Biology Kenya
Transport in Plants and Animals
Transport in Plants and Animals
Transport in Plants and Animals
Meaning and importance of transport systems
Unicellular organisms (for example amoeba), nutrients (for example oxygen and food) and waste products (for example carbon dioxide) can simply diffuse into or out of the cells from the surroundings. But in multi cellular organisms (for example humans and trees), many cells are very far away from the body surface, hence a transport system is required for the exchange of materials.
Organisms require transport systems so as to carry out various life processes. These life processes include nutrition, respiration, excretion, growth and development, movement,reproduction and coordination. For these life processes to take place, transport of materials is inevitable. Materials are transported either from environment into the organisms or from one part of an organism to another, and can also be transported from an organism into the environment.
For example, during nutrition organisms take in food substances that they need to produce energy, grow and carry out other life processes. These food substances must be taken in from the environment. The same case applies to reproduction which requires the movement of gametes(sex cells) from the sex organs to the area where fertilization occurs. Therefore, transport is very important for the survival and existence of living things.
Transport of materials is very important for the survival and development of living organisms. If transportation never existed, then no life on earth could be possible. The following is an outline of the importance of transport of materials in living things:
- It facilitates the removal of waste materials from the organism’s body, the excess of which could harm an organism.
- It ensures that essential materials like oxygen, nutrients, water, hormones and mineral salts are supplied to the cells and tissues as required.
- It enables essential substances to move from one part of the body to another. For example, food manufactured by photosynthesis in plant leaves is transported from leaves to other organs of the plant for use or storage.
Absorption of water and mineral salts
Water and mineral uptake by rootsPlants absorb water from the soil through the root and transport it to the stem, leaves and flowers. Roots have root hairs that are unicellular, thin-walled outgrowths of the epiblema (skin of the root).The root hairs are in close contact with the thin film of water surrounding the soil particles.There are mineral salts such as nitrates, chlorides, sulphates, phosphates, etc., dissolved in thiswater.Water is absorbed by osmosis, while the minerals are absorbed as ions by active transport(transport against the law of diffusion, by spending cellular energy). The cell membrane hastransport proteins that allow the ions to cross the membrane. The ions then move upward throughthe xylem, to the leaves and other aerial parts of the plant.The cell wall of each root hair is permeable to water and minerals, but its cell membrane and the membrane around the vacuole are semi permeable membranes. The root hair cells take up mineral ions by active transport.This creates a concentration difference of these ions between the root and the soil. Now, the soil solution has higher water content than the cell sap of the root hair. Hence, water from the soil diffuses into the root hair. The root hair cells now become turgid, while the adjacent cells of the cortex have lower water content.This results in the diffusion of water from the root hairs into the cortical cells (see figure below).After passing through the cortical cells by osmosis, the water reaches the endodermis (tissue separating the cortex from the vascular tissues). The endodermis forces water into the xylem tubes through passage cells.
The pressure with which water is pushed into the xylem tubes of the root is called root pressure.The water moving upwards forms a column, which is maintained up to a certain height due to root pressure.
Internal structure of root and root hairs
Water and mineral uptake by roots
Plants absorb water from the soil through the root and transport it to the stem, leaves, and flowers. Roots have root hairs that are unicellular, thin-walled outgrowths of the epiblema (skin of the root).
The root hairs are in close contact with the thin film of water surrounding the soil particles. There are mineral salts such as nitrates, chlorides, sulfates, phosphates, etc., dissolved in this water.
Water is absorbed by osmosis, while the minerals are absorbed as ions by active transport(transport against the law of diffusion, by spending cellular energy). The cell membrane transport proteins that allow the ions to cross the membrane. The ions then move upward through the xylem, to the leaves and other aerial parts of the plant.
The cell wall of each root hair is permeable to water and minerals, but its cell membrane and the membrane around the vacuole are semi-permeable membranes. The root hair cells take up mineral ions by active transport.
This creates a concentration difference of these ions between the root and the soil. Now, the soil solution has higher water content than the cell sap of the root hair. Hence, water from the soil diffuses into the root hair. The root hair cells now become turgid, while the adjacent cells of the cortex have lower water content.
This results in the diffusion of water from the root hairs into the cortical cells (see figure below). After passing through the cortical cells by osmosis, the water reaches the endodermis (tissue separating the cortex from the vascular tissues). The endodermis forces water into the xylem tubes through passage cells.
The pressure with which water is pushed into the xylem tubes of the root is called root pressure. The water moving upwards forms a column, which is maintained up to a certain height due to root pressure.
Upward movement of water within the plant
There are several processes that enable the water to move up a plant. These processes include root pressure, transpiration pull, cohesion, adhesion, and capillarity.
Root pressure
As long as the soil is damp, there will be water taken in by the root hairs. As more water is taken in, the water that is already in the xylem vessel will be pushed up the plant. This is called root push or root pressure and it helps to push water up to the leaves.
Root pressure is capable, under ideal atmospheric conditions, of pushing water one or two feet above the ground. Since root pressure is not strong enough to move water up very high, another process called transpiration pull is needed to enable the water to continue moving up the plant.
Absorption of water
Active uptake of mineral salts
Definition of transpiration
Transpiration is the evaporation of water from plants. It occurs chiefly through the leaves while their stomata are open for the passage of carbon dioxide and oxygen during photosynthesis.
Transpiration also occurs through the cuticle and lenticels. Lenticels are pores in the stems of woody plants that allow gaseous exchange between the atmosphere and the internal tissues.
Review of the structure of the leaf
Structure and function of xylem
Factors affecting transpiration
The rate of transpiration can be affected by both plant features and environmental factors.
Plant factors
These plant parameters help plants control rates of transpiration by serving as forms of resistance to water movement out of the plant. They include the following:-
Root system
Plants with extensive root systems absorb a great amount of water and therefore much water is moved up the plant. Thus, plants with extensive root systems have higher rates of transpiration than those with few roots.
Size of leaves
A plant with broad leaves tend to lose more water than that with small leaves. This is because the broad leaves have large surface areas over which transpiration takes place.
Leaf structure
The structure of a leaf has a great influence on the rate of transpiration. The following are anatomical structures of a leaf that affect the rate of transpiration:-
Number of stomata
Stomata are pores in the leaf that allow the gaseous exchange to take place, and water vapor to leave the plant. Special cells called guard cells control each pore’s opening or closing. Some plants have many stomata while others have a few stomata. The more the stomata, the higher the rate of transpiration and vice versa.
Position of stomata
Plants with few stomata on the upper surface of the leaf experience a little transpiration compared to those with many stomata on the lower leaf surface. This is because the upper surface is highly stricken by direct sunlight hence increasing the rate of transpiration.
Epidermal hairs
Epidermal hair on the leaf traps a thin layer of still air close to the leaf surface. For the water lost from the leaf to get into the atmosphere, it has to cross this resistant layer of air. The layer thus checks the excessive loss of water from the leaf. Likewise, the water vapor from the leaf is also trapped by the epidermal hairs. This prevents further loss of water vapor from the leaves and hence slows down the rate of transpiration.
Size of stomatal air spaces
Large air spaces between the cells of the spongy mesophyll and stomata called substomatal airspaces, increase the rate of transpiration. Small substomatal air spaces reduce the rate of transpiration.
Cuticle
The cuticle is the waxy layer present on all above-ground tissue of a plant and serves as a barrier to water movement out of a leaf. Because the cuticle is made of wax, it is very hydrophobic or ‘water-repelling’. Therefore, water does not move through it very easily. The thicker the cuticle layer on a leaf surface, the slower the transpiration rate. Cuticle thickness varies widely among plant species. In general, plants from hot, dry climates have thicker cuticles than plants from cool, moist climates. In addition, leaves that develop under direct sunlight will have much thicker cuticles than leaves that develop under shade conditions.
Environmental factors
Some environmental conditions create the driving force for the movement of water out of the plant. Others alter the plant’s ability to control water loss.
Light
Plants transpire more rapidly in the light than in the dark. This is large because light stimulates the opening of the stomata (a mechanism). Light also speeds up transpiration by warming the leaf.
Photosynthesis occurs in the presence of light. Higher light intensity increases the rate of photosynthesis in the guard cells. As the guard cells absorb water from the soil for photosynthesis, they become turgid, and hence the stomata are opened, and hence a higher rate of transpiration.
Temperature
Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises. At 30°C, a leaf may transpire three times as fast as it does at 20°C.
Relative humidity
The relative humidity is the amount of water vapor in the air compared to the amount of water vapor that air could hold at a given temperature. When the air is less moist, the relative humidity is low, and thus the rate of transpiration is greater. When relative humidity is high, the atmosphere contains more moisture, reducing the rate of transpiration. Therefore, transpiration increases with a decrease in relative humidity.
The rate of diffusion of any substance increases as the difference in concentration of the substances in the two regions increases. When the surrounding air is less humidity, diffusion of water out of the leaf goes on more rapidly.
Wind
The wind removes water vapor and thus increases the rate of transpiration. High winds lead to stomatal closure to stop the rapid water loss and hence bring a drop-in rate of transpiration. Moderate winds may reduce transpiration by lowering the temperature of the leaf.
When there is no breeze, the air surrounding a leaf becomes increasingly humid thus reducing the rate of transpiration. When a breeze is present, the humid air is carried away and replaced by drier air, thus increasing the rate of transpiration.
Soil water
The source of water for transpiration out of the plant comes from the soil. Plants with adequate soil moisture will normally transpire at high rates because the soil provides the water to move through the plant. Plants cannot continue to transpire without wilting if the soil is very dry because the water in the xylem that moves out through the leaves is not being replaced by the soil water. Thus, the rate of transpiration will increase when there is an adequate amount of water in the soil and will decrease when the soil contains little moisture.
Atmospheric pressure
Transpiration is high at low atmospheric pressure and it is low at high atmospheric pressure. Plants that grow naturally at higher altitudes, where the atmospheric pressure is low, have modified leaves to reduce the rate of transpiration.
Forces involved in water movement in plants: Transpiration pull, cohesion and adhesion, capillarity and root pressure
Transpiration pull
Transpiration is the loss of water through the leaves and other parts of the plant. Most transpiration occurs through openings, called stomata, on the underside of the leaves. As transpiration occurs, water is lost. This water is replaced by water in the xylem vessels. This causes an upward pull (transpiration pull or transpiration stream) on the water in the vessels. Thus, water is pulled up through the plant, and more enters by the roots to replace it.
Cohesion
Cohesion is the force of attraction between similar molecules. Transpiration pull is possible because water molecules cling to each other by cohesion. When water molecules cling to each other as they move up the stem and into the leaves, they pull up more water molecules up the plant. This process, however, is facilitated by transpiration pull since the water molecules lost through transpiration is being replaced by more water molecules absorbed by the roots.
Adhesion
Adhesion is the force of attraction between different molecules. As water molecules are stuck together by cohesion, the entire column of water in the xylem adheres to the sides of the xylem.It is said that the water in under tension as the column moves up the xylem. At the same time, the xylem tube narrows because of the tension.
Cohesion and adhesion forces maintain a continuous column of water in the xylem vessels from the roots to the leaves of plants.
Capillarity
Capillarity is the tendency of water to rise through narrow tubes. The lumen of xylem tracheids and vessels is very narrow and this enables water to rise through it by capillarity. Capillarity is assisted by adhesion and cohesion forces.
Translocation
Structure and function of phloem
Materials translocated (omit mechanisms of translocation)
Comparison between open and closed circulatory system
Mammalian Circulatory System
Blood circulation is the flow of blood from the heart to all body parts and back to the heart. Blood circulation or circulatory system, also called the cardiovascular system, is one of the three main systems in the human body which consists of organs and tissues.
The cardiovascular systems of humans are closed, so the blood never leaves the network of blood vessels. But oxygen and nutrients diffuse across blood vessel layers and enter interstitial fluid, which carries it to the target cells and carbon dioxide and wastes in the opposite direction.
The human blood circulation consists of two circulations namely the pulmonary circulation and systemic circulation.
Pulmonary circulation
Pulmonary circulation is the movement of blood from the heart, to the lungs, and back to the heart again. This is just one phase of the overall circulatory system. In this type of circulation, the blood flows from the right ventricle to the lungs and from the lungs to the left auricle. In the pulmonary circulation, the blood circulates to and from the lungs, to release the carbon dioxide and pick up new oxygen.
In the pulmonary circulation, blood from all body parts (except the lungs) enters the right auricle through the vena cava. From the right auricle, the blood descends into the right ventricle through the tricuspid valve. When the ventricle contracts, the blood is pushed into the pulmonary artery that branches into two main parts: one going to the left lung, and another to the right lung. The fresh, oxygenated blood returns to the left auricle of the heart through the pulmonary vein.
Systemic circulation
Systemic circulation is the flow of blood between the heart and the body parts. In this particular circulation, the blood flows from the left ventricle to different parts of the body and from22different parts of the body to the right auricle. The systemic circulation supplies nourishment to all of the tissues located throughout your body, with the exception of the heart and lungs because they have their own systems. Systemic circulation is a major part of the overall circulatory system. In this circulation, the blood circulates into the body’s systems, bringing oxygen to all its organs, structures and tissues and collecting carbon dioxide waste.
The systemic cycle begins when the oxygenated blood coming from the lungs enters the left auricle. As the chamber fills, it presses open the bicuspid valve and the blood flows down into the left ventricle. When the ventricles contract during a heartbeat, the blood on the left side is forced into the aorta. This largest artery of the body is an inch wide. The blood leaving the aorta brings oxygen to all the body’s cells through the network of ever-smaller arteries and capillaries. The used blood from the body returns to the heart through the network of veins. All of the blood from the body is eventually collected into the two largest veins: the superior vena cava, which receives blood from the upper body, and the inferior vena cava, which receives blood from the lower body region. Both venae cavae empty the blood into the right auricle of the heart.
The process by which blood passes through the heart twice before it returns to the other parts of the body is called double circulation.
Importance of blood circulation
Blood circulation is essential for a healthy body. Blood circulation is important because it facilitates the following processes to take place in the body:
- Every cell in the body needs to received oxygen and nutrients. Blood rich in oxygen is sent to the body organs, tissues, and cells to nourish them through blood circulation.
- It enables the transportation of waste products from body tissues to excretory organs so as to be removed from the body.
- Protects the body against diseases and infections through the white blood cells.
- Facilitates blood clotting to prevent loss of blood.
- Maintains body temperature by distributing body heat evenly from the liver and spleen to all body parts.
Structure and function of the heart, arteries, veins and capillaries
Basic structure
- Capillaries consist of anendothelium whichis only one cell thick.
- Walls of arteries and veins consist of 3 layers.
- The inner layer consists of a thin layer of endothelial cells.
- The middle layer is made up of smooth muscle with some elastic fibres. This layer controls the diameter of the vessel and hence the amount of blood and its rate of flow.
- The outer layer is composed of connective tissue; this holds the blood vessels in place in the body.
Detailed structure
Arteries
- The walls of arteries are much thicker as it carries blood away from the heart at high pressure.
- Major arteries close to the heart also have thick layers of smooth muscle in their walls to withstand the increases in pressure as the heart pumps.
- The walls also have a large proportion of elastic fibres in both the inner and middle layers – this allows for the arteries to stretch according to the increases in volume of blood. As the heart relaxes the artery walls return to their original position, hence pushing the blood along – maintaining a constant flow in one direction.
- Arteries are near the surface of the skin; the changes in the arteries diameter can be felt as a pulse.
Veins
- The walls of veins are thinner than the walls of arteries, as the blood they receive from the capillaries is at a much lower pressure.
- The walls have fewer elastic fibres and the lumen is wider (to allow for easier blood flow).
- Veins have two mechanisms for keeping the blood flow constant and in one direction. Firstly, many veins are close to muscles, hence when the muscles contract they compress the walls of the vein – pumping blood forwards. Veins also have valves which are spaced along regular intervals in veins. They work much like one-way swinging doors – as the blood is forced through the valve opens. However, once the pressure drops and the blood flow decreases, the valve shuts – preventing back flow of blood.
Capillaries
- They are extremely, tiny microscopic vessels that bring blood into close contact with the tissues, for the exchange of chemical substances between cells and the bloodstream.
- The one cell thick endothelial layer is a continuation of the lumen arteries and veins.
- Diffusion is a relatively slow process and hence the structure of capillaries is suited to slowing down the flow of blood.
- In order to maximize the exchange of substances between the blood and cells, capillaries have thin walls (for more efficient diffusion) a small lumen (that forces blood cells to pass through in single file, slowing down the rate of flow and maximizing their exposed surface area).
- They form an expansive blood flow network, such that no cells are far from blood supply
How different blood vessels are adapted for their function
| Blood vessel | Function | Adaptation |
| Artery | Carries blood away from heart at high pressure | Thick, elastic, muscular walls to withstand pressure and to exert force (pulse) |
| Vein | Returns low-pressure blood to heart | Large diameter to offer least flow resistance. Valves to prevent back flow. |
| Capillary | Allows exchange of materials between blood and tissues | Thin, permeable walls |
Differences between arteries, veins and capillaries
| Arteries | Veins | Capillaries |
| All arteries carry blood away from the heart | All veins carry blood towards the heart | Capillaries carry blood from arteries to the veins |
| With the exception of the pulmonary artery, all arteries carry oxygenated blood | With the exception of the pulmonary vein, all veins carry deoxygenated blood | Blood slowly loses its oxygen |
| They carry blood which is usually rich in digested food materials | Except for the hepatic portal vein, they carry blood which usually has little digested food materials | Blood slowly loses its food |
| Have relatively narrower lumens (see diagrams above) | Have relatively wide lumens (see diagrams above) | Have relatively narrow lumens (see diagrams above) |
| Have relatively a thick layer of muscles and elastic fibres | Have relatively a thin layer of muscles and elastic fibres | They do not have muscles and elastic fibres |
| They have thick outer walls | They have thin outer walls | Walls are only one cell thick |
| They carry blood at high pressure | They carry blood at low pressure | Pressure gradually falls as blood flows from arteries to veins |
| Do not have valves (except for the semi-lunar valves of the pulmonary artery and the aorta) | Have valves throughout the main veins of the body to prevent the back flow of blood. | Have no valves |
| Have bright red blood (because it is rich in oxygen) | Brown-red blood | Brown-red blood |
| Located deep in the to body surface | Located near to body surface | Capillaries are found inside all tissues |
| Walls are not permeable | Walls are not permeable | Walls are permeable |
| Blood flows in pulses | No pulse | Pulse gradually disappears |
Activity 1
Carry out simple experiments to determine pulse rates in human being
Diseases and defects of the circulatory system (Thronbosis, Varicose veins, Arterio-sclerosis) and how to control them
Additional notes on diseases and disorders of the circulatory system:
Hypertension
High blood pressure (hypertension) is defined as high pressure (tension) in the arteries, which are the vessels that carry blood from the heart to the rest of the body.
Blood pressure readings are given as two numbers. The systolic blood pressure (the top number)equals the pressure in the arteries as the heart contracts. The diastolic pressure (the bottom number) is the pressure in the arteries as the heart relaxes. Normal blood pressure is below120/80; blood pressure between 120/80 and 139/89 is called "pre-hypertension," and blood pressure of 140/90 or above is considered high blood pressure.
Complications of high blood pressure include heart disease, kidney (renal) disease, hardening of the arteries (atherosclerosis or arteriosclerosis), eye damage, and stroke (brain damage).
Causes and effects of diseases and disorders of the human vascular system
| Disease / Disorder | Description | Causes | Effects / Symptoms | |
| 1 | Anaemia | A reduction in the quantity of(oxygen carrying) haemoglobin in the blood and/or below normal quantity of red blood cells. |
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| 2 | Angina | Pain afterphysical effort | Narrowed coronary arteries being unable to supply increased blood flow required for increased physical exertion. (The arteries may have been narrowed by the accumulation of atheromatousplaque - see atherosclerosis, below.) | Typical symptoms include short-term discomfort such as an ache, pain or tightness across the front of the chest when or immediately following exertion or other situations in which heart rate is increased e.g. due to panic or an argument.Other less common effects & symptoms are also possible e.g. similar pain when or soon after eating. |
| Aneurysm | Balloon-like bulge or swelling in the wall of a nartery | In general, causes can be genetic or due to disease, e.g.1. a degenerative disease a syphilitic infection -causing damage to the muscular coat of the blood vessel2.a congenital deficiency in the muscular wall | Aneurysms can cause the wall of the blood vessel to weaken. When an aneurysm gets bigger the risk of rupture increases. That can lead to severe haemorrhage(bleeding) and other complications - some of which may be life threatening. | |
| 3 | Arteriosclerosis | Hardening of the arteries.(Arteriolosclerosis is the hardening of arterioles.)Artery walls thicken, stiffen and lose elasticity, a progressive condition that typically worsens overtime unless action is taken to address it.Note: Healthy arteries are flexible and elastic. | High blood pressure (also known as hypertension) is widely cited as a cause of, or at least a contributory factor to, the development of arteriosclerosis.To reduce risk, keep blood pressure within a healthy range. See also how to reduce risk of atherosclerosis (below). | Arteriosclerosis (in combination with atherosclerosis or otherwise) can reduce the flow of blood, hence the supply of oxygen, nutrients etc.,to tissues in the affected area.Arteriosclerosis can affect any artery in the body but is of greatest concern when occurs in the heart (coronary arteries) or the brain. |
| 4 | Atherosclerosis (Atheroma)- a commontype ofarteriosclerosis (see above) | •Multiple fatty plaques(consisting of e.g.cholesterol and triglyceride)accumulate on the inner walls of arteries.To reduce risk:1. Eat sensibly (see balanced diet)2.Don't smoke3. Take appropriate regular exercise4. Maintain a healthy body weight5. Do not consume excessive alcohol | A chronic disease that can remain asymptomatic for decades. However, blood flow is restricted and eventually obstructed.Various complications of advanced atherosclerosis are possible. One of the most significant risks is of an infarction due to soft plaque suddenly rupturing, causing the formation of a thrombus(blood clot) that can slow or stop blood flow leading to death of the tissues fed by the artery.Thrombosis of a coronary artery can cause a heart attack(Myocardial infarction).The same process in an artery to the brain is commonly called stroke.6. Coronary thrombosisA thrombus is a blood clot.Thrombosis is a condition in which blood changes from a liquid into aCoronary thrombosis can occur due to the accumulation of fatty deposits (plaques)inside the arteries, i.e.atherosclerosis. The hardening of arteries (arteriosclerosis)can also contribute to reducedCan lead to a | |
| 6. | Coronary thrombosis | A thrombus is a blood clot.Thrombosis is a condition in which blood changes from a liquid into a solid state, producing a 'clot'(thrombus). Coronary thrombosis is the condition in which the thrombus is formed in one of the 3 major coronary arteries that supply the heart. | Coronary thrombosis can occur due to the accumulation of fatty deposits (plaques)inside the arteries, i.e.atherosclerosis. The hardening of arteries (arteriosclerosis)can also contribute to reduced blood flow leading to coronary thrombosis. |
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| 7. | Haemophilia | Blood clots onlyvery slowly. | Deficiency of either of two blood coagulation factors:o Factor VIII(antihaemophilic factor), oro Factor IX(Christmas factor) Hereditary -symptoms in males; may be 'carried' by females who can pass itto their sons without being affected themselves. | The person might experience prolonged bleeding after any injury that causes an open wound. In severe cases of haemophilia there may be spontaneous bleeding into muscles and joints.Treatment: Bleeding incases of haemophilia has been treated by transfusions of plasma containing the missing factor, or with concentrated preparations of Factor VIII or Factor IX obtained by freezing fresh plasma. |
| 8 | Haematoma | A collection or accumulation of blood outside the blood vessels, which may clot forming as welling. |
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| 9.Haemorrhoids | Haemorrhoids(also called'piles') areswellingscontainingenlarged andswollen bloodvessels in oraround therectum and anus. |
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Activity 2
Carry out practical exercises to measure the human pulse rate and blood pressure.
The Structure and Functions of Blood
Composition of blood
Blood is the red fluid that circulates in our blood vessels. The average human body contains about 4 to 5 liters of blood. Blood is classified as connective tissue and consists of two main components:
- Plasma is a clear extracellular fluid.
- The solid component, which is made up of the blood cells and platelets
The solid component is made up of blood cells except for the platelets, which are tiny fragmentsof bone marrow cells.
The solid component consists of blood cells (corpuscles) which include:
- Erythrocytes, also known as red blood cells (RBCs)
- Leukocytes, also known as white blood cells (WBCs)
- Platelets, also known as thrombocytes
Red blood cells, most white blood cells, and platelets are produced in the bone marrow, the soft fatty tissue inside bone cavities. The white blood cells (lymphocytes) are also produced in the lymph nodes and spleen, and in the thymus gland.
Within the bone marrow, all blood cells originate from a single type of unspecialized cell called a stem cell. When a stem cell divides, it first becomes an immature red blood cell, white blood cell, or platelet-producing cell. The immature cell then divides, matures further, and ultimately becomes a mature red blood cell, white blood cell, or platelet.
Blood cells
By volume, the plasma constitutes about 55% of whole blood, and red blood cells, platelets and white blood cells about 45%.
Red blood cells
Red blood cells (RBCs) have two main functions:
- To pick up oxygen from the lungs and deliver it to tissues elsewhere.
- To pick up carbon dioxide from other tissues and unload it in the lungs.
Erythrocytes transport oxygen in the blood through the red pigment called hemoglobin. Hemoglobin contains iron and proteins joined to greatly increase the oxygen-carrying capacity of erythrocytes. The high surface area to volume ratio of erythrocytes allows oxygen to be easily transferred into the cells in the lungs and out of the cells in the capillaries of the systemic tissues. Erythrocytes are produced inside red bone marrow from stem cells at the astonishing rate of about 2 million cells every second.
White blood cells
Although the white blood cells account for only about 1% of the blood, they play a very important role in the body. Their main function is to protect the body against disease pathogens. There are two white blood cells, each of which plays a specific role in the protection of the body against illness and disease.
- Phagocytes: Engulf and digest invading bacteria and viruses (pathogens). It is the body’s main defense against germs (microbes).
- Lymphocytes: produce antibodies that neutralize antigens from bacteria or viruses. They kill microbes or make them clump together, to be removed in the lymph glands.
White blood cells are produced in the yellow marrow of the bone, spleen, thymus, and lymphatic system.
Platelets
Platelets are small fragments of bone marrow cells and are therefore not really classified as cells themselves. Platelets have the following functions:
- Secrete vasoconstrictors which constrict blood vessels, causing vascular spasms in broken blood vessels.
- Form temporary platelet plugs to stop bleeding.
- Secrete procoagulants (clotting factors) to promote blood clotting.
- Dissolve blood clots when they are no longer needed.
- Digest and destroy bacteria.
- Secrete chemicals that attract neutrophils and monocytes to sites of inflammation.
- Secrete growth factors to maintain the linings of blood vessels.
In general, the blood platelets function in the healing of the wounds when the skin gets broken. This is achieved by clumping together of the platelets to form a network of mesh, hence bleeding is stopped.
Plasma
Plasma is the non-cellular or liquid portion of the blood. Plasma is a mixture of water, proteins, and dissolved substances. Around 90% of plasma is made of water, although the exact percentage varies depending upon the hydration levels of the individual. Blood plasma has the following functions:
- Plasma serves as a transport medium for delivering nutrients to the cells of the various organs of the body.
- It transports waste products derived from cellular metabolism to the kidneys, liver, and lungs for excretion.
- It fights infections since it contains antibodies.
- It is also a transport system for blood cells, and it plays a critical role in maintaining normal blood pressure.
- Plasma helps to distribute heat throughout the body and to maintain homeostasis, or biological stability, including acid-base balance in the blood and body
- It carries and transports some hormones.
Functions of blood plasma
The structure and functions of red blood cells and white blood cells
Mechanism of blood clotting and its importance
Blood groups (ABO system and the Rhesus factor)
Human blood can be grouped into four blood groups namely groups A, B, AB, and O. They were discovered in 1900 and 1901 at the University of Vienna by Karl Landsteiner in the process of trying to learn why blood transfusions sometimes cause death and at other times save a patient. This classification is based on the type of antigens in the red blood cells and antibodies in the plasma.
Red blood cells have proteins (antigens) on their surface: A, B or A, and B. Plasma have antibodies that can cause agglutination: anti-A and anti-B.
The serum is blood plasma without fibrinogen. It can be stored without clotting and is used in transfusions.
| Blood group | Antigen | Antibodies | Agglutinates |
| A | A | Anti-B | Anti-A serum |
| B | B | Anti-A | Anti-B serum |
| AB | A and B | None | Anti-A and anti-B serums |
| O | None | Anti-A and anti-B | Neither serum |
Consider the table above. People with type A blood will have the A antigen on the surface of their red cells (as shown in the table). As a result, anti-A antibodies will not be produced by them because they would cause the destruction of their own blood. However, if B type blood is injected into their systems, anti-B antibodies in their plasma will recognize it as alien and burst or agglutinate the introduced red cells in order to cleanse the blood of alien protein.
Individuals with type O blood do not produce any antigens. Therefore, their blood normally will not be rejected when it is given to others with different blood types. As a result, type O people are universal donors for transfusions, but they can receive only type O blood themselves. Those who have type AB blood do not make any antibodies. Their blood does not discriminate against any other blood type. Consequently, they are universal receivers for transfusions, but their blood will be agglutinated when given to people with every other type because they produce both kinds of antigens.
Blood grouping
It is easy and inexpensive to determine an individual's blood type from a few drops of blood. This is how blood typing/grouping it is done: A serum containing anti-A antibodies is mixed with some of the blood. Another serum with anti-B antibodies is mixed with the remaining sample. Whether or not agglutination occurs in either sample indicates the blood type. For instance, if an individual's blood sample is agglutinated by the anti-A antibody, but not the anti-B antibody, it means that the A antigen is present but not the B antigen. Therefore, the blood type is A.
Rhesus factor
Some people have another antigen called Rhesus antigen on their red blood cells while others do not have it. Those having this antigen are referred to as Rhesus positive (Rh+) and those without are it are Rhesus negative (Rh-). Rh antigen occurs in red blood cells and the Rh antibody occurs in blood plasma.
If Rh antibody mixes with Rh antigen during a blood transfusion, agglutination will occur. Rh+ can stimulate the Rh- to produce antibodies to act against Rh+ antigens. However, the Rh- cannot stimulate the Rh+ blood to produce antibodies against Rh-. Therefore, an Rh+ person can receive blood from the Rh- donor. The donated blood below is group AB rhesus positive (AB+).
Blood transfusion
Blood transfusion is the transfer of blood from one person (donor) to another person (recipient)through blood vessels. The transfusion is done to replace lost blood due to illness, accidents, or bleeding. The donor is the person who gives blood while the recipient is the person who receives blood.
When performing blood transfusions it is important to avoid combining corresponding antigens and antibodies because they cause agglutination of red blood cells which may lead to the death of the recipient. Agglutination is the clumping of red blood cells. Blood transfusion is only possible if blood groups are compatible. Blood group compatibilities are as shown in the table below.
| Recipient | Donor | |||
| A | B | AB | O | |
| A | √ | × | × | √ |
| B | × | √ | × | √ |
| AB | √ | √ | √ | √ |
| O | × | × | × | √ |
Note: a tick (√) means compatible and a cross (×) means incompatible.
Individuals with blood group AB can receive blood from individuals of all blood groups and are known as universal recipients. Individuals with blood group O can donate blood to individuals of all blood groups and are known as universal donors.
Advantages of blood transfusions
Blood transfusion does so much for patients in need. The gift of life is donated, tested, processed, and sent to hospitals’ transfusion service departments where more important work is done to ensure it is compatible with the recipient.
Blood transfusion has a number of advantages. These are some of the benefits the donated blood can provide for patients in need:
- Increase low hemoglobin levels: low hemoglobin can cause damage to body organs and tissues due to low oxygen levels. Donated blood, with sufficient hemoglobin, can correct the problem of low hemoglobin level of the recipient.
- Help stop bleeding: bleeding may not be controlled if platelets and/or clotting factors are low. Receiving blood with high clotting factors can solve the problem.
- Keeps the heart pumping: low blood volume can lead to low pressure and the heart may not be able to maintain the circulation of blood.
- Help with serious blood infections when other methods fail. For example, a blood transfusion may serve as a treatment method for people with sickle cell anemia or blood cancer(leukemia).
- Provide red cells and platelets when the bone marrow is compromised as with blood cancers, bone marrow transplants, chemotherapy, etc.
- Provide red cells and platelets for patients with blood disorders such as sickle cell.
- Save someone’s life: people who have had a big loss of blood due to a number of reasons can have their lives saved once they receive donated blood.
- Because blood transfusion involves screening of the donor’s blood if the donor has any health problem it can be detected and hence treated before getting worse.
Disadvantages of blood transfusions
Although blood transfusions can be life-saving, they are not without risks. The following are disadvantages of blood transfusions:
Medical reactions:
- Allergic reaction: This is the most common reaction. It happens during the transfusion when the body reacts to plasma proteins or other substances in the donated blood.
- Fever reaction: The person gets a sudden fever during or within 24 hours of the transfusion. Headache, nausea, chills, or a general feeling of discomfort may come with the fever.
- Hemolytic reactions: In very rare cases, the patient's blood destroys the donor red blood cells. This is called hemolysis. This can be severe and may result in bleeding and kidney failure.
Diseases: If proper screening of the donated blood is not observed, it can cause transmission of diseases from the donor to the recipient. Examples of such transmissible diseases are HIV virus, hepatitis, and other infections.
Patients who are given too much blood can develop high blood pressure, a concern for people who have heart disease.
Blood transfusion precautions
Certain precautions and guidelines must be adhered to in blood transfusion to ensure the safety of the procedure. The precautions may include the following:
- Donated blood must carefully and thoroughly be screened for any infectious diseases before being transfused to the recipient. The blood should be screened for diseases like hepatitis B, HIV virus, and all sexually transmitted diseases (STDs).
- The donated blood must be matched with the recipient's blood type, as incompatible blood types can cause a serious adverse reaction (transfusion reaction). Blood is introduced slowly by gravity flow directly into the veins (intravenous infusion) so that medical personnel can observe the patient for signs of adverse reactions.
- During a blood transfusion, vital signs such as body temperature, heart rate, and blood pressure are carefully monitored.
- Some patients may get a sudden fever during or within 24 hours of the transfusion, which may be relieved with pain-relieving drugs such as panadol, diclofenac, or paracetamol. This fever is a common reaction to the white blood cells present in donated blood.
Immune Responses
Natural and artificial immunity
Allergic reactions
Importance of vaccinations against diseases (Tuberculosis, Poliomyletis, Measles, Diphtheria, Whooping cough)
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