4.2.1 Excretion By Ms Cullen

What is excretion?

Why excrete?

What do we need to excrete? Brainstorm: What needs to be excreted from the body in order to maintain a regulated internal environment

The Liver Largest organ in the body Largest organ in the body 1450 cm 3 of blood passes through it every minute 1450 cm 3 of blood passes through it every minute It is located under the diaphragm on the right-side of the body It is located under the diaphragm on the right-side of the body It has many different functions including many different metabolic reactions (approx 500). It has many different functions including many different metabolic reactions (approx 500). Liver

The liver and its connections to the blood system

The arrangement of liver cells into cylindrical lobules

Liver lobules under microscope

Section through a liver lobule Column of liver cells lining the sinusoid Inter-lobular vessel

The arrangement of liver cells in a lobule

Liver Cells (hepatocytes) Not very specialised Not very specialised Cuboidal shape Cuboidal shape Many microvilli on surface Many microvilli on surface Metabolic functions include: protein synthesis, transformation & storage of carbohydrates, synthesis of cholesterol & bile salts, detoxification and many others! Metabolic functions include: protein synthesis, transformation & storage of carbohydrates, synthesis of cholesterol & bile salts, detoxification and many others! Kupffer Cells Specialised macrophages Move within the sinusoids Remove bacteria Involved in breakdown and recycling of red blood cells Produce bilirubin by breaking down haemoglobin, which is secreted in bile & faeces

Structure of a liver lobule sinusoid Kupffer cell hepatocyte

The Formation of Urea- Deamination We can not store excess protein in our bodies. We can not store excess protein in our bodies. The process of deamination is the break down of amino acids (from excess proteins), which occurs in the liver. The process of deamination is the break down of amino acids (from excess proteins), which occurs in the liver. The amino group is removed, by addition of oxygen, forming ammonia NH 3. The amino group is removed, by addition of oxygen, forming ammonia NH 3. What remains is keto acid, which can be respired to release energy or converted to fat to be stored. What remains is keto acid, which can be respired to release energy or converted to fat to be stored.

The Formation of Urea - The Ornithine Cycle Ammonia is soluble and very toxic, therefore it must be removed from the body quickly. Ammonia is soluble and very toxic, therefore it must be removed from the body quickly. It is combined with carbon dioxide to form urea CO(NH 2 ) 2 It is combined with carbon dioxide to form urea CO(NH 2 ) 2 Urea is still soluble and toxic, but much less dangerous! Urea is still soluble and toxic, but much less dangerous! The liver releases urea in to the plasma of the blood where it is transported to be filtered out at the kidneys. The liver releases urea in to the plasma of the blood where it is transported to be filtered out at the kidneys.

The ornithine cycle

Using P.41 of OCR A2 Biology books write notes on the process of Detoxification in the liver

The Mammalian Kidney

Function: The kidneys have a couple of different functions. The main purpose of the kidney is to separate urea, mineral salts, toxins, and other waste products from the blood. The kidneys also conserve water, salts, and electrolytes. At least one kidney must function properly for life to be maintained.

Nitrogenous wastes: Nitrogenous wastes are a by product of protein metabolism. Amino groups are removed from amino acids prior to energy conversion. The NH 2 (amino group) combines with a hydrogen ion (proton) to form ammonia (NH 3 ). Nitrogenous wastes are a by product of protein metabolism. Amino groups are removed from amino acids prior to energy conversion. The NH 2 (amino group) combines with a hydrogen ion (proton) to form ammonia (NH 3 ). This process occurs in the liver and is called deamination. This process occurs in the liver and is called deamination. Mammals secrete urea that they form in their liver. Amino groups are turned into ammonia, which in turn is converted to urea, dumped into the blood and concentrated by the kidneys. Mammals secrete urea that they form in their liver. Amino groups are turned into ammonia, which in turn is converted to urea, dumped into the blood and concentrated by the kidneys.

Structure of the kidney:

Structure of a nephron

Dissection of kidney and microscope work Complete Activity 2 in practical guide.

Eyepiece graticules These are used to measure the size of a specimen. These are used to measure the size of a specimen. These are microscopic rulers, 1mm long and divided into 100 divisions. These are microscopic rulers, 1mm long and divided into 100 divisions. Each division is 0.01mm or 10μm. Each division is 0.01mm or 10μm. Eyepiece lens magnification Objective lens magnification Total magnification Value of 1 eyepiece division (μm) X 10X 4X 4025 X10 X X 10X 40X X 10X 100 (oil immersion lens) X

Ultrafiltration in the Nephron: The glomerulus is enclosed by the renal capsule (or Bowman’s capsule)- the first part of the nephron. The glomerulus is enclosed by the renal capsule (or Bowman’s capsule)- the first part of the nephron. The blood pressure in the capillaries of the glomerulus forces plasma out of the blood by ultrafiltration (through thousands of gaps in the endothelium). The blood pressure in the capillaries of the glomerulus forces plasma out of the blood by ultrafiltration (through thousands of gaps in the endothelium). All molecules with a molecular mass of <70k are squeezed out of the blood to form a filtrate in the renal capsule. All molecules with a molecular mass of <70k are squeezed out of the blood to form a filtrate in the renal capsule. Which molecules in the blood will be in the filtrate?

Diagram of the nephron and associated blood capillaries Blood flow in the kidneys is so high in humans that blood flows through the kidney approx. once every 5 minutes. Efficient ultrafiltration produces about 125cm 3 of filtrate every minute, about 180litres a day!

Selective Reabsorption: The proximal convoluted tubule is the longest (14mm) and widest (60µm) part of the nephron. It is situated in the cortex. The proximal convoluted tubule is the longest (14mm) and widest (60µm) part of the nephron. It is situated in the cortex. It is lined with epithelial cells containing microvilli and numerous mitochondria. It is lined with epithelial cells containing microvilli and numerous mitochondria. In this part of the nephron over 80% of the filtrate is reabsorbed into the tissue fluid and then to the blood. This ensures that all the "useful" materials that were filtered out of the blood (such as glucose and amino acids) are now returned to the blood. In this part of the nephron over 80% of the filtrate is reabsorbed into the tissue fluid and then to the blood. This ensures that all the "useful" materials that were filtered out of the blood (such as glucose and amino acids) are now returned to the blood. In humans the proximal convoluted tubule has a total surface area of about 50m 2 !

Selective Reabsorption in the proximal convoluted tubule All glucose, all amino acids and 85% of mineral ions are reabsorbed by active transport from the filtrate to the tissue fluid. They then diffuse into the blood capillaries. All glucose, all amino acids and 85% of mineral ions are reabsorbed by active transport from the filtrate to the tissue fluid. They then diffuse into the blood capillaries. Small proteins are reabsorbed by pinocytosis, digested, and the amino acids diffuse into the blood. Small proteins are reabsorbed by pinocytosis, digested, and the amino acids diffuse into the blood. The filtrate is hypotonic (has a lower solute concentration) to the blood plasma – this is because the plasma also contains plasma proteins in solution. The filtrate is hypotonic (has a lower solute concentration) to the blood plasma – this is because the plasma also contains plasma proteins in solution. As the filtrate is more dilute than the blood plasma 80% of the water is reabsorbed to the blood by osmosis. As the filtrate is more dilute than the blood plasma 80% of the water is reabsorbed to the blood by osmosis.

Loop of Henle: As the filtrate enters the Loop of Henle it is normally isotonic to the blood (has same solute concentration). As the filtrate enters the Loop of Henle it is normally isotonic to the blood (has same solute concentration). The job of the loop of Henle is to make the tissue fluid in the medulla hypertonic compared to the filtrate in the nephron. ie to reabsorb water. The job of the loop of Henle is to make the tissue fluid in the medulla hypertonic compared to the filtrate in the nephron. ie to reabsorb water. The loop of Henle does this by pumping sodium and chloride ions out of the filtrate into the tissue fluid. The loop of Henle does this by pumping sodium and chloride ions out of the filtrate into the tissue fluid. The first part of the loop (the descending limb) is impermeable to ions, but permeable to water, so some water leaves by osmosis. This makes the filtrate more concentrated as it descends. The first part of the loop (the descending limb) is impermeable to ions, but permeable to water, so some water leaves by osmosis. This makes the filtrate more concentrated as it descends.

Loop of Henle: The second part of the loop (the ascending limb) contains a Na + and Cl - pump, so these ions are actively transported out of the filtrate into the surrounding tissue fluid. Water would follow by osmosis, but it can’t, because the ascending limb is impermeable to water. So the tissue fluid becomes more salty (hypertonic) and the filtrate becomes less salty (hypotonic). The second part of the loop (the ascending limb) contains a Na + and Cl - pump, so these ions are actively transported out of the filtrate into the surrounding tissue fluid. Water would follow by osmosis, but it can’t, because the ascending limb is impermeable to water. So the tissue fluid becomes more salty (hypertonic) and the filtrate becomes less salty (hypotonic). The solute concentration at any part of the loop is lower in the ascending limb than in the descending limb. The solute concentration at any part of the loop is lower in the ascending limb than in the descending limb. This mechanism is called the (hairpin) countercurrent multiplier mechanism. This mechanism is called the (hairpin) countercurrent multiplier mechanism.

Distal convoluted tubule and collecting duct: The distal convoluted tubule is relatively short it has many microvilli, mitochondria and numerous membrane pumps for active transport. The distal convoluted tubule is relatively short it has many microvilli, mitochondria and numerous membrane pumps for active transport. The distal convoluted tubule reabsorbs varying quantities of inorganic ions according to the body’s needs. The distal convoluted tubule reabsorbs varying quantities of inorganic ions according to the body’s needs. Final Na + reabsorption occurs here and pH of the blood is also controlled. Final Na + reabsorption occurs here and pH of the blood is also controlled. If the blood is too alkaline; hydrogen carbonate ions dissociate from carbonic acid and pass into the filtrate. This raises the pH of the blood when necessary. If the blood is too alkaline; hydrogen carbonate ions dissociate from carbonic acid and pass into the filtrate. This raises the pH of the blood when necessary. If the blood is too acidic; hydrogen ions will be secreted into the filtrate. If the blood is too acidic; hydrogen ions will be secreted into the filtrate.

Distal convoluted tubule and collecting duct: The second part of the distal convoluted tubule acts as a collecting duct. The second part of the distal convoluted tubule acts as a collecting duct. Both the distal convoluted tubule and collecting duct are affected by hormones. These control the amount of water passing into the medulla, and therefore how concentrated the urine is. Both the distal convoluted tubule and collecting duct are affected by hormones. These control the amount of water passing into the medulla, and therefore how concentrated the urine is. The walls of both the distal convoluted tubule and the collecting duct are only permeable to water if the hormone ADH is present. The walls of both the distal convoluted tubule and the collecting duct are only permeable to water if the hormone ADH is present.

The loop of Henle and the collecting duct

Permeability to water of different parts of the nephron Try and complete the table below: Part of NephronPermeability Bowman’s capsule proximal convoluted tubule descending limb of Loop of Henle ascending limb of Loop of Henle distal convoluted tubule Collecting duct

Changes in the relative concentrations of certain substances as fluid passes along the nephron

Identify parts labelled A – I:

The Loop of Henle and water conservation Desert mammals have relatively thick medullas in their kidneys, these contain nephrons with long loops of Henle. Desert mammals have relatively thick medullas in their kidneys, these contain nephrons with long loops of Henle. This allows maximum reabsorption of water into the blood. This allows maximum reabsorption of water into the blood. They will excrete little water in their urine and produce highly concentrated urine. They will excrete little water in their urine and produce highly concentrated urine.

The Loop of Henle and water conservation Mammals that live in an abundance of available fresh water have kidneys with relatively thin medullas and short loops of Henle. Mammals that live in an abundance of available fresh water have kidneys with relatively thin medullas and short loops of Henle.

Osmoregulation

Osmoregulation Osmoregulation is the active regulation of the osmotic pressure of bodily fluids to maintain the homeostasis of the body's water content; that is it keeps the body's fluids from becoming too dilute or too concentrated. Osmoregulation is the active regulation of the osmotic pressure of bodily fluids to maintain the homeostasis of the body's water content; that is it keeps the body's fluids from becoming too dilute or too concentrated.

Osmoregulation The osmoreceptors responsible for detecting changes in the solute concentration of the blood, are located in the hypothalamus in the brain. The osmoreceptors responsible for detecting changes in the solute concentration of the blood, are located in the hypothalamus in the brain. The hormone responsible for conserving body water is Anti-diuretic hormone (ADH), this is released by the posterior pituitary gland in the brain. The hormone responsible for conserving body water is Anti-diuretic hormone (ADH), this is released by the posterior pituitary gland in the brain.

Osmoregulation - dehydration If you have not had a drink for a while your blood will have a low water potential. If you have not had a drink for a while your blood will have a low water potential. The osmoreceptors in the hypothalamus detect this and cause the posterior pituitary gland to release ADH into the blood. The osmoreceptors in the hypothalamus detect this and cause the posterior pituitary gland to release ADH into the blood. ADH makes the distal convoluted tubule and collecting duct more permeable to water by attaching to receptors on these cell membranes. ADH makes the distal convoluted tubule and collecting duct more permeable to water by attaching to receptors on these cell membranes. Water then causes protein channels (aquaporins) to insert themselves into these membranes and water passes through them. Water then causes protein channels (aquaporins) to insert themselves into these membranes and water passes through them. More water is then reabsorbed into the region of high solute content in the medulla. More water is then reabsorbed into the region of high solute content in the medulla.

Osmoregulation - dehydration This produces a smaller volume of more concentrated urine. This produces a smaller volume of more concentrated urine. The hypothalamus also stimulates the thirst centre in the brain. This makes you feel thirsty and prompts you to drink more, which will dilute the blood The hypothalamus also stimulates the thirst centre in the brain. This makes you feel thirsty and prompts you to drink more, which will dilute the blood

Osmoregulation If you drink lots the blood will have a high water potential. If you drink lots the blood will have a high water potential. This is detected by the osmoreceptors in the hypothalamus and less ADH is secreted. This is detected by the osmoreceptors in the hypothalamus and less ADH is secreted. The lack of ADH makes the distal convoluted tubule and collecting duct less permeable to water. The lack of ADH makes the distal convoluted tubule and collecting duct less permeable to water. Therefore less water is reabsorbed into the medulla. Therefore less water is reabsorbed into the medulla. Large quantities of dilute urine are produced. Large quantities of dilute urine are produced.

Osmoregulation – an example of negative feedback

Negative and Positive Feedback Negative feedback mechanisms restore the systems to their normal level. Negative feedback mechanisms restore the systems to their normal level. Positive feedback mechanisms takes levels further away from the norm. It is the opposite of negative feedback in that encourages a physiological process or amplifies the action of a system. Positive feedback mechanisms takes levels further away from the norm. It is the opposite of negative feedback in that encourages a physiological process or amplifies the action of a system. Examples would be platelets accumulating to clot blood, the release of oxytocin to increase muscle contractions during birth. Positive feedback can be harmful too an example would be an increase in body temperature leading to a fever.

What happens to ADH? ADH does not stay in the blood. ADH does not stay in the blood. It is a protein so will be broken down in the liver by deamination and therefore excreted. It is a protein so will be broken down in the liver by deamination and therefore excreted.

Complete Activity 4 Investigating urea concentration

Answer questions from the Excretion & Kidney Question sheet