Renal System

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The kidneys are bean-shaped organs along the back wall of the abdominal cavity.  The left kidney is slightly higher than the right because the right side of the liver is larger than the left.  The kidneys are surrounded by a layer of fat that holds them in place and protects them from physical damage.  The kidneys filter metabolic waste, excess ions and chemicals from the blood to form urine.

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We have 2 ureters and they run parallel to the vertebral column.  They are small muscular tubes which transport urine from the kidneys to the bladder. They are attached to the kidney at one end and the bladder at the other. Gravity and muscular peristaltic movements push urine from the kidneys to the bladder.  The end of the ureters are sealed by uretero-vesical valves which stop urine passing back from the bladder to the kidneys. 

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The bladder is a sac-like hollow organ which stores urine.  Urine from the ureters slowly fills the bladder and starts to stretch its elastic walls.  The bladder can hold 600-800 ml of urine. As the bladder becomes full and the walls stretch, stretch receptors send signals to the brain and spinal cord.  These signals result in the relaxation of the internal and external urethral sphincter muscles and the sensation of needing to urinate.  Urination may be delayed as long as the bladder does not exceed its maximum volume  but increasing nerve signals lead to greater discomfort and desire to urinate.  When the internal urethral sphincter muscle relaxes and the smooth muscles in the wall of the bladder contract,  urine passes from the bladder to the urethra.

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A hollow tube which leads from the bladder to the outside of the body. In males the urethra transports both semen and urine.  In females the urethra is much shorter and exits the body just above the opening of the vagina.  The external urethral sphincters is a ring of muscle at the end of the urethra which helps to control voluntary urination.

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The kidneys filter our entire blood volume approx. 20-25 times every day!

The filtration process is much like making a “proper” coffee – water is forced under pressure through a fine sieve containing ground coffee, the filtrate is the coffee (hopefully with no “bits” floating in it!) 

Specialized proteins called transporters are located on the membranes of the nephron.

These transporters grab the small molecules from the filtrate as it flows by them.

Each transporter grabs only one or two types of molecules. For example, glucose is reabsorbed by a transporter that also grabs sodium (Na).

Transporters are concentrated in different parts of the nephron. For example, most of the Na transporters are located in the proximal tubule, while fewer ones are spread out through other segments.

Some transporters require energy (active transport), while others don't (passive transport).

Water gets reabsorbed

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Maintaining a constant internal environment is called homeostasis. It is important that the body’s internal environment is controlled. 

Water concentration, temperature, and glucose concentration must be kept as constant as possible.

Homeostasis is the maintenance of constant internal conditions in an organism.

Negative feedback is an important type of control that is found in homeostasis.

A negative feedback control system responds when conditions change from the ideal or set point and returns conditions to this set point.

There is a continuous cycle of events in negative feedback. Osmoregulation is one of the key parts of homeostasis and relates to controlling water and salt levels in the body.

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The cells of the body need to grow in an isotonic environment in order to maintain their fluid and electrolyte balance. 

The kidneys maintain the body’s osmotic balance by controlling the amount of water that is filtered out of the blood and excreted into urine. When a person consumes a large amount of water, the kidneys reduce their reabsorption of water to allow the excess water to be excreted in urine. This results in the production of dilute, watery urine. In the case of the body being dehydrated, the kidneys reabsorb as much water as possible back into the blood to produce highly concentrated urine full of excreted ions and wastes. 

The changes in excretion of water are controlled by antidiuretic hormone (ADH). ADH is produced in the hypothalamus and released by the posterior pituitary gland to help the body retain water.

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How the renal symstem controls osmoregulation

ADH is carried by the blood to the kidneys.  

ADH increases the permeability of the kidney tubules, allowing water to be reabsorbed from the tubules into the blood 

If blood water concentration falls, more water re-absorption is needed so that less water is lost as urine & ADH production is increased 

If blood water concentration rises, less water re-absorption is needed so that more water is lost as urine & ADH production is decreased 

This information can be arranged into a cycle which is an example of negative feedback 

Requirements for a level 3 answer - clear, detailed and accurate discussion of 

how the renal system controls osmoregulation.  Terminology is accurate 

and the answer follows a logical sequence of events.

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Urine production

As the waste products, water and salts pass along the tubule there is a complex adjustment of the content. For example, some water and salts may be absorbed back into the bloodstream, depending on the current level of water and salt in your blood. Tiny blood vessels next to each tubule enable this fine adjustment of the transfer of water and salts between the tubules and the blood.

The liquid that remains at the end of each tubule is called urine. This drains into larger channels (ducts) which drain into the renal pelvis (the inner part of the kidney). From the renal pelvis the urine passes down a tube called a ureter which goes from each kidney to the bladder. Urine is stored in the bladder until it is passed out through the urethra when we go to the toilet. 

The cleaned (filtered) blood from each kidney collects into a large renal vein which takes the blood back towards the heart.

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Bowman’s Capsule

Performs the first step in the filtration of blood to form urine 

Allows liquids and small particles to pass through from the capillaries

Prevents larger structures (e.g. blood cells) from passing through 

Passes filtrate through to the proximal convoluted tubule 

Prevents leakage of fluid 

Enables rapid filtration 

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The glomerulus is a capillary, which looks like a ball of yarn. This is where blood is filtered through the glomerular membrane. Each glomerulus acts like a sieve that helps keep normal proteins and cells in the bloodstream and allows wastes, excess fluid and other substances to pass through.

Brings blood to the nephron

Brings waste products to the nephron 

Prevents large molecules from passing out of the blood

Enables the re-absorption of fluid / liquid into the capillaries 

Passes filtrate through to the bowman’s capsule 

Works under high pressure 

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Transports unfiltered blood to the kidneys and takes filtered blood away from the kidneys.

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Loop of Henle

Re-absorbs some filtered water 

Creates a Na concentration gradient 

Maintains osmolarity 

Passes filtrate through to the distal convoluted tubule 

Creates a counter-current multiplier system / removes ions (sodium, potassium & chloride)

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Collecting tube

These tubes collect the urine and then transport it to the ureters which will then transport it to be stored in the bladder

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Distal convoluted tubule

Re-absorbs water, glucose, sodium, potassium, and chloride

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