5:1:2 Excretion as an Example of Homeostasis

Question 1

Define excretion

Answer 1

The process by which toxic waste products of metabolism and substances in excess of requirement are removed from the body (e.g. lungs exhaling CO2). It is a key process in homeostasis

Question 2

What are the examples of metabolic waste, their source, and why an excess is bad

Answer 2

Question 3

Describe the external structure of the liver

Answer 3

• Supplied with blood from two sources (oxygenated via the hepatic artery from the heart, deoxygenated via the hepatic portal vein from the digestive system)
• The liver needs oxygenated blood to efficiently carry out its roles
• The liver needs deoxygenated blood to absorb and metabolise stress nutrients stored in the blood
• Deoxygenated blood leaves the liver from the hepatic vein
• Connected directly to the gall bladder which stores bile (bile salts and pigments) and releases it into the duodenum via the bile duct

Question 4

Describe the internal structure of the liver

Answer 4

• The liver is made up of cells known as hepatocytes which carry out all functions required
• Liver is divided into many lobules which are separated by connective tissue
• Each lobule is supplied with blood via branches from the hepatic artery and hepatic portal vein
• Blood is carried in wide capillaries known as sinusoids which are lined with an incomplete layer of endothelial cells to allow substance exchange
• Each lobule is connected to a branch of the hepatic vein to drain the blood
• Lobules are known as functional units of the liver as all the functions occur within each individual one

Question 5

What are the functions of the liver

Answer 5

• The storage of glycogen
• The formation of urea
• Detoxification

Question 6

How does the liver store glycogen

Answer 6

• Converts glucose into glycogen via glycogenesis which helps to regulate blood glucose concentration
• Insulin detects high levels of glucose in the blood, glycogenesis occurs and synthesises the glucose
• Glycogen acts as compact and efficient carbohydrate storage molecule

Question 7

Define glycogenesis

Answer 7

The synthesis of glycogen from glucose molecules, triggered by insulin

Question 8

How does the liver form urea

Answer 8

• Proteins in our diet are digested into amino acids which are then absorbed into the blood and transported to the liver
• Instead of being excreted the excess amino acids are deaminated (NH2 removed)
• The ammonia formed from this deamination is very soluble and highly toxic, so is combined with CO2 and converted by the liver into the less soluble and less toxic urea, which is transported to the kidneys
• Happens during the ornithine cycle

Question 9

Write the equation for the deamination of an amino acid

Answer 9

Question 10

What are main products and reactants in the ornithine cycle

Answer 10

• One molecule of CO2 required
• Combined with two amino groups
• One molecule of urea produced

Question 11

Define detoxification

Answer 11

Term used to describe the breakdown (by the liver) of substances that aren’t needed or are toxic (lactate, alcohol, hormones, medicinal drugs)

Question 12

Describe the process of lactate metabolism

Answer 12

• Lactate is the end product of anaerobic respiration
• It is an energy rich compound that can be restored by cardiac muscles and other tissue
• Excess lactate is absorbed by hepatocytes and metabolises
• It is converted to pyruvate in the liver
• Some pyruvate is respired in the mitochondria to provide energy to convert the rest of the lactate to glucose

Question 13

Describe the process of alcohol metabolism

Answer 13

• Alcohol (in the form of ethanol) is absorbed in the stomach and transported in the blood until it is absorbed by hepatocytes
• Ethanol is converted to ethanal then ethanoic acid by the enzyme ethanol dehydrogenase
• The ethanoic acid combined with coenzyme A to produce acetyl coenzyme A which is used in respiration
• During this process H+ atoms are released, which removed by reduce NAD
• The metabolism of alcohol takes up a lot of energy, as well as reducing a lot of NAD which helps in the metabolism of fat, which leads to the fat instead being stored which gives rise to cirrhosis

Question 14

What is cirrhosis

Answer 14

Stored fat in hepatocytes which reduces their ability to function and scars the liver. It is caused by excessive alcohol consumption

Question 15

Describe the process of hormone metabolism

Answer 15

• Protein hormones and peptide hormones are hydrolysed into amino acids
• These amino acids can be a deaminated to form urea

Question 16

How is the histology of liver studie

Answer 16

• Sections of the liver can be stained and then viewed under a microscope

Question 17

What functions are the kidneys responsible for

Answer 17

• As an osmoregulation organ, regulating the water content of the blood
• As an excretory organ, excreting the toxic waste products of metabolism (urea/salts)

Question 18

Label the structures of the kidneys, and explain their functions (renal artery and vein, kidneys, ureter, bladder, urethra)

Answer 18

Question 19

Describe the structure of the kidney

Answer 19

• It is surrounded by a tough layer known as the fibrous capsule
• The cortex
  -The medulla
• The renal pelvis

Question 20

Where is the nephron found

Answer 20

• The nephrons are the functional units of the kidney, and are responsible for the formation of urine
• Parts of the nephron are located in the cortex, whilst others are in the medulla

Question 21

Describe the ways in which the large blood supply around the nephron are organised

Answer 21

• The glomerulus is supplied with blood by an after ent arteriole
• The capillaries of the glomerulus rejoin to form an efferent arteriole
• Blood flows from the efferent arteriole into a network of capillaries that run closely alongside the rest of the nephron
• The blood eventually flows into the renal vein

Question 22

What are the two stages of urine production in the kidneys

Answer 22

• Ultrafiltration
• Selective reabsorption

Question 23

Describe how ultrafiltration works

Answer 23

• Arteriole branch off the renal artery and lead to each nephron, where they form a knot of capillaries (glomerulus) which sits inside the bowman’s capsule
• The afferent arteriole that enters the glomerulus is wider than the efferent arteriole, which results in high blood pressure
• Blood in the glomerular capillaries is separated from the lumen of the bowman’s capsule by two cell layers with a basement membrane between them
• The first cell layer is the endothelium of the capillary which is perforated with holes, then the basement membrane made of collagen and glycoproteins, then the epithelium layer of the bowman’s capsule, which have podocytes
• As blood passes through the glomerular capillaries, the high blood pressure forces substances in the blood plasma through the holes in the cell layers let substances (amino acids, water, glucose, urea, ions) into the bowman’s capsule creating glomerular filtrate
• Red and white blood cells and platelets remain as they are too large to travel through the perforations, and large proteins are stopped by the basement membrane

Question 24

What are kuppfer cells

Answer 24

Specialised types of macrophages that travel within the sinusoids of the liver and breakdown and recycle red blood cells

Question 25

Why does ultrafiltration occur

Answer 25

Due to the differences in water potential between the plasma in the glomerular capillaries and the filtrate in the Bowman’s capsule

Question 26

How does pressure affect the water potential in the glomerulus and bowman’s capsule, and what results from its change

Answer 26

• The afferent arteriole is wider than the efferent, so the blood pressure is high in the glomerular capillaries
• This raises the water potential of the blood plasma in the glomerular capillaries, above the water potential of the filtrate in the bowman’s capsule
• Water moves down the water potential gradient, from the blood plasma in the glomerular capillaries, into the bowman’s capsule

Question 27

How does solute concentration affect water potential in the glomerulus and bowman’s capsule, and what is the result

Answer 27

• Plasma proteins are too large to undergo ultrafiltration and travel through the basement membrane
• As a result, the solute concentration in the blood plasma in the glomerular capillaries is higher than that of the filtrate in the bowman’s capsule
• This makes the water potential of the blood plasma lower than that of the filtrate in the bowman’s capsule
• Water moves down the water potential gradient from the bowman’s capsule into the blood plasma in the glomerular capillaries

Question 28

What is the overall direction of water movement when blood flows through the glomerulus

Answer 28

There is an overall movement of water down the water potential gradient from the blood into the bowman’s capsule

Question 29

Describe the process of selective reabsorption in the kidneys

Answer 29

• Some substances that end up in the glomerular filtrate need to be kept and reabsorbed by the body
• Blood capillaries surrounding the surface of the proximal convoluted tubule (near the basal membranes of the tubule)
• Sodium-potassium pumps in the basal membranes move Na+ ions out of the epithelial cells and into the blood, lowering the concentration of Na+ ions in the cells, causing the Na+ ions to move down their concentration gradient through the luminal membranes
• Na+ ions travel through the luminal membranes via co-transporter proteins, allowing the Na+ ions to transport glucose and amino acids as well, and continue until they reach the blood
• All glucose in the glomerular filtrate is reabsorption
• Amino acids, vitamins and inorganic ions are also reabsorbed
• The movement of these substances from the from the proximal tubule into the capillaries increases the water potential of the filtrate and decreases in the water potential of the blood in the capillaries, causing a water potential gradient so water moves into the body

Question 30

How can the histology of kidneys be studied

Answer 30

• Staining the section of kidney tissue, and then viewing them under a microscope

Question 31

Define osmoregulation

Answer 31

The control of the water potential of body fluids, which is a key part homeostasis

Question 32

Describe the process of osmoregulation

Answer 32

• Specialised sensory neurones (osmoreceptors found in the hypothalamus) monitor the water potential of the blood
• If they detect a decrease in the water potential in blood, nerve impulses are sent along these sensory neurons to the posterior pituitary gland
• This triggers the release of antidiuretic hormone (ADH), which cause kidneys to absorb more water

Question 33

What is the effect of ADH on the kidneys

Answer 33

• Water is reabsorbed by osmosis from the filtrate in the nephron
• Reabsorption occurs in the collecting duct and loop of Henle
• ADH causes the luminal membranes of the collecting ducts cells to become more permeable to water by increasing the aquaporins (water permeable channels) in the luminal membranes of the collecting duct cells
• This is done by ADH binding to receptor proteins which leads to the phosphorylation of the aquaporin molecules, which activates them and causes the vesicles of the collecting duct to fuse with the luminal membranes and become more permeable
• As the filtrate in the nephron travels along the collecting duct, water molecules move from the collecting duct into the tissue fluid/blood plasma in the medulla down the water potential gradient

Question 34

What are the adaptations of the proximal convoluted tubule epithelial cell, and how they aid reabsorption

Answer 34

• Many microvilli: increases surface area
• Many co-transporter proteins: transport solutes across the luminal membrane
• Many mitochondria: provide energy for the sodium-potassium pumps
• Tightly packed cells: no fluid can pass between the cells

Question 35

How are water and salts reabsorbed in the nephron

Answer 35

• The ascending tubule if the loop of Henle is impermeable to water, but not ions, so ions are pumped out of it, causing a high concentration of ions around the loop of Henle
• The salts are then reabsorbed back into the blood
• The descending tubule is impermeable to ions but not water, so water is drawn out of the tubule down the concentration gradient into the high concentration of surrounding ions
• The water is then reabsorbed back into the blood

Question 36

What is kidney failure, and its results

Answer 36

• Can occur in one or both kidneys due to many reasons (e.g. diabetes)
• Can be short/long term
• Cause urea, water, salts and toxins to be retained (not excreted)
• Less blood is filtered by glomerulus, glomerulus filtration rate decreases
• Build ups of toxins in the blood
• Electrolyte balance in blood is disrupted

Question 37

Why are balanced electrolytes so important

Answer 37

• Disruption of electrolyte balance due to kidney failure can be harmful
• Excess K+ ions in the blood can lead to abdominal cramps, tiredness, muscle weakness
• If K+ concentrations continue to increase, the frequency of impulses from the sinoatrial node in the heart can decrease, causing cardiac arrest
• Na+ can’t be stored/secreted, and a buildup can cause high blood pressure, spasms and weakness

Question 38

What are the potential treatments for kidney failure

Answer 38

• Humans can survive with one kidney
• Renal dialysis: where toxins are removed from the blood via a dialysis membrane
• Kidney transplant: replacing the non-functioning kidney

Question 39

What are the two types of renal dialysis

Answer 39

• Haemodialysis
• Peritoneal dialysis

Question 40

Describe how haemodialysis works

Answer 40

• The patient requires regular treatment in the hospital or at home with a haemodialyster which acts as an artificial kidney
• The machine contains partially permeable dialysis membranes to separate the blood from the dialysis fluid
• Blood (with heparin a blood thinner) is passed through the tubes of dialysis membrane, which are surrounded by dialysate, where they flow in opposing directions
• Dialysate contains substances needed in the blood, as well as a glucose concentration that is equal to the normal blood sugar level to prevent movement of glucose, and salt concentration similar to the blood ideal, so salts only move if there is an imbalance
• Fluid in the machine is continually refreshed to keep the concentration gradients
• The dialysate contains no urea, ensuring that the urea travels down its concentration gradient from the blood to the dialysate to be disposed

Question 41

Describe how peritoneal dialysis works

Answer 41

• Dialysate enters the abdominal cavity via a catheter
• Urea and other waste products diffuse across the abdominal lining (peritoneum) into the dialysate, which is the removed

Question 42

What are the disadvantages of renal dialysis

Answer 42

• Both methods require patients to keep a specific diet
• Regular trips to the hospital which take a long time
• Machine at home is expensive

Question 43

What are the advantages of kidney transplants

Answer 43

• Good long term solutions to kidney failure
• Patient has more freedom and no time restrictions from dialysis
• Diets are less restrictive
• Removes cost of dialysis machines

Question 44

What are the disadvantages of kidney transplants

Answer 44

• Kidneys can be rejected
• Donors don’t have the same antigens on the surface, triggering the immune response of the patient, causing them to have to take immunosuppressant drugs for the rest of their lives
• Immunosuppressant drugs can have long term side effects
• Not enough donors to meet the demand for transplants

Question 45

What can urine tests be used for

Answer 45

• Glucose in the urine can be tested for, as it should be fully reabsorbed by the proximal convoluted tubule, so if there is glucose present in urine it indicates something wrong with the homeostatic control of glucose
• Ketones in urine can be tested for, as ketones are produced by people with diabetes, so if they are present the person has diabetes
• Proteins can be tested for, as if they are present in the urine, it could suggest the blood pressure is too high (pushing the large proteins through in ultrafiltration)
• Testing for pregnancy
• Testing for anabolic steroids, which can be seen in the urine via gas chromatography or mass spectrometry

Question 46

Describe how pregnancy tests work

Answer 46

• Pregnancy sticks contain monoclonal antibody molecules, which are specific to a hormone produced during pregnancy
• If the person is pregnant, the urine will contain hCG hormone which will bind to the antibodies creating a positive result

Question 47

How are false pregnancy test results minimised

Answer 47

The antibodies in the testing sticks all originate from a single clone of B lymphocyte cells which all produce the same antibody specific to hCG