Osmoregulation & Excretion *Extended Version*

Excretion & Osmoregulation

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Osmoregulation #1

Regulation of water and solute (ionic) balance

Excretion #2

Elimination of metabolic waste products from an animal’s body, including:

#2a

CO2 and H2O (cellular respiration)

#2b

Nitrogen (produced as ammonia through deamination of amino acids)

#2c

Excess solutes (ions)

Osmoregulators & Osmoconformers

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Osmoconformer #1

osmotic concentration of the body fluids of an animal equals that of the medium (animal’s environment); does not require energy

Osmoregulator #2

an animal that maintains its body fluids at a different osmotic concentration from that of its environment; requires energy

Water potential

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Water potential #1

force responsible for water movement in a system; measure of a water molecules potential for movement in a solution

Osmosis #2

movement of water molecules from a region of higher water potential to a region of lower water potential across a semi-permeable membrane

#3

Pure water has the highest water potential (=0)  all other solutions have a lower potential (negative)

#4

Water potential (Ψ) = pressure potential (Ψp) + solute potential (Ψs)

Pressure potential & solute potential

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Pressure Potential #1

pressure exerted by the rigid cell wall (plant cells) that limits further water uptake

More pressure in a cell #1a

higher water potential water will want to leave cell (high pressure  low pressure)

Solute Potential #2

the effect of solute concentration

Pure water #2a

= 0, so any solution will create a negative solute potential

#2b

As solute is added, the water potential of a solution drops (more solute = lower potential), and water will tend to move into the solution

Low solute concentration =

High water potential

High solute concentration =

low water potential

Hypo- and Hypertonic Solutions

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Hyperosmotic side

• High solute concentration

• Lower free H20 concentration

Hypoosmotic side

• Lower solute concentration

• Higher free H20 concentration

Osmoregulation in Saltwater Animals:

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#1

Animals living in seawater have body fluids with an osmotic concentration that is about a third less (hypoosmotic) than the surrounding seawater, and water tends to leave their bodies

#2

To compensate for this problem, mechanisms evolved in these animals to conserve water and prevent dehydration

Excretion in Freshwater Animals:

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#1

Freshwater animals have body fluids that are hyperosmotic with respect to their environment, and water tends to continually enter their bodies

#2

Mechanisms evolved in these animals that secrete water and prevent fluid accumulation

Osmoregulation in Terrestrial Animals:

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#1

Land animals have a higher concentration of water in their fluids than the surrounding air, so they tend to lose water to the air through evaporation; a lot of water is also lost through urine and feces

#2

Adaptive mechanisms for minimizing evaporative loss and conserving water through physiological processes

The Vertebrate Urinary System:

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#1

A variety of mechanisms have evolved in vertebrates to cope with osmoregulatory problems, most are adaptations of the urinary system

Includes #2

Kidneys, renal pelvis, ureters, bladder, and urethra

#3

Primary function include:

1. Filtration #3a

In which blood passes through a filter that retains (within the bloodstream) blood cells, proteins and other large solutes but lets small molecules, ions and urea pass through

2. Reabsorption #3b

In which selective ions and molecules are taken back into the bloodstream from the filtrate

3. Secretion #3c

Whereby select ions and end products of metabolism (ex. K, H, NH3) that are in the blood are added to the filtrate for removal from the body

Vertebrate Kidneys

The structure and function of vertebrate kidneys differ, depending on vertebrate groups and developmental stage

Reptile, Bird, & Mammal Kidneys

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#1

Reptiles, birds and mammals all possess metanephric kidneys which are by far the most complex animal kidneys (well-suited for terrestrial lifestyle and high rate of metabolism)

#2

In most reptiles, birds and mammals, the kidneys are primary regulatory organs for controlling the osmotic balance of the body fluids

Structure & Function of the Human Kidney:

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#1

Each kidney has a coat of connective tissue called the renal capsule

#2

The inner portion of the kidney is called the medulla; the region between the capsule and the medulla is the cortex

#3

Urine collects in the renal pelvis, located at the center of each kidney

#4

The metanephric kidney consists of over 1 million individual filtration, secretion, and absorption structures called nephrons (= functional unit of the kidney)

Filtration in the Glomerulus:

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#1

At the beginning of the nephron is the filtration apparatus called glomerular capsule (Bowman’s capsule)

#2

The capsules are in the cortical (outermost) region of the kidney

#3

In each capsule an afferent arteriole (branching from renal artery) enters and branches into a network of capillaries called the glomerulus

#4

The walls contain small perforations that act as filters; blood pressure forces fluid through these filters

#5

Large proteins and blood cells remain in the vessels and leave the glomerulus via the efferent (outgoing) arteriole into peritubular capillaries

#6

Peritubular capillaries wind around the nephron (eventually merge to form venules that carry blood out of the kidney > renal vein)

#7

Fluid within the nephron (now called glomerular filtrate) contains small molecules such as glucose ions, amino acids, sodium, potassium, phosphate, calcium, magnesium as well as water & urea (the primary nitrogenous waste of protein metabolism)

Reabsorption & Secretion in the Proximal Convoluted Tubule:

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#1

In the proximal convoluted tubule, various materials, (glucose, amino acids, vitamins, minerals, ions, water) are selectively reabsorbed back into the bloodstream (via peritubular capillaries), while additional waste products (ions, toxins) are secreted from the bloodstream into the filtrate

#2

Both active (ATP-requiring) and passive (via diffusion) activities are involved in the movement of these substances

Countercurrent Multiplier System in Loop of Henle:

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Loop of the nephron #1

increases the efficiency through a countercurrent multiplier system that is responsible for developing the osmotic gradient

#2

As the filtrate moves down through the descending limb of the loop of Henle, water is reabsorbed (passively) into circulation

#2a

Osmolarity of interstitial space increases as loop descends

#3

Water cannot flow out of the ascending limb because the cells of the ascending limb are impermeable to water

#4

As the filtrate passes into the ascending limb, sodium ions are actively transported out of the filtrate into the extracellular fluid (chloride ions follow passively)filtrate becomes more dilute as it ascends

Length of the Loop of Henle & Water Conservation Needs

Generally, the longer the loop of the nephron, the more water can be reabsorbed