A LEVEL: Biology (F214), Excretion

BIOLOGY: UNIT 1

EXCRETION

Excretion: Removal of metabolic waste* from the body. 

*Metabolic Waste: waste substances that may be toxic or are produced in excess by the reactions inside cells.

What is commonly excreted:

1. Carbon Dioxide from Respiration
2. Nitrogen containing compounds: Urea (excess amino acids)

Carbon dioxide is released into the bloodstream where it then travels to the lungs, and then it diffuses into the alveoli to be excreted as we breathe out. 

Urea is produced as a result of excess amino acids being broken down in a process called deamination*. 

Urea is passed into the bloodstream plasma, transported to the kidneys. In the kidneys the urea is removed from the plasma to become a part of the urine. It is then stored in the bladder, before being excreted via the urethra. 

*Deamination: The removal of the amine group from an amino acid to produce ammonia

Why is there a need for removal?

Equations:

CO2 + H20 —> H2CO3 (Carbon Dioxide + Oxygen—> Carbonic Acid)

H2CO3 —> H+ + HCO3- (Carbonic Acid —> Hydrogen ions + Bicarbonate ions)

THE LIVER

Liver Cells = Hepatocytes

Important role in homeostasis

 BLOOD FLOW TO THE LIVER:

Blood from two sources

 1. Oxygenated from the Heart:

          (hepatic artery)

Aorta ——————> Liver

Oxygen is used for metabolic processes. These require ATP, so a good supply of oxygen is necessary for metabolic processes to take place.

2. Deoxygenated from the Digestive Systems:

  (hepatic portal vein)

digestive system ————————> Liver

Blood contains all the rich products from the digestive system. Concentration of these products is uncontrolled and may be toxic. Blood leaves via the hepatic vein, to rejoin the vena cava and then get put back into normal circulation. 

Fourth Vessel: is not a blood vessel, it is a bile duct, bile is a secretion of the liver, it is both for digestive purposes and an excretory function. 

Bile duct carries bile from the liver ———> gall bladder, to be stored until required in the emulsification of fats in the small intestine (digestion).

ARRANGEMENT OF CELLS WITHIN THE LIVER

 Liver is divided into lobes, these are further divided into cylindrical lobules. 

Hepatic artery & Hepatic portal vein are divided into ever smaller vessels. They run parallel to the lobules and are known as inter lobular vessels. 

Branches of the Hepatic artery and vein will enter the lobules and the blood from the two vessels mix in a chamber called the sinusoid into the intra-lobular vessel, a brand of the hepatic vein.

The blood flows close to the liver cells. The liver cells remove molecules from the blood and pass things into the blood. 

The liver cells can also manufacture bile. 

This is released into the bile canaliculi, joining to form the bile duct, where it is transported to the gall bladder. 

LIVER CELLS

Hepatocytes, have cuboidal shape with lots of microvilli they can have many metabolic functions: Protein synthesis, transformations, storage of carbs. The cytoplasm is very dense and is specialised in terms of the certain organelles that it contains. 

Kupffer Cells 

• Specialised Macrophages*
• Move in the sinusoids
• break down and recycle old blood cells
• break down haemoglobin into bilirubin
• excretes as part of the bile and in faeces
• Bilirubin is the brown pigment in faeces

*a large phagocytic cell found in stationary form in the tissues or as a mobile white blood cell, especially at sites of infection.

FUNCTIONS OF THE LIVER

Formation of Urea

We need 40- 60g of Protein

excess amino acids cannot be stored due to the toxic amine group

Deamination 

• Produces ammonia and keto acids
• Ammonia cannot be accumulated
• Keto acids are entered directly into respiration

Ornithine Cycle

• Ammonia is very toxic and soluble
• Ammonia combines with C02 to form urea, occurs in the Ornithine Cycle
• Urea is less soluble but still toxic,
• Transported to the kidneys in the blood
• The kidney the urea is filtered out and concentrated in the urine
• Urine can be stored in the bladder until it is released

Detoxification*

*Conversion of toxic molecules to less toxic or non-toxic molecules  

liver can detoxify many molecular compounds. 

eg. alcohol can be rendered harmless

liver cells have special enzymes that can render toxic chemicals less toxic. 

catalase: breaks down hydrogen peroxide to oxygen and water.

Detoxification of Alcohols

•  Alcohol or ethanol depressed nerve activity 
• contains chemical potential energy
• can be used for respiration
• is broken down by hepatocytes 
• enzyme: ethanol dehydrogenase
• forms ethanal
• dehydrogenated further by the enzyme ethanal dehydrogenase
• final compound is ethanoate 
• this is combined with CoA ——> Acetyl Coenzyme A
• this enters the krebs cycle
• Hydrogens released combine with another coenzyme called NAD to form reduced NAD

NAD is also used to oxidise and break down fatty acids for respiration. 

If the liver detoxifies too much alcohol it has insufficient NAD to deal with the fatty acids. And are converted to lipids and stored in the hepatocytes, liver becomes enlarged, and is known as ‘fatty liver’ - leads to alcohol related hepatitis.

THE KIDNEY

Structure of the Kidney:

  Supplied with blood from the renal artery

 Blood is drained away from by the renal vein

 Kidney  removes waste products from the blood

 Produces Urine

 Urine passed down the ureter to the bladder to be stored

 Outer Region: Cortex

 Inner Region: Medulla

 Centre: Pelvis leads to the ureter

Results of the Change:

• proximal convoluted tubule: all sugars, salts and some water is absorbed (total of 85%)
• descending limb of the loop of henle, water potential of the fluid is decreased along with the addition of salts and the removal of water
• ascending limb: water potential increased as salts are removed by active transport
• collecting duct: water potential is decreased again by the removal of water, so urine has a higher concentration of solutes than is found in the blood and the tissue fluid. 

Formation of the Urine

Ultrafiltration:

afferent arteriole -> blood flows to -> the glomerulus 

afferent arteriole is wider than the efferent arteriole. 

pressure in the glomerulus is higher than the bowman’s capsule! pressure difference pushes fluid from the blood into the bowman’s capsule. 

barrier separating the blood in the capillary and the lumen of the bowman’s capsule:

1. capillary wall
2. basement membrane
3. epithelial cells

capillary endothelium: has gaps between the cells so blood plasma and substances dissolved within it can pass through

basement membrane: fine mesh of collagen fibres and glycoproteins, act as a filter to prevent the passage of molecules with a relative molecular mass of  > 69,000 

epithelial cells: are called podocytes -> they have finger like projections

                                                             -> make sure that there are gaps between the cells 

What is filtered from the blood? 

• water
• amino acids
• glucose 
• urea
• inorganic ions

Selective Reabsorption 

 cell surface membrane is highly folded to form microvilli this creates a large surface area. 

 special cotransporter proteins transports glucose and amino acids with Na+ ions from the tubule into the cells -> facilitated diffusion 

 on the opposite membrane: it is also highly folded and it also increases the surface area. 

 this membrane has a sodium-potassion pump, Na+ out and K+ in.

 cell cytoplasm has a lot of mitochondria so ATP is produced.           

 

so how does reabsorption occur?

WATER REABSORPTION

Role of the Loop of Henle: to create a low water potential so that more water is reabsorbed from the fluid in the collecting duct. 

water potential becomes much lower further down the medulla. 

How is this achieved?

 Descending limb:

• water moves out by osmosis into the surrounding tissue
• diffusion of sodium and chloride ions into the tubule from the surrounding tissue 

Ascending Limb:

• base of tubule: sodium and chloride ions diffuse out of the tubule into the tissue fluid 
• sodium and chloride ions are actively pumped out higher up the tubule. 
• wall of the ascending limb is impermeable to water, water cannot leave the tubule
• fluid loses salts but not water 

hairpin countercurrent  multiplier.

Collecting Duct: 

after the loop of henle: 

the fluid passes through the short distal convoluted tubule

active transport is used to adjust the concentration of the salts

as fluid flows into the collecting duct there is still a high water potential, the collecting duct carries the fluid down the medulla to the pelvis.

tissue fluid around in the medulla has a lower water potential. water moves by osmosis out of the duct into the surrounding tissue. 

OSMOREGULATION

altering the permeability of the collecting duct:

ADH! IS THE HORMONE THAT THE WALLS OF THE COLLECTING DUCT RESPOND TO!

The cells in the wall have membrane bound receptors for the ADH!

-> when ADH binds to receptors a chain of enzyme controlled reactions occur. 

Result? put vesicles containing water permeable channels (aquaporins) into the cell surface membrane. 

more ADH means more water permeable channels are inserted, and more water reabsorbed, 

less ADH means the cell surface membrane folds inwards, creating new vesicles that make the the walls less permeable because they remove water permeable membranes from the membrane. 

Adjusting the concentration of ADH in the blood

• water potential is monitored by osmoreceptors in the hypothalamus 
• cells respond to effects of osmosis, so where there is a low water potential in the bloom, osmoreceptor cells osmose water out of the cell and they shrink
• this causes them to stimulate neurosecretory cells in the hypothalamus 
• neurosecretory cells produce and release ADH, 
• ADH then flows down the axon to the terminal bulb in the posteriori pituitary gland and stored until needed
• when stimulated the neurosecretory cells will send action potentials down the axon and ADH is released
• ADH enters blood capillaries running through the posterior pituitary gland 
• it then acts on the collecting ducts of the nephrons
• when water potential within the blood rises again, less ADH is released. 
• ADH is then broken down, half life of 20 minutes.  

KIDNEY FAILURES

Common causes:

• diabetes mellitus 
• hypertension
• infection

Treating Kidney Failure

Dialysis: 

it removes excess waste, fluid and salt -> does so by -> passing it through a dialysis machine 

This has a partially permeable membrane that allows for the exchange of materials between the blood and the dialysis machine. 

the other options are kidney transplants, although the old kidneys are sometimes left in place, unless they run a risk of infection or are cancerous. 

it is a major surgery and the person is under anaesthesia, surgeon implants the new organ into the lower abdomen and then connects it to a major blood supply and the bladder. Most will feel much better, but because the body will recognise the new body as foreign, the immune system will either weaken or react. 

patients must continue taking immunosuppressant drugs that lower the immune system or WBC count.