kidneys

Biosystems maintenance

The kidney, excretion and osmoregulation

Excretion

• What is excretion and why is it important?• Excretion is getting rid of metabolic waste. • Metabolic waste is unwanted material

produced from cellular reactions and processes.

• Excretion is important because many of these materials are toxic and poisonous.

Excretion cont'd

• The two main excretory products in humans are carbon dioxide and urea.

• Carbon dioxide is produced by respiring cells. It is taken to the lungs where it is breathed out.

• Urea is produced only in the liver from excess amino acids. Urea is removed from the blood by the kidneys. This will be explored in the following slides.

Deamination

• Deamination is the process by which an amino group is removed from a compound, in this case amino acids.

• If excess protein is ingested from the diet, it cannot be stored as is.

• It would however be wasteful to excrete the excess since amino acids contain useful energy.

Deamination cont'd

• Deamination occurs in the liver.• The process is outlined below.1.The amino (NH2) group along with an extra

hydrogen atom is removed from an amino acid.

2.The amino group combines with the hydrogen atom to form ammonia.

3.What is left of the amino acid is called a keto acid.

Deamination cont'd

• Ammonia is highly toxic and very soluble. • It must therefore be gotten rid of very quickly.

• In aquatic animals it is gotten rid of by

diffusion into the environment. • In terrestrial organisms such as humans it has

to be gotten rid of by other means lest it causes immense damage.

Ornithine cycle

• In humans, urea is formed from the ammonia.• This is done by combining ammonia with

carbon dioxide.• Ornithine, an amino acid not used in protein

synthesis, is used as a carrier and facilitates the combination of carbon dioxide and ammonia. ATP is required.

Urea

• A normal adult produces approximately 25-30 grams of urea per day.

• Urea is the main nitrogenous compound produced in humans.

• This is gotten rid of via urine.

The kidney

• As stated before, urea is removed from the blood by the kidneys.

• Let’s recap the structure.• The kidney is covered by a fibrous capsule. • Three main areas will become obvious if a

longitudinal section is cut. • Moving from outside in, these are the cortex,

medulla and pelvis.

Longitudinal section of a mammalian kidney

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Structure of kidney and nephron

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The kidney cont'd

• The functional unit of the kidney is the nephron. Let’s look at the structure.

Structure of the nephronhttp://www.google.com.jm/imgres?q=structure+of+the+nephron&hl=en&sa=X&qscrl=1&nord=1&rlz=1T4SKPT_enJM414JM415&biw=1280&bih=582&tbm=isch&prmd=imvns&tbnid=00WaHI9F8Wo2dM:&imgrefurl=http://www.tutorvista.com/content/biology/biology-ii/excretion-and-osmoregulation/excretion-osmoregulation-man.php&docid=unrBdQBuZzQdVM&imgurl=http://images.tutorvista.com/content/excretion-and-osmoregulation/nephron-structure.jpeg&w=442&h=322&ei=JA4pT-jYCIrg2AWloLHTAg&zoom=1&iact=hc&vpx=635&vpy=139&dur=7067&hovh=192&hovw=263&tx=160&ty=106&sig=103164267484295617731&page=1&tbnh=112&tbnw=154&start=0&ndsp=21&ved=1t:429,r:3,s:0

The nephron

• Nephrons are also called Uriniferous tubules• One end of the nephron forms a cup shaped

structure called the Bowman’s or renal capsule.

• The tube leaving the Bowman’s capsule forms a twisted region called the proximal convoluted tubule.

• A long hairpin loop extends from the proximal convoluted tubule called the loop of Henle.

The nephron cont'd

• Another twisted region is formed by the tube after leaving the loop of Henle. This is known as the distal convoluted tubule.

• The distal convoluted tubule joins the collecting duct.

• The Bowman’s capsule, proximal and distal convoluted tubules are in the cortex.

• The loop of Henle is in the medulla.

The nephron cont'd

• The collecting duct runs through the medulla and pelvis.

The kidney and nephron

• The kidney is supplied with blood by the renal artery.

• The renal vein returns blood from the kidney. • Urine made in the kidney is taken to the

bladder by the ureter. • The urine is taken from the bladder to outside

by the urethra.

The kidney and nephron cont'd

• A branch of the renal artery, the afferent arteriole, supplies the renal capsule with blood.

• The afferent arteriole splits to form a mass of capillaries in the ‘cup’ of the capsule.

• This mass of capillaries is called a glomerulus. • The capillaries of the glomerulus rejoin to

form the efferent arteriole.

The kidney and nephron cont'd

• The efferent arteriole forms a network of capillaries which run along the remainder of the nephron and eventually rejoin to form a branch of the renal vein.

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Urine formation

• Urine is formed by two main processes:1.Ultrafiltration2.Reabsorption

Ultrafiltration

• During this process small molecules, including urea, filter out into the renal capsule.

• The filtrate flows along the nephron. • Focus on the diagrams as we go through this

process. • I’m sure you know of filtration from CSEC. But

what exactly controls what leaves the blood and is filtered out?

Ultrafiltration cont'd

• Two cell layers and a basement membrane separate the blood in the glomerular capillaries and the lumen of the renal capsule.

• The basement membrane lies between the two cell layers.

• One cell layer is the endothelium of the capillary.

• These capillaries have more gaps in their walls than other capillaries.

Ultrafiltration cont'd

• The basement membrane is made of collagen and glycoproteins.

• The next cell layer is formed by epithelial cells of the wall of the renal capsule.

• The epithelial cells of the renal capsule are called podocytes.

• They have fingerlike projections and also have gaps between them.

Ultrafiltration cont'd

• The holes present in both cell layers are large enough to allow large protein molecules to pass through.

• However, in a normal person any molecule with a relative molecular mass of 69000 or more is never found in the filtrate.

• It is the basement membrane that acts as a filter .

Ultrafiltration cont'd

• Large molecules such as plasma proteins and blood cells do not pass through the basement membrane.

• What causes the filtrate to leave the blood?

Glomerular filtration rate

• This is the rate at which fluid seeps from the capillaries of the glomerulus.

• In a normal human the rate is approximately 125cm3min-1. This is for all the glomeruli in both kidneys.

• The differences in water potential determine the rate of filtration.

Glomerular filtration rate cont'd

• What is water potential?• Remember that both solute and pressure affect

water potential. • Water potential is increased by high pressure

and lowered by the presence of solutes. • The pressure inside the glomerular capsule is

relatively high because the diameter of the afferent arteriole is larger than the diameter of the efferent arteriole and the capillaries.

Glomerular filtration rate cont'd

• This relatively high pressure raises the water potential of the contents of the renal capsule.

• The solute potential of the contents of the blood capillaries is however higher than that of the renal capsule.

• This is because the large plasma proteins cannot pass through.

• The pressure potential is greater than the

Glomerular filtration rate cont'd

solute potential. There is an overall increase in the water potential of the material in the glomerular capillaries. • Water then moves down the water potential

gradient, from the blood into the capsule.

Reabsorption

• We’ll be looking at reabsorption in three sections. This is because the process of reabsorption is controlled differently in these areas.

1.Reabsorption in the proximal convoluted tubule.

2.Reabsorption in the loop of Henle.

Reabsorption cont'd

3. Reabsorption in the distal convoluted tubule and collecting duct.

Reabsorption in the proximal convoluted tubule

• The filtrate within the renal capsule is almost identical to the blood plasma except that it does not contain large protein molecules.

• Many of the materials in the filtrate are needed by the body.

• These are selectively reabsorbed. • Most of the selective reabsorption takes place

in the proximal convoluted tubule.

Reabsorption in the proximal convoluted tubule cont'd

• Sodium ions are actively transported out of the cells of the basal membrane.

• The basal membrane is the membrane nearest the blood and furthest from the lumen.

• The sodium ions are carried away by the blood. • Consequently, the concentration of sodium ions

inside the cell is lowered.

Reabsorption in the proximal convoluted tubule cont'd

• As a result, sodium ions move down a concentration gradient from the fluid in the lumen of the tubule into the basal cells.

• The sodium ions move by facilitated diffusion. • These transporter molecules also transport

other molecules into the cells and into the blood. eg. Glucose

Reabsorption in the proximal convoluted tubule cont'd

• All the glucose is reabsorbed here, so there should be no glucose in the urine.

• Amino acids, vitamins, chloride and sodium ions are actively reabsorbed here.

• Active reabsorption of these molecules lower solute concentration of the filtrate.

• Therefore, water follows the molecules by osmosis as they move out of the filtrate.

Reabsorption in the proximal convoluted tubule cont'd

• The overall concentration of the filtrate remains the same.

• Approximately 65% of water from the filtrate is reabsorbed here.

• Because urea is a small molecule, quite a lot (approx. 50%) is reabsorbed here.

Reabsorption in the proximal convoluted tubule cont'd

• All the reabsorption in the proximal convoluted tubule greatly reduces the the volume of liquid remaining.

• In a normal adult, the reduction is approximately by 64%. i.e from 125 cm3 to 45 cm3.

Reabsorption in the loop of Henle

• The function of the loop of Henle is to conserve water.

• It does this by creating a very high concentration of salts in the tissue fluid in the medulla of the kidney.

• A high solute potential of tissue fluid in the medulla of the kidney lowers the water potential and cause water to be absorbed from the filtrate.

Reabsorption in the loop of Henle cont'd

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

• We’ll first focus on the action of the ascending limb.

• The walls of ascending limb are impermeable to water.

Reabsorption in the loop of Henle cont'd

• Sodium and chloride ions are actively transported out of the fluid in the tube into the tissue fluid between the cells lying between both limbs.

• This transport is done by the cells in the wall of the ascending limb.

• The concentration created there can be up to 4 times greater than the normal concentration of tissue fluid.

Reabsorption in the loop of Henle cont'd

• The wall of the descending limb is permeable to water, sodium and chloride ions.

• As fluid moves down the tube, water moves out by osmosis.

• At the same time sodium and chloride ions move down a concentration gradient into the tube.

Reabsorption in the loop of Henle cont'd

• The fluid at the bottom of the hair pin loop is very concentrated.

• It has more sodium and chloride ions and less water than it did at the top.

• A longer loop of Henle therefore results in a more concentrated fluid.

• As the fluid turns the corner and moves up the ascending limb, the process can be repeated.

Reabsorption in the loop of Henle cont'd

• Both limbs of the loop of Henle operate on a counter-current multiplier principle.

• This is so because both limbs run parallel to each other with fluid flowing in opposite directions.

• This allows the maximum concentration to be built up inside and around the tube of the loop.

Question

• Desert animals such as the kangaroo rat have very long loops of Henle. Can you suggest why?

• In humans, only a third of the loops of Henle reach the medulla. Suggest a reason.

Reabsorption in the distal convoluted tubule and collecting duct.

• Water is reabsorbed from the fluid in the collecting duct as is passes through the medulla where water potential is very low.

• The amount of water that is absorbed is controlled by ADH (Anti Diuretic Hormone). This will be explored later.

• The first region of the distal convoluted tubule behaves in a similar way as the ascending limb of the loop of Henle.

Reabsorption in the distal convoluted tubule and collecting duct cont'd

• The second part behaves like the collecting duct.

• In the collecting duct and distal convoluted tubule, sodium ions are actively pumped from the fluid in the tubule into the tissue fluid and eventually into the blood.

• Potassium ions are actively transported into the tubule.

Reabsorption in the distal convoluted tubule and collecting duct cont'd

• Movement of these ions help to regulate the amount of these ions in the blood.

• Regulation of these ions translates to regulation of blood pH.

• Blood pH is maintained by tubular secretions in the distal convoluted tubule.

• If blood pH falls too low, the cells of the tubule combine water with carbon dioxide to form carbonic acid

Reabsorption in the distal convoluted tubule and collecting duct cont'd

• This acid dissociates to form hydrogen ions and hydrogen carbonate ions.

• The hydrogen carbonate ions diffuse into the blood.

• Hydrogen ions are pumped into the lumen where they combine with hydrogen phosphate ions to form dihydrogen phosphate ions.

Reabsorption in the distal convoluted tubule and collecting duct cont'd

• These ions are passed out in the urine.• Sodium ions move into the blood to balance

the loss of hydrogen ions. • Both hydrogen and sodium ions restore

neutrality. • If the urine is very acidic, the ammonium

mechanism can be employed.

Reabsorption in the distal convoluted tubule and collecting duct cont'd

• The tubule cells have enzymes that can convert glutamine to ammonia.

• The ammonia combines with hydrogen ions to form ammonium ions which is excreted.

• The cells of the distal convolution are similar in structure to those of the proximal convolution.

• They have a brush border and numerous mitochondria.

Control of water content

• Why is it necessary to control the water content of the blood?

• Do you remember the term which describes the maintenance of a constant internal environment?

• The kidneys play a part in that process. • The part the kidneys play is known as

osmoregulation.

Control of water content cont'd

• Osmoregulation works by negative feedback mechanism.

• Remember that in negative feedback mechanism there is always a receptor and an effector.

• The receptor detects that there is a change in the parameter and the effector reverses the change.

Control of water content cont'd

• I’ll explain what happens and then you tell me what the receptor and effectors are.

• Within the hypothalamus are special cells called osmoreceptors.

• These cells constantly monitor the water content of the blood.

• It is believed that they lose water when water levels are low and gain water when water levels are high.

Control of water content cont'd

• The loss of water triggers a stimulation of the nerve cells in the hypothalamus.

• The cell bodies of these nerve cells produce Anti Diuretic Hormone (ADH).

• This small polypeptide (made of just 9 amino acids) passes along to their nerve endings in the posterior lobe of the pituitary gland.

Control of water content cont'd

• When the osmoreceptors cause a stimulation of these nerve cells, ADH is released into the blood of the capillaries within the posterior pituitary gland.

• The blood then transports the ADH. • ADH controls how much water is reabsorbed

from the filtrate in the kidneys.

The action of ADH

• ADH causes the cells of the walls of collecting duct more permeable to water.

• It does this by increasing the number of water permeable channels in the plasma membrane of the cells in that region.

• Inside the cells are ready-made vesicle which are surrounded by membranes full of water permeable channels.

The action of ADH cont'd

• A receptor on the plasma membrane picks up the ADH molecule.

• This activates an enzyme within the cell. • The activation causes these special vesicles to

fuse with the plasma membrane of the cells thus increasing their permeability to water.

• As the fluid flows by through the collecting duct, water moves freely out of the tubule into the tissue fluid.

The action of ADH cont'd

• Movement is enhanced by the highly concentrated tissue fluid.

• Because there is further loss of water from the filtrate, small amounts of concentrated urine is produced.

• As the name suggests, ADH prevents diuresis (production of dilute urine)

• The action of ADH is similar on the distal convoluted tubule.

What happens when there is an increase in blood water content?

• When there is high water content in the blood, the osmoreceptors are not stimulated.

• Therefore the secretion of ADH slows down. • There is a corresponding effect on the walls of

the collecting ducts. • The water permeable channels are removed

to be stored in the cytoplasm.

What happens when there is an increase in blood water content?

• This makes the collecting duct less permeable to water.

• Consequently, large volumes of dilute urine are produced.

• There is not an immediate response of the collecting duct to the reduction in secretion of ADH.

What happens when there is an increase in blood water content? cont'd

• This is because ADH takes some time to be broken down and there would be some in the blood.

• Approximately half of the amount is broken down every 15-20 minutes.

• Once the ADH stops arriving at the kidneys, the channels are removed in about 10-15 minutes.

What happens when there is an increase in blood water content? cont'd

• Diabetes insipidus results when the body does not produce enough ADH to control urine production.

• Consequently, there is excessive urine production and thirst.

• To treat this disease, drugs and synthetic hormones are used to reduce urine production and changes to the diet are made.

Osmoregulation

• So you should know by now what the receptors and effectors are.

• Construct a flow diagram to show how blood water concentration is controlled. Show clearly the receptors and effectors, and show how negative feedback is involved.