11.3 The Kidney

Topic 11 Human Health & Physiology

The Kidney 11.3.1 Define excretion. Excretion is the removal

from the body of the waste products of metabolic pathways.

11.3.2 Draw and label a diagram of the kidney. (Include the cortex, medulla, pelvis, ureter and renal blood vessels).

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

The Kidney 11.3.4 Explain the process of ultrafiltration,

including blood pressure, fenestrated blood capillaries and basement membrane.

11.3.5 Define osmoregulation. Osmoregulation is the control of the water balance of the blood, tissue or cytoplasm of a living organism.

Aim 7: Data logging using colorimeters to measure the response of blood cells to changing salt concentrations is possible.

The Kidney 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.

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.

(Details of the control of ADH secretion are only required in option H).

The Kidney 11.3.8 Explain the differences in the concentration

of proteins, glucose and urea between blood plasma, glomerular filtrate and urine.

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

Kidney Structure The main parts of a kidney you need to know are:

Cortex Medulla Pelvis Ureter Renal Artery Renal Vein

Kidney Structure

Ref: IB Biology, Allott

Processes of the Kidney There are two main process that occur in the Kidney:

Ultrafiltration Filtering out of the waste products from the blood

Reabsorption Taking back into the blood of substances still needed by the body.

The Nephron The Nephron is the functional unit of the kidney. The parts of a nephron include:

The afferent and efferent blood vessels. The glomerulus. The Bowman's capsule. Proximal and distal convoluted tubules. Loop of Henle – descending and ascending limbs. The collecting duct.

The Nephron

Ref: Biology, Rowland

The Glomerulus The function of the glomerulus is to produce a filtrate from

the blood. This process is called Ultrafiltration.

It is passive. It is unselective.

In the glomerulus, about 20% of the blood plasma escaped through the walls of the capillaries into the bowman’s capsule.

This is because of two reasons: The blood pressure is very high in the glomerulus.

Efferent blood vessel has a much smaller diameter than the afferent. The capillaries in the glomerulus are fenestrated.

They have many pores in them.

Structure of Part of a Glomerulus

Ref: IB Biology, Allott Ref: IB Biology OSC

Ultrafiltration Ultrafiltration of solutes occurs through the fenestrations

(gaps) in the blood capillaries. Large molecules in the blood are able to pass through the

gaps in the capillary cells, but are unable to pass through the basement membrane.

The basement membrane is a protein membrane outside the cells. It contains no pores and acts as a dialysis membrane to stop the blood cells and large proteins getting out of the blood vessel.

The capillaries are surrounded by podocyte cells, the cells of the inner wall of the bowman’s caspule.

These cells wrap around the capillaries and have gaps between them to allow the filtrate to pass through quickly.

Osmoregulation Osmoregulation is the control of water (and hence solute)

levels in the blood, tissues or cytoplasm of a living organism.

Osmoregulation is performed by the kidney. The kidney controls the amount of water passed out of the

body in the urine.

Reabsorption in the Convoluted Tubule As well as waste products, the glomerular filtrate contains

substances the body still needs. Most of this selective reabsorption occurs in the proximal

convoluted tubule. The inside of the PCT has microvilli, to increase the

surface area for re-absorption. Glucose and salts are reabsorbed by active transport. Water is reabsorbed by osmosis. All glucose is reabsorbed (under normal conditions) About 80% of water is reabsorbed leaving only about

20% to pass onto the loop of henle.

Reabsorption in the Convoluted Tubule

Ref: IB Biology, Allott

Reabsorption in the Loop of Henlé Glomerular filtrate flows deep into the medulla in the

descending limbs of the loops of Henlé. The cells of the ascending limb actively transport salt out. In theory water should follow by osmosis, but the walls in

the ascending limb are impermeable to water. The walls of the descending limb are permeable to water

and some leaves the descending limb due to the osmotic gradient.

Thus the loop of Henlé acts to create an area of high solute concentration in the medulla with an increasing concentration deeper into the medulla

Reabsorption in the Loop of Henlé

Ref: IB Biology OSC

ADH and the Collecting Duct If the water content of the blood is too low, the pituitary

gland (in the brain) secretes anti diuretic hormone (ADH). This hormone makes the cells of the collecting duct wall

produce membrane channels called aquaporins, which make the collecting duct more permeable to water.

The high solute concentration in the surrounding medulla, causes most of the water in the filtrate to be re-absorbed by osmosis into the medulla and then the blood vessels.

A small amount of concentrated urine is produced. If the water content of the blood is high. ADH is not

secreted and water passes through into the pelvis. A large amount of dilute urine is produced.

ADH and the Collecting Duct

Ref: IB Biology OSC

A Comparison of Blood The blood in the renal artery is different from blood in the

renal vein. Renal artery:

higher in urea, salt, water.

Renal vein: lower in urea, salt, water.

Levels of glucose and blood proteins should be the same, unless blood glucose levels are high and excess has been removed in the kidney.

Comparing Glomerular Filtrate and Urine Glomerular Filtrate:

High in glucose Low in urea

Urine: No glucose (it has been reabsorbed) High in urea (it has been concentrated)

Diabetes The urine of non-diabetic patients should contain no

glucose as it is selectively reabsorbed from the filtrate in the proximal convoluted tubule

Diabetics have higher levels of blood glucose due to either a lack of insulin secretion (type I) or insensitivity to insulin secretions (type II)

Because of this, not all of the glucose in diabetics is reabsorbed into the blood (protein pumps in tubule wall become saturated)

This results in the presence of glucose in the urine of untreated diabetics, which can be detected using test strips

IBO guide: 11.3.1 Define excretion. Excretion is the removal

from the body of the waste products of metabolic pathways.

11.3.2 Draw and label a diagram of the kidney. (Include the cortex, medulla, pelvis, ureter and renal blood vessels).

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

IBO guide: 11.3.4 Explain the process of ultrafiltration,

including blood pressure, fenestrated blood capillaries and basement membrane.

11.3.5 Define osmoregulation. Osmoregulation is the control of the water balance of the blood, tissue or cytoplasm of a living organism.

Aim 7: Data logging using colorimeters to measure the response of blood cells to changing salt concentrations is possible.

IBO guide: 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.

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.

(Details of the control of ADH secretion are only required in option H).

The Kidney 11.3.8 Explain the differences in the concentration

of proteins, glucose and urea between blood plasma, glomerular filtrate and urine.

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