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40.2A: The Role of Blood in the Body - Biology

40.2A: The Role of Blood in the Body - Biology


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The many roles of blood include delivering nutrients and oxygen to cells, transporting waste from cells, and maintaining homeostasis.

Learning Objectives

  • Identify the variety of roles played by blood in the body

Key Points

  • Blood plays an important role in regulating the body’s systems and maintaining homeostasis.
  • Other functions include supplying oxygen and nutrients to tissues, removing waste, transporting hormones and other signals throughout the body, and regulating body pH and core body temperature.
  • Blood is composed of plasma, red blood cells, white blood cells, and platelets.
  • Blood platelets play a role in coagulation (the clotting of blood to stop bleed from an open wound); white blood cells play an important role in the immune system; red blood cells transport oxygen and carbon dioxide.
  • Blood is considered a type of connective tissue because it is made in the bones.

Key Terms

  • hydraulic: pertaining to water
  • coagulation: the process by which blood forms solid clots
  • homeostasis: the ability of a system or living organism to adjust its internal environment to maintain a stable equilibrium

The Role of Blood in the Body

Blood is a bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells. The components of blood include plasma (the liquid portion, which contains water, proteins, salts, lipids, and glucose ), red blood cells and white blood cells, and cell fragments called platelets.

Blood plays an important role in regulating the body’s systems and maintaining homeostasis. It performs many functions within the body, including:

  • Supplying oxygen to tissues (bound to hemoglobin, which is carried in red cells)
  • Supplying nutrients such as glucose, amino acids, and fatty acids either dissolved in the blood or bound to plasma proteins (e.g., blood lipids)
  • Removing waste such as carbon dioxide, urea, and lactic acid
  • Immunological functions, including circulation of white blood cells and detection of foreign material by antibodies
  • Coagulation, which is one part of the body’s self-repair mechanism (blood clotting by the platelets after an open wound in order to stop bleeding)
  • Messenger functions, including the transport of hormones and the signaling of tissue damage
  • Regulating body pH
  • Regulating core body temperature
  • Hydraulic functions, including the regulation of the colloidal osmotic pressure of blood

Medical terms related to blood often begin with hemo- or hemato- (also spelled haemo- and haemato-), which is from the Greek word α (haima) for “blood”. In terms of anatomy and histology, blood is considered a specialized form of connective tissue, given its origin in the bones.


Vasodilation

Vasodilation is the dilation, or widening, of blood vessels. (The word dilatation is also sometimes used instead of dilation when talking about a hollow, tubular structure.) Vasodilation causes increased blood flow through the blood vessels and decreased blood pressure. Substances that cause vasodilation are called vasodilators. The opposite of vasodilation is vasoconstriction, which is when blood vessels become narrower.

This is a simple diagram of vasoconstriction and vasodilation.


6.1 + 6.2 IB biology study guide

h. through nerves/named example of nerve/autonomic/sympathetic/ parasympathetic nervous system ✔ In mph, only accept vagus nerve for slowing heart rate and sympathetic nerve for accelerating it.

i. one nerve increases the rate and the other decreases it ✔

j. epinephrine/adrenaline increases heart rate/force of contraction ✔

b. secreted by salivary glands/pancreas ✔

c. active/released into the mouth/small intestine ✔

d. acts on starch/polysaccharides ✔

e. breaks «glycosidic» bond by hydrolysis/adding water ✔

f. converts insoluble/large molecule to soluble/small molecules ✔

b. have specific active sites to which specific substrates bind ✔

c. enzyme catalysis involves molecular motion and the collision of substrates with the active site ✔ OWTTE

d. enzymes break macromolecules into monomers/smaller molecules indigestion ✔

e. smaller molecules/monomers more readily absorbed ✔

f. <<pancreas>> secretes enzymes into the «lumen of» small intestine ✔

g. the small intestine has an alkaline pH ✔

h. enzymes have maximum action at specific pHs
OR
enzymes can be denatured at other pHs ✔

i. amylase breaks down starch into sugars/disaccharides ✔

j. lipase breaks lipids/triglycerides into monoglycerides/fatty acids and glycerol ✔

k. endopeptidase/protease breaks «peptide» bonds in proteins/polypeptides ✔

b. open valves allow blood to flow through
OR
opening and closing of valves controls timing of blood flow «during cardiac cycle» ✔

c. closed «semilunar» valves allow ventricles/chambers to fill with blood
OR
closed «semilunar» valves allow pressure in ventricles to rise «rapidly» ✔

d. valves open when pressure is higher upstream/OWTTE/converse for closed valves ✔

e. AV/bicuspid/tricuspid/mitral valves prevent backflow from ventricle to atrium
OR
AV/bicuspid/tricuspid/mitral valves open when pressure in atrium is higher «than in the ventricle»/when atrium is pumping/contracting ✔

f. semilunar/aortic/pulmonary valves prevent backflow from artery to ventricle
OR
semilunar/aortic/pulmonary valves open when pressure in ventricle is higher «than in the artery»/when ventricle is pumping/contracting ✔

b. example of simple diffusion, eg: fatty acids

c. facilitated diffusion of nutrients involves movement through channel proteins

d. example of nutrient for facilitated diffusion eg: fructose

e. active transport of nutrients against a concentration gradient / involving protein pumps

f. example of active transport, eg: (iron) ions/glucose/amino acids

g. endocytosis / by means of vesicles

b. nutrients move into tissues

c. gas exchange / Oxygen and carbon dioxide exchange between tissues and blood/capillaries

d. (nitrogenous) wastes/excess water move from cells/tissues into blood/capillaries

b. pressure is high in arteries/pressure is low in veins

c. arteries receive blood from ventricles/heart / carry blood away from heart

d. lumen of artery is small to keep pressure high

e. arteries have thick (muscular) walls (with elastic fibres) to withstand pressure

f. elastic fibres recoil in response to ventricle/heart contraction

g. muscle / elastic fibres help maintain pressure between heartbeats
OR
muscle / elastic fibres help propel blood toward capillary beds

h. veins receive blood from capillaries/capillary beds / carry blood to heart

i. large lumen of veins so there is less resistance to blood flow

b. oxygen diffuses from air to blood and carbon dioxide diffuses from blood to air

c. oxygen binds to hemoglobin in red blood cells

d. pressure inside/volume of alveoli increases/decreases / air enters/exits alveoli during inspiration/expiration/ventilation

e. blood flow through capillaries / concentration gradients of gases/oxygen/CO2 maintained

f. type II pneumocytes secrete fluid/surfactant / secretion of surfactant to prevent sides of alveolus adhering

b. heart is a double pump / heart has separate pumps for lungs and other systems / left and right sides of heart are separate / no hole in heart (after birth)

c. deoxygenated blood pumped to the lungs and oxygenated to other organs/tissues/whole body (apart from lungs)

d. each side of the heart has an atrium and a ventricle

e. left ventricle/side pumps blood to the systems/tissues and right ventricle/side pumps blood to the lungs

f. left atrium receives blood from the lungs and right atrium receives blood from systems/tissues

g. left ventricle pumps blood via the aorta and right ventricle pumps blood via the pulmonary artery

h. left atrium receives blood via the pulmonary vein and right atrium receives blood via the vena cava

i. lungs require lower pressure blood / high pressure blood would damage lungs

j. high pressure required to pump blood to all systems/tissues apart from lungs

k. pressure of blood returning from lungs not high enough to continue to tissues / blood has to be pumped again after returning from lungs

l. oxygenated blood and deoxygenated blood kept separate / all tissues receive blood with high oxygen content/saturation


Inflammation in the body is caused by tissue damage and is usually a result of infection or injury. Neutrophils play a key role in the inflammatory response and are the first white blood cells to arrive at the site of tissue damage. They rapidly enter the injured tissue from the bloodstream where their primary function is to prevent wound infection by killing off invading microbes. If a wound becomes infected, a number of the neutrophils involved in fighting the pathogens will die and accumulate in the infected area. These dead cells are a major constituent of pus.


The Blood

The components of blood are plasma, red blood cells, white blood cells, and platelets. Some homeostatic functions of blood are the transport of nutrients and wastes, defending the body against invaders, and distributing heat to regulate body temperature.


The image above shows the formed elements of the blood.


Roles of Absorption in Different Nutrients

All carbohydrates are absorbed as monosaccharide’s in stomach and jejunum. Glucose and galactose are absorbed by active trans­port. Sodium pump of the cell membrane helps in its active take up.

Fructose is ab­sorbed by facilitated transport. Glucose, galactose and fructose are absorbed into the blood capillaries. The most rapidly transported monosaccharide is galactose with glucose running a close second.

2. Absorption of Amino acids:

Amino acids are absorbed by active transport and some amino acids are absorbed by facili­tated transport. It occurs mainly in the duodenum and jejunum. Normally 95-98% of amino acids are absorbed in the small intestine. They also enter the blood stream (Fig. 16.22).

3. Absorption of Fatty acids and Glycerol (= Absorption of fat) and fat soluble vitamins:

All these nutrients are absorbed via simple diffusion. Fatty acids and glycerol are insoluble in water, therefore, they cannot reach the blood stream directly. They are first incorporated into small, spherical, water soluble droplets called micelles with the help of the bile salts and phospholipids in the intestinal lumen.

A micelle is an aggregate of many molecules. From the micelles fatty acids, glycerides, sterols and fat soluble vitamins are absorbed into the intestinal cells by diffusion where they are resynthesized in the ER and are converted into very small fat molecules (droplets) called chylomicrons.

The latter are released from the intestinal cells into the lymph present in the lymphatic capillaries, the lacteals. Small quantities of short chain fatty acids are absorbed directly into the blood by diffu­sion rather than into the lymph. Fatty acids, glycerol and vitamins are absorbed in jejunum.

4. Absorption of Water:

About 90% of all water absorption occurs in the small intes­tine by osmosis from the lumen of the small intestine through epithelial cells and into the blood capillaries in the villi. The absorption of water from the small intestine is associated with the absorption of electrolytes and digested food in order to maintain an osmotic balance with the blood. Absorption of water also occurs in the stomach and the large intestine.

5. Absorption of Salts (Electrolytes):

Sodium is absorbed from small intestine by active transport. This process is coupled to the movement of glucose, as mentioned earlier. Several other ions, including calcium, potassium, magnesium, iron and phosphate are ab­sorbed by active transport. Calcium absorption is enhanced by vitamin D and parathormone (hormone secreted by parathyroid glands).

Chloride ions can be absorbed by diffusion or active transport. Salts are also absorbed into the blood capillaries. Most ions are actively absorbed throughout the small intestine. Calcium absorption is mainly limited to the duode­num. Almost all iron absorption occurs in the duodenum. Bile salts are absorbed in ileum.

6. Absorption of Water Soluble Vitamins:

Most of water soluble vitamins such as the vitamin В complex, vitamin С and vitamin P are absorbed by simple diffusion into the blood capillaries. But reabsorption of vitamin B12 requires combination with Castle’s intrinsic factor produced by the stomach for its absorption.

7. Absorption of Alcohol:

Because alcohol is lipid soluble, it begins to be absorbed in the stomach. However, the surface area for absorption is much greater in the small intestine than in the stomach, so when alcohol passes into the duodenum, it is absorbed more rapidly.

Thus amino acids, monosaccharide’s, short chain fatty acids, minerals, water soluble vitamins, and water are absorbed into the blood and fatty acids, glycerol, glycerides and fat soluble vitamins are absorbed into the lymph.


Red Blood Cells and Hemoglobin

Only a small amount of the oxygen needed for life can dissolve directly in plasma. Oxygen transport instead relies on red blood cells. At any one time, there are more than 25 trillion RBCs in circulation in an adult, more than the combined total of all other cell types in the body. As RBCs develop, they extrude their cell nucleus , so that at maturity they have almost nothing inside their membranes except the oxygen-carrying protein, hemoglobin . The absence of a nucleus contributes to the RBC's short life, as does the constant physical stress it experiences squeezing through capillaries that are narrower than it is. The average RBC circulates for approximately 120 days before being destroyed in the liver, bone marrow, or spleen. The iron from hemoglobin is recycled, while the cyclic nitrogen compound that holds it, called heme, is converted to bilirubin. Bilirubin is transported to the liver for elimination from the body as bile. Liver disease can cause jaundice, a yellowing of the skin due to bilirubin in the blood.

The iron in hemoglobin is critical for oxygen transport. Lack of dietary iron is one cause of anemia, a condition in which the blood cannot carry enough oxygen. The heme group binds oxygen tightly when the concentration of O 2 is high (as it is in the lungs), but quickly releases it when the concentration is low, as it is in the tissues. The iron can also bind carbon monoxide (CO), which is produced by car engines and other combustion sources. CO binds much more tightly than oxygen does and prevents oxygen binding, making CO a deadly poison.

A genetic variant of the hemoglobin gene causes a single amino acid change in the hemoglobin molecule. This change causes the red blood cell to become sickle-shaped at low oxygen concentrations, so that it tends to become lodged in small capillaries, depriving tissues of oxygen. A person with one such variant hemoglobin gene does not suffer ill effects, but with two variants will develop sickle-cell anemia. Despite this, the sickling variant is common in populations historically exposed to malaria, because having one variant helps protect against malaria infection.


Tissue Fluid: Formation and Functions | Plasma | Blood | Biology

In this article we will discuss about:- 1. Definition and Sources of Tissue Fluid 2. Composition of Tissue Fluid 3. Functions 4. Aggregation.

Definition and Sources of Tissue Fluid:

Tissue fluid is formed from the plasma by process of diffusion and filtration. This fluid occupies the intracellular space and forms the connecting link in the transport of nutrition, gases and the metabolic end products between blood capillaries, tissue cells and the lymph. It constitutes the internal environment of the body, which surrounds tissue cells.

Tissue fluid is derived from two sources:

The amount of tissue fluid formed from blood depends upon:

(b) The difference of pressure between the capillary and the tissue fluid, and

(c) The difference of colloidal osmotic pressure of blood and tissue fluid.

It is obvious that anything that increases the capillary permeability will increase the amount of tissue fluid formed. Regarding blood pressure and osmotic pressure, it is known that at the arterial end of capillaries, the average blood pressure is about 32 mm of Hg and at the venous end, 10 mm of Hg.

The colloidal osmotic pressure at both ends is same (25 mm of Hg on the average). At the arterial end, the net filtration pressure which is the difference between the two is 7 mm of Hg towards the tissue (interstitial) fluid. At the venous end due to fall in blood or hydrostatic pressure, the filtration pressure is 15 mm of Hg to the opposite side, i.e., from tissue fluid to the capillary (Fig. 5.2).

The amount of tissue fluid formed from the tissue cells depends upon the degree of metabolic activity of the cells. Tissue cells produce water as an end product of metabolism. This metabolic water is added to the already existing tissue fluid. More the degree of activity more will be the metabolic water formed and consequently the amount of tissue fluid will increase.

Two important exceptions to the capillary pressure are:

(a) In capillaries of the lungs, hydrostatic pressure about 6 mm of Hg, and

(b) In capillaries of the kidneys, glomerular hydrostatic pressure about 60 to 70 mm of Hg.

If the hydrostatic pressure is increased within capillaries, then it will interfere the return of materials to the lymphatic’s or capillaries and will result in excess accumulation of tissue fluid (i.e., oedema).

Composition of Tissue Fluid:

It is very difficult to obtain a pure sample of tissue fluid hence, its exact composition is not known. It is believed that its composition is same as that of lymph, excepting that its protein content is negligible and as such, its colloidal osmotic pressure is very low.

The composition and volume of tissue fluid is regulated by constant interchange with blood and lymph. It has been mentioned above that filtration of tissue fluid takes place at the arterial end of the capillaries. At the venous end of the capillary the blood pressure is very low—about 10 mm of Hg and the colloidal osmotic pressure is much higher. These two factors help in drawing away just as much fluid comes out from the arterial side. As we know that water content of tissue fluid is derived from two sources—blood and tissue cells.

The amount of water that goes out of blood is drawn in again at the venous side of the capillaries. But vascular capillaries cannot draw away the amount of metabolic water formed by the tissue cells. It is for the drainage of this excess water that the lymphatic system has developed. Thus it will be seen that blood and lymph remain as if on two sides of tissue fluid and try to keep it constant in volume and composition by continuous interchange.

Specific gravity of the tissue fluid is about 1.015 to 1.023. It may contain a few erythrocytes. But regarding the white cells, the tissue fluid contains a good number of lymphocytes and a small number of granulocytes. Blood proteins and nutrient contents of it are very low. It does not contain platelets and may also clot, but with a very slow process. It contains higher concentration of waste products but glucose, salt and water contents are more or less same as those are present in blood.

Functions of Tissue Fluid:

i. It constitutes the internal medium in which the tissue cells are bathed. The cells draw in oxygen and nutrition from the tissue fluid and excrete their metabolites into it. Hence, tissue fluid may be regarded as the medium which supplies all the immediate requirements of the cell.

ii. It acts as a great reservoir of water, salts, nutrition, etc. This function is very important. Under any condition, in which the blood volume is increased or diminished, physical forces are set up by which the blood volume is kept constant with the help of the tissue reserve. For example in haemorrhage, the capillary pressure becomes very low and goes below the colloidal osmotic pressure in the capillary which remains same.

Due to this higher O.P in the capillaries, water is drawn in from the tissue spaces, so that blood volume is restored. When water is drawn away from blood, such as due to diuresis, excessive sweating or diarrhoea, blood volume and blood pressure will be lowered, but the plasma proteins will be more concentrated. This will increase the colloidal O.P. of blood. This increased osmotic pressure of plasma and reduced blood pressure will increase the rate of absorption from the tissue fluid, and thus blood volume will be kept constant.

On the other hand, when blood volume increases, as for instance, by intravenous injection of large quantities of isotonic saline, fluid will pass out into the tissue spaces due to two causes:

a. Saline will dilute the colloids and reduce the colloidal osmotic pressure.

b. Increased volume of blood will raise the blood pressure and cause more filtration. Both these factors will cause more fluid to run out into the tissue spaces, until blood volume comes back to the original level.

Aggregation of Tissue Fluid:

Swelling or oedema observed sometimes in different parts of the body is due to the aggregation of the tissue fluid.

This might result from several factors:

i. Increased capillary permeability resulting from dilated, damaged or inflammated capillary.

ii. Increase in the capillary pressure which might be due to changes in posture (in lower extremities it is due to continued standing), obstruction to veins or rise in the venous pressure as observed in the cardiac failure.

iii. Blockage of lymphatic nodes or vessels, as a result of inflammation of the node or blockages by very small worms like that of Filaria.

iv. Loss of the plasma proteins whether due to malnutrition or excessive loss resulting from the renal damage, causes decrease in plasma osmotic pressure and excessive aggregation of the tissue fluid.

v. Renal disease causes impairment of excretion of urine and the resulting water retention causes increase in the tissue fluid.

vi. Unfamiliar exercise might cause swelling due to accumulation of metabolites.

vii. Ingestion of a large amount of salts results in retention of water. Adrenal cortical extract also produces similar effects.


Blood

Blood is an important fluid that keeps us alive. We cannot live without it. The heart pumps blood to all parts of the body and brings them oxygen and food. At the same time blood carries all the substances we don&rsquot need away from us. Blood fights infections, keeps our body temperature the same and carries chemicals that control body functions. Finally, blood has substances that repair broken blood vessels so that we don&rsquot bleed to death.

What blood is made of

Blood is a mixture of fluid and solid matter.

Plasma is the liquid part of our blood. It makes up about 50 &ndash 60 % of it. Plasma consists mostly of water but many other substances are in it. It contains dissolved food, chemicals that control our growth and do other jobs, proteins, minerals and waste products.

Red blood cells look like flat round discs. They contain haemoglobin, a protein that carries oxygen to the body and gives blood its red colour. Each drop of blood has about 300 million of these red cells.

White blood cells, also called leukocytes, fight infections and harmful substances that invade the body. Most of these cells are round and colourless. They have different sizes and shapes. White blood cells are not as numerous as red ones. For every 700 red blood cells there is only one white blood cell.

Platelets are tiny bodies that are much smaller than red blood cells. They stick to the edges of a cut and form blood clots to stop bleeding. The blood of a normal adult has about 2 trillion platelets.

How blood works in the body

The circulatory system carries blood to all parts of your body. The heart pumps blood through big blood vessels called arteries and veins. In our body there are also millions of small blood vessels called capillaries. Oxygen, food and other substances pass through the thin walls of these capillaries into the tissue.

When you inhale air oxygen passes through your lungs and and is picked up by haemoglobin which transports it to your whole body. It is released into cells which produce energy. In return cells produce carbon dioxide which enters your blood stream and is transported back to your lungs where it is exhaled.

The blood stream of our body

Food also reaches your body by means of blood. It is digested in your stomach and important substances like fat, sugar, proteins, vitamins and minerals are separated. These nutrients enter your blood stream and are moved to the cells and muscles where they are needed in order to give you energy or fuel. The work of the muscles and other tissue creates heat. Blood is the transporting system which carries heat throughout your body and warms you. The things that you don&rsquot need are transported to your intestines and kidneys and leave your body again.

White blood cells play an important role in your immune system. When harmful substances invade your body an alarm goes off and white blood cells are activated. Then they work to destroy the invaders. They fight off viruses, harmful bacteria and begin anti-body production.

Blood also carries hormones to places where they are needed. When a hormone reaches a part of the body it controls growth, how the body uses food and other things.

You would bleed to death from a small cut if your blood didn&rsquot clot. When a blood vessel breaks platelets rush to the damaged area and stick to one another , forming a plug.

The blood supply

Blood cells come from bone marrow. They begin as stem cells and then develop into red or white blood cells, or platelets. They don&rsquot live forever and must be replaced by new ones. Red blood cells live an average of 120 days before wearing out. Then they are captured and destroyed in the liver and spleen. Platelets live only for about 10 days.

The amount of blood in your body depends on your size, weight and the altitude at which you live. An adult who weighs 80 kg has about 5 litres of blood, a 40 kg child about half the amount. People who live in high areas where the air is thinner need more blood to deliver more oxygen to the body.

Blood groups

Blood groups are very important in order to find out if a person can donate blood or receive blood in case of an accident or another disease. Almost everyone&rsquos plasma has antibodies that that may not work together with another person&rsquos blood.

There are four main blood groups :

  • Type 0 is the most common blood group. In an emergency type 0 blood can be transfused to anybody.
  • Type AB is the most seldom group. People with this blood group can receive any other blood in case of an emergency.
  • Type A can only be received by donors with A or 0.
  • Type B can only be received by donors with B or 0.
Blood Groups

Blood transfusion

If an adult suddenly loses a litre or more of blood he may die unless the blood in his body can be replaced. Over the years blood transfusions have saved countless lives. Transfusions can also help patients who cannot produce enough blood cells to survive. They also help during operations when patients lose some blood.

Blood banks collect blood from donors and put it in sterile bags. It is cooled down and can be stored for up to 50 days. Laboratory workers screen blood for infectious diseases like AIDS and hepatitis. Only clean and safe blood can be given to patients.

Blood diseases

When a person suffers from anaemia there are not enough red blood cells to supply the body with the oxygen he needs. Leukaemia is a kind of cancer of the bone marrow, in which not enough or abnormal white blood cells are produced. Without white blood cells diseases can enter your body without being controlled.

When your body does not have enough platelets blood cannot clot well. Even small injuries can lead to a loss of blood because bleeding doesn&rsquot stop.


The biology of fats in the body

When you have your cholesterol checked, the doctor typically gives you levels of three fats found in the blood: LDL, HDL and triglycerides. But did you know your body contains thousands of other types of fats, or lipids?

In human plasma alone, researchers have identified some 600 different types relevant to our health. Many lipids are associated with diseases--diabetes, stroke, cancer, arthritis, Alzheimer's disease, to name a few. But our bodies also need a certain amount of fat to function, and we can't make it from scratch.

Researchers funded by the National Institutes of Health are studying lipids to learn more about normal and abnormal biology. Chew on these findings the next time you ponder the fate of the fat in a French fry.

Fat Functions

Triglycerides, cholesterol and other essential fatty acids--the scientific term for fats the body can't make on its own--store energy, insulate us and protect our vital organs. They act as messengers, helping proteins do their jobs. They also start chemical reactions involved in growth, immune function, reproduction and other aspects of basic metabolism.

The cycle of making, breaking, storing and mobilizing fats is at the core of how humans and all animals regulate their energy. An imbalance in any step can result in disease, including heart disease and diabetes. For instance, having too many triglycerides in our bloodstream raises our risk of clogged arteries, which can lead to heart attack and stroke.

Fats help the body stockpile certain nutrients as well. The so-called "fat-soluble" vitamins--A, D, E and K--are stored in the liver and in fatty tissues.

Using a quantitative and systematic approach to study lipids, researchers have classified lipids into eight main categories. Cholesterol belongs to the "sterol" group, and triglycerides are "glycerolipids." Another category, "phospholipids," includes the hundreds of lipids that constitute the cell membrane and allow cells to send and receive signals.

Breaking It Down

The main type of fat we consume, triglycerides are especially suited for energy storage because they pack more than twice as much energy as carbohydrates or proteins. Once triglycerides have been broken down during digestion, they are shipped out to cells through the bloodstream. Some of the fat gets used for energy right away. The rest is stored inside cells in blobs called lipid droplets.

When we need extra energy--for instance, when we exercise--our bodies use enzymes called lipases to break down the stored triglycerides. The cell's power plants, mitochondria, can then create more of the body's main energy source: adenosine triphosphate, or ATP.

Recent research also has helped explain the workings of a lipid called an omega-3 fatty acid -- the active ingredient in cod liver oil, which has been touted for decades as a treatment for eczema, arthritis and heart disease. Two types of these lipids blocked the activity of a protein called COX, which assists in converting an omega-6 fatty acid into pain-signaling prostaglandin molecules. These molecules are involved in inflammation, which is a common element of many diseases, so omega-3 fatty acids could have tremendous therapeutic potential.

This knowledge is just the tip of the fat-filled iceberg. We've already have learned a lot about lipids, but much more remains to be discovered.



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