Option H Assessment statements

Abi S. Eleah W.

Richard M.John R.

Hormonal Control

• A hormone is a form of chemical communication. Hormones are secreted by endocrine cells placed throughout the body and transported in the blood to target cells. Hormones can be steroids, which pass through the cell membrane and affect the genetic expression of a cell, or proteins, which bind to receptors on the surface of cell membranes to release secondary messengers inside cells. The most prominent center for hormonal control is in the brain, with the pituitary gland and the hypothalamus. The hypothalamus is linked to the pituitary through the portal vein (to the anterior lobe), which carries hormones that trigger the release of other hormones, and also through neurosecretory cells (the posterior lobe) receive impulses that trigger hormone release.

• The secretion of ADH is an example of hormone control of a process in the body. Osmoreceptor cells in the hypothalamus monitor the osmolarity of the blood, and when an increase is detected, the hypothalamus sends signals to the posterior pituitary to release ADH into the bloodstream, where it travels to the kidney and allows more water to be collected. As the osmolarity of the blood returns to homeostasis, negative feedback reduces the amount of ADH released.

Digestion

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

• Exocrine glands are responsible for the release of digestive fluids and secrete their products into ducts. They have a duct portion and a glandular portion. At the end of each branch is an acinus formed by serous cells (secrete proteins), and mucous cells (secrete mucus).

• Saliva consists of water, electrolytes, salivary amylase, mucus, and lysozyme.

• Gastric juice consists of water, mucus, enzymes such as pepsin and rennin, and HCl.

• Pancreatic juice consists of water, bicarbonate, enzymes (amylase, lipase, carboxypeptidase, trypsinogen)

• Some of the enzymes (maltase, lactase and sucrase) are immobilized on the membranes of the intestinal epithelium cells in the intestinal villi. The active sites of theses enzymes are oriented toward the lumen of the intestine and remain functional after the cells have been sloughed off into the lumen.

• Human lack the enzyme cellulase, and thus the capacity to digest cellulose. In some animals the symbiotic relationship developed with cellulose-digesting bacteria.

 • Pepsinogen and trypsinogen are initially inactive to prevent self-digestion of

the cells that produce the enzymes. Pepsinogen is converted into pepsin by the acidity of the hydrochloric acid in the stomach. Trypsinogen is converted into trypsin by the action of enteropeptidase (the enzyme that is bound to the membranes of the small intestine) in the pancreas.

• 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. About 80 per cent of ulcers are caused by the bacterium Helicobacter pylori, a corkscrew-shaped bacterium that survives in the stomach by producing the enzyme urease, which neutralizes stomach acid and allows the bacterium to colonize the stomach's mucous lining, opening up the stomach wall to erosion bt digestive fluids, to the point of creating an ulcer or vulnerability to stomach cancer.

• Lipids tend to coalesce, in an aqeuous environment, due to their water insolubility, and are only accessable to lipase at a lipid-water interfaces. When the lipids clump it decreases the surface area-volume ratio, meaning the lipase have less surface to attach to.

• Bile molecules have a hydrophobic end and a hydrophilic end which emulsifies the lipids, exposing the maximum lipid surface area to lipases. Lipase is water-soluble, but has a hydrophobic active site.

Absorption

• Ileum- last section of the small intestines.• Absorption of nutrients.• Facilitated diffusion is need for substances when absorption

occurs.

• Epithelial cells:– Villi- huge surface area– Microvilli- face lumen of gut– Mitochondria- ATP– Pinocytotic vesicles- fluids up-taken or released in tiny

vesicles. – Tight Junctions-binds epithelial cells so only way in

tissue is through the epithelium.

Liver Function• 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.

• 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.

• 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.

• 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).

• 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.

• 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 acidsand → heme groups - iron – bile pigment

• 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

• The flow of blood through the heart is influenced by the valves and nodes that regulate heartbeat. First, the sinoatrial node, which receives signals from the brain about how quickly to contract, signals the atria to contract. Blood moves from the high pressure in the atria to low pressure in the ventricles, and the atrioventricular valves close to prevent blood from flowing back into the atria. Then, the atrioventricular node, which receives a signal from the SA node, signals the ventricles to contract, pushing blood out of the heart and causing the semilunar valves to close.

• If the body is not taken care of, heart disease can occur. The most common issues are atherosclerosis, or buildup of plaque in blood vessels, and coronary heart disease. Plaque can build up when there is so much cholesterol and lipids in the diet that they stick to the walls of blood vessels, and if red blood cells catch on the plaque and release clotting factors, a clot can form that blocks off the vessel. Coronary heart disease can occur based on genetic predisposition, age, smoking, obesity, low exercise, and eating a diet high in fat and cholesterol.

Gas Exchange

Partial pressure- exerted on an object by an individual gas; measure of oxygen.

• Hemoglobin in adult and fetal– (HBF)- fetal higher oxygen curve to left– (HB)- Adult low oxygen curve to right.

• Myoglobin- muscle mass– Delivers extra oxygen to actively respiring muscles– On curve- to left of hemoglobin and rises steeply and

levels off

• Co2 produced by body tissue diffuses into interstitial fluid and into plasma.

• Less than 10 % remains in plasma• 70% diffuses into red blood cells • 20% picked up and transported by hemoglobin.• CO2 reacts with H2Oin red blood cells to form carbonic acid.• Hemoglobin binds with most h+ preventing from acidifying

the blood.