Biology (5.2): Excretion as an example of homeostatic control

excretion

excretion

removal of metabolic waste from the body

metabolic waste

a substance that is produced in excess by the metabolic processes in the cell that may become toxic

What are the main excretory products? (3)

1. carbon dioxide

2. nitrogenous waste (e.g. urea)

3. bile

What are the excretory organs? (4)

1. lungs

2. liver

3. kidneys

4. skin

Why are the lungs excretory organs?

in the lungs carbon dioxide diffuses into the alveoli to be excreted when you breathe out

Why are the kidneys excretory organs?

Urea is removed from the blood to become part of urine in the kidneys

Why is the liver an excretory organ? (2)

1. substances produced in metabolic roles are passed into the bile for excretion

2. converts excess amino acids to urea by deamination

How is the skin involved in excretion?

sweat contains substances like salts, urea, water, uric acid and ammonia, some of which are excretory products

Why is excretion important?

some metabolic products are toxic, so interfere with cell processes by altering pH or acting as inhibitors

Why is excess hydrogen ions in the blood plasma dangerous?

excess hydrogen ions can reduce the pH of the plasma

What are amino acids converted into?

ammonia then urea

What is bad about ammonia?

very soluble and highly toxic

Why is ammonia converted into urea?

urea is less soluble and less toxic

deamination equation

amino acid + oxygen →keto acid + ammonia

Formation of urea equation

ammonia + carbon dioxide →urea + water

liver cells

hepatocytes

What are the sources of blood for the liver? (2)

1. hepatic artery

2. hepatic portal vein

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

liver cells are very active as they carry out metabolic processes which require ATP

Hepatic artery

oxygenated blood from the aorta travels to the liver via the hepatic artery

Hepatic portal vein

deoxygenated blood from the digestive system enters the liver via the hepatic portal vein

Hepatic vein

deoxygenated blood leaves the liver via the hepatic vein

Parts of the liver (6)

1. liver

2. gall bladder

3. bile duct

4. hepatic portal vein

5. hepatic artery

6. hepatic vein

bile duct

carries bile from the liver to the gall bladder where it is stored

liver lobules

cylindrical structures made of hepatocytes that are arranged in rows radiating out from the centre

What do each lobule contain?

a central vein in the middle that connects to the hepatic vein and many branches of the hepatic artery, hepatic portal vein and bile duct are also connected to each lobule

sinusoids

the hepatic artery and the hepatic portal vein are connected to the central vein via capillaries called sinusoids

Kupffer cells

remove bacteria and break down old red blood cells

Hepatocyte functions (4)

1. protein synthesis

2. transformation and storage of carbohydrates

3. synthesis of cholesterol and bile salts

4. detoxification

Ornithine cycle

a series of biochemical reactions that convert ammonia to urea

metabolic functions of the liver (7)

1. control blood glucose levels

2. synthesis of bile, plasma proteins & cholesterol

3. synthesis of red blood cells in the fetus

4. storage of vitamins A, D and B12, iron & glycogen

5. detoxification of alcohol & drugs

6. breakdown of hormones

7. destruction of red blood cells

How is glycogen stored?

glycogen forms granules in the cytoplasm of hepatocytes

Enzymes in the liver cells that make toxic molecules less toxic (2)

1. catalase

2. cytochrome P450

Catalase function

converts hydrogen peroxide to oxygen and water

cytochrome P450 function

breakdown drugs

enzyme involved in the detoxification of alcohol

ethanol dehydrogenase

stages of the detoxification of alcohol

1. ethanol →ethanal (using ethanol dehydrogenase) using 2H to convert NAD to reduced NAD

2. ethanal → ethanoic acid (using ethanol dehydrogenase) using 2H to convert NAD to reduced NAD

3. ethanoic acid → acetyl coenzyme A

What is acetyl coenzyme A used for?

respiration

What happens when the liver has to detoxify too much alcohol?

it uses up its stores of NAD and can’t deal with the fatty acids, so fatty acids are converted to lipids and stored as fats

What processes are involved in the formation of urea? (2)

1. deamination

2. ornithine cycle

deamination equation

amino acid + oxygen → keto acid + ammonia

ornithine cycle summary

ammonia + carbon dioxide → urea + water

outer region of the kidney

cortex

inner region of the kidney

medulla

centre of the kidney

pelvis (leading to ureter)

where are the kidneys positioned?

each side of the spine, just below the lowest rib

renal artery

supplies the kidney with blood

renal vein

drains blood from the kidney

role of the kidney

excretion - removing waste products from the blood and producing urine

parts of the kidney (8)

1. nephron tubules

2. capsule

3. cortex

4. medulla

5. branch of renal vein

6. branch of renal artery

7. pelvis

8. ureter

nephron

each nephron starts in the cortex at the Bowman’s capsule and passes through the cortex and forms a loop down the medulla back to the cortex before connecting to a collecting duct that passes back down into the medula

ultrafiltration

filtration of the blood at a molecular level under pressure

what surrounds each glomerulus?

the Bowman’s capsule

What does the renal artery split into?

many afferent arterioles

What is carried out in the Bowman’s capsule?

ultrafiltration

What is forced out of the blood in ultrafiltration?

everything except larger proteins, red blood cells and most white blood cells

What causes substances to be forced out of the blood?

high pressure caused by the difference in diameter between the afferent and efferent arterioles

Which arteriole is wider?

afferent

What are the three layers of the barrier between the blood and lumen of the Bowman’s capsule?

1. endothelium of the capillary

2. basement membrane

3. epithelial cells of the Bowman’s capsule

endothelium of the capillary

narrow gaps between cells of the endothelium and capillary wall that contain pores (fenestrations) which allow blood plasma and substances dissolved in it to pass out of the capillary

basement membrane

consists of a fine mesh of collagen fibres and glycoproteins which acts as a filter to prevent the passage of molecules that are too big (so most proteins and blood cells are held in the capillaries of the glomerulus)

epithelial cells of the Bowman’s capsule name

podocytes

shape of podocytes (2)

1. finger-like projections (major processes)

2. minor/foot processes

What do the projections in podocytes ensure?

there are gaps between the cells for fluid from the blood in the glomerulus to pass through

what are the three parts of the tubule after the Bowman’s capsule? (3)

1. proximal convoluted tubule

2. loop of Henle

3. distal convoluted tubule

What is filtered out of the blood into the lumen of the Bowman’s capsule? (5)

1. water

2. amino acids

3. glucose

4. urea

5. inorganic mineral ions (Na, Cl, K)

What is left in the capillary?

blood cells and proteins

What is reabsorbed in the proximal convoluted tubule? (3)

1. all the sugars

2. most mineral ions

3. some water

What is an adaptation of the proximal convoluted tubule?

the epithelium of the wall of the PCT has microvilli to provide a large SA for reabsorption of useful substances from the filtrate into the blood

Which limb is permeable to water in the loop of Henle?

descending limb

which limb is impermeable to water?

ascending limb

What moves out of the descending limb in the loop of Henle?

water by osmosis

What moves out near the bottom of the ascending limb?

Na⁺ and Cl^- ions diffuse out

What leaves near the top of the ascending limb?

Na⁺ and Cl^- ions by active transport

What is urine made up of? (4)

1. water

2. dissolved salts

3. urea

4. other substances (e.g. hormones and excess vitamins)

What doesn’t urine usually contain? (5)

1. proteins

2. blood cells

3. glucose

4. amino acids

5. vitamins

Where is water reabsorbed?

distal convoluted tubule and the collecting duct

Why would an animal have a longer loop of Henle?

They can absorb more water, so they save as much water as possible

How is the proximal convoluted tubule specialised to achieve reabsorption? (4)

1. microvilli

2. cell surface membrane contains cotransported proteins (that transport glucose or amino acids & Na⁺ ions from the tubule into the cell)

3. opposite membrane of the cells also folded & contains sodium/potassium pumps

4. many mitochondria

Antidiuretic hormone (ADH)

a hormone that controls the permeability of the collecting duct walls

What kind of system arrangement is the loop of Henle?

hairpin countercurrent multiplier system

osmoreceptor

a sensory receptor that detects changes in water potential

What does ADH do?

Cause vesicles containing aquaporins to fuse with the cell surface membrane and make the walls more permeable to water

What happens when there is an increase in the water potential of the blood? (5)

1. increase is detected by osmoreceptors in hypothalamus

2. less ADH released from posterior pituitary

3. collecting duct walls less permeable

4. less water reabsorbed into blood (& more urine produced)

5. decrease in water potential of blood

What happens when there is a decrease in the water potential of the blood? (5)

1. decrease detected by osmoreceptors in hypothalamus

2. more ADH released from posterior pituitary

3. collecting duct walls more permeable

4. more water is reabsorbed into blood (and less urine produced)

5. increase in water potential of blood

What monitors the water potential of the blood?

the osmoreceptors in the hypothalamus

When is ADH released?

when there is a low water potential in the blood

Glomerular filtration rate (GFR)

how much fluid passes into the nephrons each minute

Kidney failure

when the kidneys are unable to regulate the levels of water and electrolytes in the body or remove waste products from the blood

Causes of kidney failure (4)

1. diabetes mellitus

2. heart disease

3. hypertension

4. infection

Treatments of kidney failure (2)

1. renal dialysis

2. kidney transplant

Types of renal dialysis (2)

1. haemodialysis

2. peritoneal dialysis

renal dialysis

a mechanism used to artificially regulate the concentrations of solutes in the blood

how does dialysis work?

Waste products are removed from the blood by passing it over a partially permeable dialysis membrane that allows the exchange of substances between the blood and dialysis fluid

What does the dialysis fluid contain?

the correct concentrations of mineral ions, urea, water and other substances found in the blood plasma

What kind of system is haemodialysis?

countercurrent (the artificial capillaries surrounded by dialysis fluid a flow in the opposite direction to the blood)

Where is peritoneal dialysis carried out?

can be carried out at home or work

Advantages to kidney transplants (4)

1. freedom from time-consuming renal dialysis

2. feeling physically fitter

3. improved quality of life (able to travel)

4. improved self-image

Disadvantages to kidney transplants (4)

1. need to take immunosuppressant drugs

2. need for major surgery under general anaesthetic

3. need for regular checks for signs of rejection

4. side effects of immunosuppressant drugs

What are the side effects of immunosuppressant drugs? (3)

1. fluid retention

2. high blood pressure

3. susceptibility to infections

What can urine be tested for? (5)

1. glucose

2. alcohol

3. recreational drugs

4. hCG (pregnancy testing)

5. anabolic steroids