The Urinary SystemThe kidneys and the balance of water

We know it’s important…

• We all understand that water is important as it has a few main functions in the human body:

The kidney

• The kidney is the main organ responsible for the regulation of water in the body

• However, because water retention is so essential for the human body, there are other systems that have either minor or major involvement:

Maintenance of water balance

• Therefore, becuase it is an essential substance in the body, the regulation of water levels in the body is essential to the survival of a human being

Losing water

• It is, however, worthwhile to note that as important as water retention is, water loss is necessary

• While egestion (defecation) in the gastrointestinal tract is one method by which we dispose of solid waste,

Urinary purpose

• Therefore, the urinary system carries out three main purposes:

Linked to blood

• Therefore, it is important to note that the urinary system is linked to the cardiovascular system

• Water transport between these two systems occur through OSMOSIS

Not just water

• Before continuing, it is also important to note how important the role of other substances dissolved in blood plasma is to the process of water filtration in the kidney

• Other substances such as

Salt is a key player

• It is important to note that out of all of these, salt is the most important accessory substance in fluid regulation

• Mainly this refers to NaCl, but other dissolved ions from salts such as potassium play a role

• Although consumption of excess salt has earned a bad rep for causing health problems, it is worthwhile to note that the opposite problem, HYPONATREMIA can be just as harmful to the human body

Gross anatomical structure

Gross anatomical structure

The Nephron

• The kidney is the organ – however, the functional unit within the kidney that carries out filtration is known as the NEPHRON

• The nephrons in the kidney are composed of:

The nephron

Blood supply

• The renal artery feeds blood into the kidney• This blood supply provides the kidney with the

necessary blood flow for function, however,

Bowman’s capsule

Filtration

• Water from the blood plasma filters into the glomerulus via blood pressure in the capillaries

• Remember that capillaries have FENESTRAE or small holes in them

• As the heart beats, it produces pressure; this forces water out of the fenestrae

Losing stuff

• So the first process of filtration results in important substances such as water and salts being removed from the blood plasma

• Therefore, the kidney must be able to retrieve these substances and control their removal

A word on osmosis

• Osmosis will cause water to move towards area of low water concentration (though sometimes we don’t think about it this way – since water is usually a solute)

• The easiest way to think about this is:

Overview of kidney function

• http://health.howstuffworks.com/human-body/systems/kidney-urinary/adam-200032.htm

• http://www.aboutkidshealth.ca/HowTheBodyWorks/Urine-Formation.aspx?articleID=10245&categoryID=XK-nh1-04

• Check out this tutorial at home:• http://www.aboutkidshealth.ca/En/

HowTheBodyWorks/KidneysandBladderOverview/TheKidneysOverview/Pages/UrineFormation.aspx

Renal tubule

• More than just a pipe, the renal tubule is lined with cells that can move substances across concentration gradients

• The renal tubule is divided into 4 major parts that cross the 3 different layers of the kidney:

Some vocabulary

• Absorption/Reabsorption:

• Secretion:

The basics

• What you should notice is a net movement of water and NaCl out of the renal tubule

• The absorption of water is facilitated by the absorption of NaCl

• This is the idea behind electrolyte drinks: the body is more efficient at reabsorbing water if it has salt to help it do so!

Physiology• The ascending loop of Henle is impervious to water – this means

that no water is lost from this loop –

• The descending limb is more permeable to water and less permeable to salt

• Since the ascending limb doesn’t allow for water secretion and only salt secretion, this creates a more hypotonic fluid entering into the collecting duct –

• The collecting duct is generally impervious to water, but can increase its permeability (hormones/drugs can change this)

• Water, NaCl and other ions move out of proximal tubule• As water moves into descending loop of Henle into the outer

medulla, the fluid is low in solutes due to loss of ions, so water moves out of the tube into the salty interior of the medulla

• As the fluid in the descending loop of Henle enters the loop of Henle in the inner medulla, the fluid is now higher in salt – so NaCl passively moves out of the loop of Henle

• As the fluid moves up into the ascending loop of Henle, salt is actively pumped out the tubule into the outer medulla, helping to create the salty environment that drives water reabsorption in the descending loop

• In the distal tubule, salt is actively pumped out allowing water to follow it

• In the collecting duct that passes through the outer medulla, salt is actively pumped out leaving the fluid entering the inner medulla to be less salty – creating conditions for the last reabsorption of water before the fluid leaves the collecting duct and drains into the ureter for excretion

Structure Absorbs Secretes

Proximal tubule Via active transport:NaCl, nutrients such as glucoseVia passive transport:Bicaronate, K+, water

Via active transport:H+Via passive transport:Ammonia

Descending loop of Henle Via passive transport:Water

Thin segment of ascending loop of Henle in outer medulla

Via passive transport:NaCl

Thick segment of ascending loop of Henle in inner medulla

Via active transport:NaCl

Distal tubule Via active transport:NaCl, bicarbonateVia passive transport:water

Via active transport:H+, K+

Collecting duct in inner medulla Via active transport:NaCl

Via passive transport:Urea, water

Control

• The cells in certain parts of the renal tubule are responsive to hormones and drugs that can control the rate of absorption of water and salt

Fluid regulation in a small range

• One center of fluid control is initiated by your brain• Arteries carrying blood to your brain must pass

through your hypothalamus

• (Caffeine is a DIURETIC – does the opposite – increases water secretion; therefore coffee = need to pee)

HIGH WATER, LOW SALT = low osmotic pressure (low plasma-osmality)

LOW WATER, HIGH SALT = high osmotic pressure (high plasma-osmality)

Fluid regulation in a larger range

• More serious responses are needed for rapid changes in blood pressure

• For example, severe bleeding is a good example of this

• Losing blood means a severe drop in blood volume which can lead to a severe drop in blood pressure

JGA (Juxtoglomerular Apparatus)

• The JGA is composed of the glomerulus and the blood vessels that feed it

• The JGA can sense changes in blood pressure

The adrenal gland

• The adrenal gland then secretes ALDOSTERONE in response to angiotensin II

Decrease in blood pressuresensed by the JGA