5.2 - Excretion

Question 1

What is excretion?

Answer 1

The removal of metabolic waste from the body.

Question 2

What is secretion?

Answer 2

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

Question 3

What is egestion?

Answer 3

The removal of undigested matter from the body.

Question 4

Name excretory organs and the products they excrete.

Answer 4

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.

Question 5

Describe the importance of excretion.

Answer 5

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

Question 6

Describe the impact of carbon dioxide on the blood.

Answer 6

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.

Question 7

Describe what happens if blood pH drops below pH 7.35.

Answer 7

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.

Question 8

Describe how nitrogenous compounds are excreted.

Answer 8

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.

Question 9

Describe the blood flow to and from the liver.

Answer 9

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.

Question 10

What is the role of bile?

Answer 10

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.

Question 11

Describe the structure of the liver.

Answer 11

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.

Question 12

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

Answer 12

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.

Question 13

Describe the structure of hepatocytes.

Answer 13

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.

Question 14

Describe the role of Kupfer cells.

Answer 14

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

Question 15

Describe the roles of the liver.

Answer 15

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.

Question 16

Describe the structure and storage of glycogen.

Answer 16

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.

Question 17

Name two enzymes found in liver cells and their roles.

Answer 17

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.

Question 18

Describe the detoxification of alcohol.

Answer 18

Catalysed by alcohol dehydrogenase to form ethanal. Ethanal dehydrogenated 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.

Question 19

Describe the impact of excess alcohol on the liver.

Answer 19

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.

Question 20

Describe the deamination of amino acids and protein.

Answer 20

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.

Question 21

Outline the ornithine cycle.

Answer 21

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.

Question 22

What is the role of the kidneys?

Answer 22

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

Question 23

Describe the structure of the kidney.

Answer 23

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

Question 24

Describe the fine structure of the kidney.

Answer 24

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.

Question 25

Describe the stages of ultrafiltration.

Answer 25

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.

Question 26

Summarise ultrafiltration.

Answer 26

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.

Question 27

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

Answer 27

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.

Question 28

What remains in the capillaries of the glomerulus?

Answer 28

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

Question 29

What is the role of the nephrons?

Answer 29

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.

Question 30

Describe how the convoluted tubule is adapted for selective reabsorption.

Answer 30

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.

Question 31

Outline the mechanism of reabsorption.

Answer 31

Sodium ions actively pumped from cells of tubule into tissue fluid and then diffuse into capillary. Concentration of sodium ions within tubule cell decreases, concentration gradient established between cell and fluid in tubule lumen. Sodium ions diffuse from fluid lumen into tubule cell, down concentration gradients, passive diffusion through cotransport protein. Glucose/amino acids diffuse with sodium ions. Water potential of tubule cell is lowered. Water moves by osmosis, down concentration gradient, from lumen of tubule into tubule cell. Glucose and amino acids diffuse down concentration gradient from tubule cell into blood.

Question 32

Describe the reabsorption of water in the loop of Henle.

Answer 32

Fluid moves from the PCT into the descending limb. Water diffuses from the descending limb. Sodium and chloride ions diffuse from the tissue fluid into the descending limb. As the descending limb extends down into the medulla towards the pelvis the water potential becomes more negative. Fluid moves along the ascending limb
In the lower ascending limb sodium and chloride ions diffuse into the tissue fluid. The upper ascending limb is impermeable to water. Sodium and chloride ions are actively pumped from the ascending limb into the tissue fluid. The water potential of the fluid in the ascending limb is less negative. The water potential of the tissue fluid of the medulla becomes more negative as the loop of Henle passes deeper into the medulla.

Question 33

What is a hairpin countercurrent multiplier?

Answer 33

Fluid passing along the descending limb of the loop of Henle has a decreasing water potential. Fluid passing along the ascending limb of the loop of Henle has an increasing water potential. This leads to a more negative water potential in the bend of the ‘hairpin’ as the loop moves from descending to ascending. The longer the loop of Henle, the more negative the water potential of the fluid and so the more water is reabsorbed back into the blood.

Question 34

Describe the movement of fluid through the collecting duct.

Answer 34

Fluid passes from the top of the ascending limb into the DCT. Active transport is used to adjust ion concentrations between fluid in the DCT and the tissue fluid. The DCT has a high water potential relative to the surrounding tissue fluid. The collecting duct passes down into the medulla towards the pelvis where the water potential becomes more negative as it gets deeper. Water moves by osmosis, down its concentration gradient, into the tissue fluid. The permeability of the collecting duct walls is regulated by antidiuretic hormone, ADH.

Question 35

Describe the concentration changes in the tubule fluid as it passes through the nephron.

Answer 35

Structure: PCT. Concentration of substance in fluid: glucose concentration low as it is reabsorbed into blood.
Structure: Descending limb of loop of Henle. Concentration of substance in fluid: sodium ions diffuse into limb, concentrations rise.
Structure: Ascending limb of loop of Henle. Concentration of substance in fluid: sodium ions pumped out of limb, concentrations fall.
Structure: DCT. Concentration of substance in fluid: water removed, urea actively transported from tissue fluid into DCT, urea concentration rises.
Structure: DCT. Concentration of substance in fluid: sodium ions removed but water removed so overall sodium ion concentration increases, potassium ions actively transported in, concentration rises.

Question 36

What is osmoregulation?

Answer 36

The control of water potential in the body. Regulated by ADH by negative feedback.

Question 37

Why must water potential be regulated?

Answer 37

Maintain water balance between cells and tissue fluid. If water potential is lower in cells than in tissue fluid water will move into the cells, down its concentration gradient, by osmosis. Cells will swell and may undergo lysis. If water potential is higher in cells than in tissue fluid water will move out of the cells and into the tissue fluid, down its concentration gradient, by osmosis. Cells will shrivel, or crenate.

Question 38

What are the sources of water?

Answer 38

Food; Drink; Metabolism - water produced during respiration.

Question 39

Outline the mechanisms of osmoregulation if water potential of blood is too high.

Answer 39

Osmoreceptors in hypothalamus monitor water potential of blood. If water potential is too high, blood contains too much water. Less/no antidiuretic hormone, ADH, is released from posterior pituitary into the blood. Less/no ADH binds to cell surface receptors on walls of collecting duct. Walls of collecting become less permeable to water. Less water reabsorbed into blood, more urine produced. Water potential of blood returns to normal.

Question 40

Outline the mechanisms of osmoregulation if water potential of blood is too low.

Answer 40

Osmoreceptors in hypothalamus monitor water potential of blood. If water potential is too low, blood does not contain enough water. More antidiuretic hormone, ADH, is released from posterior pituitary into the blood. More ADH binds to cell surface receptors on walls of collecting duct. Walls of collecting become more permeable to water. More water reabsorbed into blood, less urine produced. Water potential of blood returns to normal.

Question 41

Describe the action of an increase in ADH on the collecting duct.

Answer 41

More ADH binds to receptors on cell surface receptors on collecting duct. Series of enzyme catalysed reactions moves vesicles containing aquaporins to cell surface membrane. Vesicles fuse with cell surface membrane, inserting aquaporins into wall of collecting duct. More water absorbed from lumen of collecting duct into blood stream.

Question 42

Describe the action of a reduction in ADH on the collecting duct.

Answer 42

Less ADH binds to receptors on cell surface receptors on collecting duct. Cell surface membrane containing aquaporins folds in - invaginates. Vesicles containing aquaporins form. Wall of collecting duct less permeable. Less water absorbed, more urine produced.

Question 43

Describe the release of ADH from the pituitary.

Answer 43

ADH made in neurosecretory cells, specialised neurones. Manufactured in cell body and moves down axon to terminal bulb. Stored in vesicles. Stimulation by osmoreceptors triggers release.

Question 44

How long does ADH persist in the body?

Answer 44

It has a half-life of 20 min (the concentration of ADH reduces by 50% every 20 min).

Question 45

What is kidney failure?

Answer 45

The inability for kidneys to regulate levels of water and electrolytes in the body. The inability to remove waste products, such as urea from the blood. Can lead to rapid death.

Question 46

How is kidney function assessed?

Answer 46

Analysis of urine for substances such as proteins, glucose and blood cells. Estimation of glomerular filtration rate, GFR. This is the measure of fluid through the nephron. Normal rate 90-120 ml/s. Below 60 ml /s indicates chronic kidney failure.

Question 47

What are the causes of kidney failure?

Answer 47

Diabetes mellitus, type 1 and type 2; Heart disease; Hypertension, high blood pressure; Infection.

Question 48

What are the treatments for kidney failure?

Answer 48

Transplant or renal dialysis; haemodialysis, peritoneal dialysis.

Question 49

Describe haemodialysis.

Answer 49

Blood from artery or vein passed to dialysis machine. Network of artificial capillaries increase surface area for exchange. Heparin added to prevent clotting. Artificial capillaries surrounded by dialysis fluid, flowing in opposite direction, countercurrent exchange. Dialysis fluid contains correct concentrations of minerals, urea, water to mirror normal blood plasma. Substances in excess in the patient’s blood diffuse across the artificial capillaries, down their concentration gradients into the dialysis fluid. Substance that are too low in the patient’s blood diffuse from the dialysis fluid, across the artificial capillaries, down their concentration gradients, into the blood. Performed 2 or 3 times a week, taking several hours at a hospital.

Question 50

Describe peritoneal dialysis

Answer 50

Patients abdominal membrane, peritoneum, used as dialysis membrane. Tube permanently implanted in the abdomen. Dialysis fluid added to abdomen via tube, filling space between abdominal wall and organs. Fluid left for several hours for diffusion to occur. Substances in excess in the patient’s blood diffuse across the peritoneum down their concentration gradients into the dialysis fluid. Substances that are too low in the patient’s blood diffuse from the dialysis fluid, across the peritoneum, down their concentration gradients, into the blood. Patient can carry procedure out at home and is able to walk around.

Question 51

Describe the advantages and disadvantages of a kidney transplant.

Answer 51

Advantages: No need for dialysis; greater feeling of health and fitness; improved quality of life; improved self image.
Disadvantages: Immunosuppressant drugs taken can have other implications; major surgery requiring general anaesthetic so not suitable for all; ongoing regular checks for rejection; side effects of immunosuppressant drugs - fluid retention, high blood pressure, susceptibility to infection; some patients also find it difficult to reconcile the presence of another person’s organ.

Question 52

Describe the substances detected by urine analysis.

Answer 52

Any molecule less than RMM 69000.
Glucose - diabetes.
Alcohol - drivers.
Recreational drugs - safety at work.
Anabolic steroids - athletes.
Human chorionic gonadotrophin, hCG - pregnancy.

Question 53

Describe the mechanism of pregnancy testing.

Answer 53

Human embryo implants in uterine wall and releases hCG. hCH has RMM 36700 so is detected in urine. Test stick has mobile and fixed monoclonal antibodies. hCG minds to mobile antibodies attached to blue bead. Mobile antibodies move down test stick. If hCH present it binds to fixed antibodies, blue line forms. Mobile antibodies without bound cHG attache to second binding site further down test stick. Second blue line shows test is working.

Question 54

Describe testing for anabolic steroids.

Answer 54

Anabolic steroids increase muscle mass. They have a half-life of 16h and remain in the blood for days. Have low RMM so enter nephron, then urine, easily. Urine sample tested using gas chromatography.