Excretion

Chapter Content

The Liver

The liver needs a good blood supply in order for the

hepatocytes (liver cells) to carry out metabolic processes and

to be involved in homeostasis. There are two sources of blood

supply…

Oxygenated blood from the heart enters via the hepatic

artery. This is oxygen is essential for aerobic respiration

and so ATP can be produced for the metabolic processes.

Deoxygenated blood from the digestive system enters via

the hepatic portal vein. It is rich in digestion products.

Blood leaves via the hepatic vein to rejoin the vena cava.

The Bile duct carries bile from the liver to the gall bladder

where it is stored until it needs to aid the digestion of fats in the

small intestine.

Functions of the Liver

Control of blood glucose, amino acid and lipid levels

Synthesis of red blood cells in the fetus bile, plasma

proteins and cholesterol

Storage of vitamins A, D, B12, iron and glycogen

Detoxification of alcohol and drugs

Breakdown of hormones

Destruction of red blood cells

Urea Formation

A high intake of proteins results in excess amino acids, which

can’t be stored. Instead, these amino acids are converted to

urea by two processes…

Amino acid Ammonia Urea

Deamination Ornithine Cycle

The 2 Processes

Deamination: this is the removal of the amine group from

the amino acid, which produces ammonia.

Ornithine Cycle: the ammonia produced during

deamination is very toxic and highly soluble, so it must be

converted to urea.

Citrulline

Ornithine Arginine

NH3

H2O

Urea H2O

NH3 and CO2H2O

The Equations

Deamination:

Ornithine Cycle:

2NH3 + CO2 CO (NH2)2 + H2O

Detoxification of Alcohol

Ethanol is oxidised to ethanal (catalysed by ethanol

dehydrogenase). Ethanal is dehydrogenated, and the two

hydrogen atoms are accepted by NAD which becomes

reduced. Ethanal is then oxidised to ethanoic acid/acetate

(catalysed by ethanal dehydrogenase). The ethanoic acid is

degydrogenated, and the two hydrogen atoms are accepted by

NAD, which becomes reduced. Ethanoic acid is then converted

to Acetyl Coenzyme A, which enters the Krebs Cycle in

respiration

Excess Alcohol

NAD is required to oxidise and break down fatty acids so

they can be used in respiration. If the liver has too much

alcohol to detoxify, it will have insufficient NAD to deal

with fatty acids. These fatty acids are then converted

back to lipids and stored in the hepatocytes which

enlarges the liver (a ‘fatty liver’). This is known as

cirrhosis.

The Kidney

The Nephron

How does the composition of fluid change?

1. Glucose is actively transported out of the tubule

2. Sodium ions enter the descending and leave via the

ascending limb

3. Urea concentration rises as water is removed from the

tubule

4. Water is reabsorbed from the tubule, which increases the

relative concentration of sodium ions and potassium ions

are then actively transported into the tubule to removed in

urine

Selective Reabsorption

As fluid moves along the nephron, substances are removed from the fluid and reabsorbed into the blood. Most reabsorption (85% of the filtrate) occurs at the proximal convoluted tubule (PCT). All of the amino acids, and some salts and water are reabsorbed here. The cells lining the PCT are specialised to achieve this…

Microvilli: a large surface area for contact with the filtrate

The cell cytoplasm has many mitochondria to produce ATP for the active transport

The membrane has co-transporter proteins to transport glucose or amino acids in association with sodium ions (facilitated diffusion)

Ultrafiltration

Is filtration at molecular level at the basement membrane. Blood

flows into the glomerulus from the afferent arteriole. The

afferent arteriole is wider than the efferent arteriole, and this

difference in diameter ensures a high hydrostatic pressure. This

pressure pushes the fluid into the Bowmans capsule. The

endothelium has narrow gaps (fenestrations) for the blood

plasma to pass through. Also, the basement membrane

consists of a fine mesh of collagen fibres and glycoproteins

which act as a filter to prevent the passage of molecules with a

relative molecular mass of >69 000.

What is filtered out?

Water

Urea

Amino acids

Glucose

Inorganic ions (Na, Cl, K)

What is left in the capillary?

Blood cells

Proteins

The presence of these proteins means the blood has a

low, very negative, water potential. This ensures some of

the fluid is retained in the blood which contains some of

the things normally filtered out. This also helps reabsorb

water.

Reabsorption

1. The sodium-potassium pumps remove sodium ions from the cells lining the PCT, reducing the concentration of Na ions in the cell cytoplasm

2. Sodium ions (associated with glucose and amino acids) are transported along the cell by facilitated diffusion

3. As the glucose and amino acid concentration rises inside the cell, these substances can diffuse into the tissue fluid

4. From the tissue fluid, they diffuse into the blood are are carried away

5. The reabsorption of salts, glucose and amino acids reduces the water potential in the cell and increases the water potential in the tubule fluid. Water will thereby enter the cell and be reabsorbed back into the blood by osmosis

6. Larger molecules will be reabsorbed by endocytosis

The Loop of Henle

The role of the loop of Henle is to create low, very negative,

water potential in the tissue of the medulla. This ensures even

more water can be reabsorbed from the fluid in the collecting

duct.

It consists of a descending limb (into the medulla) and an

ascending limb (out into the cortex). It allows salts to be

transferred from the ascending to the descending limb.

How is this achieved?

The loop of Henle has a countercurrent mechanism (a tube that

turns back on itself). This increases the efficiency of the

exchange of salts.

Sodium and chloride ions move by active transport and diffuse

out of the filtrate as it moves along the ascending limb

This decreases the water potential of the tissue fluid in the tips of

the pyramids of the medulla

The ions pass back into the descending limb and are recycled,

so they can be concentrated in the medulla

The low water potential outside the collecting duct allows water

to leave the collecting duct by osmosis (if ADH is present)

The Collecting Duct

From the DCT, fluid flows into the collecting duct (the fluid

has a high water potential). The collecting duct carries the

fluid through the medulla to the pelvis. As the tubule fluid

passes down the collecting duct, water moves by osmosis

into the surrounding tissue fluid, it then enters the

capillaries by osmosis and is carried away.

Osmoregulation

Osmoregulation is done by the kidney and is the control and

regulation of the water potential of the blood and body fluids.

This is monitored by osmoreceptors in the hypothalamus of

the brain.

The walls of the collecting duct can be altered. They respond to

the levels of the antidiuretic hormone (ADH) which is released

from the posterior pituitary gland. Cells in the wall of the

collecting duct have receptors for ADH to bind to. Vesicles

(containing aquaporins) are inserted into the cell surface

membrane. This makes the walls permeable to water, so more

water is reabsorbed by osmosis resulting in less urine with a

low water potential being produced.

Kidney Failure

Common causes of kidney failure include diabetes,

hypertension, infection. Once the kidneys have failed, the body

can’t remove excess water or waste products, nor can thy

regulate water or salt level.

One treatment for kidney failure is dialysis. This is the use of a

partially permeable membrane to filter the blood. The waste,

excess fluid and salt pass over a dialysis membrane which

allows the exchange of substances between the blood and

dialysis. There are two types of dialysis…

Haemodialysis Peritoneal Dialysis (PM)

Blood from a vein is passed into a

machine that contains an artificial

dialysis machine. Heparin is added to

avoid clotting and any bubbles are

removed before the blood returns to

the body. This is usually carried out a

clinic 3x a week although some

patients carry it out at home

The filter is the body’s own

abdominal membrane (the

peritoneum). A surgeon implants a

permanent tube into the abdomen

and dialysis fluid is poured through

the tube. The fluid fills the space

between the organs abdomen wall.

After several hours, the fluid is

drained. This is usually carried out

several times a day, but the patient

can move around (ambulatory)

Kidney Transplant

The second form of treatment is to have a transplant. This

has many advantages and disadvantages…

Advantages Disadvantages

• No time consuming dialysis

• Diet is less limited

• Feel better physically

• Better quality of life

• Don’t see yourself as chronically ill

• Need immuno-suppresants for life

and this can cause a susceptibility

to anti infection

• Majory surgery under general

anaesthetic

• Surgery risks (infection, bleeding

etc.)

• Frequent checks for signs of

rejection

Pregnancy Testing

The embryo secretes human chorionic

gonadotrophin (hCG). The pregnancy test stick is

marked with monoclonal antibodies which will only

bind to hCG. Any hCG in the urine will attach to an

antibody (they are tagged with blue beads). The hCG-

antibody complex moves up the strip until is sticks to a

band of immobilised enzymes. A blue line is formed

from the antibody-hCG complex being held in one

place (there is always a control line)

Anabolic Steroid Testing

Anabolic steroids increase protein synthesis so there is a

build up of cell tissue (especially in muscle cells). They

have a half-life of around 16hrs but can remain in the

blood for many days. Analysing one involves gas

chromatography. The sample is vaporised in the

presence of a gaseous solvent and passed down a long

tube lined with an absorbing agent. Each susbstance

dissolves differently in the gas and stays there for a

unique time (retention). Eventually, it comes out of the

gas and is absorbed onto the lining, creating a

chromatogram.