Excretion as an example of Homeostatic Control (2)

Done

What happens to blood as it passes through the sinusoids?

• Hepatocytes convert toxic substances into less harmful molecules.

• Kupffer cells break down old red blood cells and remove bacteria from the bloodstream.

Describe the outline of the ornithine cycle (all we need to know. Not whole process)

1) Amino acids are deaminated: the amino group is removed, forming ammonia and organic acids. (not ornithine cycle)

2) The organic acids are respired or stored as glycogen.

3) Ammonia is combined with carbon dioxide, which converts it into urea in a cyclical reaction.

4) Urea is released into the bloodstream.

5) It is then filtered by the kidneys and excreted from the body in the urine.

What is excretion?

• The removal of metabolic wastes from the body (eg co2)- would cause tissue damage if allowed to accumulate

• Excretion maintains homeostasis by keeping substances within a healthy range.

• Enables organisms to maintain pH balance and regulate osmotic pressure

Main metabolic waste products

* Carbon dioxide from respiration

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* bile pigments

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* nitrogen-containing compounds such as urea

Why is it important to remove metabolic waste products?

• To stop toxic products such as CO2 and ammonia building up

• The products interfere with cell processes by altering the pH- prevents normal metabolism

• Other metabolic products may act as inhibitors- reduces the activity of essential enzymes

Name 4 excretory organs

* skin

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* liver

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* lungs

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* kidneys

What is the kidneys function in excretion?

• Removes the urea from the bloodstream to become a part of the urine

• Urine is stored in the bladder before being excreted from the body via the urethra

How is hydrogen carbonate formed?

1) CO2 + H2O = H2CO3 (carbonic acid)

The carbonic acid dissociates to release hydrogen ions

2) H2CO3= H^+ + HCO3^-

Where does the formation of hydrogen carbonate occur?

* Occurs inside red blood cells under the influence of the enzyme carbonic anhydrase

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* Can also occur in the blood plasma

What is the skins function in excretion?

• Sweat contains salts, urea, water, uric acid and ammonia which are all excretory products

• The loss of water and salts is an important part of homeostasis- maintaining body temperature and water potential of the blood

What affect do hydrogen ions have on red blood cells?

1) They affect the pH of the cytoplasm in RBCs

2) The hydrogen ions interact with bonds within haemoglobin, changing its 3D shape

3) Change of the 3D shape reduces the affinity of haemoglobin for oxygen

4) Hydrogen ions can then combine with haemoglobin to form haemoglobinic acid

What affect do hydrogen ions have on blood plasma?

• Reduce the pH

• Maintaining pH is essential because

Why is it important to maintain the pH of blood plasma?

Changes could alter the structure of many proteins that help transport a wide range of substances around the body

What do proteins in the blood act as?

Buffers to resist the change in pH

What can happen to the CO2 that isn’t converted into hydrogen carbonate ions?

Can combine directly with haemoglobin to produce carbaminohaemoglobin

What are extra hydrogen ions detected by when the change in pH is small?

* Detected by the respiratory centre in the medulla oblongata of the brain

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* Causes an increase in breathing rate to help remove the excess CO2

What is the effect of both carbaminohaemoglobin and haemoglobinic acid?

• They are unable to combine with oxygen as normal

• This reduces oxygen transport further

What is the livers function in excretion?

• Excretes some metabolites as bile

• Converts excess amino acids to urea

How are amino acids converted to urea in the liver?

• They are broken down by the process of deamination

• The nitrogen-containing part of the molecules is then combined with CO2 to make urea

What is the lungs function in excretion?

* CO2 is passed from cells of respiring tissue into the bloodstream

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* It is then transported mostly in the form of hydrogen carbonate ions to the lungs

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* In the lungs CO2 diffuses into the alveoli to be excreted as you breath out

Explain the details of the excretion of bile pigmentation

• Formed from the breakdown of haemoglobin in the liver

• Excreted in the bile from the liver via the small intestine and gall bladder

Explain the details of the excretion of nitrogenous waste products

• Formed from the breakdown of excess amino acids by the liver

• All mammals produce urea to remove nitrogenous waste

General structure of the liver

• Made up of lobes which divide into lobules

• Has four vessels

Why are the cells, blood vessels and chambers in the liver arranged the way they are?

To ensure greatest possible contact between the blood and liver cells

Vessels in the liver

* Hepatic artery

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* hepatic vein

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* bile duct

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* hepatic portal vein

Role of the hepatic artery

• Supplies the liver with oxygenated blood from the heart- good supply of O2

• This supplies O2 that is essential for aerobic respiration

Why is it important that the liver has a good supply of O2?

* Liver cells are very active because they carry out many metabolic processes

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* Many of these processes require energy in the form of ATP- Need good supply of O2 for aerobic respiration

Role of the hepatic vein

To carry deoxygenated blood from the liver to the heart by rejoining the vena cava

Role of the hepatic portal vein

To carry blood containing products of digestion from the intestines to the liver

What does the Bile duct do?

• Carries bile from the liver to the gall bladder

• It is stored here until its required to aid the digestion of fats in the small intestine

What is the name for liver cells?

Hepatocytes

Structure of hepatocytes

• Large nuclei

• Prominent Golgi apparatuses

• Lots of mitochondria

• Many microvilli on the surface

• Dense cytoplasm

What are Inter-lobular vessels?

• The smaller vessels that the hepatic artery and hepatic portal vein split into once they enter the liver

• They run between and parallel to the lobules

Describe the composition of each liver lobule.

• Have a central vein in the middle that connects to the hepatic vein

• Many branches of the hepatic artery, hepatic portal vein and bile duct are connected to each lobule

About the Sinusoid

• The blood from the hepatic artery and hepatic portal vein is mixed and passes along this chamber

• It is lined with liver cells

• It contains Kupffer cells

• As blood flows along the sinusoid it is in close contact with the liver cells

• They have canaliculi

What is the role of liver cells in the liver lobules?

• They remove harmful substances and oxygen from the blood that passes past them in the sinusoids

• They break down the harmful substances into less harmful substances that then re-enter the blood

• Produce and secrete bile into the bile canaliculi

What do the central veins from all the lobules connect up to form?

The hepatic vein

What do the sinusoids connect?

They connect the hepatic artery and the hepatic portal vein to the central vein

Reason for mixing blood in the sinusoids

Increases the oxygen content of the blood from the hepatic portal vein

Role of Kupffer cells

• To breakdown and recycle old RBCs- The haemoglobin of RBC’s is broken down into bilirubin

• To ingest foreign particles and help protect against disease

How the vessels are arranged in the lobules

• Hepatic artery, portal vein and bile duct are known as interlobular vessels

• Interlobular vessels are found on the outside of the lobules

• A branch of hepatic veins known as the intra-lobular vessel is found at the centre of each lobule

What are some functions of the liver?

• Breaks down amino acids into urea – this involves deamination followed by the ornithine cycle

• Detoxifies the blood - alcohol, drugs and hormones in the blood are broken down by the liver - products are excreted in the urine.

• Stores glycogen – the liver converts excess glucose into glycogen.

• Makes bile – bile aids lipid digestion by emulsifying fats.

• Destruction of red blood cells

What is the role of the liver in storing glycogen?

• Hepatocytes respond to the binding of insulin to receptors in the cell membrane by converting glucose into glycogen for storage.

• Glycogen can then be released when blood glucose levels are low.

What enzymes do liver cells use for detoxification?

* Catalase

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* Cytochrome P450

The role of catalase in detoxification

• Converts hydrogen peroxide to oxygen and water

• It has a high turnover* number of 5 million

*the number of molecules of hydrogen peroxide that 1 molecule of catalase can render harmless in 1 second

The role of cytochrome P450 in detoxification

• Its a group of enzymes used to break down drugs such as cocaine and medicinal drugs

• Their role in metabolising drugs can interfere with other metabolic roles and can cause the unwanted side effects of some medicinal drugs

Arrangement of liver cells into cylindrical lobules

Arrangement of cells in a lobule

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What does the liver store sugar as?

Glycogen

Where is alcohol broken down?

• In the hepatocytes

• By the action of the enzyme ethanol dehydrogenase

What is the resulting compound after alcohol is broken down?

• Ethanal

• This gets broken down further into Ethanoate by the enzyme ethanal dehydrogenase

What compound is produced once ethanal is broken down?

* Ethanoate (acetate)

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* This is then combined with coenzyme A to form acetyl coenzyme A which enters the process of aerobic respiration

What happens to the hydrogen atoms released from alcohol?

They're combined with a coenzyme called NAD to form reduced NAD

Describe the detoxification of alcohol diagram

Equations showing the process of the breakdown of ethanol

Ethanol -> Ethanal

Ethanal -> Ethanoate

Ethanoate + Coenzyme A -> Acetyl coenzyme A

What is required for the break down of ethanol and fatty acids?

NAD

Describe how you get fatty liver

• If the liver has to detoxify too much alcohol it uses up its stores of NAD

• There are insufficient stores left to deal with fatty acids

• The fatty acids are then converted back into lipids and stored as fat in the hepatocytes

• This storing causes the liver to become enlarged

Types of reaction that the liver can do to detoxify

• Oxidation

• Reduction

• Methylation

• Combination

What 2 processes do amino acids go through to remove and excrete the amino component?

1) Deamination

2) Ornithine cycle

Formation of urea

Amino acid -> ammonia + keto acid

Ammonia+ keto acid → urea

Transamination

Converting between amino acids

Deamination

• Removes the amino group from excess amino acids and produces ammonia

• Also produces keto acid which can enter respiration directly to release its energy

Product of deamination

Ammonia and keto acid

Equation for deamination

Amino acid + Oxygen -\> Ammonia + Keto acid

Equation for the ornithine cycle

Ammonia + CO2 -\> Urea + H2O

Why do we produce urea instead of leaving it as ammonia?

• Its less soluble and toxic

• It can be passed back into the blood and transported around the body to the kidneys

What happens to urea in the kidneys?

It is filtered out of the blood and concentrated in the urine

What can keto acids be used for?

• Respiration

• Lipid storage

Sequence of events in the ornithine cycle (don’t think i need to know this much detail)

1) Ornithine

2) Ammonia in, carbon dioxide in, water out, citrulline is made

3) Ammonia in, water out, arginine is made

4) Water in, urea out, ornithine is made

goes back to the start

Reasoning behind the process of nitrogenous compound removal

Amino acids contain lots of energy so wasteful to excrete them

Give an overview of how the kidneys excrete waste products

1) Blood enters the kidneys via the renal artery

2) The blood then passes through capillaries in the cortex of the kidneys

3) As it passes through the capillaries, substances are filtered out of the blood -ultrafiltration

4) Useful substances are reabsorbed back into the blood from the tubules in the medulla and cortex- selective reabsorption

5) The remaining unwanted substances pass along the tubules and ureter to the bladder- expelled as urine

6) Filtered blood leaves via the renal vein

What supplies blood to the kidneys?

Renal artery

What drains the blood from the kidneys?

Renal vein

What are the roles of the kidneys?

• Excretion- they remove waste products(urea) from the blood and produce urine

• They also regulate the water potential of the blood (remove excess water, excess ions)

Parts of the structure of the kidney

• Capsule (surrounds the 3 regions)

• Cortex (outer region)

• Medulla (inner region)

• Renal vein

• Renal artery

• Pelvis (centre) which leads into the ureter

Capsule of the kidney

• Hard outer layer of the kidney

• Protects against mechanical damage

Medulla of the kidney

• Beneath the cortex

• Contains the loop of Henle and the collecting duct

Pelvis of the kidney

• Where urine is emptied into

• Made of connective tissue

Parts of the nephron

• Afferent arteriole

• Efferent arteriole

• Glomerulus

• Bowman's capsule

• Loop of Henle

• Proximal convoluted tubule

• Peritubular capillaries

• Distal convoluted tubule

• Collecting duct

What processes occur in the nephron in order?

1) Ultrafiltration

2) Selective reabsorption

3) Reabsorption of water

About nephrons

• Each one starts in the cortex at the Bowmans capsule

• The remainder of the nephron is a coiled tubule that passes through the cortex

• It then forms a loop down into the medulla and back to the cortex

• It then joins a collecting duct that passes back down into the medulla

Which parts of the nephron are in the medulla?

• Loop of Henle

• Collecting duct

What do the renal arteries split into?

Afferent arterioles- carry blood into the glomerulus

What happens to the blood in the glomerulus?

• It continues into an efferent arteriole

• The efferent arteriole carries the blood to more capillaries surrounding the rest of the tubule

• These capillaries eventually flow into the renal vein

What is each glomerulus surrounded by?

The Bowman’s capsule

What pushes the fluid from the blood into the Bowman’s capsule?

The process of ultrafiltration

Type of epithelial cells in the renal capsule

Squamous epithelial cells

Why might tubules look different sizes down a microscope?

• Different planes of cut

• Distorted by preparation of the slide

What doesn’t leave the blood during ultrafiltration?

Larger molecules

• Proteins

• RBC’s

Why is there a high pressure in the glomerulus?

• Blood comes in via the afferent arteriole which has a wide lumen

• Blood leaves via the efferent arteriole which is narrower

What does the high pressure in the glomerulus lead to?

It forces liquid and small molecules in the blood out of the capillary into the Bowmans capsule

What is the filter?

The barrier between the blood in the capillary and the lumen of the Bowman’s capsule

What are the 3 layers that make up the filter?

* The endothelium of the capillary

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* The basement membrane

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* Podcytes

Basement membrane

• Consists of a fine mesh of collagen fibres and glycoproteins

• The mesh acts as a filter to prevent the passage of larger molecules

• This means that most proteins and all blood cells are held in the capillaries of the glomerulus

Podocytes

• Have many finger-like projections called major processes

• On each major process there are minor processes

  
  

• The minor processes hold the cells away from the endothelium of the capillary

• These projections ensure that there are gaps between the cells

• Fluid from the blood in the glomerulus can pass between these cells into the lumen of the Bowman’s capsule

What are the 3 parts of the rest of the tubule that the Bowman’s capsule leads to?

• Proximal convoluted tubule

• Loop of Henle

• Distal convoluted tubule

Where does the fluid from the nephrons go?

It enters the collecting ducts which pass through the medulla to the pelvis

The endothelium of the capillary

• There are narrow gaps between cells of the endothelium of the capillary

• The cells of the endothelium contain pores called fenestrations

• The fenestrations allow blood plasma and the substances dissolved in it to pass out of the capillary

Describe the process of ultrafiltration

1) The diameter of the afferent arteriole is wider than that of the efferent arteriole

2) This means the blood in the glomerulus is under high pressure

3) The high pressure forces liquid and small molecules out of the capillary into the Bowmans capsule

4) The fluid is filtered by the 3 layers

5) Filtrate moves into the proximal convoluted tubule

6) Plasma proteins and cells remain in the blood- allows water reabsorption later

What is Ultrafiltration?

Filtration of the blood at a molecular level under pressure