrium via pulmonary vein.

Differences between Cardiac Muscle and Skeletal Muscle :

Cardiac Muscle	Skeletal Muscle
Involuntary	Voluntary
Never fatigued	Gets fatigued
Not attached to bones of skeleton	Attached to bones of skeleton
One nucleus per cell	Many nuclei scattered within fibre
Myogenic	Neurogenic

ARTERIES

Structure of Arteries

·                Small lumen

·                Smooth endothelium / Tunica interna

·                A thick tunica media constitutes a muscle layer to contract forcefully enough to push blood on its way

·                The tunica media is elastic to recoil as blood passes through it

·                Tunica externa contains a layer of collagen and is fibrous

·                Rhythmic contractions create pulse

·                This helps maintain high blood pressure

Note : Arteries have no valves as they carry blood at high pressure. Blood is recently pumped by the heart. So, no backflow occurs.

Structure of Arterioles

·                Muscle fibres allow size of lumen to be changed.

·                This allows blood to be diverted from skin capillaries during vasoconstriction.

VEINS

Structure

·                Large lumen

·                Wall is less muscular, thus, thin

·                Less elastic tissue in its walls

·                Valves are present to prevent backflow of blood

·                Blood flows at low pressure

Similarities between Arteries and Veins :

·                Both carry blood between tissues / organs and the heart.

·                Both are tubular.

·                Both have a lumen.

·                Both have tunica media.

·                Both have smooth endothelium.

·                Both have tunica externa

Differences between Veins and Arteries :

Arteries	Veins
No valves	Valves are there
Blood flows away from heart	Blood flows towards heart
Blood flows at high speed	Blood flows at low speed
Blood pressure is high	Blood pressure is low
They are deeper	They are less deep
Pulsatile flow occurs	No pulsatile flow occurs
Oxygenated blood (except in umbilical and pulmonary arteries)	Deoxygenated blood (except in umbilical and pulmonary veins)

Note : Arterial blood has higher concentration of oxygen than venous blood as oxygen is used by respiring cells. So, oxygen is removed from arterial blood as it passes through capillaries.

CAPILLARY:

Structure :

·                Small

·                Thin walled due to being one cell thick

·                This allows gas exchange by diffusion

·                Glucose and oxygen leaves the capillaries by diffusion

·                Carbon dioxide contained in plasma enters capillaries by diffusion

Pressure at the venous end of capillaries is lower than the pressure at the arterial end as volume of liquid in capillary is reduced and due to friction of fluid with the capillary wall. Tissue fluid is forced back into capillaries through the venous end as pressure outside capillaries is greater than the pressure inside capillaries. Tissue fluid carries waste substances, like carbon dioxide, from the body cells into capillaries.

Functions :

·                Capillaries supply cells with oxygen and glucose for respiration.

·                Capillaries allow removal of carbon dioxide.

TISSUE FLUID :

Source :

·                Blood plasma is the liquid part of blood.

·                When it is forced out of blood capillaries, it forms tissue fluid. Tissue fluid bathes / surrounds the body cells.

·                Some tissue fluid drains back to blood capillaries.

·                Rest drains into the lymphatic system to become lymph.

·                Lymph drains into lymph vessels called lymphatics.

·                Lymphatics join to form larger vessel, which passes through lymph nodes.

·                The lymphatic system rejoins the circulatory system near the heart.

·                Lymph is returned to blood plasma.

Functions :

·                Tissue fluid transfers materials into and out of cells by diffusion as it bathes body cells / tissues. Glucose, amino acids and oxygen diffuse into cells from tissue fluid and waste substances, like carbon dioxide diffuse out of the cells into tissue fluid.

·                Lymph returns surplus tissue fluid to blood system. Lymph drains into lymph vessels called lymphatics, which join to form larger vessels. Lymph passes through lymph nodes and rejoins the circulatory system near the heart, returning lymph to blood plasma.

·                Lymph transports fatty acids and glycerol. Fatty acids and glycerol are absorbed into lacteals in the villus in the small intestine.

·                Lymph plays a major role in the body’s defence mechanism by producing lymphocytes.

Substances which are more in lymph than in blood plasma :

·                fats : fatty acids and glycerol are absorbed in lymphatics

·                antibodies : lymphocytes are made in the lymphatic system, which secrete antibodies

·                vitamins A and D are absorbed by lymph in lacteals in small intestine

Note : If the diet of a person is too high in fats, fats would block the coronary arteries leading to heart attack.

Differences between tissue fluid and blood plasma :

TISSUE FLUID	BLOOD PLASMA
Outside blood vessels as it surrounds cells	Contained in blood vessels as it is the liquid part of blood
Has little / no plasma proteins, like fibrinogen	Has plasma proteins, like fibrinogen
Not red as it has no red blood cells	Is red due to the presence of red blood cells
Fewer white blood cells	Lots of white blood cells

Exercise :

Vigorous exercise requires more energy as they respire rapidly. Cells need more glucose and oxygen (for aerobic respiration). These are carried in blood. High pulse rate helps to force blood along blood vessels. Higher pressure means blood circulates more rapidly. Delivery of oxygen and glucose to body cells is rapid and removal of waste, like carbon dioxide, is also more rapid. Anaerobic respiration occurs if oxygen is short in supply. This produces lactic acid, which needs to be removed. High carbon dioxide level is detected by the carotid bodies. Nerve impulses are sent to the heart. Heart beats faster to provide greater blood flow.

Carbon dioxide levels in the blood increase during exercise. Nerve impulses from the medulla cause intercostal muscles and diaphragm muscles to contract more rapidly and more strongly. Volume of thorax increases and decreases more rapidly, causing greater gaseous exchange.

Exercise requires more energy and thus more oxygen. An increased rate and depth of breathing increases capacity of the lungs to take in more oxygen. Oxygen releases energy from glucose by respiration. Extra oxygen is obtained from air in the lungs. This allows aerobic respiration to continue. Built up of lactic acid is prevented as no anaerobic respiration occurs. Carbon dioxide is released by respiration. Extra carbon dioxide is excreted via air in lungs.

When body is less active, such as, asleep, less energy is required.

Breathing rate is high immediately after exercise as lactic acid builds up. Oxygen debt must be paid as lactic acid needs oxygen to be broken down. Breathing rate returns to the resting rate long after exercise as oxygen debt is paid back.

Note : A larger heart means more cardiac muscle in the heart. Such a heart is more powerful. The heart has greater stroke volume, that is, it can pump more blood per beat. Blood containing glucose and oxygen is carried to the cardiac muscle of the heart. Oxygen and glucose are used up in respiration to release energy.

Heart Rate

·                Exercise : Increased energy demand. So, supply of oxygen and glucose to muscles increases. Heart beat is increased.

·                Illness / Fever / High temperature : More heat to be got rid of. Increased blood flow to skin surface. Pyrogens

·                Shock causing release of adrenalin : More blood to muscles. Increased heart rate to compensate for this.

Effects of weak heart beat :

·                Less blood pressure in arteries

·                Difficult to maintain adequate blood supply to organs / tissues.

·                For example, loss of blood supply to the brain can lead to dizziness and fainting.

·                Kidney filtration becomes less efficient

·                Very easily tired and cannot sustain muscle activity

CARDIOVASCULAR DISEASE :

Causes of Heart Attack (in the population) :

·                Change in diet – more intake of fatty, high cholesterol junk food

·                Less exercise due to use of vehicles

·                Increase in smoking

·                Ageing population

Heart Attack :

·                Cholesterol narrows lumen of coronary arteries.

·                A blood clot is formed.

·                Blockage in coronary arteries cuts off blood supply to an area of the heart muscle.

·                Less blood, containing oxygen and glucose reaches the cardiac muscle.

·                Less respiration occurs.

·                Less energy is released.

·                Lactic acid causes muscle fatigue. So the muscle cells stop contracting.

·                The heart muscle is unable to contract efficiently, causing angina and heart attack.

·                Cells affected may die.

·                Rise in blood pressure in coronary artery.

Permanently Raised Blood Pressure :

Effects:

·                Damage to blood capillaries leads to bleeding.         

·                Internal bleeding due to damage to blood capillaries in the brain can cause coma.

·                It can cause headaches.

·                It can damage the retina of the eye.

Cure:

·                Reduce salt in diet as salt raises blood pressure

·                Avoid stress

·                Take gentle exercise

·                Stop smoking

·                Reduce alcohol intake as too much can damage the heart

·                Reduce weight by reducing fat intake, especially, saturated cholesterol as it increases risk of heart attack

Food for a healthy heart:

·                Avoid too much saturated fat as it increases risk of heart attack

·                Have adequate protein to maintain heart muscle

·                Reduce salt intake as salt raises blood pressure

·                Take no or less alcohol as too much can damage the heart

·                Avoid too much caffeine as it raises heart rate and blood pressure

·                Have more fibre

Stroke Volume : Volume of blood forced from the heart at each ventricular contraction is called stroke volume.

A Rise in Carbon Dioxide Concentration of Blood : This can due to more respiration as a result of more activity.

·                Carbon dioxide is an acidic gas, and as it dissolves in blood plasma, the pH of blood falls.

·                This is detected by the chemoreceptors in the carotid body.

·                This is detected by the hypothalamus of the brain.

·                Nerve impulses are sent to the medulla.

·                Nerve impulses are sent to the diaphragm muscles and intercostal muscles to increase the rate and depth of breathing.

·                The diaphragm muscles and intercostal muscles contract more strongly and more forcefully.

·                Volume of thorax increases and decreases more rapidly to allow increased gaseous exchange to occur.

·                The excess carbon dioxide passes out of the lungs into the air.

RED BLOOD CELLS :

Function : They transport oxygen from the alveoli of the lungs to the cells of the body tissues.

Note : If water is removed by osmosis from red blood cells because of lower water potential in plasma. Red blood cells shrink.

Adaptations for transporting oxygen :

·                haemoglobin

·                biconcave shape

·                large surface area to volume ratio

·                no nucleus

Features for identification of Red blood cells :

·                Biconcave shape (disc shaped)

·                Absence of nucleus

WHITE BLOOD CELLS :

Infection by microorganisms causes an increase in body temperature to increase activity of white blood cells.

Phagocytes :

·                Phagocytes can move out of blood and between the cells of the body.

·                Phagocytes move towards bacteria.

·                Bacteria are engulfed by phagocytes before they can reproduce.

·                Bacteria are enclosed in a vacuole.

·                Enzymes produced by the phagocyte enter the vacuole and digest bacteria.

·                Bacteria are destroyed.

·                Bacteria are prevented from spreading / increasing in population.

Lymphocytes :

·                Lymphocyts move to the site of the infection.

·                Lymphocytes produce antibodies in response to detecting specific surface antigens.

·                Specific antibodies, like agglutinins, destroy bacteria by causing them to clump together for easier digestion by phagocytes.

·                Antitoxins neutralise toxins produced by bacteria.

·                Lysins break down bacterial cell membrane.

·                Bacteria cannot reproduce.

·                They become inactive.

·                Memory cells are then formed that retain the ability to make specific antibody.

·                Only mild symptoms are experienced.

·                The next infected pathogen is destroyed by antibody production before the population of the pathogen rises to large numbers to cause symptoms.

·                This prevents the specific disease.

PLATELETS :

Role in blood clotting :

·                Platelets are inactivated by exposure to air.

·                Platelets trigger blood clotting.

·                They release the enzyme, thrombokinase, in presence of calcium ions / Vitamin K.

·                Thrombokinase converts soluble prothrombin to thrombin.

·                Thrombin acts on the soluble plasma protein fibrinogen and converts it to insoluble fibres of fibrin.

·                Fibrin forms a mesh over the wound called a clot (thrombosis).  

·                The clot forms a seal over the wound.

·                Red blood cells are trapped in the clot.

·                This dries to form a solid scab.

·                This prevents excessive bleeding.

·                Blood washes out bacteria and toxins.

·                The scab acts as a barrier to the entry of pathogens as there is no gap in the skin.

·                White blood cells move to wound and gather under scab to destroy bacteria.

·                Living cells divide to repair wound and form new layer.

·                Vasoconstriction / Serotonin reduces blood flow.

Note :

·                Antibiotics need to be added to deep wounds where bacteria can enter the living layers of the skin and reproduce there. Antibiotics kill bacteria or help prevent bacterial reproduction.

·                Thrombosis (clotting) occurs only when blood vessel is cut, so that, they are not blocked and cause heart attack / stroke.

·                A wound caused by a nail should be rinsed with running cold water for several minutes as this removes bacteria. Cold water constricts blood vessels. The wound must be covered to prevent entry of pathogens into damaged tissues.

PLASMA :

Definition : It is the liquid part of blood.

Functions :

·                Plasma transports materials, like glucose, oxygen, digested foods, to tissues.

·                Plasma carries waste materials, like urea and carbon dioxide to the excretory organs, like the lungs and the kidneys

Structural differences between red blood cells and phagocytes :

·                Red blood cells have no nucleus, phagocytes have a lobed nucles.

·                Red blood cells contain haemoglobin, phagocytes don’t.

·                Red blood cells have a regular biconcave shape, phagocytes have an irregular shape.

·                Red blood cells don’t have a granular cytoplasm, but phagocytes do.   

Vaccine :

Definition : Weakened / dead pathogens / antigens / toxins are injected / taken orally by a person.

In case of polio, an attenuated strand or the dead polio virus / toxins are injected into the body. Antigens are proteins on the surface of the viruses and they stimulate lymphocytes (white blood cells) to produce specific antibodies that kill the treated pathogens and remain in the blood for some time. Antibodies agglutinate pathogens for easier phagocytosis. Memory cells are then formed that retain the ability to make specific antibody. Only mild symptoms are experienced. The next infected pathogen is destroyed by antibody production before the population of the pathogen rises to large numbers to cause symptoms. This prevents the specific disease. Vaccination provides active, artificial immunity.

A person may feel unwell after getting vaccinated as pathogens reproduce in the body and produce toxins that may cause symptoms. It takes several days for antibody levels to rise. When the antibody levels do rise, the pathogens are destroyed.

If antibody level falls below safe level, the person can catch the disease. Vaccination increases antibody level, so that it remains above unsafe level, making the person immune to the disease.

The active components of vaccination are toxins and attenuated organisms.

After infection, vaccination is of little value as the white blood cells are already stimulated as antigens are already present. It takes some time for antibodies to be formed.

IMMUNITY

Definition : Immunity is defined as the presence of antibodies / very rapid production of antibodies, so that pathogens that enter the body are destroyed before symptoms occur. So that, only mild symptoms can appear, but the person does not suffer from the disease.

Antibodies are present in mother’s blood from the infections / immunisations she has had. Antibodies cross placenta to the child and provides natural passive immunity. They are active in the child at birth. Memory cells do not cross the placenta. The child’s organs destroy antibodies and excrete them within months after birth. Presence of future antigens need to trigger the child’s own immune system.

White blood cells (lymphocytes) are needed to form the body’s immune system. If white blood cells are inactive, the baby is unable to form antibodies, and therefore, unable to destroy pathogens. Even normally mild infections can become fatal.

White blood cells (lymphocytes) are formed in the bone marrow. Tissue matching is necessary for donor tissue to avoid rejection. Successful transplant of bone marrow cells will form new active / healthy white blood cells (lymphocytes) that produce antibodies to destroy pathogens.

Smoking related diseases are not causes by pathogens, but by pollutants which enter the body and cause damage. White blood cells cannot respond to these as no antigens exist. No antibodies are produced. No memory cells are made. So, the body cannot be immune to such diseases.

Artificial Passive Immunity : Serum is injected into the body. Antibodies or antitoxins, formed in another animal, are present in serum. It has an immediate effect, but this is short lasting / temporary.

Natural Passive Immunity : Ready made antibodies or antitoxins diffuse across placenta from maternal blood to fetal circulation. They are present in breast milk or colostrum. The antibodies are however, excreted by the body, so the effect is temporary.

Natural Active Immunity : The person suffers from the disease. The white blood cells are stimulated to produce antibodies to destroy pathogens. Antibodies form memory cells, which are long lasting. They respond rapidly if infection by the same microorganism occurs, as antibodies are produced quickly. The new infection is rapidly dealt with.

Artificial Active Immunity : The person is vaccinated, where the antigen of the pathogen or a weakened or dead strand of the microorganism is injected. This causes the lymphocytes to produce antibodies and memory cells, that remain in the blood for a long time.

Differences between Active and Passive Immunity :

Active	Passive
Presence of pathogen / antigen in body. So lymphocytes produce antibodies	Antibodies are produced in another organism and are either injected or are passed across placenta or in breast milk
Memory cells retain ability to produce antibodies if re-infection occurs.	Body does not learn to make antibodies
Long term immunity	Short term immunity
Occurs by having disease or vaccination	Occurs by antibodies crossing placental membrane, found in colostrums / breast milk and injecting serum

Differences between Immunity and Resistance :

·                Immunity refers to white blood cells producing antibodies rapidly and in large numbers due to infection by microorganism.

·                Immunity is due to vaccination, resistance to chemicals is due to natural selection.

·                Resistance is due to mutation of genes (alleles).

Notes :

·                If there is a hole in the heart between the right and left atria, the oxygenated and deoxygenated blood may mix in the heart. Blood leaving the left ventricle may not be fully oxygenated. Muscles receive limited oxygen, so less respiration occurs and less energy is released. The person will find exercising difficult and will get tired very easily due to lack of energy. His/Her skin and lips may develop a blue tinge.

·                It is better to use stem cells, found in the bone marrow, of the same person to replace dead heart muscle cells to reduce the risk of rejection. Genes may be different in different people, and thus, result in different blood groups, antibodies and antigens.