Excretion

Excretion

(a) define the term excretion

Excretion – the removal of metabolic waste from the body, of by-products or unwanted substances from normal cell processes.

Functions of the Liver:

control of: blood glucose levels, amino acid levels, lipid levels
synthesis of: red blood cells in the fetus, bile, plasma proteins, cholesterol
storage of: vitamins A, D and B₁₂, iron, glycogen
detoxification of: alcohol, drugs
breakdown of hormones
destruction of red blood cells

(b) explain the importance of removing metabolic wastes, including carbon dioxide and nitrogenous waste, from the body

Carbon Dioxide:

Carbon dioxide is produced by every living cell in the body as a result of respiration. It’s passed from the cells of respiring tissues into the bloodstream where it is transported, mostly in the form of hydrogencarbonate ions, to the lungs where the CO₂ diffuses into the alveoli to be excreted as we breathe out.

Affecting Haemoglobin:
CO₂ is mostly transported in the form of hydrogencarbonate ions which forms hydrogen ions.The hydrogen ions combine with haemoglobin and they compete with oxygen for space, reducing oxygentransport.
CO₂ can also directly combine with haemoglobin to form carbaminohaemoglobin, which has a loweraffinityfor oxygen than normal haemoglobin.
Respiratory Acidosis:

Excess CO₂ dissolves in the blood plasma and combines with water to form carbonic acid.

CO₂+ H₂O à H₂CO₃

The carbonic acid dissociates to release hydrogen ions.

H₂CO₃àHCO₃^– + H⁺

The H⁺ ions lower the pH and make the blood more acidic.
If the change in pH is small then the extra hydrogen ions are detected by the respiratory centre in the medullaoblongata of the brain. Breathing rate increases to help remove excess CO_2.
If the blood pH drops below 7.35 it results in slowed or difficult breathing, headache, drowsiness, restlessness, tremor, confusion, rapid heart rate and changes in blood pressure – respiratory acidosis. It can be caused by diseases or conditions that affect the lungs themselves such as emphysema, chronic bronchitis, asthma or severe pneumonia, as well as blockage of airways due to swelling, a foreign object or vomit.

Nitrogenous Compounds (Urea):

Urea is produced in the liver from excess amino acids bring broken down, called deamination (the removal of the amine group from an amino acid to produce ammonia). It’s passed into the bloodstream to be transported to the kidneys where the urea is removed from the blood to become part of the urine.

The body can’t store proteins or amino acids, but they contain almost as much energy as carbohydrates so it would be wasteful to excrete excess amino acids.

Excess amino acids are transported to the liver and the potentially toxic amino acid is removed (deamination). The amino group initially forms the very soluble and highly toxic compound, ammonia.

Deamination: amino acid + oxygen → keto acid + ammonia

The ammonia is converted to a less soluble and less toxic compound called urea, which can be transported to the kidneys for excretion.

Formation of Urea: ammonia + carbon dioxide → urea + water

2NH₃ + CO₂ → CO(NH₂)₂ + H₂O

The remaining keto acid can be used directly in respiration to release its energy or it may be converted to a carbohydrate or fat for storage.

(c) describe, with the aid of diagrams and photographs, the histology and gross structure of the liver

Hepatic Artery– supplies liver with oxygenated blood.

Hepatic Vein– deoxygenated blood leaves the liver which rejoins the vena cava and the blood returns to normal circulation.

Hepatic Portal Vein–has capillaries at both ends deoxygenated blood travels to the liver from the digestive system. The blood is rich in the products of digestion.

Bile Duct– transports bile from the liver to the gall bladder where it is stored until required to aid the digestion of fats in the small intestine. (Bile emulsifies fat, producing a larger surface area, so they are broken down more easily.)

Arrangement of Cells in the Liver:

The liver is divided into lobes, which are further divided into cylindrical lobules.

As the hepatic artery and hepatic portal vein enter the liver they split into smaller and smaller vessels, which run between, and parallel to, the lobules and are known as inter-lobular vessels. Blood from the two vessels is mixed and passed along a sinusoid which is lined by liver cells. The sinusoids empty into the intra-lobular vessel, a branch of the hepatic vein. The branches of the hepatic vein from different lobules join together to form the hepatic vein, which drained from the liver.

As blood flows along the sinusoid it is in very close contact with the liver cells. They are able to remove molecules from the blood and pass molecules into the blood.

The liver manufactures bile and is released into the bile canaliculi (small canals) which join together to form the bile duct and transports the bile to the gall bladder.

The Different Cells in the Liver:

Liver cells (hepatocytes) have a simple cuboidal shape with many microvilli on their surface. Their metabolic functions include protein synthesis, transformation and storage of carbohydrates, synthesis of cholesterol and bile salts, detoxification, etc. = cytoplasm is very dense and is specialised in the amounts of certain organelles that it contains. For example, they contain lots of mitochondria as they need ATP.
Kupffer cells are specialisedmacrophages – move about in the sinusoids and are involved in the breakdown and recycling of old red blood cells. The breakdown of haemoglobin produces bilirubin, which is excreted as part of the bile and in faeces giving the brown pigment.

(d) describe the formation of urea in the liver, including an outline of the ornithine cycle

Most people in the developing world eat more than the 40-60g of protein we need every day. Excess amino acids cannot be stored, as the amine groups make them toxic. However, the amino acid molecules contain a lot of energy so it would be wasteful to excrete the whole molecule. In order for the amino component to be excreted, it must undergo the processes of deamination and the orithine cycle.

(e) describe the roles of the liver in detoxification

The liver is able to detoxify many compounds, e.g. hydrogen peroxide which is produced in the body or alcohol which is consumed as part of our diet. Toxins can be rendered harmless by oxidation, reduction, methylation or combination with another molecule.

(f) describe, with the aid of diagrams and photographs, the histology and gross structure of the kidney

(g) describe, with the aid of diagrams and photographs, the detailed structure of a nephron and its associated blood vessels

(h) describe and explain the production of urine, with reference to the processes of ultrafiltration and selective reabsorption

Ultrafiltration:

The afferent arteriole is wider than the efferent arteriole which is wider than the capillaries. Therefore there is a higher blood pressure in the capillaries to push fluid from the blood into the Bowman’s capsule.

The barrier between the blood in the capillary and the lumen of the Bowman’s capsule consists of 3 layers:

The endothelium of the capillaries – has narrow gaps(pores) between its cells that blood plasma and the substances dissolved in it can pass.
The basement membrane – consists of a fine mesh of collagen fibres and glycoproteins. They act as a filter to prevent the passage of molecules with a relative molecular mass of greater than 69000 (proteins and erythrocytes).
The epithelial cells of Bowman’s capsule, called podocytes (finger-like projections). These ensure that there are gaps between cells.

Selective Reabsorption:

The cells lining the proximal convulated tubule are specialised to achieve reabsorption:

Microvilli– increases the surface area for reabsorption.
Cotransporter proteins– transport glucose or amino acids, in association with sodium ions from the tubule into the cell – facilitated diffusion.
Sodium-potassium pumps– pump sodium ions out the cell and potassium ions into the cell.
Many mitochondria– indicates an active or energy-requiring process, which needs lots of ATP.

At the capillary end of the proximal convulated tubule cell, sodium ions are actively transported out of the cells into the blood by sodium-potassium pumps, reducing the concentration of sodium ions in the cell.
At the filtrate end of the proximal convulated tubule, sodium ions move into the cell along with glucose or amino acid molecules by facilitated diffusion using co-transporter proteins, and the levels of sodium ions increase in the cell.
The glucose and amino acids in the cells are able to just diffuse into the tissue fluid and into the blood, from a high concentration inside the cell to a low concentration in the capillaries.
The movement of sodium ions, glucose and amino acids reduces the water potential in the cells so water will enter the cell by osmosis. The blood in the capillaries has an even lower water potential so water moves into the capillaries by osmosis.
Larger molecules (e.g. small proteins that may have entered the tubule) will be reabsorbed by endocytosis.

(i) explain, using water potential terminology, the control of water content of the blood, with reference to the roles of the kidney, osmoreceptors in the hypothalamus and the posterior pituitary gland

Water Reabsorption:

The role of the loop of Henle is to create a low (very negative) water potential in the tissue of the medulla. This ensures that even more water can be reabsorbed from the fluid in the collecting duct.

As the fluid in the descending limb gets deeper into the medulla, the water potential becomes lower (more negative) due to the increasing concentration of sodium and chloride ions down the descending limb.

The wall of the descending limb is permeable to water so water is lost by osmosis to the surrounding tissue.
Sodium and chloride ions can diffuse into the tubule from the surrounding tissue.

As the fluid in the ascending limb moves back up towards the cortex, the water potential becomes higher (less negative) due to the decreasing concentration of sodium and chloride ions up the ascending limb.

At the base of the tubule, sodium and chloride ions diffuse out of the tubule into the tissue fluid.
Higher up the tubule, sodium and chloride ions are actively transported out into the tissue fluid.
The wall of the ascending limb is impermeable to water so the fluid in the tubule loses salts but not water as it moves up the ascending limb.

The arrangement of the loop of Henle is known as a hairpin countercurrent multiplier system. This is where one part of the tubule passes close to another part of the tubule with the fluid flowing in opposite directions, allowing exchange between the contents. The arrangement increases the efficiency of salt transfer from the ascending limb to the descending limb, and causes a build-up of salt concentration in the surrounding tissue fluid.

Animals that live in drier habitats, have longer loop of Henles. It provides them with a longer countercurrent mechanism that can increase the salt concentration in the medulla more than in other mammals. Therefore more water can be reabsorbed, which is important as there’s not much water is available for them to drink.

Osmoregulation:

When there’s a decrease in water potential, the osmoreceptors in the hypothalamus of the brain, loses water by osmosis. They shrink and stimulate neurosecretory cells in the hypothalamus.
The neurosecretory cells are specialised neurones that produce and release ADH. The ADH is manufactured in the body of the cells. ADH flows down the axon to the terminal bulb in the posterior pituitary gland – stored there until it is needed.
The ADH enters the blood capillaries and transported around the body to the collecting duct. They bind to the complementary receptors on the walls of the collecting duct causing a chain of enzyme controlled reactions forming vesicles containing water-permeable channels (aquaporins), making the walls of the collecting duct more permeable.
More water is reabsorbed by osmosis into the blood and less urine is produced – negative feedback.

(j) outline the problems that arise from kidney failure and discuss the use of renal dialysis and transplants for the treatment of kidney failure

Common causes of kidney failure are:

hypertension – high blood pressure can damage the small blood vessels in your kidneys and stop them working properly.
diabetes mellitus – blocks the small blood vessels of your kidney and makes them leaky so you kidneys work less efficiently.
Infection.

Once the kidneys fail completely the body is unable to remove excess water and certain waste products from the blood. This includes urea and excess salts. It is also unable to regulate the levels of water and salts in the body. This will rapidly lead to death.

Dialysis is the most common treatment for kidney failure. It removes wastes, excess fluid and salt from blood by passing the blood over a dialysis membrane, which is partially permeable, allowing the exchange of substances between the blood and the dialysis fluid.

In a kidney transplant the old kidneys are left in place unless they are likely to cause infection or are cancerous. The donor kidney can be from a living relative who is willing to donate one of their healthy kidneys or from someone who has died. A kidney transplant is a major surgery. While the patient is under anaesthesia, the surgeon implants the new organ into the lower abdomen and attaches it to the blood supply and the bladder.

(k) describe how urine samples can be used to test for pregnancy and detect misuse of anabolic steroids

Substances or molecules with a relative molecular mass of less than 69 000 can enter the nephron. If these substances are not reabsorbed further down the nephron they can be detected in the urine