Homeostasis and Kidney Function

Homeostasis is the maintenance by organisms of a constant internal environment.

Homeostasis allows cells to function normally despite changes in the external environment - for example, changes in pH, temperature and water potential.

These changes fluctuate around a set point. Homeostasis is the ability to return to that set point.

Negative feedback is a primary mechanism of homeostasis. When there is a change in a monitored physiological variable a response is triggered to counteract the initial function restoring it to its normal level.

The control of any self regulating system involves a series of stages:

• Set point - the desired level at which the system operates
• Receptor - detects any deviation from the set point
• Co-oridnator - communicates with one or more effectors which carry out the corrective procedures
• Effector - muscles or glands that bring about changes to the system in order to return it to the set point

Once the correction is made and the factor returned to normal, information is fed back to the detector which then 'switches off'.

• Removes nitrogenous waste from the body
• Osmoregulation - regulates the balance of water and dissolved solutes

Urea is a poisonous chemical made by the liver.

If there is too much protein in the diet, any excess has to be broken down.

Amino acids, which make up proteins, are deanimated in the liver.

The reaction produces ammonia which is highly toxic, so it is quickly converted to urea. Urea is released into the blood and travels round the body until it is removed by the kidneys.

Humans have two kidneys. They are the main organs that filter waste products from the blod.

Each kidney receives blood from the renal artery and returns blood to the general circulation via the renal vein.

The ureter carries urine from the kidneys to the bladder.

The kidney is covered in a capsule.

There are three main areas in a kidney:

• Cortex
• Medulla
• Pelvis

The kidney is made up of thousands of minute tubes called nephrons.

Blood vessels are in close association with the nephrons.

Blood supply to the nephron begins as an afferent arteriole serving the glomerulus. From the glomerulus the blood is carried by the efferent arteriole to two other capillary structures:

• Capillaries serving the proximal and distal convoluted tubules
• Capillaries running beside the Loop of Henle - vasa recta

Ultrafiltration is filtration under pressure that separates small, soluble molecules from the blood plasma. It is the process by which small molecules such as water, glucose, urea and salts are filtered from the glomerulus into the Bowman's capsule.

High blood pressure is maintained in the glomerulus by the contraction of the heart (hydrostatic pressure), the afferent arteriole having a wide diameter than the efferent arteriole and the large surface area of the capillaries of the glomerulus.

The blood is separated from the lumen of the capsule by two cell layers and a basement membrane.

The first cell layer is the wall of the capillary. This single layer contains many small gaps (pores)

The basement membrane between the two cell layers acts as a molecular 'sieve' and forms the selective barrier between the blood and the nephron. This allows small molecules to pass through but retains in the capillaries the blood proteins and cells.

The second cell layer makes up the wall of the Bowman's capsule. The epithelial cells in this layer are called podocytes and fit together with filtration slits between the feet of the podocytes.

Selective reabsorption is the process by which useful products such as glucose and salts are reabsorbed back into the blood as the filtrate flows along the nephron.

All the glucose and most of the water and salt are reabsorbed in the proximal convoluted tubule. The cells are adapted by having:

• Microvilli providing a large surface area
• Mitochondria providing ATP for the active transport of glucose and salts

Water is reabsorbed passively by osmosis following the transport of salt.

Most of the water is reabsorbed in the collecting duct.

The numerous loops of Henle collectively concentrate salts in the tissue fluid of the medulla of the kidney. The high concentration of salt then causes an osmotic flow of water out of the collecting ducts thereby concentrating the urine.

The first part of the loop of Henle is called the descending limb and the second part is called the ascending limb.

The walls of the ascending limb are impermeable to water. The cells in the wall actively transport sodium and chloride ions out of the fluid in the tubule and into the tissue fluid between the two limbs. This produces an area of low water potential.

The walls of the descending limb are permable to water and also to sodium and chloride ions. As fluid flows down, water passes out by osmosis into the region of low water potential. At the same time, sodium and chloride ions diffuse into the descending limb, so as it flows down it contains progressively less water and the solution is most concentrated at the bottom of the hairpin.

When the fluid reaches the collecting duct it runs back down into the medulla passing through the region of low water potential.

Water therefore passes out of the collecting duct by osmosis towards this region.

The water is then transported away in the blood capillaries surrounding the Loop of Henle and into the general circulation.