Author Archives: Alila Medical Media

Anaphylaxis, with Animation

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Anaphylaxis (incl. anaphylactic shock): etiology, pathophysiology, symptoms and treatment. Anaphylaxis versus anaphylactoid reactions.

Anaphylaxis is a sudden, potentially life-threatening allergic reaction that involves multiple system dysfunction. It is caused by a massive release of inflammatory mediators from mast cells and basophils into the circulation. These mediators are normally responsible for the body’s protective response against infections or injuries. They dilate blood vessels, increase their permeability, allowing immune cells to seep through to arrive at the site of infection. But when released systemically, they can lead to extensive vasodilation and smooth muscle spasms, causing blood pressure to drop and airways to narrow to a dangerous level.

Common triggers include certain medications, foods, insect stings, animal venoms, and latex. Symptoms typically begin within minutes to one hour of exposure, and may include widespread itching, hives, swelling, wheezing and difficulty breathing, nausea, abdominal cramps, diarrhea, dizziness, a fast heart rate and low blood pressure. Shock may develop within minutes, patients may have seizures or faint. There is also a late phase response, usually less severe, within several hours to one day.

Classically, anaphylaxis is defined as a type I hypersensitivity, which involves immunoglobulin E, IgE, and only occurs in presensitized individuals. Patients must have had a previous contact with the allergen, which produced no symptoms, but during which the body had produced IgE antibodies against the allergen. IgE molecules bind to their receptors on the surface of mast cells and basophils. Upon reexposure to the same allergen, or sometimes a similar allergen, the allergen binds to adjacent IgE molecules, bringing their receptors together, triggering a signaling cascade that induces the release of inflammatory chemicals.

There are also anaphylactoid reactions which are clinically indistinguishable from anaphylaxis but do not involve IgE and do not require prior sensitization. They occur via direct stimulation of mast cells or basophils, in the absence of immunoglobulins, and have different triggers. These reactions are now classified as “non-immunologic anaphylaxis”, as they are equally serious and must be treated the same way, with the same urgency.

Immediate injection of epinephrine is the cornerstone treatment for anaphylaxis. Epinephrine increases blood flow, widens airways and may help relieve all symptoms, at least temporarily. Other treatments may include antihistamines, oxygen therapy or intubation, intravenous fluids, beta-agonists, or vasopressors.

The best way to prevent anaphylaxis is to avoid the triggers. People with serious reactions to unavoidable allergens may benefit from immunotherapy. In immunotherapy, patients are injected weekly with gradually increasing doses of the allergen, starting with a tiny amount. This process desensitizes the immune system, reducing reactions to the allergen, but may take several years to complete.

All images/videos by Alila Medical Media are for information purposes ONLY and are NOT intended to replace professional medical advice, diagnosis or treatment. Always seek the advice of a qualified healthcare provider with any questions you may have regarding a medical condition.

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Bell’s Palsy Pathophysiology, Symptoms, Diagnosis and Treatment, with Animation

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Bell’s palsy: pathophysiology, symptoms, causes, risk factors, diagnosis and treatment. How to differentiate Bell’s palsy from stroke.

Bell’s palsy is a form of facial muscle weakness or paralysis, typically on one side of the face. It results from dysfunction of the facial nerve, also known as the seventh cranial nerve. The facial nerve has many branches and diverse functions. It controls the muscles of facial expression, including those involved in eye blinking and closing; it carries nerve impulses to tear glands, salivary glands; and conveys taste sensations from the anterior two-thirds of the tongue. There are two facial nerves, one on each side of the face. Typically, only one nerve, and hence one side of the face, is affected. The malfunction of the facial nerve is thought to result from its inflammation. The swollen nerve is compressed as it exits the skull within a narrow bony canal. Symptoms develop suddenly, usually within a couple of days, and can range from mild weakness to total paralysis of face muscles. Other symptoms may include drooping of mouth, drooling, inability to close one eye, facial pain or abnormal sensation, distorted sense of taste, and intolerance to loud noise. By definition, Bell’s palsy is idiopathic, meaning it has no known cause, but it has been associated with certain viral infections. In particular, reactivation of a dormant virus, triggered by stress, trauma or minor illness, is often thought to be the culprit. Risk factors include diabetes, hypertension, obesity, pregnancy, and upper respiratory infections. Diagnosis is based on clinical presentation after other possible causes of facial paralysis are excluded. Patients usually present with rapid development of symptoms, reaching a peak in severity around 72 hours from the time of onset. In most cases, muscle weakness can be observed with both upper and lower facial muscles, including the forehead, eyelid, and mouth. If forehead muscle strength is not affected, a central cause, especially stroke, should be suspected. This is because the upper facial muscles, unlike the lower ones, receive nerve impulses from both hemispheres of the brain, so a lesion in one side will not affect their function. An electromyography test can be used to confirm nerve damage and determine the extent of severity. Imaging studies can help rule out structural causes, such as a tumor or skull fracture. Because Bell’s palsy impairs the eyelid’s ability to close and blink, the affected eye is exposed to drying and potential injury. Patients must keep the eye moist with lubricating eye drops, and protect it from injury with an eye patch, especially at night. Without treatment, Bell’s palsy resolves spontaneously in about 2 thirds of patients. Symptoms usually start to improve after a few weeks, and complete recovery is achieved in about six months. Corticosteroids, when started early, can reduce inflammation and improve recovery. Some patients may benefit from physical therapy or facial massage. Decompression surgery to relieve pressure on the nerve is rarely needed and not usually recommended. Severe cases may take longer to resolve. A small number of patients with complete paralysis may continue to have some symptoms for life.

All images/videos by Alila Medical Media are for information purposes ONLY and are NOT intended to replace professional medical advice, diagnosis or treatment. Always seek the advice of a qualified healthcare provider with any questions you may have regarding a medical condition.

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All Types of COVID-19 Vaccines, How They Work, with Animation.

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How it works. mRNA vaccine (Pfizer, Moderna), DNA & Viral vector vaccines (Johnson & Johnson (J&J, JNJ), Oxford-AstraZeneca, Inovio, Sputnik V); protein/peptide vaccine (Novavax, EpiVacCorona), conventional inactivated (CoronaVac of Sinovac, Covaxin). Mechanism of each type of coronavirus vaccines explained. Vaccine-induced immune response as compared to natural immunity.

During a natural viral infection, infected cells alert the immune system by displaying pieces of viral proteins on their surface. They are said to present the viral antigen to immune cells – cytotoxic T-cells, and activate them.
Debris of dead cells and viral particles are picked up by professional antigen-presenting cells, (dendritic cells…). Dendritic cells patrol body tissues, sampling their environment for intruders. After capturing the antigen, dendritic cells leave the tissue for the nearest lymph node, where they present the antigen to another group of immune cells – helper T-cells. Viral particles also activate B-cells.
These cells mount 2 types of immunity specific to the viral antigen: cell-mediated immunity and antibody-mediated immunity.
Vaccines deliver viral antigens to trigger immune responses without causing the disease. The events of a vaccine-induced immune response are similar to that induced by a natural infection, although some types of vaccines may induce only antibody-mediated immunity (B cell immunity, not T cell (cellular) immunity).
Many existing vaccines contain a weakened or an inactivated virus. Because the whole virus is used, these vaccines require extensive safety testing. Live attenuated vaccines may still cause disease in people with compromised immune systems. Inactivated vaccines (Sinovac/China, Covaxin/India) only induce humoral (B cell) immunity.
Subunit vaccines contain only part of the virus, usually a spike protein (peptide – EpiVacCorona/Russia). These vaccines may not be seen as a threat to the immune system, and therefore may not elicit the desired immune response. For this reason, certain substances, called adjuvants, are usually added to stimulate the antigen-presenting cells to pick up the vaccine.
Nucleic acid vaccines contain genetic information for making the viral antigen, instead of the antigen itself. Naked DNA vaccines (Inovio, phase 2/3 clinical trials) require a special delivery method to reach the cell’s nucleus (electroporation). Alternatively, a harmless, unrelated virus may be used as a vehicle to deliver the DNA. In this case, the vaccine is also known as viral-vector vaccine (Sputnik V/Russia, Oxford-AstraZeneca, Johnson & Johnson’s). For example, the Oxford-AstraZeneca Covid-19 vaccine uses a chimpanzee adenovirus as a vector. The adenoviral genome is modified to remove viral genes, and the coronavirus spike gene is added. This way, the viral vector cannot replicate or cause disease, but it acts as a vehicle to deliver the DNA. Why a non-human adenovirus is used?
Do DNA vaccines change human DNA?
mRNA vaccines (Pfizer, Moderna) are delivered within a lipid covering that will fuse with the cell membrane. The mRNA is translated into viral antigen, which is then displayed on the cell surface. mRNA vaccines are extremely unlikely to integrate into human genome.
All images/videos by Alila Medical Media are for information purposes ONLY and are NOT intended to replace professional medical advice, diagnosis or treatment. Always seek the advice of a qualified healthcare provider with any questions you may have regarding a medical condition.

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Cerebral Venous Sinus Thrombosis, CVST, with Animation

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CVST is a type of brain stroke caused by blood clots in a vein. This rare blood clot disorder prompted the current pause of Johnson & Johnson (J&J) COVID-19 vaccine, as well as Astrazeneca vaccine.

Pathophysiology, signs and symptoms, risk factors, diagnosis, treatment and prognosis.

Other names: Cerebral vein thrombosis, Cerebral sinovenous thrombosis, Cerebral venous thrombosis (CVT), Cerebral venous and sinus thrombosis, Cerebral venous sinus thrombosis (CVST), Cerebral sinovenous thrombosis (CSVT), Cerebral vein and dural sinus thrombosis, Sinus and cerebral vein thrombosis.

Cerebral venous sinus thrombosis, CVST, occurs when a blood clot forms and blocks a vein in the brain. Blood is transported to the brain in arteries. After delivering oxygen and nutrients, it leaves in veins. Small veins of the brain are called cerebral veins. They drain into large veins, called sinus veins, or venous sinuses. Sinus veins empty into jugular veins, which carry the blood back to the heart. A blockage in a vein causes the blood to back up in the brain, increasing pressure, causing headache, which is often severe. The increased pressure may damage the surrounding brain tissue, producing stroke symptoms such as blurred vision, confusion, loss of consciousness, loss of movement control, seizure or coma. The engorged blood vessel may also rupture, bleeding into the brain, a condition known as “venous hemorrhagic stroke”. Unlike arterial thrombosis that causes the typical brain stroke, venous thrombosis usually develops slowly. This is due to the slow growth of blood clots in veins, and the ability of the venous system to form new vessels to bypass an obstruction, maintaining more or less normal flow at first. In most cases, symptoms develop gradually, over days, weeks or even months, but sudden onset may also occur. CVST is a rare type of stroke that can affect all age groups, including infants. Risks factors include: having inherited blood disorders, systemic conditions, cancers; use of certain medications, and some infections. Women of reproductive age are more at risk due to pregnancy and use of birth control pills. Infants with difficult birth, or whose mothers had certain infections, are also more vulnerable. CVST is often misdiagnosed due to its rarity, wide spectrum of symptoms, and the fact that symptoms can appear suddenly or gradually. The standard MRI or CT scans used to detect stroke are often normal in CVST. To diagnose CVST, the veins must be specifically examined in a procedure called magnetic resonance venography. CVST must be suspected in patients of any age who have severe headache that doesn’t go away, and any risk factors for clotting disorders. Timely diagnosis and prompt treatments are essential for survival. Immediate treatment includes blood thinners, typically intravenous heparin, or subcutaneous low-molecular-weight heparin. The goal is to prevent the enlargement of existing clots and formation of new clots, while letting the body’s own system dissolves the existing clots slowly, typically over weeks or months. However, patients who have bleeding must be monitored closely to ensure it does not worsen. If the patient deteriorates despite heparin, catheter-directed procedures to breakdown blood clots may be considered. Once the patient is out of danger, an oral anticoagulant such as warfarin is typically given for 3 to 6 months, although patients with known clotting disorders may need to take warfarin for life. About 3 in 4 patients fully recover, but it may take some time to get back to normal.

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Sepsis and Septic Shock, with Animation

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Sepsis is a life-threatening condition that occurs when the body’s excessive response to an infection causes damage to its own tissues. Sepsis may progress to septic shock, a body-wide deficiency of blood supply that leads to oxygen deprivation, buildup of waste products, and eventual organ failure. Without timely treatment, mortality rates are high.
With sepsis, patients typically experience fever, weakness, sweating, and a rapid heart rate and breathing rate. As septic shock develops, blood pressure decreases, and signs of organ damage, such as confusion and reduced urine output, can be observed. The skin is initially warm or flushed, then becomes cold, sweaty, mottled or bluish.
While any infection can lead to sepsis, bacterial infections in the lungs, digestive and urinary organs, are the most common causes. Sepsis may also develop from a post-surgery infection or an infected catheter.
Septic shock occurs more often in newborns, the elderly and pregnant women. Other risk factors include having a compromised immune system or chronic diseases, extended hospital stays, having invasive devices, and overuse of antibiotics or corticosteroids.
The pathogenesis of septic shock is not fully understood. In most cases, the immune system is overwhelmed by an infection that gets out of control, and responds with a systemic cytokine release that causes widespread vasodilation and fluid leakage from capillaries. These cytokines also activate the coagulation process, producing tiny blood clots that clog blood vessels, reducing blood flow. Bleeding may also develop because excessive coagulation depletes clotting factors. Poor capillary flow reduces oxygen supply and impairs removal of carbon dioxide and waste products, resulting in organ dysfunction and eventually failure.
Diagnosis is primarily clinical but requires confirmation of an ongoing infection.
An elevated blood lactate level serves as an indicator of shock. This is because in the absence of oxygen, the body switches to anaerobic metabolism, which breaks down glucose only partially, producing lactic acid. Blood tests may also indicate signs of organ damage, and infection. Other specimens such as urine, respiratory or wound secretions, may be taken for culture to detect infection. Imaging tests may also help identify the source of infection. Other causes of shock should be ruled out.
Early and aggressive treatment is critical for survival. Treatments include:
– Intravenous fluids, and possibly vasopressors, to restore blood flow.
– Broad-spectrum antibiotics while waiting for culture results. Once the causative organism is identified, more specific antibiotics will be used.
– Other measures to control infection.
– Supportive care such as supplemental oxygen, and in case of organ failure, mechanical ventilation or dialysis.

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Mitral Stenosis, with Animation

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The mitral valve serves to ensure one-way blood flow from the left atrium to left ventricle of the heart. It opens when left atrial pressure is higher than left ventricular pressure, allowing blood to fill the left ventricle; and closes when the ventricles contract, to prevent blood from flowing back to the atrium. The mitral valve has 2 flaps, or leaflets, supported by a fibrous ring.
Mitral stenosis occurs when these leaflets thicken and become stiff, causing the valve opening to narrow, reducing blood flow. As a result, blood volume and pressure in the left atrium increases, and, over time, this may have several consequences.
First, the left atrium enlarges and becomes a risk factor for developing atrial fibrillation, a condition in which the atria beat rapidly and irregularly. The atrium quivers rather than contracts, and does not empty completely into the ventricle. Ineffective pumping causes the blood to stagnate, facilitating the formation of blood clots. These clots may then pass into the bloodstream, get stuck in small arteries and block them, resulting in stroke and other problems.
Second, because the left atrium receives blood from the lungs, pulmonary pressure may increase, causing secondary pulmonary hypertension, which in turn, may lead to right ventricular heart failure, as well as tricuspid or pulmonary valve regurgitation.
Mitral stenosis is most commonly caused by rheumatic fever, a complication of untreated strep throat or scarlet fever during childhood. For this reason, it is most prevalent in developing countries where rheumatic fever is more common. Rarely, mitral stenosis may develop with age, as a result of accumulated calcium deposits on the valve. Mitral stenosis can also be congenital.
Symptoms progress slowly, over years or even decades, so patients may not be aware until atrial fibrillation or heart failure develops. Symptoms may appear or worsen with increased heart rates, such as during exercise or stress. Women may suddenly discover they have the condition as they become pregnant.
Mitral stenosis produces a characteristic heart murmur that can be heard with a stethoscope. Diagnosis is confirmed with echocardiography, which uses ultrasound to visualize cardiac structures and blood flow. Echocardiography also helps determine the severity of the disease by measuring the mitral valve area. ECG recordings and chest X-ray may show signs of left atrial enlargement.
Because most cases of mitral stenosis are caused by rheumatic fever, prompt treatment of strep throat with antibiotics effectively prevents both rheumatic fever and mitral stenosis.
Treatment is not needed for asymptomatic patients. Patients with mild symptoms may be treated with diuretics to reduce blood pressure; beta-blockers or calcium channel blockers to control heart rates; and anticoagulants to prevent blood clots.
Valve repair or replacement surgery may be indicated for moderate to severe cases.
In percutaneous valvuloplasty, a catheter with a balloon is threaded through a vein and into the heart. The balloon is inflated to widen the opening of the valve, then deflated and removed.
Patients with heavy calcification may require open heart surgery to repair the valve. Valve replacement is considered when repair is not possible. Artificial valves can be mechanical or bio-prosthetic. Mechanical valves last longer but usually require life-long anticoagulation to prevent formation of blood clots.

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COVID-19 Tests Explained

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There are 2 major types of COVID-19 tests: diagnostic tests for active infection, and antibody tests for past infection.
Diagnostic tests look for components of the virus in a sample taken from the nasal cavity, throat, or saliva. Sample taken from the nasopharynx, the upper part of the throat behind the nose, is preferred when higher accuracy is required.
There are 2 types of diagnostic tests: molecular tests detecting viral RNA, and antigen tests detecting viral proteins.
Antigen tests use a technology similar to that of a pregnancy test. Some are made available as at-home test kits. The test is fast and less expensive, but is less sensitive. Antigen test gives positive results only with high viral loads, when the person is near the peak of infection, so it’s more likely to miss an active infection. In other words, the rate of false negative – a test that says you don’t have the virus when you actually do, is high. Symptomatic patients who test negative with rapid antigen test must be confirmed with a more sensitive molecular test. On the other hand, positive results are highly accurate, but false positive – a test that says you have the virus when you actually don’t, can still happen, most commonly due to errors in sample handling.
Molecular tests detect viral RNA. They are also called nucleic acid amplification tests, NAAT, because they amplify viral nucleic acids until there are detectable levels. Different tests are based on different technologies, with polymerase chain reaction, PCR, being just one of them. PCR is the gold standard for diagnostic testing but it requires specific equipment and takes longer to deliver results.
Molecular tests are much more sensitive than rapid antigen tests, but they can still produce false-negative results early in the infection. On the other hand, the high sensitivity may sometimes pick up the low viral load in a patient who has recovered and is no longer contagious. Positive results are highly accurate, most false positives are due to lab contamination or other errors with sample handling.
Antibody tests, also called serology tests, detect antibodies that the body produced in response to the infection. A blood sample is taken for this test. Because antibodies can take a couple of weeks to develop and may stay in the blood for weeks or months after recovery, a positive test result only proves that the person has been exposed to the virus. It gives no indication about active infection and should not be used to diagnose COVID-19.

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Protein Metabolism Overview, with Animation

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Proteins are responsible for nearly all bodily and cellular functions: from structural proteins in bones; contractile proteins in muscles; transport proteins in blood plasma; to hormones, antibodies, cell receptors, ion channels, and enzymes that catalyze almost every chemical reactions in biological systems. Proteins are polymers of amino-acids linked together by peptide bonds. An amino-acid consists of an amino group, a carboxyl group, and a unique side chain, connected to a central carbon, the α-carbon. Instead of being an extended chain of amino-acids, a protein usually folds into a three-dimensional conformation that is critical for its functions. The structure forms as a result of interactions between the side chains of amino-acids, and is thus dictated by the amino-acid sequence. Of the 20 amino-acids that make up proteins, nearly half are essential, meaning the body cannot synthesize them and must get them from the diet. Animal proteins are usually considered high-quality, complete proteins, because they have similar amino-acid composition as human proteins, and can thus provide all the required amino-acids, but a combination of a variety of plant foods may also do the job. Proteins in foods are digested in the stomach and small intestine, by the action of stomach acid, which denatures proteins, and several enzymes that hydrolyze peptide bonds. Together they break down proteins into individual amino acids, which are then absorbed into the bloodstream and transported to the liver. The liver uses these amino-acids to synthesize new proteins, most of which are plasma proteins. The liver also distributes free amino-acids to other tissues, for synthesis of tissue-specific proteins. Proteins are synthesized based on genetic information of the cell, using the genetic code, and regulatory signals. Each cell has a characteristic collection of proteins, specific to its functions. Body proteins are constantly renewed. Older proteins are broken down into free amino-acids, which are recycled, they combine with dietary amino-acids to make new proteins. Unlike carbohydrates and lipids, proteins cannot be stored for later use. Once the cellular requirement for proteins is met, excess amino-acids are degraded and used for energy, or converted into glucose or fatty acids. Use of amino-acids for energy production also occurs when there is energy shortage, such as during prolonged exercise or extended fasting. Since there are no nitrogenous compounds in the energy production pathways, the first step in amino-acid degradation is the removal of the amino group, by deamination or transamination, to produce keto-acids. Some amino-acids can be directly deaminated, while others must transfer their amino group to α-ketoglutarate to form glutamate, which is then deaminated to recycle α-ketoglutarate. Depending on their side chains, keto-acids from different amino-acids may enter the metabolic cycles at different points. They may be converted to pyruvate, acetyl-CoA, or one of the intermediates of the citric acid cycle. Some of these reactions are reversible. When amino-acids are in short supply, citric acid intermediates can be aminated to create new amino-acids for protein synthesis. Deamination produces ammonia, which is toxic if accumulated. The liver converts ammonia to urea to be excreted in urine. Extreme diets that are excessively high in proteins may overwhelm the kidneys with nitrogenous waste and cause renal damage.

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Lipid Metabolism, with Animation

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Although the term “lipid” includes several types of molecules, lipid metabolism usually refers to the breakdown and synthesis of fats. Fats are triglycerides, they are esters of glycerol and three fatty acids. Fats can come from the diet, from stores in adipose tissue, or can be synthesized from excess dietary carbohydrates in the liver.
Dietary fats are digested mainly in the small intestine, by the action of bile salts and pancreatic lipase. Bile salts emulsify fats. They act as a detergent, breaking large globules of fat into smaller micelles, making them more accessible to lipase. Pancreatic lipase then converts triglycerides into monoglycerides, free fatty acids, and glycerol. These products move into the cells of intestinal epithelium – the enterocytes, inside which they re-combine again to form triglycerides. Triglycerides are packaged along with cholesterol into large lipoprotein particles called chylomicrons. Lipoproteins enable transport of water-insoluble fats within aqueous environments. Chylomicrons leave the enterocytes, enter lymphatic capillaries, and eventually pass into the bloodstream, delivering fats to tissues. The walls of blood capillaries have a surface enzyme called lipoprotein lipase. This enzyme hydrolyzes triglycerides into fatty acids and glycerol, enabling them to pass through the capillary wall into tissues, where they are oxidized for energy, or re-esterized for storage.
Fats that are synthesized endogenously in the liver are packed into another type of lipoprotein, the VLDL, to be transported to tissues, where triglycerides are extracted in the same way.
When required, fat stores in adipose tissue are mobilized for energy production, by the action of hormone-sensitive lipase, which responds to hormones such as epinephrine.
Lipid metabolism pathways are closely connected to those of carbohydrate metabolism. Glycerol is converted to a glycolysis intermediate, while fatty acids undergo beta-oxidation to generate acetyl-CoA. Each round of beta-oxidation removes 2 carbons from the fatty acid chain, releasing one acetyl-CoA, which can then be oxidized in the citric acid cycle. Beta-oxidation also produces several high-energy molecules which are fed directly to the electron transport system. Fats yield more energy per unit mass than carbohydrates.
When acetyl-CoA is produced in excess, it is diverted to create ketone bodies. During glucose starvation, ketone bodies are an important source of fuel, especially for the brain. However, ketone bodies are acidic, and when produced in excess, can overwhelm the buffering capacity of blood plasma, resulting in metabolic acidosis, which can lead to coma and death. Ketoacidosis is a serious complication of diabetes, in which cells must oxidize fats for fuel as they cannot utilize glucose. Extreme diets that are excessively low in carbohydrates and high in fat may also result in ketoacidosis.
On the other hand, diets that are high in carbohydrates generate excess acetyl-CoA that can be converted into fatty acids. Synthesis of fatty acids from acetyl-CoA is stimulated by citrate, a marker of energy abundance, and inhibited by excess of fatty acids. Fatty acids can be converted into triglycerides, for storage or synthesis of other lipids, by combining with glycerol derived from a glycolysis intermediate.

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Carbohydrate Structure and Metabolism, with Animation.

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Carbohydrates are biomolecules that consist of carbon, hydrogen and oxygen atoms, usually in the ratio of 1:2:1. Carbohydrates play crucial roles in living organisms. Among other functions, they serve as major sources of energy, and structural components.
Carbohydrates are made of base units called monosaccharides. Monosaccharides consist of a carbon chain with a hydroxyl group attached to all carbons except one, which is double-bonded to an oxygen. This carbonyl group can be in any position along the chain, forming either a ketone or an aldehyde. Some monosaccharides share the same molecular formula, but are different in structure due to different positions of atoms. These seemingly small structural details result in completely different sugars, with different properties and metabolism pathways.
Monosaccharides exist in open-chain form and closed-ring form. The ring forms can connect to each other to create dimers, oligomers and polymers, producing disaccharides, oligosaccharides and polysaccharides, respectively. Examples of disaccharides are sucrose, maltose, and lactose. Common polysaccharides include glycogen, starch and cellulose, all of which are polymers of glucose. Their differences arise from the bonds between monomers. Glycogen and starch serve as energy storage in animals and plants, respectively. Their monomers are bonded by alpha-linkages. Some monomers can make more than one connection, producing branches. Starch in food can be digested by breaking these bonds, with the enzyme amylase.
Cellulose, the major structural component of plants, consists of unbranched chains of glucose bonded by beta-linkages, for which humans lack the enzyme to digest. Cellulose and other non-digestible carbohydrates in food do not supply energy, but are an important part of human diet, known as dietary fibers. Fibers help slow digestion, add bulk to stool to prevent constipation, reduce food intake, and may help lower risk of heart diseases.
During digestion, digestible carbohydrates are broken down into simple sugars. Digestion of starch starts with amylase in the saliva and continues in the small intestine by other enzymes. Sucrose and lactose are hydrolyzed by their respective intestinal enzymes. Simple sugars are then absorbed through the intestinal wall and transported in the bloodstream to tissues, for consumption or storage.
Foods rich in simple sugars deliver glucose to the blood quickly, and can be helpful in case of hypoglycemia, but regular diets of simple sugars produce high spikes of glucose and may promote insulin insensitivity and diabetes. Complex carbohydrates take longer to digest and release simple sugars. Eating complex carbohydrates helps dampen the spikes of blood glucose and reduce diabetes risk.
Glucose is central to cellular energy production. Cells break down glucose when energy reserves are low. Glucose that is not immediately used is stored as glycogen in liver and muscles. Glycogen is converted back to glucose when glucose is in short supply.
Energy production from glucose starts with glycolysis, which breaks glucose into 2 molecules of pyruvate, releasing a small amount of energy. Glycolysis involves multiple reactions and is tightly regulated by feedback mechanism.
In the absence of oxygen, such as in the muscles during exercise, pyruvate is converted into lactate. This anaerobic pathway produces no additional energy, but it regenerates NAD+, which is required for glycolysis to continue.
When oxygen is present, pyruvate is further degraded to form acetyl-CoA. Significant amounts of energy can be extracted from oxidation of acetyl-CoA to carbon dioxide, by the citric acid cycle and the following electron transport system. When present in excess, acetyl-CoA is converted into fatty acids. Reversely, fatty acids can breakdown to generate acetyl-CoA during glucose starvation.
When blood sugar level is low and glycogen is depleted, new glucose can be synthesized from lactate, pyruvate, and some amino-acids, in a process called gluconeogenesis, which is almost the reverse of glycolysis.
Metabolism of other simple sugars converges with the glycolytic pathway at different points. For example, fructose feeds into the pathway at the level of 3-carbon intermediate, and thus bypasses several regulatory steps. Fructose entrance to glycolysis is therefore unregulated, unlike glucose. This means production of acetyl‐CoA from fructose, and its subsequent conversion to fats, can occur unchecked, without regulation by insulin.

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