F214: Communication, Homeostasis and Energy4.2.1 The Kidney

• describe, with the aid of diagrams and photographs, the histology and gross structure of the kidney;

• describe, with the aid of diagrams and photographs, the detailed structure of a nephron and its associated blood vessels;

The Structure of the Kidney

Task• You are going to complete a kidney dissection

and explore the structure of the kidney. You will need a dissection kit and a copy of Cambridge Biology 2 p47.

• Read the instructions on the sheet provided and complete the activities.

Cortex: Lighter brown colour contains the Malpighian bodies which are the capsules that contains Bowman's capsule and a glomerulus at the expanded end of a nephron. There are also the proximal and distal convoluted tubules and the upper sections of collecting ducts.

Medulla: The darker, redder region composed of loops of henle and the lower sections of the collecting ducts. Notice that it seems to form triangular regions which are called the pyramids.

Pelvis: This Is a cavity which collects the urine that emerges from the open ends of the collecting ducts. The nephrons open on the margin of the pyramids and pelvis. The white tissue forms a funnel called the ureter which conducts the urine to the bladder.

The photograph is of a large pig kidney. There are three distinct regions based on the distribution of the different sections of the nephron. The human kidney contains approx 106 nephrons.

Task

• Now that you have dissected the kidney, use the textbooks to make any drawings or notes that you feel you need to be able to describe the structure of a kidney

• Ensure that you include the structure of nephrons and any associated blood vessels

Basic Kidney Structure

In a longitudinal section of the kidney, we can see:

• outer region called the cortex, surrounded by the capsule

• inner region called the medulla

• the pelvis in the centre leading to the ureter

The Nephron

The bulk of the kidney consists of tiny tubules called nephrons- about a million in each kidney

The NephronEach nephron starts in the cortex.

The capillaries form a knot called the glomerulus

This is surrounded by the Bowman’s capsule

Fluid is pushed from the capillaries into the Bowman’s capsule by a process called ultrafiltration

The Bowman’s capsule leads to the rest of the nephron- proximal convoluted tubule, loop of Henle, distal convoluted tubule and the collecting duct

The NephronAs fluid moves along the nephron, selective reabsorption occurs.

Substances are reabsorbed back into the tissue fluid and blood capillaries surrounding the nephron tubule

The final product is urine

This passes into the pelvis and down the ureter to the bladder

Selective Reabsorption

All sugars, most salts and some water is reabsorbed

water potential of the fluid is decreased by addition of salts and removal of water

Water potential increased as salts are removed by active transport

Water potential decreased again by the removal of water- ensuring that urine has a low water potential. Urine has a higher concentration of solutes than blood and tissue fluid

Ultrafiltration

Blood flows from the afferent arteriole, into the glomerulus, and leaves through the efferent arteriole, which is narrower, meaning that blood in the glomerulus is at high pressure

As the blood in the glomerulus is at higher pressure than in the Bowman’s capsule, fluid from the blood is pushed into the Bowman’s capsule

The barrier between the blood in the capillaries, and the lumen of the Bowman’s capsule consists of:

• Endothelium- having narrow gaps between its cells that plasma can pass through• Basement Membrane- made of a fine mesh of collagen fibres and glycoproteins which act as a filter to stop molecules with a relative molecular mass of 69000 getting through (most proteins and all blood cells)• Podocytes- epithelial cells of the Bowman’s capsule containing finger like projections

called major processes. These ensure that there are gaps between the cells allowing fluid to pass into the lumen of the Bowman’s capsule

What is filtered out of the blood?

• Blood plasma which includes

• Water• Amino acids• Glucose• Urea• Inorganic ions (sodium, chloride, potassium)

What is left in the capillary?• Blood cells• Proteins

This makes the blood have a low (very negative) water potential which ensures some fluid is retained in the blood

The very low water potential of the blood in the capillaries helps to reabsorb water at a later stage

Selective Reabsorption

• Most reabsorption occurs from the proximal convoluted tubule where 85% of filtrate is reabsorbed

• All glucose and amino acids, some salts and some water are reabsorbed

Specialised for Selective Reabsorption• Microvilli on the cell surface membrane of

the tubule provides a large surface area• Co-transporter proteins in the membrane

transport glucose and amino acids in association with sodium ions by facilitated diffusion

• The opposite membrane is folded to increase surface area and contains sodium-potassium pumps that pump sodium out and potassium in

• Cell cytoplasm has many mitochondria indicating that energy is required as ATP

How does Selective Reabsorption Occur?• Sodium ion concentration is reduced as Sodium-potassium pumps remove

sodium ions from the cells lining the proximal convoluted tubule• Sodium ions transported into the cell with glucose or amino acids by

facilitated diffusion• As concentration rises, they are able to diffuse out of the opposite side of

the cell into the tissue fluid- active transport may also support this process• from the tissue fluid, they diffuse into the blood and are carried away• Reabsorption of salts, glucose and amino acids reduces the water

potential in the cells (makes it more negative) and increases the water potential in the tubule fluid (towards zero)- this means water will enter the cells from the tubule fluid and then be reabsorbed into the blood by osmosis

The Kidney

1. Create a movie on the kidney…Success criteria:Your movie must include: • Diagrams showing the structure of the kidney• Information about the blood vessels and

nephrons• The stages in Ultrafiltration