Communication,Homeostasis and Energy

Excretion:The removal of metabolic wastes from the body

• Carbon dioxide must be removed as, when it dissolves in water is produces hydrogencarbonate ions. These ions compete with oxygen for space on the haemoglobin. This causes a reduction in oxygen transport.
• Carbon dioxide can also combine directly with haemoglobin to form carbaminohaemoglobin, which has a low affinity for oxygen.
•  Can cause respiratory acidosis; breathing difficulties, headaches, drowsiness , restlessness etc caused by Carbon Dioxide dissolving in the blood plasma and combining with water to produce carbonic acid, which dissociates to release hydrogen ions. This lowers the pH.
• Nitrogenous wastes must be removed because the amino group is highly toxic, but proteins and amino acids are very high in energy, so it would be wasteful to excrete them.
• In the orthinine cycle, the amine group is removed to form ammonia, which forms urea, water and a keto acid when added to oxygen and carbon dioxide. The keto acid can be used in respiration and the urea is transported to the kidneys for excretion.

• The hepatic arteries supply the liver with oxygenated blood from the heart, so the liver has a good supply of oxygen for respiration, providing plenty of energy. The hepatic vein takes deoxygenated blood away from the liver - which rejoins the vena cava and normal circulation will proceed. Bile duct is where the substance bile is secreted, which is carried to the gall bladder where it is stored until it is required in the small intestines.
• The hepatic portal vein brings blood from the small intestine, the blood is rich in the products of digestion, and this means that any harmful substances ingested will be broken down quickly by the liver cells (hepatocytes). The liver is made up of lobules, which consists of cells called hepatocytes that are arranged in rows.
• Each Lobule has a Central vein in the middle that connects to the hepatic vein. Every single lobule has branches of the hepatic artery, hepatic portal vein and bile duct. Hepatic artery and hepatic vein are connected to the central vein via capillaries called sinusoid.
• The blood flows past every hepatocytes via the sinusoid, this ensures that the harmful stuff are broken down quickly. Also the blood provides the liver cells with oxygen. The central veins from all the lobules join up to form the hepatic vein.

• Amino acid + Oxygen →Keto acid + Ammonia
• Ammonia + Carbon dioxide → Urea + Water
•  2NH3+CO2→CO(NH2)2+H2O

Catalase can convert 5 million molecules of H2O2 into harmless substances in a minute. Alcohol contains a lot of chemical potential energy which can be used in respiration. Ethanol Dehydrogenase catalyses the detoxification of alcohol in hepatocytes. 

Ethanol →Ethanal → Ethanoic Acid → Acetyl CoA Ethanal and Ethanoic acid are dehydrogenated, and the hydrogen reduces NAD. If too many NADs are busy detoxifying alcohol, there will be too few NAD to break down fatty acids for use in respiration, so the fatty acids are converted back to lipids, which are stored in hepatocytes, making the liver enlarged- Fatty liver.

Supplied with blood from the renal artery and is drained by the renal vein. The Kidney is surrounded by a tough capsule, the outer region is the cortex and the inner is the medulla. The central region if the pelvis, which leads into the ureter. 

The nephron starts in the cortex, where the capillaries form a knot called the glomerulus, surrounded by the Bowman’s capsule. Fluid from the blood is pushed into the capsule by ultrafiltration. The fluid leaves the capsule and flows through the nephron, starting with the proximal convoluted tubule, and then into the medulla for the loop of Henle, which is a hairpin counter current multiplier. Here the composition of the fluid is altered by Selective reabsorption. Substances are reabsorbed back into the tissue fluid and capillaries surrounding the nephron tubule. The fluid then passes into the Distal convoluted tubule, and then into the collecting duct as urine.

Ultrafiltration:- Kidney
• Blood flows into the glomerulus via the afferent arteriole which is at a higher pressure than the blood that leaves through the efferent arteriole due to the difference in size of the diameters of the lumen
• Blood enters the glomerulus and must pass through 3 distinct layers in order to enter the Bowman’s capsule
• Endothelium of capillaries- contains gaps from which blood passes through as well as the substances dissolved in it
• Basement membrane- fine mesh of collagen fibres and glycoproteins that do not allow molecules with an RMM larger the 69000 to pass through (usually proteins)
• Epithelium of Bowman’s capsule- contain finger like projections (podocytes) that fluid from the glomerulus can pass through into the Bowman’s capsule

Selective Reabsorbtion: - (Proximal Convoluted Tubule)
• Na ions are actively transported out of the wall of the convoluted tubule and enter the surrounding tissue fluid
• Sodium is transported into the cell with Amino Acids or Glucose by facilitated diffusion. 
• As the concentrations of Amino Acids or glucose rise, they diffuse into the tissue fluid; they may also be actively removed.
• They then diffuse into the blood and are carried away.
• The reabsorbtion of salts, glucose or amino acids reduces the water potential of the cell and increases the water potential in the tubule fluid, so water will enter the cells and be reabsorbed into the blood by osmosis.
• Larger molecules, such as proteins, will be reabsorbed by osmosis.

• Microvilli - increase the surface area for re-absorption.
• Co-transporter proteins - contained in the cell surface membrane that is in contact with the tubule fluid.
•  Transports glucose or amino acids.
•  Na/K pumps - contained in the cell surface membrane opposite to the fluid tubule.
•  Actively transports Na+ and K+ against their concentration gradient.
•  Many Mitochondria - provides the energy needed to drive the selective re-absorption process.
• Many mitochondria= a lot of ATP.

• In the loop of Henle, salts are transferred from the ascending limb to the descending limb. This means that the tissue fluid in the medulla has a very negative water potential, as so water is lost by osmosis, particularly in the collection duct.
• 1. The water potential of the blood is monitored by osmoreceptors in the hypothalamus of the brain
• 2. When the water potential is very low, they shrink, and stimulate neurosecretory cells in the hypothalamus.
• 3. These produce and release anti diuretic hormone which flows down the axon to the posterior pituitary gland where it is stored until needed.
• 4. When the neurosecretory glands are stimulated thy send action potentials down their axons and cause the release of ADH
• 5. It enters the capillaries running through the posterior pituitary gland. It is transported around the body and acts on the cells of the collecting ducts.
• 6. When it binds to the receptors, it causes a chain of enzyme catalysed reactions, the end result of which is the insertion of vesicles containing water-permeable channels (aquaporins) in the walls of the cells, so they are more permeable to water.
•  7. More water is reabsorbed, by osmosis, into the blood
• 8. Less urine, with a lower water potential is release. 9. Less ADH is released when the water potential rises again.
• 10. The ADH is slowly broken down and the collecting ducts receive less stimulus.

Problems:
• Unable to remove excess water & waste products from the body
o E.g. urea & excess salts
• Inability to regulate urea and salt levels
• Death
Dialysis Waste, excess fluids and salts are removed from the body by passing the blood over a dialysis membrane. This allows the exchange of substances between the blood and the dialysis fluid, which has the same concentration of substances as blood plasma. Substances diffuse from both sides to create the correct concentration of substances.

Haemodialysis:
• Blood is passed through a machine that contains an artificial dialysis membrane. Heparin is used to avoid clotting. Thrice weekly trips to hospital lasting several hours.

 Peritoneal dialysis
• The body’s own abdominal membrane is used as a filter. 

Advantages
o No dialysis
o Less limited diet
o Better physical feeling
o Better quality of life
o No longer ‘chronically ill’

Disadvantages
o Need immunosuppressants for life of kidney
o Major surgery
o Risk of infection
o Need frequent checks in case of rejection
o Side effects of medication

Pregnancy
• A human embryo secrets human chorionic gonadotrophin (hGC) as soon as it is implanted on the uterine lining. The hormone can be detected in the mother’s urine after as few as 6 days.
• Pregnancy tests contain monoclonal antibodies which are tagged with a blue bead and bind only to hCG.
• The hCG-antibody complex moves along the ***** until it sticks to a band of immobilised antibodies, so forms a blue line
• One blue line is a control, so two means pregnancy

 Anabolic Steroids
• Urine samples are tested using gas chromatogrzaphy
1. The sample is vaporised in the presence of a gaseous solvent
2. It is passed down a long tube lined with an absorbing agent.
3. Each substance dissolves differently in the gas and stays there for a unique, specific time- the retention time
4. Eventually, the substance leaves the gas and is absorbed by the lining
5. It is then analysed to make a chromatogram
6. Standard samples of drugs and urine samples are run so drugs can be identified and quantified in the chromatogram.