IB Biology/Option H - Further Human Physiology

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Hormonal control[edit]

State that hormones are chemical messengers secreted by endocrine glands into the blood and transported to specific target cells.

  • Hormones are chemical messengers secreted by endocrine glands into the blood and transported to specific target cells.

State that hormones can be steroids, proteins and tyrosine derivatives, with one example of each.

  • Steroid hormone: estrogen
  • Protein hormone: insulin, glucagon
  • Tyrosine derivative: thyroxine (T4)

Distinguish between the mode of action of steroid hormones and peptide hormones.

  • Steroid hormones:
  1. Pass easily through lipid bilayer of plasma membrane.
  2. Directly affects the expression of genes.
  • Peptide hormones:
  1. Attachment to receptor protein on outer cell membrane (Glycoprotein)
  2. The attachment triggers action of a secondary messenger
  3. The secondary messenger in cytoplasm alters the action of the cell

Outline the relationship between the hypothalamus and the pituitary gland.

  • Anterior Pituitary Lobe
  • Hormones are sent from the hypothalamus to the anterior pituitary via a blood vessel called the portal vein.
  • The hypothalamus acts as the endocrine gland and produces releasing hormones (ex gonadotrophin releasing hormones, GnRH)
  • Hormone travels in blood through portal vein to the cells of the anterior pituitary
  • Releasing hormone causes secretion of specific hormones (such as GnRH stimulates LH and FSH secretion)
  • Posterior Pituitary Lobe
  • Neurosecretory cells connect the hypothalamus and the posterior pituitary lobe
  • Hormones secreted from the posterior lobe are produced in the hypothalamus
  • Nerve impulses travel down the axon into the posterior pituitary. This causes the release of the vesicles of hormones into the blood stream at the posterior pituitary e.g. Oxytocin, ADH

Explain the control of ADH (vasopressin) secretion by negative feedback.

  • The homeostatic regulation of water (osmoregulation) is controlled by secretion of anti-diuretic hormone (ADH).
  • Osmoreceptor cells monitor the water content of the blood as it passes through the hypothalamus
  • Neurosecretory cells in the hypothalamus synthesize ADH and transport this along the axon of their nerves for storage in their synaptic end.
  • Osmoreceptor cells send action potential through the neurosecretory cells to the posterior pituitary if water content in low, secreting ADH and reabsorbing water into the bloodstream
  • ADH is secreted and targets the collecting duct, making it more permeable to water (meaning more water is reabsorbed into medulla)
  • If the water content is high, no signal is sent for ADH secretion.

Digestion[edit]

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.

  • 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.

Explain the structural features of exocrine gland cells.

  • Exocrine glands are responsible for the release of digestive fluids. Exocrine glands secrete into ducts. Exocrine gland has a duct portion and a glandular portion. At the end of each branch is an acinus formed at secretory cells of two types: serous cells (which secrete proteins such as enzymes), and mucous cells (which secrete mucus).

Compare the composition of saliva, gastric juice and pancreatic juice.

  • saliva (from salivary glands) consists of: water, electrolytes, salivary amylase, mucus, lysozyme (which works as antibacterial fluid)
  • gastric juice (from stomach) consists of: water, mucus, enzymes (pepsin, rennin), HCl
  • pancreatic juice (from pancreas) consists of: water, bicarbonate, enzymes (amylase, lipase, nuclease, carboxypeptidase, trypsinogen)

Outline the control of digestive juice secretion by nerves and hormones, using the example of secretion of gastric juice.

  • The initial release of gastric juice occurs under nerve stimulation after sight or smell of food.
  • The sustained release occurs under the influence of gastrin secreted when food is in the stomach.

Outline the role of membrane-bound enzymes on the surface of epithelial cells in the small intestine in digestion.

  • Some of the enzymes are immobilized on the membranes of the intestinal epithelium cells. The active sites of the enzymes are oriented toward the lumen of the intestine. They remain functional even when epithelial cells have been sloughed off into the lumen.

Outline the reasons for cellulose not being digested in the alimentary canal.

  • Human lack the digestive enzyme cellulase, the dietary cellulose remains undigested and is egested in the feces. In some animals the symbiotic relationship developed with cellulose-digesting bacteria.

Explain why pepsin and trypsin are initially synthesized as inactive precursors and how they are subsequently activated.

  • Pepsinogen and trypsinogen are inactive forms of enzymes. This is done to prevent self-digestion of the cells that produce those inactive enzymes (known as the zymogens).
  • Pepsinogen (in stomach) is converted into pepsin by the acidic conditions, the hydrochloric acid (HCl) of the stomach.
  • Trypsinogen (in pancreas) is converted into trypsin by the action of enteropeptidase (the enzyme that is bound to the membranes of the small intestine).

Discuss the roles of gastric acid and Helicobacter pylori in the development of stomach ulcers and stomach cancers.

  • A stomach ulcer is an open sore in the stomach wall, where digestive juices - mostly acid and the enzyme pepsin - have begun to eat away the stomach lining.
  • It is now recognized that about 80 per cent of ulcers are caused by infection from a bacterium called Helicobacter pylori (the remaining 20 per cent is caused by over-use of anti inflammatory drugs such as aspirin and ibuprofen).
  • The corkscrew-shaped bacterium survives in the stomach by producing an enzyme called urease, which neutralizes stomach acid and allows the bacterium to colonize the stomach's mucous lining, opening up the stomach wall to attack form digestive fluids.
  • The theory that ulcers was the consequence of an infection was put forward in the early 1980's by Barry Marshall and Robin Warren.

Explain the problem of lipid digestion in a hydrophilic medium and the role of bile in overcoming this.

  • Lipids tend to coalesce (lump together), in an aqeuous environment, due to their water insolubility. When the lipids clump it decreases the surface area-volume ratio, meaning the lipase have less surface to attach to. They can only bind to the lipids on the surface, not to ones in the middle of the "bubble"
  • Bile molecules have a hydrophobic end and a hydrophilic end which emulsifies (prevents from coalescing) the lipids.
  • Lipase (just like all enzymes) is water-soluble, but it does have a hydrophobic active site (for its substrate, lipids)
  • The increased surface area allows lipase greater access to its substrate.

Absorption of digested foods[edit]

Draw and label a diagram showing a transverse section of the ileum as seen under a light microscope.

Explain the structural features of an epithelial cell of a villus as seen in electron micrographs, including microvilli, mitochondria, pinocytotic vesicles, and tight junctions.

  • Villi – provide a huge surface area for absorption
  • Epithelium cells – single layer of small cells, packed with mitochondria – the source of ATP (metabolic energy) for active uptake across the plasma membrane
  • Mitochondria – these organelles are present in large numbers, suggesting a significant demand for ATP in these cells.
  • Microvilli – these tiny, finger-like infoldings of the cell surface facing the lumen of the gut greatly increase the surface area in contact with material to be absorbed.
  • Pump proteins in the plasma membrane of epithelium cells – actively transport nutrients across the plasma membrane into the villi
  • Mucus from goblet cells in epithelium – lubricates movement of digested food among the villi and protects plasma membrane of epithelial cells
  • Tight junctions – these bind together the individual epithelial cells, so that the only way into the tissues of the body is through the epithelium.
  • Network of capillaries – large surface area for uptake of amino acids, monosaccharides, and fatty acids and glycerol into blood circulation
  • Lacteal – branch of the lymphatic system into which triglycerides (combined with protein) pass for transport to body cells
  • Pinocytotic vesicles – these are the site of pinocytosis by which fluid is taken up or released in tiny vesicles, across the plasma membrane of a cell.

Explain the mechanisms used by the ileum to absorb and transport food, including facilitated diffusion, active transport and endocytosis.

Facilitated diffusion. Some substances need a little assistance to enter and exit cells. The transmembrane protein helps out by changing shape.

Active transport. Some substances need a lot of assistance to enter cells. Similar to swimming upstream, energy is needed for the substance to penetrate against an unfavorable concentration gradient.

Endocytosis. Cells can use their cell membranes to engulf a particle and bring it inside the cell. The engulfing portion of the membrane separates from the cell wall and encases the particle in a vesicle.

List the materials that are not absorbed and are egested.

The materials that are not absorbed and are egested are cellulose and lignin from plant matter, the remains of intestinal epithelial cells, bile pigments, and bacteria.

Functions of the liver[edit]

Outline the circulation of blood through liver tissue, including the hepatic artery, hepatic portal vein, sinusoids and hepatic vein.

The liver is served by the hepatic artery, which delivers oxygenated blood, and it is drained by the hepatic vein. In addition, there is a portal vein, the hepatic portal vein that brings blood to the liver directly from the small intestine.

The blood brought by the hepatic portal vein is deoxygenated, because it has already flowed through the wall of the stomach of the intestines. The level of nutrients in this blood varies considerably, depending on the amount of digested food that is being absorbed.

Inside the liver the hepatic portal vein divides up into vessels called sinusoids. These vessels are wider than normal capillaries and have more porous walls, consisting of a single layer of very thin cells, with many pores or gaps between the cells and no basement membrane. Blood flowing along the sinusoids is therefore in close contact with the surrounding hepatocytes. The sinusoids drain into wider vessels that are branches of the hepatic vein. Blood from the liver is carried by the hepatic vein to the right side of the heart via the inferior vena cava.

The hepatic artery supplies the liver with oxygenated blood from the left side of the heart via the aorta. Branches of the hepatic artery join the sinusoids at various points along their length, providing the hepatocytes with the oxygen that they need for aerobic cell respiration.

Explain the role of the liver in regulating levels of nutrients in the blood.

The normal level of blood glucose in humans is about 90mg per 100cm3(90mg 100cm-3). On arrival in the liver sinusoids, excess glucose is withdrawn from the plasma solution and used in metabolism or stored as glycogen. Glycogen reserves are also stored elsewhere in the body, particularly in the skeletal muscles. Respiring tissues of the body receive glucose supplies from the blood circulation. For most tissues, it is a principal substrate for respiration. As the level of blood glucose falls due to respiration in tissues, glycogen reserves in the liver are converted back to glucose to maintain the normal plasma concentration.

The liver cells also adjust the level of amino acids as the blood passes along the liver sinusoids. A pool of amino acids is maintained in the plasma, in the liver and in other tissues undergoing rapid protein synthesis. Amino acids are constantly being built up into proteins, which then function as enzymes, components of membranes, and structural components (e.g. collagen fibres, keratin). The demand for new proteins on a daily basis is very high. Most proteins are short-lived, but the body cannot store amino acids. Instead, excess amino acids are deaminated in the liver. The organic acid part of each amino acid is removed and respired, or converted to fat or carbohydrate.

By this deamination process, the liver ensures that soluble ammonia is not formed and released in the tissues. Urea is removed from the blood in the kidneys. The fatty acids (and glycerol) that reach the liver are combined to form triglycerides. These are combined with proteins in the liver, and may be stored there. Alternatively they are transported in the blood plasma, mostly as low-density lipoproteins (LDLs), to the tissues. Here lipids may be stored as food reserves (fat), or immediately broken down and respired as a source of energy.

Outline the role of the liver in the storage of nutrients, including carbohydrate, iron, vitamin A and vitamin D.

When certain nutrients are in excess in the blood, hepatocytes absorb and store them, releasing them when they are at too low a level. For example, when the blood glucose level is too high, insulin stimulates hepatocytes to absorb glucose and convert it to glycogen for storage. When the blood glucose is too low, glucagon stimulates hepatocytes to break down glycogen and release glucose into the blood. Iron, retinol (vitamin A) and calciferol (vitamin D) are also stored in the liver.

State that the liver synthesizes plasma proteins and cholesterol.

The liver is the site of synthesis of all the blood proteins, including globulins, albumin, prothrombin and fibrinogen. Also, most of the cholesterol required by the body on a daily basis is manufactured in the liver (but the remainder is taken in as part of the diet).

State that the liver has a role in detoxification.

The liver detoxifies harmful substances such as alcohol (see below), or renders drugs and toxins that have entered the blood stream into harmless forms for excretion from the blood circulation in the kidneys. Drugs such as the antibiotics penicillin and erythromycin are handled in this way, as are sulphonamides. Hormones such as thyroid hormone, and steroid hormones such as oestrogen, testosterone, and aldosterone are similarly inactivated, ready for removal from the blood.

Describe the process of erythrocyte and hemoglobin breakdown in the liver, including phagocytosis, digestion of globin and bile pigment formation. Erythrocytes, also called red blood cells, have a fairly short lifespan of about 120 days. The plasma membrane becomes fragile and eventually ruptures, releasing the hemoglobin into the blood plasma. The hemoglobin is absorbed by phagocytosis, chiefly in the liver. Some of the cells in the walls of the sinusoids are phagocytic. They are called Kupffer cells. Inside the Kupffer cells hemoglobin splits into heme groups and globins. The globins are hydrolysed to amino acids, which are released into the blood. Iron is removed from the heme groups, to leave a yellow-coloured substance called bile pigment or bilirubin. The iron and the bile pigment are released into the blood. Much of the iron is carried to bone marrow, where it is used in the production of hemoglobin in new red blood cells. The bile pigment is absorbed by hepatocytes and forms part of the bile.

Hemoglobin → globins – amino acids
and → heme groups - iron – bile pigment

Explain the liver damage caused by excessive alcohol consumption.

Cirrhosis of the liver – a chronic inflammation of the liver in which liver cells are destroyed and replaced by fibrous or adipose (lipid-containing) connective tissue

The transport system[edit]

Explain the events of the cardiac cycle, including atrial and ventricular systole and diastole, and heart sounds.

The atria contracts, causing a higher pressure in the atria than in the ventricles. The blood moves from the atria to the ventricles through atrioventricular valves until it finishes filling the ventricles. When the ventricles contract, there is an increase of pressure in the ventricle, causing the atrioventricular valves close (making the 1st vibration sound). When the pressure in the ventricle is higher than in the pulmonary artery and aorta, the semilunar valves open and blood moves from the ventricles to the pulmonary artery and aorta. As ventricular systole occurs, the atria are relaxed and filling with blood from the pulmonary vein/vena cava. In ventricular diastole, the blood pressure in the ventricle falls below that of the pressure in the aorta, causing the semilunar valves to close (2nd heart beat). During this phase, the atria continues to fill with blood from the pulmonary veins and vena cava. The next atrial systole will start when the pressure in the ventricles fall lower than the pressure in the atria.

Analyze data showing pressure and volume changes in the left atrium, left ventricle and the aorta, during the cardiac cycle.

Outline the mechanisms that control the heartbeat, including the roles of the SA (sinoatrial) node, AV (atrioventricular) node and conducting fibers in the ventricular walls.

The SA node is stimulated by two nerves: a sympathetic nerve that accelerates the heart, and the vagus nerve that restores the heartbeat to a homeostatic level (parasympathetic). It is located towards the top of the right atrium, so that the atria beat from the top down. The AV node receives the signal from the SA node and retards it a tenth of a second before firing, to allow the atria to finish contracting before ventricular systole begins. The impulse from the AV node travels down the common bundle, then is carried up the walls of the ventricles by Purkinje fibers. This ensures that the ventricles contract from the bottom up.

Outline atherosclerosis and the causes of coronary thrombosis.

Atherosclerosis happens when arteries become damaged and form scar tissue. Cholesterol and other lipids build up on the scar tissue, forming a plaque. This plaque can induce platelets to release chemicals that cause a clot to form over the plaque, called a thrombus.

Discuss factors that affect incidence of coronary heart disease.

Risk factors affecting incidence of coronary heart disease are genes (family history of heart attacks), age, being male, smoking, obesity, a diet high in saturated fats and cholesterol, and a lack of exercise.


Gas exchange[edit]

Define partial pressure.

Partial pressure: (the pressure exerted on an object by an individual gas) The measure of how much oxygen is present in a system

Explain the oxygen dissociation curves of adult hemoglobin, fetal hemoglobin and myoglobin.

Fetal haemoglobin (HbF) has a higher oxygen affinity than adult Haemoglobin (Hb) due to a slightly different sequence in amino acid structure. The oxygen dissociation curve of fetal haemoglobin is to the left of adult haemoglobin. This allows for oxygen to leave the blood in the mother's placenta and travel to the baby's, in particular the haemoglobin of the mother to the baby's haemoglobin. A baby has no myoglobin because it possesses no muscle mass. Both adult and fetal haemoglobin have four oxygen molecule affinities (the ability to posses 4 oxygen molecules) due to 4 heme groups containing iron, hence, explaining the slight sigmoid shape of the graph. Whereas myoglobin, which delivers extra oxygen to actively respiring muscles, has one affinity (carries one oxygen molecule) and thus has a single heme group. The graph of myoglobin is to the left of haemoglobin, for it either is completely saturated or not; the graph rises steeply up and levels off.

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 produced by body tissues diffuses into the interstitial fluid and into the plasma. Less than 10% remains in the plasma as dissolved CO2. The rest (70%) diffuses into red blood cells, where some (20%) is picked up and transported by hemoglobin. Most of the CO2 reacts with H20 in the red blood cells to form carbonic acid. Red blood cells contain the enzyme carbonic anhydrase, which catalyzes this reaction. Carbonic acid dissociates into a bicarbonate ion and hydrogen ion (H+). Hemoglobin (a plasma protein) binds most of the H+, preventing them from acidifying the blood. The reversibility of the carbonic acid- bicarbonate conversion also helps buffer the blood, releasing or removing H+ depending on the pH. Chlorine goes into the red blood cells when bicarbonate comes out. This is referred to as the chloride shift.

Explain the role of the Bohr shift in the supply of oxygen to respiring tissues.

The Bohr shift helps the body release more O2 to respiring tissues when the blood pH is more acidic. During exercise, a lot of CO2 is produced, which results in larger amounts of hydrogen ions that acidify the blood. Thus, the Bohr shift lets the body know it's exercising.

Explain how and why ventilation rate varies with exercise. When you exercise, you use more oxygen, and you produce more carbon dioxide. When there is more carbon dioxide in the blood, the pH level lowers. There are chemosensors which are found in the aorta and the carotid arteries and aorta that will detect this change. These sensors send impulses to the breathing center in the brain which include the expiratory and inspiratory centers. This center compares the incoming information with the normal value, and if the level of the pH is too low (below 7.4), impulses are sent to the intercostal muscles and the diaphragm to increase the rate and depth of lung ventilation to repay oxygen debt after anaerobic cell respiration.

Outline the possible causes of asthma and its effects on the gas exchange system.

Asthma is a disease of the respiratory system. During an asthma attack, the air passages constrict, and it becomes very difficult to breathe and get enough air into the lungs. Asthma is caused by allergens, and other factors such as vigorous exercise. These factors are the direct cause of the attack, and the cause may also be hereditary. During an attack the bronchioles will tighten and the air passages will become mucus filled and inflamed, making it very difficult to breathe. Effects of asthma include inflammation and constriction of the bronchial tubes, which can lead to wheezing, coughing, and respiratory distress. It can be controlled by medication and trying to avoid triggers. Recent research has found that clean homes can increase the risk of asthma since the immune system will react against harmless substances causing allergies to develop.

Explain the problem of gas exchange at high altitudes and the way the body acclimatizes.

Mountain sickness may occur when a person travels quickly from a low to high altitude. Over a period of time the person becomes acclimatized: red blood cell production and ventilation rate increase. People living permanently at high altitude have greater lung surface area and larger vital capacity than those living at sea level.