State that hormones are chemical messengers secreted by endocrine glands into the blood and transported to specific target cells.
The endocrine system along with the nervous system works to maintain homeostasis.
Endocrine glands secrete chemical messengers known as hormones that have a physiological effect on cells in the body
The hormones are carried on the bloodstream to the cells they will have an effect on; these are known as target cells. These cells have special receptors that react to the presence of the hormone and can be located a significant distance from the location of the endocrine gland.
State that hormones can be steroids, proteins and tyrosine derivatives, with one example of each
Hormones can be steroids, proteins and tyrosine derivatives.
- Steroids - Testosterone, estrogen, progesterone
- Protein - Insulin, ADH, FSH, LH
- Tyrosine - thyroxin
Distinguish between the mode of action of steroid hormones and protein hormones.
Steroid hormones and protein hormones used two different mechanisms to effect change. Steroid hormones are often produced from cholesterol and are classified as a lipid. The plasma membrane is a lipid bilayer therefore steroids can pass easily across the membranes of their target cells. This is important because the hormone receptors are inside the target cell. Protein hormones are also known as peptide hormones. Unlike steroid hormones the protein hormone does not enter the cell. Instead it binds with a receptor protein on the surface of the plasma membrane of the target cell. Once the binding of the protein hormone and the receptor protein occurs a secondary messenger molecule is released inside of the cell and it causes the changes in cellular activity usually through the activation or inhibition of an enzyme.
Outline the relationship between the hypothalamus and the pituitary gland.
The hypothalamus is an important section of the human brain that acts as a link between the endocrine system and the nervous system. The pituitary gland is located just below the hypothalamus and though it is often referred to as the master gland, the pituitary itself is controlled by the hypothalamus. The pituitary gland consists of an anterior and posterior lobe. Each lobe is connected to the hypothalamus in a different way. The posterior pituitary is connected to the hypothalamus via neurosecretory cells. These long cells have dendrites and cell bodies in the hypothalamus and their axons extend down into the posterior pituitary. As the neurosecretory cells produce hormones from the cell body the hormones travel down the axons and into the posterior pituitary. The posterior pituitary is now responsible for the secretion of the hormone that was actually produced by the hypothalamus. An example of hormones secreted this way would be oxytocin and ADH (antidiuretic hormone). The neurosecretory cells of the hypothalamus also secrete hormones called releasing hormones into a capillary bed within the hypothalamus itself. These capillary beds join to form a portal vein that leads to capillaries within the anterior pituitary. The releasing hormones can then move out of the blood to their target cells within the anterior pituitary. This causes the cells of the anterior pituitary to secrete specific hormones that are produced by and released from the anterior pituitary. One such releasing hormone is known as GnRH (gonadotropin releasing hormone) and it causes the secretion of FSH (follicle stimulating hormone) and LH (lutenizing hormone) from the anterior pituitary).
Explain the control of ADH (vasopressin) secretion by negative feedback.
ADH is a hormone called antidiuretic hormone. It is produced by the hypothalamus and secreted by the posterior pituitary. It is often referred to as vasopressin and it is a hormone that controls how much water is reabsorbed from the collecting duct back into the bloodstream. If ADH is secreted by the posterior pituitary the collecting duct becomes permeable to water and water will leave the collecting duct via osmosis into the medulla of the kidney. Water is then reabsorbed back into the bloodstream. If ADH is not secreted the collecting duct remains impermeable to water and the urine produced by the kidneys will contain high water content. The hypothalamus regulates and detects changes in the concentration of blood plasma using osmoreceptors. If the blood plasma becomes too concentrated impulses are relayed to the down the axon of the neurosecretory cell and ADH will be released into the blood stream from the stores in the posterior pituitary. ADH will travel in the blood stream to the kidneys where it will increase the permeable of the collecting ducts to water so that it may be released into the bloodstream. If blood plasma becomes too dilute there is no ADH released which means less water is reabsorbed and more dilute urine is produced. This results in the increased concentration of the blood plasma and is an example of negative feedback.
State that digestive juices are secreted into the alimentary canal by glands, including salivary glands, gastric glands in the stomach wall, the pancreas and the wall of the small intestine.
The alimentary canal of the digestive system includes the mouth, esophagus, stomach, small intestine, and large intestine. Food travels in one direction through the alimentary canal and is digested along the way, nutrients are absorbed and waste products are formed. The digestive system also contains accessory organs which contribute secretions that aid in the chemical digestion of food. These accessory glands include the salivary glands, gastric glands, pancreas, liver and glandular cells in the intestinal wall. Remember the primary role of digestion is to convert the large macromolecules we eat into their much smaller subunits that can then be absorbed into our cells. These accessory glands contribute digestive secretions or “juice” that aid in the breakdown of macromolecules.
Explain the structural features of exocrine gland cells.
An exocrine gland is a collection of cells that produce a product and release it into a very specific location. An exocrine gland cell contains ducts which transfer a secretion from the gland to a specified location. More often than not the secretion is a protein either in the form of a digestive enzyme or a hormone. Due to the fact that the secretions from an exocrine gland are often proteins these glands tend to contain high numbers of organelles that are involved in the synthesis and processing of proteins. This would include high numbers of ribosomes, endoplasmic reticulum, Golgi bodies, vesicles, and mitochondria. The pancreas is an example of an exocrine gland that secretes enzymes into a duct. The exocrine cells of the pancreas are grouped around the end of a very small branch of the much larger pancreatic duct known as a ductule. The ductules empty into larger and larger ducts until the secretions reach the pancreatic duct and can then be released into the small intestine. The grouping or arrangement of the exocrine glands around the ductule is called an acinus.
Compare the composition of saliva, gastric juice and pancreatic juice.
- solvent in water; amylase; mucus
- Gastric Juice - solvent in water; mucus; Hydrochloric acid; Pepsin in the form of pepsinogen
- Pancreatic Juice - solvent in water; amylase; bicarbonate; trypsin in the form of trypsinogen; lipase
Outline the control of digestive juice secretion by nerves and hormones, using the example of secretion of gastric juice.
Digestive juices are not continuously secreted. They are only released when they are needed to hydrolyze molecules. For example, digestive enzymes in the stomach are released once a stimulus is received. The sight and smell of food can stimulate the release of gastric juices. Presence of food in the stomach stimulates receptors in the stomach wall to send a message to the brain which in turn signals the secretion of more gastric juices. There is also a hormone called gastrin involved in the control of the secretion of gastric juices. When the stomach is distended (widened due to presence of food) it promotes the production of gastrin. Gastrin is a hormone that causes a sustained release of gastric fluid, most of all hydrochloric acid.
Outline the role of membrane-bound enzymes on the surface of epithelial cells in the small intestine in digestion.
Most enzymes which enter the alimentary canal catalyse their specific hydrolytic reaction by mixing with the substrates that have been consumed. They often have a limited molecular lifespan and are normally digested themselves or eliminated as wastes. There are some exceptions which are membrane-bound digestive enzymes which are produced by and remain in the membranes of the cells (epithelial) lining the small intestine. An example of such an enzyme would be maltase. Maltase breaks down the disaccharide maltose into two glucose molecules. Maltase remains embedded within the membranes of the inner epithelial cells of the villi and microvilli. As maltose comes in contact with the active site of maltase the enzymes catalyses the hydrolysis reaction. The advantage of the membrane bound enzymes is that once the reaction has been catalysed the end products are exactly where they need to be in order to be absorbed.
Outline the reasons for cellulose not being digested in the alimentary canal.
Despite the fact that many mammals are herbivores, no species of mammal including humans produces an enzyme that can digest cellulose, a polysaccharide composed of thousands of glucose molecules and a major structural component of plant cell walls. Mammals known as grazers (i.e. cows) contain a huge number of mutualistic bacteria in their intestines which produce an enzyme known as cellulase that can hydrolyze cellulose into glucose. Despite the number of bacteria present these grazers still cannot get a high yield of energy from plant material so they must continually ingest large amounts of plant matter. Due to the fact that we do not contain cellulase producing bacteria and we cannot produce cellulase on our own, most plant material we consume exits the body in our feces.
Explain why pepsin and trypsin are initially synthesized as inactive precursors and how they are subsequently activated.
Pepsin and trypsin are known as proteases; enzymes which catalyze the hydrolysis of peptide bonds in proteins. The difficulty is that proteases cannot distinguish between a protein that has been ingested and a structural protein that is part of the human body. In order to control the hydrolysis of essential proteins these proteases are initially released in an inactive molecular form. Pepsin is released as pepsinogen and trypsin is released as trypsinogen. The inactive molecular form of pepsin has 44 additional amino acids attached to the primary structure of the enzyme. When pepsinogen is released into the stomach it is exposed to hydrochloric acid which removes the extra 44 amino acids converting pepsinogen into pepsin, the active protease. There is a lining of mucus that protects the lining of the stomach from being digested by pepsin and hydrochloric acid. Trypsinogen is released from the pancreas via the pancreatic duct and enters the small intestine at the duodenum. When the partially digested food from the stomach enters the small intestine it stimulates the release of an enzyme known as enterokinase. Enterokinase converts trypsinogen into its active form trypsin.
Discuss the roles of gastric acid and Helicobacter pylori in the development of stomach ulcers and stomach cancers
It was believed for a long time that stomach ulcers were caused by too much hydrochloric acid in the stomach perhaps brought on by stress. Until very recently scientists believed that the acidic level in the human stomach was too high for any organism to survive there. Dr. Barry J. Marshall and Dr. J. Robin Warren isolated bacterial cells from the intestines of patients with stomach ulcers in 1982-83. This has lead us to what we now know and understand about this strain of bacteria known as Helicobacter pylori. These bacteria can survive in the human stomach by burrowing beneath the mucus layer and infecting the cells of the lining of the stomach. They use the enzyme urease to create ammonia which neutralizes the stomach acids. The infection in the cells leads to gastritis and stomach ulcers. Patients can be treated with antibiotics but those who have had gastritis for 20-30 years have a significant increase in their risk of stomach cancer as compared to the general population.
Explain the problem of lipid digestion in a hydrophilic medium and the role of bile in overcoming this.
Lipids serve many important functions within the human body (think phospholipids!) however they are difficult to digest due to their insolubility in water. As we saw earlier the solute for digestive enzymes is water and lipids are not water soluble. This means we have hydrophobic molecules in a hydrophilic medium. When lipids are exposed to an aqueous environment they tend to stick together or coalesce. When the molecules clump together it decreases the surface area in comparison to the volume of molecules. This means that the lipid molecules on the outside of this cluster of lipids can be digested but the enzymes cannot reach the lipid molecules in the interior. This problem is overcome by the addition of bile to the small intestine. Bile is produced by the liver and stored in the gall bladder. Bile molecules have both a hydrophobic and hydrophilic end which allows them to be partially soluble in both water and lipids. This means that bile molecules can wedge themselves inside that large lipid globule, break them apart and prevent them from coalescing again into the large lipid molecule. This process is known as the emulsification of lipids. The now much smaller lipid molecules can be digested by lipase.
Draw and label a diagram showing a transverse section of the ileum as seen under a light microscope.
The small intestine is divided into three sections; the duodenum
and the ileum
. The duodenum is the first section where secretions from the pancreas and liver are added to the chyme which has entered the small intestine from the stomach. The jejunum and the ileum are much longer sections where digestion is completed and nutrients are absorbed. The longitudinal
smooth muscle of the intestines move in rhythmic contractions known as peristalsis which move food through the small intestine. The intestinal mucosa
is the innermost lining of the small intestine that shapes itself into villi; invaginations of the mucosa that greatly increase the surface area of the small intestine for absorption of nutrients. Digested food molecules come into direct contact with the mucosa which is responsible for the absorption of the nutrients. Each villus contains a network of capillaries
and a lacteal
Explain the structural features of an epithelial cell of a villus as seen in electron micrographs, including microvilli, mitochondria, pinocytotic vesicles and tight junctions.
Epithelial cells line the outside of the villus and it is through these cells that the digested nutrients are absorbed into either the capillaries or the lacteal. The epithelial cells contain a few different structural features which makes them efficient for absorption. The microvilli cover the entire surface of the villus. The microvilli extend into the lumen of the small intestine and significantly increase the surface area of the villus for absorption. This means nutrients can be absorbed faster (increased rate of diffusion and facilitated diffusion). The epithelial cells will also contain high numbers of mitochondria so that food nutrients which cannot enter by simple diffusion can be brought into the cells via active transport. Active transport uses ATP produced by the mitochondria to move particles inside the cells. Molecules such as amino acids, glucose, and mineral ions can be absorbed into the cell using active transport. Pinocytotic vesicles are also used in the epithelial cells as another form of active transport to move molecules from the lumen of the small intestine to the interior of the cells. The pinocytotic vesicles contain droplets of fluid from the small intestine. Once inside the cell the particles can move out of the vesicle and into the cytoplasm of the epithelial cell. Due to the fact that most cells in the body are surrounded by fluid, molecules can sometimes slip between cells. It is important however that this does not occur in the lining of the small intestine. Food molecules must be completely digested before they can be absorbed and the plasma membrane of the epithelial cells ensures that absorption is controlled and that only fully digested molecules are moving into the capillaries and the lacteal. To ensure that this is what occurs the epithelial cells are packed close together and are sealed to one another by structures called tight junctions. The adjacent membranes share some membrane proteins and this help to keep them tightly packed together.
Explain the mechanisms used by the ileum to absorb and transport food, including facilitated diffusion, active transport and endocytosis.
Both active and passive transport are used to move substances from the lumen of the small intestine inside the epithelial cells. All molecules that move into the cells have been digested and are now small enough to pass inside of the cells. Some molecules such as those that are soluble in lipids pass easily inside the epithelial cells via simple diffusion. Facilitated diffusion is used to transport molecules whose polarity makes it difficult for them to pass through the hydrophobic center of the plasma membrane. The channel proteins can help moves these molecules inside due to their non-polar outer perimeter and their polar interior channel. Active transport using membrane pumps can be used to absorb molecules for which a concentration gradient does not exist. The membrane proteins of the plasma membrane in the microvilli are used to pump molecules inside using ATP from the mitochondria. Endocytosis or more specifically pinocytosis can also be used to bring in small droplets of fluid which can later be released into the cytoplasm.
List the materials that are not absorbed and are egested.
- Cellulose – found in plant cell walls
- Lignin – also found in plant cell walls
- Bile pigments – give color to feces
- Bacteria – found in large intestine
- Intestinal cells – break off as food moves through lumen
Outline the circulation of blood through liver tissue, including the hepatic artery, hepatic portal vein, sinusoids and hepatic vein.
The liver is the largest organ in mammals. It has an excellent blood supply received from two major blood vessels; the hepatic artery and the hepatic portal vein. The hepatic artery delivers oxygenated blood from the aorta. The hepatic portal vein delivers blood to the liver from the digestive tract. This blood is low in oxygen and the pressure in the vein is also low because it has already been through the capillary bed of the intestines and stomach. Although it is low in oxygen at times it can contain high levels of digested nutrients. The levels of nutrients in the blood will vary based on the timing of ingestion and the time it takes to digest and then absorb the nutrients. The blood leaves the liver through the hepatic vein which carries the deoxygenated blood away from the liver and back to the heart via the inferior vena cava. The function of the liver is to remove some substances from the blood and to add other substances to the blood. One important function is to monitor and regulate the concentration of nutrients in the blood before it flows through the entire body. Liver cells known as hepatocytes are responsible for the removal and addition of substances to and from the blood. The liver is divided into many lobes and the hepatic artery and hepatic portal vein branch into the lobes. Blood from both of these vessels flow through the sinusoids of the liver and then drain into wider vessels that lead to the hepatic vein. In the sinusoids, exchange of materials occurs between the blood and the hepatocytes. This can occur because blood is flowing slowing through the sinusoids and the spaces between the endothelial cells that line the sinusoids facilitate the exchange. Although the sinusoids are often considered the capillary beds of the liver they differ in many ways. The lumen of the sinusoids is larger than that of capillaries. They are lined with gapped endothelial cells that facilitate the exchange of materials between the blood stream and the hepatocytes. They contain Kupffer cells that aid in breaking down worn out red blood cells and they receive a mixture of oxygenated and deoxygenated blood at the same time.
Explain the role of the liver in regulating levels of nutrients in the blood.
The levels of nutrients in the blood rise and fall depending on how recently a meal was ingested and the rate at which the body is using the nutrients. The liver plays a very important role in regulating blood nutrient levels. As the blood passes through the sinusoids the hepatocytes absorb and store excess nutrients and will release those nutrients when the levels drop too low in the blood. Glucose, commonly called blood sugar, is an example of a nutrient whose levels are controlled by the liver. Glucose is filtered out of the blood and stored as a polysaccharide called glycogen in the liver. This helps to maintain a normal concentration of glucose in the blood. However, when blood glucose levels drop too low the glycogen is converted back into glucose and released into the blood stream. Two hormones produced by the pancreas help the liver maintain proper blood glucose levels. Insulin (a hormone produced in the beta cells of the Islets of Langerhans in the pancreas) stimulates the hepatocytes to absorb glucose when levels are too high. Glucagon (a hormone produced in the alpha cells of the Islets of Langerhans in the pancreas) stimulates the hepatocytes to convert glycogen into glucose and release it into the blood stream.
Outline the role of the liver in the storage of nutrients, including carbohydrate, iron, vitamin A and vitamin D.
Carbohydrates are metabolized and stored in the liver in the form of glycogen. The hormones insulin and glucagon stimulate the cells in the liver to either store or release glucose.
As erythrocytes (red blood cells) flow through the sinusoids they are broken down by the Kupffer cells. Iron is a component of hemoglobin, a protein found in red blood cells, and as those cells are broken down the iron is removed and stored in the liver. Most of the iron will eventually be used by the bone marrow to produce hemoglobin for new erythrocytes.
Also known as retinol, vitamin A is found in dairy products and carrots. It is stored in the liver and released when there is a deficit in the blood. A deficiency of retinol can lead to “night blindness”.
Also known as calciferol, vitamin D is found in cod liver oil and dairy products. It is stored in the liver and released when there is a deficit in the blood. A vitamin D deficiency can lead to rickets in small children. Your body can also produce vitamin D in your skin with the help of UV light,
State that the liver synthesizes plasma proteins and cholesterol.
There are several plasma proteins that are produced or synthesized in the rough endoplasmic reticulum of the hepatocytes. These include albumin, fibrinogen, and globulins. Albumin plays a role in the regulation of osmotic pressure in the body and fibrinogen plays a role in blood clotting. Globulins are a diverse group of plasma proteins and not all of them are produced in the liver. Although we ingest cholesterol in our food, the hepatocytes synthesize cholesterol on a daily basis. Most cholesterol is used to produce bile salts while some is used as a component of cell membranes.
State that the liver has a role in detoxification
The hepatocytes of the liver absorb toxic substances from the blood such as ethanol, food preservatives, pesticides and herbicides and convert them into non-toxic or less toxic substances.
Describe the process of erythrocyte and hemoglobin breakdown in the liver, including phagocytosis, digestion of globin and bile pigment formation.
Erythrocytes have a short life span (120 days or 4 months). Due to the fact that they do not contain a nucleus they cannot undergo mitosis and must therefore be replaced by new cells from the bone marrow. As a red blood cell nears the end of its life span the membrane weakens and ruptures. This normally occurs in the spleen or bone marrow but has the potential to happen anywhere in the bloodstream. The rupture of the cell membrane releases hemoglobin molecules into the bloodstream and as the blood flows through the sinusoids of the liver the Kupffer cells ingest the hemoglobin molecules via phagocytosis. Hemoglobin is a globular protein consisting of four polypeptides (globin) and a prosthetic group known as the heme group which contains iron. Inside the Kupffer cells the hemoglobin is broken down into globins and heme groups and iron. The globins are broken down via hydrolysis into amino acids which are released into the bloodstream. The iron is removed from the heme groups and stored in the liver before being sent to the bone marrow to be used in the production of new erythrocytes. Once the iron has been removed from the heme group the remaining substance is known as bilirubin or bile pigments. The bile pigments are absorbed by the hepatocytes and used in the production of bile.
Explain the liver damage caused by excessive alcohol consumption
- The liver plays a key role in the detoxification of the blood. As alcohol is consumed it is absorbed from the small intestine into the blood stream and delivered to the liver. The liver cells absorb the alcohol and break it down. Not all alcohol can be absorbed in one passage through the liver so it may pass back through the sinusoids several times before it can all be absorbed. Each time it passes through the hepatocytes absorb more and more of the alcohol and begin breaking it down. Excessive consumption of alcohol leads to the break down of the hepatocytes which are replaced by fatty tissue. As liver cells die they cannot be replaced so liver function is significantly impaired. Long term exposure to alcohol results in the following three primary effects on the liver. Cirrhosis: irreversible damage to liver tissue characterized by normal tissue (hepatocytes and blood vessels) being replaced by scar tissue.
- Fat Accumulation: Fatty deposits build up in place of normal liver tissue.
- Inflammation: damaged liver tissue swells due to overexposure to alcohol. This is often referred to as alcoholic hepatitis.
- Excessive exposure to alcohol over an extended period of time significantly damages the liver and results in cirrhosis, the development of scar tissue. Liver function is significantly impaired and death may result from liver failure. Studies show that females are more susceptible to liver damage than males. It has also been shown that some liver damage that is not too severe can be partially reversible if excessive drinking ceases.
Explain the events of the cardiac cycle, including atrial and ventricular systole and diastole, and heart sounds
The cardiac cycle
is a series of repeating events or sequences that are commonly referred to as one heartbeat. It includes all events that occur between the beginnings of one heartbeat to the beginning of the next one. When we refer to an individual’s heart rate in beats per minute (bpm) we are actually talking about how many cardiac cycles are occurring in each minute. When the chambers of the heart contract it increases pressure inside the chamber and blood flows out of the chamber and this is called systole
. When the chambers of the heart are relaxed and filling with blood this is called diastole
. Both atria undergo systole and diastole concurrently and the same holds true for the ventricles. The valves in the heart keep blood flowing in one direction only. The two atrioventricular valves (tricuspid and bicuspid) allow blood to flow from the atria to the ventricles. The two semi lunar valves (pulmonary and aortic) allow blood to flow from the ventricles into the arteries. These valves open and close depending on the pressure of blood on either side of the valve. The sound of the heart is commonly described as a “lub-dub” sound. Each cardiac cycle or heartbeat is one lub-dub and for the most part what is heard is the sound of the valves in the heart closing. Keep in mind that what happens on the right side is happening on the left at the same time so although there are 4 valves we only hear two separate sounds. The lub portion of the cardiac cycle is the atrioventricular valves closing while the dub portion is the semi lunar valves closing.
- Timing within the cardiac cycle
- Origin of sound
- Heard 0.1 seconds into cardiac cycle at the end of atrial systole and the beginning of ventricular systole
- Atrioventricular valves closing
- Heard 0.4 seconds into the cardiac cycle at the end of ventricular systole.
- Semi lunar valves closing
- Lack of sound for a total of 0.5 seconds; 0.4 seconds of the first cardiac cycle and 0.1 seconds of the next cardiac cycle.
Analyse data showing pressure and volume changes in the left atrium, left ventricle and the aorta, during the cardiac cycle
During the portion of the cardiac cycle when both chambers are in diastole pressure and volume is low in both chambers. The atrium is currently receiving slow flowing blood from the pulmonary veins while the ventricle has just sent blood out into the aorta. In this case atrial pressure is slightly higher than ventricular pressure which allows the atrioventricular valve to remain open and blood can move passively through. Pressure in the aorta however is much higher which keeps the semi lunar valve closed and prevents backward flow of blood. When the atrium is in systole and the ventricle is in diastole the pressure produced by the atria is not very high. Remember the walls of the atria are relatively thin and can therefore not create a lot of pressure. Also, a lot of blood has already passively collected in the ventricle so at this point the systole function of the atrium is to move any remaining blood into the ventricle. When the atrium is in diastole and the ventricle is in systole blood pressure significantly increases in the ventricle. Pressure in the ventricles surpasses that of the atria and causes the atrioventricular valve to close. Pressure in the aorta is still higher than the ventricle so the semi lunar valve remains closed. There is a much larger volume of blood in the ventricle now and as the muscular walls of the ventricle begin to contract the pressure in the ventricle exceeds that of the aorta causing the semi lunar valve to open and blood flows into the aorta. As the blood empties, the pressure in the ventricle once again drops below that of the aorta and the semi lunar valve closes. Both the atria and ventricle are now in diastole and the cardiac cycle begins again.
Outline the mechanisms that control the heartbeat, including the roles of the SA (sinoatrial) node, AV (atrioventricular) node and conducting fibres in the ventricular walls.
Changes in heart rate can occur due to factors such as exercise or emotion and controlled by hormones and the brain. During times of rest your cardiac cycle is being controlled by the heart itself and this is known as myogenic control. The muscle in your heart does not need nervous stimulation to contract. Instead the mass of tissue known as the pacemaker stimulates the heart muscle cells causing them to contract in unison. Although individual cardiac muscle cells contract in an independent rhythm, they will synchronize their contractions when they come in contact with one another. The sinoatrial node (pacemaker) is located in the upper wall of the right atrium close to the opening from the superior vena cava. The SA node is a cluster of modified cardiac cells that have the capacity of producing action potentials on a regular basis. Action potentials produced by the SA node are generated at the beginning of each cardiac cycle and they spread out resulting in the systole of the atrium. The action potentials also have an effect on another cluster of cells known as the atrioventricular node or AV node. The AV node is located in the lower wall of the right atrium in the septum between the right and left atria. Once it has been stimulated by the action potential from the SA node it sends out its own action potential which travels to both ventricles. Due to the thickness of the ventricles there are conducting fibres in the septum that extend from the AV node and deliver the action potential to the ventricles causing them to contract.
Outline atherosclerosis and the causes of coronary thrombosis.
- Humans begin life with very healthy cells and tissues which begin deteriorating over time. Some damage is due to the natural aging process while some damage can be attributed to lifestyle. Arteries are designed to expand with the movement of blood and increases in pressure. Over time, the slow build up of plaque begins in the arteries. Plaque is composed of lipids, cholesterol, cell debris and calcium. This build up is most noticeable in the large and medium size arteries and takes many years to develop into a serious problem. That serious problem is known as atherosclerosis, the slow build up of plaque in the arteries which then begins to harden and results in arteries that are much less flexible. Build up usually begins at a damaged area on the inner lining of the artery. White blood cells known as phagocytes migrate to the damaged area and release growth hormones to help repair the artery wall. Low density lipoproteins (LDL) can release cholesterol in the damaged area which begins to build up large deposits causing the narrowing of the lumen. There is also a loss of elasticity in the artery due to the plaque. The coronary arteries branch off of the aorta and supply the cardiac muscle with much needed oxygen and nutrients. If these arteries become narrowed due to atherosclerosis, the deposits can often cause blood clots to form. The formation of a clot is known as thrombosis and these clots can break away from the artery walls and block smaller arteries. If a coronary artery or one of its branches becomes blocked then the heart tissue it normally serves will be deprived of oxygen. The cells of this cardiac tissue will not be able to carry out cell respiration and they will stop contracting and this is what we commonly call a heart attack or coronary thrombosis. Symptoms of a heart attack include chest pain which often radiates out to the left arm, constriction of throat, difficulty breathing, severe dizziness and often fainting.
Discuss factors that affect the incidence of coronary heart disease.
Risk factors include having parents who have experienced heart attacks (genetic), age, being male, smoking, obesity, eating too much saturated fat and cholesterol, and lack of exercise.
Define partial pressure.
Partial pressure can be defined as the pressure exerted from a single type of gas when it is found in a mixture of gases. Each gas within the mixture will exert its own partial pressure which combined with the others forms the total pressure of the mixture. If we consider oxygen within a mixture of gases for example, the pressure exerted due to oxygen is known as the partial pressure of oxygen. The partial pressure exerted by the gas within the mixture is the same as the pressure it would exert if it occupied the same volume alone at the same temperature.
Explain the oxygen dissociation curves of adult hemoglobin, fetal hemoglobin and myoglobin.
Hemoglobin is a protein molecule found within red blood cells and has the capability of carrying oxygen and carbon dioxide molecules. Hemoglobin is a conjugated protein. It consists of four polypeptides each with a heme group attached near its center. The heme group contains an iron molecule and it is the iron that binds with an oxygen molecule and allows hemoglobin in the blood to carry oxygen. Due to the fact that each of the four polypeptides in a hemoglobin molecule contains a heme group, one hemoglobin molecule has the capacity to carry four oxygen molecules. Hemoglobin can alter its three dimensional shape as oxygen bonds with the heme group. The hemoglobin molecule has four possible shapes depending on how many oxygen molecules it is carrying and each shape affects the hemoglobin’s ability to bind with oxygen molecules. This is referred to as hemoglobin’s affinity for oxygen
and it increases with each addition of an oxygen molecule. In other words, a hemoglobin molecule that is already carrying three oxygen molecules has the highest affinity for oxygen. This is due to the fact that the addition of each oxygen molecule changes the shape of the hemoglobin which increases its affinity for another molecule of oxygen.
And now to oxygen dissociation curves...
An oxygen dissociation curve
is a graph that displays how various forms of hemoglobin or myoglobin perform under various conditions. It shows the percent saturation of hemoglobin with oxygen at each partial pressure of oxygen. In other words, it shows the tendency of hemoglobin to both bind to oxygen and also to dissociate from it. The x axis
of the graph measures the partial pressure of oxygen
while the y axis
shows the percent saturation of hemoglobin with oxygen
. Hemoglobin is saturated with oxygen when it is carrying four oxygen molecules. The oxygen dissociation curve pictured below is for adult hemoglobin.
- The S-shape curve of the graph represents the affinity of hemoglobin for oxygen at various partial pressures. At low pressures there is little oxygen already bound to hemoglobin and therefore it has a low affinity for oxygen. At moderate partial pressure, oxygen will dissociate with adult hemoglobin. At higher partial pressure some oxygen has already been bound to the hemoglobin and this causes a change in shape of the hemoglobin molecule and further increases the molecule’s affinity for oxygen. The plateau of the graph represents the saturation of the hemoglobin molecules with oxygen. The dissociation of oxygen from hemoglobin is very important to humans. Hemoglobin binds with oxygen in the lungs to form oxyhemoglobin and delivers it to the body tissues. The oxygen needs to dissociate form the hemoglobin in order to diffuse into the body’s cells where it is needed.Adult hemoglobin releases oxygen over a narrow range of partial pressure. If you refer back to the oxygen dissociation curve for adult hemoglobin, the dotted lines represent the range of normal partial pressures within the human body. The upper end of the normal range (approx. 75 mm Hg) represents the partial pressure found within the lungs. At this point we can see that hemoglobin is more than 90% saturated with oxygen. The lower end of the normal range (approx. 35 mm Hg) represents the partial pressure typical of body tissues and hemoglobin is only about 50% saturated with oxygen. Myoglobin is also an oxygen binding protein that consists of one globin (polypeptide) and one heme group. It is found in the muscles and its function is to store oxygen in the muscles. Myoglobin holds onto oxygen until it is needed by the muscles when they enter an anaerobic situation.
Due to its structure myoglobin can only hold onto one oxygen molecule and it has a very high affinity for oxygen. As you can see by the oxygen dissociation curve for myoglobin, it will only release oxygen when partial pressure in the muscle tissues is very low. This allows myoglobin to hold onto its oxygen until it is needed and its release will help delay the onset of anaerobic respiration when you are exercising heavily. Hemoglobin in fetal blood is different than hemoglobin in adult blood. It has a different amino acid sequence that changes its molecular composition from that of adult hemoglobin and therefore increases its affinity for oxygen. It is very important that fetal hemoglobin have such a high affinity for oxygen so that it can bind with oxygen molecules released by the placental capillaries and not dissociate until it reaches the respiring tissues of the fetus. In the following oxygen dissociation curve for fetal hemoglobin you can see that it is consistently to the left of the adult curve which shows that at any point in the curve, adult hemoglobin has less oxygen attached at any given partial pressure.
Describe how carbon dioxide is carried by the blood, including the action of carbonic anhydrase, the chloride shift and buffering by plasma proteins.
Carbon dioxide is a waste product produced by cellular respiration. As it is produced it eventually diffuses out of the cells and into capillaries or into tissue fluids that are drawn into the capillaries. Once in the blood stream, carbon dioxide must be delivered to the lungs in one of the following three ways. A small percentage (about 7%) of carbon dioxide will remain as is dissolved in the blood plasma and will be carried to the lungs. Some carbon dioxide (15-20%) will bind with hemoglobin and be carried to the lungs. Hemoglobin can only carry one carbon dioxide molecule. When carbon dioxide binds to hemoglobin it forms carbaminohemoglobin which will dissociate in the lungs so that carbon dioxide can be released. When hemoglobin binds with carbon dioxide it lowers its affinity for oxygen. The majority of the carbon dioxide (about 70%) will enter the erythrocytes where it will be converted into hydrogen carbonate ions which will then be transported in blood plasma. The conversion of carbon dioxide into hydrogen carbonate ions can happen in a split second once it enters the red blood cells. The erythrocytes contain an enzyme called carbonic anhydrase which catalyses a reaction in which carbon dioxide is combined with water to form carbonic acid. The carbonic acid will quickly dissociate into a hydrogen carbonate ion and a hydrogen ion. The hydrogen carbonate ions produced in the red blood cells will leave the cells via facilitated diffusion through specialized protein channels. One hydrogen carbonate ion will be exchanged for a chloride ion from the blood plasma. This exchange of ions balances the charges on either side of the red blood cell and is known as the chloride shift. The hydrogen ions that were formed must also be accounted for or there would be a major shift in blood pH. A process known as pH buffering occurs in which some of the hydrogen ions temporarily bind with hemoglobin and some hydrogen ions are released from the RBC into the blood plasma where they bind with plasma proteins. The plasma proteins act as pH buffers.
Explain the role of the Bohr shift in the supply of oxygen to respiring tissues.
- Hemoglobin must dissociate with oxygen in the respiring tissues and this is achieved through an effect known as the Bohr shift. Hemoglobin has a high affinity for oxygen but this affinity is reduced when the hemoglobin is in an environment where the partial pressure of carbon dioxide is high.
- The partial pressure of carbon dioxide is high in respiring tissues which means it will bind with hemoglobin. When carbon dioxide binds with hemoglobin it causes a conformation change which promotes the dissociation of oxygen molecules.
- The partial pressure of carbon dioxide is low in the lungs so oxygen binds with hemoglobin and carbon dioxide dissociates.
Explain how and why ventilation rate varies with exercise
- Ventilation was covered in topic 6.4 and we learned it is the exchange of stale air for fresh air. The human body can change the ventilation rate depending on the energy demands being placed on the body.
- During exercise, increased demands are placed on the muscles that move the body. The movement of muscles requires the use of ATP therefore the rate of cellular respiration increases along with the need for oxygen. As you know, cellular respiration produces ATP for energy and carbon dioxide as a waste product. The body must compensate and increase the rate of transport of both oxygen and carbon dioxide to meet the increased demand.
- As the carbon dioxide levels in the blood increase this causes a decrease in blood pH. Normal blood pH is 7.4 (slightly alkaline) and when the level drops it is detected very quickly by cells in the walls of the arteries such as the aorta and carotid arteries. These cells are known as chemosensors and once they detect the drop in blood pH they send action potentials (nerve impulses) to the medulla of the brain.
- The medulla oblongata of the brain is the breathing center. The breathing centers control the ventilation rate and contain the same type of chemosensors found in the arteries. As blood flows through the capillaries in the medulla the chemosensors detect carbon dioxide levels and blood pH.
- During exercise the concentration of carbon dioxide in the blood increases. Chemosensors in the arteries send messages to the breathing center which is also monitoring the blood levels. In response to the increased carbon dioxide levels the medulla sends action potentials to the diaphragm and intercostal muscles. The nerve impulses increase muscular contractions which increases the ventilation rate. The increased ventilation rate helps to remove the carbon dioxide from the blood and increase the uptake of oxygen.
- Once you stop exercising the carbon dioxide levels decrease in the blood and once again the chemosensors detect the change and decrease the ventilation rate.
Outline the possible causes of asthma and its effects on the gas exchange system.
- Asthma is a chronic inflammatory disease of the airways which affects the bronchi and its associated bronchioles. When an individual is suffering from an asthma attack the muscular bronchi contract excessively and become inflamed and swollen, often producing excess mucus. The swelling narrows the bronchi and gas exchange becomes very difficult.
- Asthma attacks have no set pattern but there are many varying triggers that can induce an asthma attack such as:
- -Allergens such as pollen, moulds and animal dander
- -Certain arthropods such as dust mites and cockroaches
- -All types of smoke
- -Scented products
- -Exercise which increases ventilation rate
- -Stress and strong emotions
- -Cold air
- -Some medications
- -Some food preservatives
- There is evidence that shows there is a genetic link with asthma. Children of adults who have asthma are more likely to have asthma then those that do not. There is currently no cure for asthma just treatment of symptoms.
Explain the problem of gas exchange at high altitudes and the way the body acclimatizes.
- At higher altitudes the partial pressure of oxygen is lower than partial pressure at sea level. Many people believe the common misconception that there is less oxygen by percentage in the air at higher altitudes, however it is the pressure that changes so the molecules are more spread out. What this means is that ventilation at higher altitudes results in less oxygen entering the body so hemoglobin is not as saturated with oxygen as it would be at sea level.
- Due to the fact that less oxygen is entering the blood stream people often suffer from altitude sickness or mountain sickness. Symptoms include muscular weakness, vision problems, fatigue, nausea, abnormally high pulse rate, and headaches. If altitude sickness is severe it can lead to an accumulation of fluid in the brain and lings and can be life threatening.
- You can avoid altitude sickness by gradually ascending to a higher altitude and allowing the body to acclimatize. If given time the body can respond to the higher altitude and adopt physiological changes such as and increase in the number of red blood cells and hemoglobin, increase in the capillary network of the lungs and muscles, increase in lung size and surface area, and an increase in the myoglobin within the muscle tissues.