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What is excretion?

The removal of metabolic waste from the body.


What is secretion?

The release of a product from a gland into the bloodstream.


What is egestion?

The removal of undigested matter from the body.


Name excretory organs and the products they excrete.

Organ: lungs. Product: Carbon dioxide, removed from respiring cells, into bloodstream, transported as hydrogen carbonate ions, diffuse into alveoli and exhaled
Organ: liver. Product: Breakdown of bilirubin from red blood cells; deamination of amino acids into urea; breakdown of alcohol and toxins.
Organ: kidneys. Product: Urea from bloodstream, dissolved in plasma, forms part of urine, stored in bladder, excreted via urethra.
Organ: skin. Product: Largest excretory organ; sweating removes salts, water, uric acid, ammonia; role in homeostasis of temperature and water potential.


Describe the importance of excretion.

Prevent the buildup of potentially harmful products. Removal of potential enzyme inhibitors.


Describe the impact of carbon dioxide on the blood.

Carbon dioxide dissolves in plasma forming carbonic acid, catalysed by carbonic anhydrase. Carbonic acid dissociates to form hydrogen carbonate ions and hydrogen ions. Hydrogen ions lower pH of blood, altering tertiary structure of haemoglobin, reducing affinity for oxygen. Oxygen released, hydrogen ions associated with haemoglobin forming haemoglobinic acid. Carbon dioxide can also bind directly to haemoglobin forming carbaminohaemoglobin. Proteins in plasma buffer hydrogen ions to maintain pH. Small changes in pH noted by medulla oblongata in brain, and breathing increases to remove carbon dioxide more rapidly.


Describe what happens if blood pH drops below pH 7.35.

Causes respiratory acidosis. Headaches, tremors, drowsiness, confusion. Rapid rise in heart rate, change in blood pressure. Can be caused by respiratory disorders; emphysema, chronic bronchitis, asthma, pneumonia.


Describe how nitrogenous compounds are excreted.

Excess amino acids cannot be stored, contain energy. Transported to liver for deamination. Toxic amino group removed and converted to ammonia, less toxic. Ammonia converted to water soluble urea and transported to kidneys. Remaining keto acid enters aerobic respiration pathway. Or converted to fat or carbohydrate for energy store.


Describe the blood flow to and from the liver.

Hepatic artery - oxygenated blood from aorta carried to liver; highly metabolically active so oxygen essential for aerobic respiration.
Hepatic portal vein - deoxygenated blood carried from digestive system to liver; liver adjusts levels and concentrations of the products of digestion before blood circulates rest of body.
Hepatic vein - blood leaves liver and joins vena cava; circulation around rest of body continues.


What is the role of bile?

Made in liver cells. Released into bile canaliculi. Bile canaliculi fuse to become bile duct. Secreted from bile duct into gallbladder for storage. Emulsifies lipids
Aids in digestion and excretion.


Describe the structure of the liver.

Cells and tissues arranged to provide large surface area: volume. Divided into lobes. Lobes divided into lobules. Hepatic artery and hepatic portal vein split into smaller interlobular vessels. Blood from both vessels mixes in sinusoid. Sinusoid line with liver cells, hepatocytes. Liver cells remove unwanted substances from blood.


What happens to blood as it reaches the end of the sinusoid?

Concentrations of components from digestion modified and regulated. Blood empties into intra-lobular vessels; branch of the hepatic vein. Branches of intralobular vessels fuse to become hepatic vein.


Describe the structure of hepatocytes.

Cuboidal with many microvilli - large surface area. Metabolic functions include protein synthesis; transformation and storage of carbohydrates; synthesis of cholesterol and bile; detoxification. Cytoplasm dense with specialised organelles e.g. Golgi, rough and smooth ER, vesicles, ribosomes.


Describe the role of Kupfer cells.

Specialised macrophages found in sinusoid. Breakdown and recycle red blood cells. Haemoglobin broken down into bilirubin.


Describe the roles of the liver.

Control of blood glucose; amino acids levels; lipid levels.
Synthesis of bile; cholesterol; plasma proteins.
Synthesis of RBC in fetus.
Storage of Vit A, D, B12; iron; glycogen.
Detoxification of alcohol and drugs.
Breakdown of hormones.
Destruction of RBC.


Describe the structure and storage of glycogen.

Alpha glucose, 1,4 and 1,6 glycosidic bonds. Highly branched. Insoluble. Stored as granules in hepatocytes. Can be hydrolysed to provide aloha glucose for respiration and the control of blood glucose concentrations.


Name two enzymes found in liver cells and their roles.

Catalase - hydrolysis of hydrogen peroxide to water and oxygen. High turnover number of 5 million molecules per second.
Cytochrome P450 - breakdown of cocaine and other medicinal drugs; also used in electron transport chain during aerobic respiration.


Describe the detoxification of alcohol.

Catalysed by alcohol dehydrogenase to form ethanal. Ethanol dehydrogenated to by ethanal dehydrogenase to form acetate. Acetate combines with CoA to form acetyl CoA which enters aerobic respiration. Hydrogen atoms released combine with NAD to form NADH.


Describe the impact of excess alcohol on the liver.

NAD needed to breakdown fatty acids during beta oxidation. Excess alcohol breakdown depletes NAD
Insufficient NAD for beta oxidation of fatty acids. Fatty acids converted to triglycerides. Stored in hepatocytes. Causes fatty liver, leads to alcohol related hepatitis or to cirrhosis.


Describe the deamination of amino acids and protein.

Amino group removed from amino acid as ammonia. Ammonia is toxic and soluble so must be converted to a less toxic urea during the ornithine cycle. Remaining organic keto acid enters aerobic respiration.


Outline the ornithine cycle.

Ammonia combines with carbon dioxide and ornithine to form citrulline. More ammonia is added to citrulline to form arginine. Arginine converted to ornithine by removal of urea. Urea carried in blood to kidneys for excretion.
2NH3 + CO2 ------> CO(NH2)2 + H2O.


What is the role of the kidneys?

Excretion - removal of urea and toxins from blood. Forms urine. Osmoregulation.


Describe the structure of the kidney.

Outer cortex. Inner medulla. Central pelvis leading to ureter. Oxygenated blood from renal artery. Deoxygenated blood leaves via renal vein.


Describe the fine structure of the kidney.

Nephrons. Start in cortex with Bowman’s capsule. Bowman’s capsule holds knot of capillaries, glomerulus. Coiled tube proximal convoluted tubule, PCT, loops down into medulla and back to cortex. Loop of Henle connects PCT to distal convoluted tubule, DCT. DCT connects to collecting duct which leads to ureter.


Describe the stages of ultrafiltration.

Occurs in glomerulus and Bowman’s capsule. Renal artery splits to form afferent arterioles as it enters glomerulus. Blood under high pressure forces fluid from blood into Bowman’s capsule. Lumen of Bowman’s capsule lined with three membranes. Endothelium of capillary- narrow pores, fenestrations, allow plasma and dissolved substances to pass. Basement membrane- collagen mesh filters molecules of >69000 RMM. Epithelial cells of Bowman’s capsule-specialised cells, podocytes, hold epithelial and endothelial cells part to allow gaps for fluid to pass.


Summarise ultrafiltration.

Plasma and dissolved substances forced from afferent capillaries of glomerulus into Bowman’s capsule through fenestrations. Basement membrane and podocytes filter fluid further so only molecules >69000RMM remain in filtrate. RBC, proteins and molecules <69000 RMM remain in arterioles of glomerulus. Blood leaves glomerulus via efferent capillaries. These capillaries wrap the tubule and eventually flow back to renal vein.


What is filtered from the blood and into the glomerular filtrate?

Water; amino acids; glucose; urea; inorganic mineral ions - sodium, chloride, potassium. The water potential of the filtrate is less negative than that of the blood.


What remains in the capillaries of the glomerulus?

Blood cells; proteins; dissolved substances including glucose, urea and mineral ions. This makes the water potential of the blood very negative.


What is the role of the nephrons?

In PCT all sugars and most mineral ions absorbed. 85% of water reabsorbed. In descending limb of loop of Henle water potential of fluid is decreased as ions are added and water is removed. In ascending loop of Henle water potential is increased as mineral ions are actively removed. In collecting duct water potential is decreased by removal of water. Urine is produced.


Describe how the convoluted tubule is adapted for selective reabsorption.

Structure: Cell surface membrane in touch with fluid in tubule lumen. Function: Highly folded to form microvilli, increased surface area for absorption.
Structure: Cell surface membrane has cotransport proteins. Function: Cotransport of glucose and amino acids in association with sodium ions, movement from lumen of tubule into cell.
Structure: Cell surface membrane in touch with tissue fluid and capillary. Function: Folded to increase surface area for absorption, contains sodium-potassium ion pumps that move sodium ions out of cell and potassium ions into cell, this uses active transport, ATP.
Structure: Cell cytoplasm high in mitochondria. Function: Active processes and transport require energy in form of ATP.