Circulation
1. What This Chapter Covers
Every organism has to move nutrients, gases and liquids to all parts of its body. In unicellular organisms these do not travel far; in multicellular ones they may travel a long way — as much as 100 feet in the tallest plant on earth.
Organisms like Amoeba and Hydra move everything by simple processes such as diffusion and osmosis. But a body of trillions of cells relying on diffusion alone would take far too long, so a separate, faster and more efficient system evolved: the circulatory system.
The index gives this chapter 10 periods in July and runs it from page 51 to page 77. It opens with a genuine question — we eat solids, drink liquids and breathe gases, so is it possible to transport all three types of material through a single system?
2. Heart Beat and Pulse (Textbook 3.1)
When a doctor holds a patient's wrist and watches a clock for a minute, the doctor is counting the heart beat.
Activity 1. Place your index and middle fingers on your wrist below the thumb. You feel something pushing your fingers rhythmically up and down; that rhythm is the pulse. Count it for a minute, then run on the spot for a minute and count again, recording readings for at least two friends.
Pulse rate varies from person to person and from situation to situation. It goes up when you are afraid or excited, and while climbing stairs or running.
Activity 2. Make your own pulse indicator from an injection bottle lid or a shirt button with a matchstick pushed into it. Place it on your wrist and watch the matchstick move while your other hand feels your chest.
Variation in pulse rate. Newborn, 0 to 3 months: 100-150. Infants, 3 to 6 months: 90-120. Infants, 6 to 12 months: 80-120. Children, 1 to 10 years: 70-130. Children over 10, adults and senior citizens: 60-100. Well-trained adult athletes: 40-60.
Activity 3. In 1816, Rene Laennec invented the stethoscope. Before it, doctors listened by putting an ear on the patient's chest. Laennec found a paper tube let him hear the beat perfectly, then used bamboo instead, and named the device.
Make your own: a paper tube 10 inches long and one inch in diameter, one end at your ear and the other on a friend's chest. Count heart beats for a minute and count the same friend's pulse, for at least ten students. The relationship that emerges — the book's Table 2 shows Eshwar at 72 and 72 — is that the pulse rate equals the number of heart beats.
3. The Heart (Textbook 3.2)
The heart lies between the lungs, protected by the rib cage, and is about the size of your fist.
Lab Activity — dissecting a mammalian heart
All mammalian hearts are similar in structure, so a freshly collected sheep's or goat's heart is used. Materials: soda straws, used pen refills, a sharp long blade or scalpel, a dissection tray, a jug of water, dissection scissors and forceps.
Wash the heart thoroughly first so the blood drains completely from the chambers. Insert soda straws into the stumps of the blood vessels and note the shape, the number of covering layers, the number of large vessel stumps, and which end is broader.
Then keep the heart in the tray with the large arch-like tube facing upwards — that is the ventral side — and open it with the scalpel so the chambers are exposed.
What you find
The heart is pear shaped, triangular in outline, wider at the anterior end and narrower at the posterior end.
It is covered by a two-layered membrane, the pericardium, and the space between the two layers holds pericardial fluid, which protects the heart from shocks.
It has four chambers: two upper atria (auricles) and two lower ventricles. The walls of the ventricles are thicker than the atrial walls. The blood vessels in the walls of the heart itself are the coronary vessels, which supply the heart muscle.
The vessels attached to it
Arteries originate from the heart and supply blood to organs. They have thick walls. The largest is the aorta; the relatively small pulmonary artery carries blood from the heart to the lungs.
Veins bring blood from all body parts to the heart and have relatively thin walls. The superior vena cava, at the anterior end on the right, collects blood from the head and neck; the inferior vena cava brings it from the hands, legs and other posterior parts.
Septa and valves
The two atria and the two ventricles are separated by muscular partitions called septa, and the openings between atria and ventricles are guarded by valves.
| Structure | Name |
|---|---|
| Valve on the right auriculo-ventricular septum | Tricuspid valve |
| Valve on the left auriculo-ventricular septum | Bicuspid valve, also called the mitral valve |
| Major vessel from the right ventricle | Pulmonary aorta, with pulmonary valves |
| Major vessel from the left ventricle | Systemic aorta, with systemic valves |
In the right atrium are the openings of the superior and inferior venae cavae, bringing deoxygenated blood from the body. In the left atrium are the openings of the pulmonary veins, bringing oxygenated blood from the lungs.
The aorta arises from the upper part of the left ventricle and supplies oxygenated blood to the body. The pulmonary artery arises from the upper part of the right ventricle and takes deoxygenated blood to the lungs. Valves are present in both.
4. How Circulation Was Worked Out (Textbook 3.3)
It was not until the 16th century that anyone understood how blood vessels function.
Girolamo Fabrici, an Italian doctor, was studying the veins of the leg in 1574 when he noticed they contain small valves that permit blood to flow towards the heart — one-way valves that check backflow, helped along by the movement of the leg muscles. But everyone believed blood leaving the left ventricle always moved away from the heart, so Fabrici missed the importance of his own discovery.
William Harvey (1578-1657), an English doctor, went to Italy to study under Fabrici. He dissected the hearts of dead people and examined the valves between each atrium and its ventricle, finding they too were one-way: blood passes freely from atrium to ventricle, but when the heart contracts it cannot flow back, and is pushed into the arteries instead.
Harvey then returned to his teacher's leg valves. He tied off and blocked different veins in animals, and found the veins always bulged on the side of the block away from the heart, as though the blood were trying to flow towards it. In arteries the blood bulged on the heart side of any block.
The conclusion
The heart pushes blood into the arteries and it returns by the veins, and it does this for both ventricles. From the right ventricle blood leaves by arteries to the lungs and returns by veins to the left atrium — pulmonary circulation.
From the left ventricle it leaves by the aorta to the rest of the body and returns by veins to the right atrium — systemic circulation. Because the blood passes the heart twice per complete round, this is double circulation.
Harvey also disproved the older idea that blood flowing out of the heart was used up and new blood made. He measured how much blood the heart pumps in one contraction and counted the contractions per minute, and found that in one hour the heart pumps out a quantity of blood three times the weight of a man. No body could make blood at that rate, so the same blood must circulate and be used over and over.
The missing link
One problem remained: the arteries and veins that can be seen had to be joined by vessels too small to see. In the 1650s scientists learned to combine lenses so that invisible objects could be magnified, and Marcello Malpighi (1628-1694) used the microscope to look at the thin membranes of bats' wings. There he saw that the smallest arteries and veins were connected by very fine vessels.
He named them capillaries, from the Latin word for hair, because they were as thin as the finest of hairs. With that, the idea of the circulation of the blood was complete.
Harvey's demonstration, repeated
Tie a tourniquet just above the elbow of a friend whose blood vessels are prominent, and have them hold a firm support so the vessels show more clearly. Locate a prominent bluish vessel.
At the end of the vessel farthest from the elbow, apply steady pressure to close its cavity. Then press from the elbow towards the palm and watch what happens to the vessel. Release, then press from the palm towards the elbow instead, and compare.
The question to answer is why the vessels just below the skin bulge on the side away from the heart when the hand is tied — which is Harvey's own observation reproduced with no microscope and no modern equipment.
Trace the loops in the order Harvey found them. Right ventricle to lungs to left atrium is pulmonary; left ventricle to body to right atrium is systemic; and because the heart sits in both, the blood passes it twice.
5. Arteries, Veins and Capillaries (Textbook 3.4)
Blood capillaries are microscopic vessels made of a single layer of cells. They allow diffusion of various substances, the leucocytes can squeeze out through the capillary wall, and they establish continuity between arteries and veins.
The book leaves Table 3 as an exercise. Every row can be filled from statements the chapter itself makes:
| Structure or function | Artery | Vein |
|---|---|---|
| Thickness of walls | Thick | Thin |
| Valves | Absent | Present |
| Shape when blood is absent | Rigid, retains its shape | Cannot retain its shape |
| Direction of blood flow | Heart to organs | Body organs to heart |
| Pressure on the vessel | High | Low |
| Type of blood carried | Oxygenated, except the pulmonary artery | Deoxygenated, except the pulmonary veins |
Those last two exceptions are the ones examiners rely on, and the book states them in its own summary.
Activity 4 offers two ways to see this for yourself. Sit with one leg dangling and the other resting on it so the back of one knee rests on the other knee; after a time the upper leg gives a small movement with each heart beat, and held too long it reduces blood flow and produces pins and needles.
Or swing your arm round several times to fill the veins, hold it vertically down, and stroke a prominent vein gently in the reverse direction to the blood flow, towards the hand. The swellings you see are blood pushed up against the valves.
The book's own discussion prompts are worth answering: why artery walls are strong and elastic, why an artery is compared with a tree dividing into smaller branches, and why the lumen is bigger in a vein than in an artery.
6. The Cardiac Cycle (Textbook 3.5)
The human heart starts beating around the 21st day of embryonic development and beats rhythmically until death.
One contraction and one relaxation of atria and ventricles is one cardiac cycle. The book takes it in five steps:
- All four chambers are relaxed — diastole.
- Blood from the venae cavae and the pulmonary veins enters the right and left atria.
- The atria contract, forcing blood into the ventricles.
- The filled ventricles start contracting while the atria are still contracted. The aperture between atria and ventricles is closed by the valves, and closing them forcibly produces the first sharp sound, lub. The pressure moves blood into the aorta and pulmonary artery.
- As the ventricles relax the pressure in them falls, and blood already in the aorta tries to come back. The semilunar valves in the aorta close to prevent it, producing the second sound, dub. Meanwhile the atria fill again.
The cycle has an active phase, systole, and a resting phase, diastole, for both atria and ventricles.
| Phase | Time |
|---|---|
| Atrial systole | 0.11 to 0.14 seconds |
| Ventricular systole | 0.27 to 0.35 seconds |
| Joint diastole of auricles and ventricles | About 0.4 seconds |
| Whole cycle | About 0.8 seconds |
Blood is pumped into the vessels at regular intervals, so tissues receive it not continuously but in spurts. The pressure of that spurt felt at the wrist is the pulse, and its rate equals the number of heart beats.
Heart rate across animals. Blue whale: body 1,50,000 kg, heart 750 kg, 7 beats per minute. Elephant: 3000 kg, 12-21 kg, 46. Human being: 60-70 kg, 300 g, 72. Coaltit, a bird: 8 g, 0.15 g, 1200. The smaller the animal, the faster the heart.
7. Single and Double Circulation (Textbook 3.6)
Trace the book's two flow charts with a pointer and count how many times you pass the body parts, the heart, and the respiratory organs.
In the single-circulation chart the blood flows through the heart only once to complete one circulation; the route passes the gills and then the body. In the double-circulation chart it flows through the heart twice, once on the lung loop and once on the body loop.
8. The Lymphatic System (Textbook 3.7)
After an overnight journey spent sitting without moving, your footwear feels tight; in elders the lower legs visibly swell. That state is oedema.
As blood flows through tissues and capillaries, some fluid and certain solid materials are constantly flowing out at different junctions, and these have to be collected and returned to the circulation.
The liquid portion of the blood, carrying nutrients, flows out of the capillaries to supply the cells. That is tissue fluid, and it travels through the lymphatic system, mainly returning to the blood stream.
Lymph is the vital link between blood and tissues, by which essential substances pass from blood to cells and excretory products from cells to blood. The book's distinctions are worth memorising as a set: blood contains solid and liquid particles; lymph is blood without the solid particles; tissue fluid is lymph present in the tissues; and serum is the liquid portion left after a blood clot has formed.
The skeletal muscles act as pumps: when they contract, the pressure they exert pushes lymph along the lymphatic vessels and blood along the veins, both towards the heart. Valves in the lymphatic vessels and veins stop reverse flow.
9. Evolution of the Transport System (Textbook 3.8)
When unicellular organisms separated themselves from the sea by forming a limiting membrane, the problem of transportation arose. Nature's solution was to create a microscopic ocean with its own currents.
In Amoeba the protoplasm shows natural Brownian movements, which distribute nutrients and oxygen evenly. This simplest intracellular system has been retained in multicellular animals including humans — protoplasm in any of our cells is mobile, and protoplasmic currents exist even in nerve cells.
| Group | Transport arrangement |
|---|---|
| Parazoa, sponges | Use sea water, creating their own currents by beating flagella |
| Cnidarians, Hydra and jellyfish | A blind-sac gastrovascular cavity doing both digestion and transport |
| Platyhelminthes, Fasciola hepatica | A highly branched digestive system supplying every cell directly, with the excretory system collecting from each cell |
| Nemathelminthes, roundworms | The pseudocoelom collects and distributes materials |
| Annelids, earthworm | The first eucoelomates; a pulsatile vessel moves the fluid, and the medium is blood |
| Arthropods | A pulsatile organ, the heart; blood floods the tissues directly |
That last case defines the two types. In an open circulatory system the transporting fluid supplies the tissues directly — arthropods, many molluscs and lower chordates — and in arthropods oxygen is supplied directly by the respiratory system instead. In a closed system the blood does the delivering and flows inside vessels — annelids, cephalopod molluscs such as the octopus, and all the higher chordates.
The human circulatory system moves one ml of blood from the heart to a foot and back, about 2 metres, in roughly 60 seconds. By diffusion the same journey would take more than 60 years.
10. Blood Pressure (Textbook 3.9)
Moving blood through the network of vessels requires a great deal of pressure. It is highest when the ventricles contract and drops as they refill for the next beat.
Blood pressure is the pressure exerted by the blood on the walls of the blood vessels, developed by the contraction of the ventricles. It varies through the body, so it is usually measured in the upper arm artery, with a sphygmomanometer.
There are two readings. Systolic pressure is measured while blood is forced out of the ventricles, around 120 mm of Hg for a healthy young adult. Diastolic pressure is taken during the resting period as the ventricles refill, around 80 mm of Hg.
Blood pressure changes with activity — resting, walking, running. People whose blood pressure is high during the resting period are said to have hypertension.
11. Coagulation of Blood (Textbook 3.10)
After an injury, blood flows out for only a short time and then the cut fills with a reddish solid, the blood clot. Without clotting, even a slight wound would bleed profusely.
The sequence the book gives is:
- Platelets in the oozing blood release an enzyme, thrombokinase.
- Thrombokinase acts on prothrombin, converting it into thrombin.
- Thrombin acts on fibrinogen, which is present in dissolved state, converting it into insoluble fibrin.
- Blood cells become entangled in the fibrin fibres, forming the clot.
- The fibrin fibres attach to the edges of the wound and pull them together.
- The yellowish straw-coloured fluid left after the clot forms is serum.
Prothrombin --(thrombokinase)--> Thrombin
Fibrinogen --(thrombin)--> Fibrin
Blood from a wound normally clots in 3 to 6 minutes. In some people vitamin K deficiency makes it take longer. A genetic disorder in which blood may not clot at all after injury is haemophilia, which the book notes is common in children born of marriages between very close relatives. Thalassemia is another inherited blood disorder.
The book's two annexures
The rhesus factor. Another red-cell antigen, present in 85 per cent of people in Britain and first found in Rhesus monkeys. Those who have it are Rh positive, those who do not are Rh negative, and Rh-negative people do not normally carry the antibody. If Rh-positive blood is transfused into an Rh-negative person, antibodies form that can destroy Rh-positive red cells.
The hazard arises in pregnancy. If an Rh-positive man marries an Rh-negative woman some children are likely to be Rh positive, and blood mixes at birth, so antibodies form in the mother. With later pregnancies the antibody level often rises and may pass into the baby's blood, producing serious anaemia or death.
Such cases are not frequent, and the baby can be given a complete transfusion soon after birth, or now even before it. The mother can also be injected shortly after her first child to prevent the harmful antibody forming.
Thalassemia. A group of inherited blood disorders with mild to severe anaemia, caused by abnormally low production of haemoglobin. The two main types are alpha and beta thalassemia, each with a different part of the haemoglobin protein defective. Symptoms include anaemia, enlarged liver and spleen, susceptibility to infections, slow growth, thin and brittle bones, and heart failure.
The book's figures: 4.5 per cent of the world population, 250 million people, have thalassemia minor; about 35 million Indians carry the abnormal gene; about 1,00,000 infants are born worldwide each year with major haemoglobinopathies, and 10,000 to 12,000 thalassemic children are born in India each year. Prevention depends on awareness and pre-marital or pre-conceptual screening followed by antenatal diagnosis.
Treatment means diagnosing thalassemia major as early as possible. Regular transfusion of concentrated red cells every three to four weeks is the treatment of choice, aiming at a median haemoglobin of 115-120 grams per litre; thalassemia major can today be cured by stem cell transplantation from a sibling of identical HLA type.
12. Transport in Plants (Textbook 3.11)
Water absorbed by the roots and food prepared by the leaves are supplied to the rest of the plant by vascular bundles containing xylem and phloem.
One structural detail is worth noting because it is easy to get backwards: in the root, xylem develops from periphery to centre; in the stem, it develops from centre to periphery in each vascular bundle.
How water is absorbed (3.11.1)
Activity 5. Germinate bajra or mustard seeds. Examine a seedling with a hand lens: the mass of fine threads is the roots, and on them are microscopic root hairs. Squash a portion of root hair between a slide and cover slip with a drop of water and look under a compound microscope, noting how thin the walls are.
Root hairs grow out into the spaces between soil particles and are surrounded by moisture. Every living cell acts as an osmotic system, with the cytoplasm lining the cell wall acting as the semipermeable membrane.
Soil water is an extremely dilute solution of salts — more dilute than the cell sap in the root hair — so water passes into the root hair's vacuole by osmosis. That entry dilutes the root hair's contents until they are more dilute than the neighbouring cell, so water passes on, and cell by cell it finally reaches the xylem vessels.
Because a vast number of root hairs and root cells are involved, a pressure develops in the xylem vessels that forces water upwards. This is root pressure. The book is careful to say root pressure is not the main cause of water movement in xylem, only one factor.
Activity 6. Cut the stem of a regularly watered potted plant 1 cm above ground level and connect a glass tube of the same size by strong rubber tubing, bound tightly so no water escapes. Pour water in until the level shows above the rubber tube and mark it as M1. Leave it two to three hours and mark the new level M2. The difference is the water raised by root pressure.
How water moves up (3.11.2)
Root pressure is seldom high and in some seasons is nil, so it cannot explain how water reaches the top of a eucalyptus 180 metres high.
Evaporation of water through leaves is transpiration, happening through the stomata of leaves and the lenticels of stems. When the leaves transpire there is a pulling effect on the continuous columns of water in the xylem vessels.
The tops of those vessels are surrounded by the leaf's mesophyll cells, which contain cell sap, so the water is continuous from the xylem right up to the mesophyll cell walls, from which it evaporates into the air spaces — and that is what causes the pull. The water column does not break because of its continuous molecular attraction, a property you demonstrate every time you drink through a straw.
So the complete picture is: water absorbed by osmosis from the soil through root hairs, passed into a continuous system of xylem tubes through root and stem into the leaves, evaporated there into the atmosphere. Evaporation provides the main pull; root pressure gives a variable, minor push from below. The result is a continuous column of moving water, the transpiration stream.
An oak tree can transpire as much as 900 litres of water per day, so forests significantly raise the saturation of the air above them. Air that arrives already nearly saturated becomes fully saturated over a forest and comes down as rain, which is why forest areas often have higher rainfall than areas nearby.
Each fully grown maize plant transpires 15 litres per week, so one acre of maize may transpire more than 13,25,000 litres in a hundred-day growing season. A big mango tree transpires from 750 to more than 3500 litres per day in the growing spring season.
Transport of mineral salts (3.11.3)
Mineral salts come from the soil solution through the root hairs, and they are in the form of electrically charged ions: sodium chloride as Na⁺ and Cl⁻, magnesium sulphate as Mg²⁺ and SO₄²⁻.
They are not absorbed by simple diffusion. The process involves the use of energy by the cytoplasm. Once absorbed, the ions travel with the water in the xylem vessels to the growing points, where they are used for growth, and they may also pass laterally from xylem to phloem.
13. Transport of Food in Plants (Textbook 3.12)
Sugar is made in the green parts, mainly leaves, but has to reach every living cell, especially actively growing cells and cells that store food. The veins of a leaf contain xylem and phloem continuous with the stem.
The aphid experiment
Phloem sieve tubes are extremely small and their contents hard to analyse, so biologists used aphids, the greenfly that cluster round young stems feeding on plant juices. An aphid pierces the tissues with a long needle-like organ, the proboscis.
When a feeding aphid is killed and the stem carefully sectioned, the proboscis is found to penetrate only as far as a phloem sieve tube. That gives a ready-made sampling tube: kill the aphid while it is feeding, cut the body away leaving the hollow proboscis in place, and because the sieve tube contents are under slight pressure the fluid slowly exudes in drops. Analysed, it contains sugars and amino acids.
Aphids absorb so much sugar that they cannot assimilate it all, and excrete the excess as a sticky syrup, honey dew. Leaves attacked by aphids often feel sticky for this reason.
The ringing experiment
Remove a ring of bark from a shoot to expose the wood, taking out all tissues from the centre outwards including the phloem.
After a few days, analysis shows food has accumulated above the ring and is absent below it. Left longer, the stem thickens immediately above the ring and no growth occurs below it.
So damage to the phloem all round a stem prevents food passing down to the roots and the tree eventually dies. This has real economic weight: mammals scratch bark to get at the food stored in the phloem, especially in hard winters when food is scarce. Voles do this to young saplings at ground level and rabbits damage older ones, so foresters find it worthwhile to enclose new plantations with wire netting.
Key words from the chapter
Circulation, auricles, ventricles, pulse, artery, vein, stethoscope, aorta, capillary, systole, diastole, cardiac cycle, blood pressure, lymph, single circulation, double circulation, coagulation of blood, sphygmomanometer, prothrombin, thrombin, fibrinogen, fibrin, root hair, radical, root pressure, plant nutrients, xylem, phloem, vascular bundles.
14. Summary
The pulse rate equals the heart beat, and it can be counted without any instrument. Rene Laennec discovered the first stethoscope.
The heart is covered by two pericardial membranes with pericardial fluid between them, which protects it from shocks. It has four chambers: two upper atria and two lower ventricles.
The rigid vessels attached to the heart are the arteries, supplying the body and the lungs; the less rigid ones are the veins, bringing blood back from body parts.
An atrium and the ventricle on the same side are connected by an atrio-ventricular aperture. The atria are separated by the interatrial septum and the ventricles by the interventricular septum, and the atrio-ventricular apertures are guarded by valves, as are the aorta and pulmonary artery.
The right side of the heart receives blood from the body and sends it to the lungs; the left side receives it from the lungs and sends it to the body. Arteries carry oxygenated blood except the pulmonary artery, and veins carry deoxygenated blood except the pulmonary veins.
One contraction and relaxation of the heart is a cardiac cycle. If blood passes through the heart once before reaching all the body parts it is single circulation; if twice, double circulation.
Vitamin K deficiency leads to delayed coagulation of blood.
Plants absorb soil water through the roots by osmosis. Water travels through the xylem and food through the phloem, and there is a direct relation between transport and transpiration. Biologists worked out what phloem sieve tubes carry with the help of aphids.
