Excretion (OCR A Level Biology): Flashcards

Metabolic waste containing nitrogen, including ammonia, urea and uric acid.

The liver is a key organ in producing all of the excretory substances except carbon dioxide.

True

False — bile pigments are produced during the breakdown of haemoglobin.

It carries out the breakdown of toxic substances and the production of excretory waste, both of which require a rich blood supply to deliver and remove substances.

The vessel that carries oxygenated blood from the heart to the liver, providing oxygen for aerobic respiration to fuel the metabolic activity of liver cells.

The vessel that carries blood from the digestive system to the liver, allowing the liver to absorb and metabolise nutrients absorbed into the blood in the small intestine.

The hepatic vein carries deoxygenated blood out of the liver and back to the heart.

• Store bile, a liquid containing bile salts (for lipid digestion) and bile pigments (from the breakdown of haemoglobin)\n\n- Release bile into the duodenum via the bile duct

• Bile salts for lipid digestion\n\n- Bile pigments from the breakdown of haemoglobin

The main cells of the liver, which carry out almost all of the liver's functions.

The structures into which liver cells (hepatocytes) are arranged; each is supplied with blood by branches of the hepatic artery and hepatic portal vein.

Wide capillaries within a liver lobule where blood from the hepatic artery and hepatic portal vein mixes, and where substances are exchanged with nearby hepatocytes.

Within each lobule, blood from the hepatic artery and hepatic portal vein mixes inside wide capillaries known as sinusoids.

Each lobule is connected to a branch of the hepatic vein, which drains blood away from the lobule and into the main hepatic vein.

The liver is mainly made up of cells known as hepatocytes, which carry out almost all of its functions.

False — the hepatic artery carries oxygenated blood to the liver.

True

• Storage of glycogen

• Formation of urea

• Detoxification

The conversion of glucose into glycogen, which occurs in the liver.

Insulin (released after the pancreas detects a rise in blood glucose) triggers glycogenesis, the conversion of glucose into glycogen.

The glycogen produced is stored inside hepatocytes.

Glycogen is a compact storage molecule that allows for easy release of glucose when it is needed.

Excess amino acids reach the liver from the gut via the hepatic portal vein.

• Deamination

• The ornithine cycle

The removal of the amino group (NH₂), together with an extra hydrogen atom (H⁺), from an amino acid; these combine to form ammonia (NH₃).

A keto acid remains.

Ammonia is very soluble and highly toxic, so it can be damaging if it builds up in the blood.

It is converted into urea, which is less toxic than ammonia.

Ammonia is combined with carbon dioxide to form urea.

One molecule of urea is produced from one molecule of carbon dioxide and two amino groups.

Urea diffuses through the phospholipid bilayer of the hepatocytes and is transported to the kidneys dissolved in the blood plasma, where it is excreted.

The breakdown of substances that are not needed, or are toxic (e.g. alcohol, hydrogen peroxide, lactate and medicinal drugs) by the liver.

Ethanol is absorbed in the stomach and transported in the blood to the hepatocytes.

Inside the hepatocytes the enzyme alcohol dehydrogenase converts ethanol into ethanal.

Ethanal is then converted into other molecules that enter respiration.

The metabolism of ethanol generates ATP, so hepatocytes do not metabolise as much fat as usual and instead store it, causing fatty liver.

Stored fat reduces the ability of hepatocytes to carry out their other functions, and can eventually lead to cirrhosis (scarring of the liver caused by excessive alcohol consumption).

It removes glucose from the bloodstream, decreasing blood glucose concentration back to within the normal range.

It can:

• enter the Krebs cycle to be respired

• be converted to glucose

• be converted to glycogen or fat for storage

False — urea is less toxic than ammonia.

True

The branch of biology that studies the microscopic anatomy of biological tissues.

• Microscopic anatomy

• Microanatomy

By staining sections of liver tissue and viewing them under a microscope.

A microscope.

For drawing and labelling of the tissue.

Staining makes the otherwise transparent structures of the tissue visible, so the sample can be examined, drawn and labelled.

Histology is the branch of biology that studies the microscopic anatomy of biological tissues.

The histology of the liver can be studied by staining sections of liver tissue and viewing them under a microscope.

True

True

False — they are viewed under a microscope.

Humans have two kidneys.

It regulates the water content of the blood, which is vital for maintaining blood pressure.

It excretes the toxic waste products of metabolism (such as urea) and substances in excess of requirements (such as salts).

The tough outer layer that surrounds the kidney.

• The cortex\n\n- The medulla\n\n- The renal pelvis

The glomerulus, the Bowman's capsule, the proximal convoluted tubule and the distal convoluted tubule.

The loop of Henle and the collecting duct.

The area where the ureter joins the kidney.

The functional unit of the kidney, responsible for the formation of urine. Each kidney contains thousands of these tiny tubes.

The functional unit of the kidney, responsible for the formation of urine, is the nephron.

A network of capillaries found within the Bowman's capsule of each nephron, supplied with blood by the afferent arteriole.

• Blood enters via the afferent arteriole (carrying blood from the renal artery)\n\n- It flows into the capillaries of the glomerulus\n\n- These rejoin to form the efferent arteriole\n\n- Blood then flows into a network of capillaries running alongside the rest of the nephron\n\n- Blood eventually leaves via the renal vein

• Osmoregulation – regulating the water content of the blood

• Excretion – removing toxic waste products of metabolism (e.g. urea) and substances in excess of requirements (e.g. salts)

False — the cortex is the outermost region, with the medulla lying beneath it.

True

The functional unit of the kidney, responsible for the formation of urine.

• Ultrafiltration – small molecules are filtered out of the blood into the Bowman's capsule, forming glomerular filtrate

• Selective reabsorption – useful molecules are taken back from the filtrate and returned to the blood

The process by which small molecules are forced out of the glomerular capillaries and into the Bowman's capsule under high blood pressure, forming glomerular filtrate.

The afferent arteriole (entering the glomerulus) is wider in diameter than the efferent arteriole (leaving the glomerulus).

This creates high blood pressure within the glomerulus, forcing small molecules out of the capillaries and into the Bowman's capsule.

• Amino acids

• Water

• Glucose

• Urea

• Inorganic ions (mainly Na⁺, K⁺ and Cl⁻)

Blood cells and large proteins remain in the blood.

• The capillary endothelium – gaps between endothelial cells allow small molecules through

• The basement membrane – a mesh of collagen and glycoproteins with holes small molecules pass through

• The epithelium of the Bowman's capsule – made of podocytes with gaps between them

The epithelial cells of the Bowman's capsule that have many finger-like projections; the gaps between these projections allow small molecules to pass through during ultrafiltration.

The process by which only certain useful substances (e.g. water, salts, glucose and amino acids) are reabsorbed from the glomerular filtrate back into the blood as the filtrate passes along the nephron.

Most selective reabsorption occurs in the proximal convoluted tubule of the nephron.

• Many microvilli on the luminal membrane – increase surface area for reabsorption

• Many co-transporter proteins in the luminal membrane – each transports a specific solute (e.g. glucose or an amino acid)

• Many mitochondria – provide energy for the sodium-potassium (Na⁺/K⁺) pumps

• Tightly packed cells – so all reabsorbed substances must pass through the cells, not between them

1. Sodium-potassium pumps actively transport sodium ions out of the epithelial cells and into the blood

1. This lowers the sodium ion concentration inside the cells, so sodium ions in the filtrate diffuse down their concentration gradient into the epithelial cells

1. Sodium ions move via co-transporter proteins, which transport glucose or an amino acid at the same time

1. Once inside the cells, these solutes diffuse down their concentration gradients into the blood

The movement of ions, sugars and amino acids into the surrounding tissues lowers the water potential of the tissues.

Water therefore leaves the proximal convoluted tubule by osmosis, moving down the water potential gradient.

Sodium and chloride ions are pumped out of the filtrate in the ascending limb into the surrounding medulla, lowering the water potential of the medulla.

The ascending limb is impermeable to water, so water cannot leave here by osmosis.

The low water potential in the medulla then draws water out of the descending limb and the collecting duct by osmosis.

They are too large to pass through the holes in the capillary endothelial cells.

False — the afferent arteriole is wider than the efferent arteriole, creating high blood pressure in the glomerulus.

True

The branch of biology that studies the microscopic anatomy of biological tissues.

Also known as microscopic anatomy or microanatomy.

The functional unit of the kidney.

They allow the external and internal structures of organs to be examined, so that theories can be made about how the organs function.

• The external (gross) structure of the kidney

• The internal structure, seen in a vertical section

By staining sections of kidney tissue and viewing them under a microscope.

The stained samples can then be drawn and labelled to identify the different structures within the nephron.

Because nephrons are irregular structures that do not lie in any one plane within the kidney.

The parts of many adjacent nephrons, rather than a single complete nephron.

• The cortex

• The medulla

Histology is the branch of biology that studies the microscopic anatomy of biological tissues.

True

True

The control of the water potential of body fluids. It is a key part of homeostasis.

Specialised sensory neurones that monitor the water potential of the blood. They are found in the hypothalamus of the brain.

Osmoreceptors are located in the hypothalamus.

• Nerve impulses are sent along the sensory neurones to the posterior pituitary gland

• This stimulates the posterior pituitary gland to release antidiuretic hormone (ADH)

• ADH enters the blood and travels to the kidneys

• The kidneys reabsorb more water, reducing the loss of water in the urine

A hormone released by the posterior pituitary gland that causes the kidneys to reabsorb more water, reducing the volume of water lost in the urine.

ADH is released from the posterior pituitary gland, which lies just below the hypothalamus.

Water is reabsorbed by osmosis from the filtrate.

ADH increases the number of aquaporins (water-permeable channels) in the luminal membranes of the collecting duct cells.

• ADH molecules bind to receptor proteins on the collecting duct cells

• This activates a signalling cascade leading to phosphorylation of the aquaporin molecules

• This activates the aquaporins, causing vesicles (whose membranes contain aquaporins) to fuse with the luminal membranes

• This increases the membrane's permeability to water

Water-permeable channel proteins in cell membranes. In the collecting duct, ADH increases their number in the luminal membranes to allow more water reabsorption.

Water moves out of the collecting duct (high water potential), through the aquaporins, into the tissue fluid and blood plasma in the medulla (low water potential).

This movement occurs by osmosis, down a water potential gradient.

As the filtrate loses water it becomes more concentrated.

A small volume of concentrated urine is produced.

• Osmoreceptors in the hypothalamus are not stimulated

• No nerve impulses are sent to the posterior pituitary gland, so no ADH is released

• Aquaporins are moved out of the luminal membranes, so collecting duct cells are no longer permeable to water

• The filtrate loses no water, so a large volume of dilute urine is produced

They detect changes in the water potential of the blood.

Water is reabsorbed as the filtrate passes through the collecting ducts.

True

False — high ADH causes a small volume of concentrated urine to be produced.

Any four of:

• Blood loss in an accident

• High blood pressure

• Diabetes

• Overuse of certain drugs (e.g. aspirin)

• Certain infections

They are retained and not excreted, leading to a build-up of toxins in the blood.

Less blood is filtered by the glomerulus, so the GFR decreases.

As well as retaining toxins, kidney failure disrupts the electrolyte balance of the blood, so the concentrations of ions and charged compounds are not maintained.

Can cause abdominal cramps, tiredness, muscle weakness and even paralysis.

If concentrations keep rising, the frequency of impulses from the sinoatrial node may decrease, potentially leading to arrhythmia and cardiac arrest.

Sodium plays an important role in neuromuscular function, fluid balance and acid/base balance.

• Renal dialysis - toxins, metabolic waste and excess substances are removed from the blood by diffusion across a dialysis membrane

• Kidney transplant - the non-functioning kidney(s) is replaced with a functioning kidney

A treatment for kidney failure in which small and large molecules are separated across a partially permeable membrane, using a machine (haemodialyser) that acts as an artificial kidney to remove wastes from the blood.

The dialysis fluid in a haemodialyser. It contains substances needed in the blood (e.g. glucose and sodium ions) at normal blood concentrations, but contains no urea.

Because glucose is at a concentration equal to normal blood sugar, there is no concentration gradient, so there is no net movement of glucose out of the blood.

Salts are at the ideal blood concentration, so salts only move where there is an imbalance (diffusing into the blood if it is too low, out of the blood if it is too high).

The dialysate contains no urea, so urea diffuses down its concentration gradient from the blood into the dialysate.

The fluid is continually refreshed to maintain the gradient, and blood and dialysate flow in opposite directions (countercurrent) so a gradient is maintained along the whole dialyser, removing almost all the urea.

Heparin is an anticoagulant (blood thinner) that prevents the formation of blood clots.

• Regular trips to hospital for treatment lasting at least three hours each time

• Having to carefully control their diet to minimise urea production and salt intake

The donor kidney is often a different tissue type, so the patient's immune system may reject it (the donor will not have the same cell-surface antigens).

Any two of:

• Much more freedom, as the patient is not tied to regular dialysis sessions

• Diet can be much less restrictive

• Removes the high cost of using dialysis machines

• A long-term solution, whereas dialysis only works for a limited time

Disorientation, muscle spasms, higher blood pressure and general weakness.

They must be taken for life, can have long-term side effects and leave the patient vulnerable to infections.

False — they flow in opposite directions (countercurrent), maintaining the concentration gradient along the whole dialyser.

True — this means urea diffuses down its concentration gradient out of the blood and into the dialysate.

All of the glucose in the glomerular filtrate should be reabsorbed by the proximal convoluted tubule (PCT), so none remains to be excreted in the urine.

It suggests something is wrong with the person's homeostatic control of glucose, in particular a problem with the functioning of insulin.

It suggests the person has diabetes mellitus.

It can indicate that:

• The person's blood pressure may be too high

• There is a kidney infection

• There is something wrong with their blood filtration mechanism

It indicates the person has a bacterial infection in the urinary tract.

A hormone produced during pregnancy, secreted by the early embryo after it has implanted in the uterus. Its presence in the mother's urine is detected by pregnancy tests.

They contain monoclonal antibodies that are specific to hCG, a hormone that becomes present in the mother's urine during pregnancy. The antibodies bind hCG to give a positive result.

Because they all originate from a single clone of B lymphocytes, they produce the same antibody specific to hCG. This specificity minimises the chance of false results.

The antibodies in a pregnancy testing stick are specific to the hormone human chorionic gonadotropin (hCG), which is secreted by the early embryo after implantation.

Anabolic steroids can be used to rapidly build muscle mass by stimulating protein synthesis, so athletes are regularly tested to detect their illegal use.

Anabolic steroids are detected in urine via:

• Gas chromatography

• Mass spectrometry

All the glucose in the glomerular filtrate should be reabsorbed by the proximal convoluted tubule (PCT), so there should be no glucose present in the urine.

Ketones, such as acetone (propanone) and acetoacetate, are produced by the metabolism of people who have diabetes.

True — all glucose in the glomerular filtrate is reabsorbed by the proximal convoluted tubule (PCT).

False — nitrate ions in the urine indicate a bacterial infection in the urinary tract; protein is the marker linked to high blood pressure.