The Kidney

The Structure of the Kidney

There are three distinct regions based on the distribution of the different sections of the nephron. The human kidney contains approx nephrons.

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 Kidney

This diagram shows which elements of the nephron are present in kidney cortex and medulla regions

The Nephron 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 collecting duct 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 Nephron As 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

The Kidney

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 and is called glomerular filtrate.

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 large molecules 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

Q: What is filtered out of the blood? A: 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 This very low water potential of the blood in the capillaries helps to reabsorb water at a later stage (as we shall see!)

Image for Q1a Further Practice

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 (assuming healthy working kidneys)

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? 1.Sodium ion concentration is reduced as Sodium-potassium pumps remove sodium ions from the cells lining the proximal convoluted tubule 2.Sodium ions transported into the cell with glucose or amino acids by facilitated diffusion 3.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 4.from the tissue fluid, they diffuse into the blood and are carried away 5.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

TASK: The Kidney 1. Create a presentation 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