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11.3 – The Kidney

11.3 – The Kidney

11.3.1 – Define excretion

The removal of the waste products of metabolic pathways from the body

All living things excrete their waste, along with any excess metabolites.

11.3.2 – Draw and label a diagram of the kidney

The two kidneys act as filters for the blood, removing harmful toxins. Blood flow through the kidneys is extremely high because of the large number of capillaries. The medulla has a high osmolarity because it contains lots of salt. The nephrons are surrounded by veins, but they do not touch.

11.3.3 – Annotate a diagram of a glomerulus and associated nephron to show the function of each part

The glomerulus is a group of branching capillaries.

11.3.4 – Explain the process of ultrafiltration, including blood pressure, fenestrated blood capillaries and basement membrane

Ultrafiltration takes place in the Malpighian body. Here, there are many porous capillaries which are not selective and therefore allow most substances through.

Blood Pressure

Unfiltered blood is transported to the nephron in the renal artery. The blood pressure increases in the glomerulus because the branches become narrower. The high blood pressure forces water, urea, salts and other solutes form the blood in the glomerulus to the lumen of the Bowman’s capsule.

The porous capillaries and podocytes, or special cells, filter blood by being permeable to water and small solutes but not blood and other plasma proteins. The process in non-selective, and the resulting filtrate contains salts, glucose, vitamins and nitrogenous waste. On the end of the nephron, there is an invagination that accommodates the glomerulus.

On the end of the nephron, there is an invagination that accommodates the glomerulus.

Fenestrated Blood Capillaries

The fenestrated blood capillaries form a path of low resistance out of the glomerulus as they have gaps between the cells that form the vessels. These gaps are only visible through an electron microscope.

Basement Membranes

Not all of the contents of the blood are forced out during the process, which is prevented by basement membranes. These act as filtration barriers, preventing cells and large proteins or polypeptides from passing through. These are instead retained in the circulating blood.

11.3.5 – Define osmoregulation

The control of the water balance of the blood, tissue or cytoplasm of a living organism.

The collecting duct is important for the process of osmoregulation

11.3.6 – Explain the reabsorption of glucose, water and salts in the proximal convoluted tubule, including the roles of microvilli, osmosis and active transport

The proximal convoluted tubule has microvilli, which give it greater surface area to absorb glucose, amino acids and mineral ions from the filtrate back into the capillary network. It is the longest section of the nephron. Urea is also transported by diffusion.

The mineral ions are transported by active transport, facilitated diffusion and exchange of ions.

About 80% of the water is reabsorbed by osmosis.

Active transport is used for the transport of glucose and amino acids, so there are a large number of mitochondria to provide the ATP required.

11.3.7 – Explain the roles of the loop of Henle, medulla, collecting duct and ADH (vasopressin) in maintaining the water balance of the blood

Loop of Henlé

In the descending limbs of the Loop of Henle are permeable to water, but not to sodium ions. The sodium ions are instead diffused into the loop. The water is drawn out of the filtrate by osmosis. This decreases the concentration of the fluids in the medulla, whilst the filtrate becomes more concentrated.

Water loss from the body is minimised by expelling more concentrated urine. The glomerular filtrate flows through the medulla, the loop of Henle, and the out through the cortex. The loop of Henle increases the solute concentration of the medulla, as it creates and area of high solute concentration in the cells and tissues of the medulla. The volume of filtrate is eventually reduced.

On the other hand, the ascending limbs are not permeable to water, but to sodium ions, which are pumped from the filtrate into the medulla via active transport. As a result, the concentration of sodium ions in the medulla is increased. The concentration of the filtrate is reduced.

ADH

If this hormone is released, the pores of the distal convoluted tubule and collecting duct open, making them permeable to water.

The blood moves into the distal convoluted tubule where ions are exchanged, and then on to the collecting duct. The permeability of these is increased by the hormone vasopressin (ADH), which opens pores in the cell membranes of these tubules.

11.3.8 – Explain the differences in the concentration of proteins, glucose and urea between blood plasma, glomerular filtrate and urine

All of these products move in the following sequence:

During this process, urea is eliminated from the body through the urine. However, not all of the urea that leaves the collecting duct will be taken up by the loop of Henle, so there is only 50% total reabsorption. Glucose, on the other hand, is reabsorbed from the filtrate back into the blood. There is 100% reabsorption of it. Whilst proteins are found in the blood plasma, they do not move into the glomerular filtrate. If proteins are found in urine, then this means that blood pressure is too high and there is damage nephritis of the Bowman’s capsule.

11.3.9 – Explain the presence of glucose in the urine of untreated diabetic patients

People with diabetes experience erratic blood glucose levels, and will often rise above normal levels, especially after meals. As a result, the kidney is not able to reabsorb all the glucose from the blood, so some still remains in the urine. This is because the pumps of the proximal convoluted tubule are not able to reabsorb such high concentrations of glucose (higher than 90mg per 100mL).

As a result, the concentration of glucose in the person’s urine can indicate whether they are diabetic. If they are not treated, glucose will continue to be present.