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Focus on

Osmoregulatory organs

contractile vacuole, antennal glands,

protonephridia/flame-bulb system,

metanephridia, malphigian tubules

Basic function of kidney

Types of nitrogenous waste

ammonia, urea and uric acids

Evolution of the vertebrate kidne

Freshwater fish, Marine fish, Amphibians and

Reptiles, Birds and Mammals

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In order for the system to function properly, the relative concentrations of water and solutes in this environment must be maintained within somewhat narrow limits.

Homeostasis is one of the fundamental characteristics of living things. It is the maintenance of the internal environment within tolerable limits.

Living organisms achieve homeostasis by monitoring and regulating a variety of internal parameters and by behavioral changes.

Excretory systems help in maintaining homeostasis.

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Human homeostasis refers to the body's ability to regulate its internal physiology to maintain stability in response to fluctuations in the outside environment.

The liver and kidneys help maintain homeostasis.

The liver is responsible for metabolizing toxic substances and maintaining carbohydrate metabolism.

The kidneys are responsible for:

regulating blood water levels

re-absorption of valuable substances into the blood

maintenance of salt and ion levels in the blood

regulation of blood pH

excretion of urea and other wastes.

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Osmoregulation Osmoregulation is the control of the levels of

water and mineral salts in the blood. It is a homeostatic mechanism.

There are three important homeostatic mechanisms:

osmoregulation

thermoregulation

regulation of blood sugar levels.

Homeostasis is important because it results in our cells being bathed in tissue fluid which has the correct amount of water, mineral salts, glucose and temperature

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Osmoregulation is important in regulating

solute concentrations and balances the

gain and loss of water.

Osmoregulation is based largely on

controlled movement of solutes between

internal fluids and the external

environment.

Cells require a balance between osmotic

gain and loss of water. Water uptake and

loss are balanced by various mechanisms

of osmoregulation in different

environments Mimi Sophia 6

Osmotic Challenges

Osmolarity is a measure of the osmotic pressure exerted by a solution across semi-permeable membrane (one which allows free passage of water and completely prevents movement of solute) compared to pure water.

Osmolarity is also expressed as solute concentration expressed as molarity.

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Fig. 2.1 Osmolarity

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Osmoconformers, which are only marine animals are isoosmotic with their surroundings and do not regulate their osmolarity.

Osmoregulators used up their energy to control water uptake and loss in a hyperosmotic or hypoosmotic environment.

Most animals are said to be stenohaline (organism that cannot handle a wide fluctuation in the salt content of water) and cannot tolerate substantial changes in external osmolarity.

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Euryhaline animals such as mollusks and crustaceans can survive large fluctuations in external osmolarity.

Euryhaline organisms are commonly found in habitats such as estuaries where the salinity changes regularly.

However, some organisms are euryhaline because their life cycle involves migration between freshwater and marine environments, as is the case with salmon and eels.

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Freshwater animals constantly take in water from their hypoosmotic environment and lose salts by diffusion.

They maintain water balance by excreting large amounts of dilute urine. Salts lost by diffusion are replaced by foods and uptake across the gills.

Fig. 2.3 Osmoregulation in

freshwater fish

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• Freshwater animals show adaptations that reduce water

uptake and conserve solutes. They must rid themselves

of excess water.

Freshwater fish

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Marine animals consist of marine invertebrates and marine vertebrates. Most marine invertebrates are osmoconformers.

Most marine vertebrates and some invertebrates are osmoregulators.

Marine bony fishes are hypoosmotic to sea water and lose water by osmosis and gain salt by both diffusion and from food they eat. These fishes balance water loss by drinking seawater.

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Marine animals

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Fig. 2.2 Osmoregulation in saltwater fish

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Animals That Live in Temporary Waters

Some aquatic invertebrates living in temporary ponds can lose almost all their body water and survive in a dormant state. This adaptation is called anhydrobiosis

a) Hydrated tardigrade (b)Dehydrated tardigrade

Fig. 2.4 Anhydrobiosis. Tardigrade

(water bears) inhabit temporary pond

and droplets of water in soil and on

moist plants

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Land Animals

Land animals are able to manage their water budgets by drinking, eating moist foods and using metabolic water.

Fig. 2.5 Water balance in two terrestrial mammals

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Desert animals Get major water savings from simple anatomical features.

EXPERIMENT

Knut and Bodil Schmidt-Nielsen and their colleagues from Duke University observed that the fur of camels exposed to full sun in the Sahara Desert could reach temperatures of over 70°C, while the animals’ skin remained more than 30°C cooler. The Schmidt-Nielsen reasoned that insulation of the skin by fur may substantially reduce the need for evaporative cooling by sweating. To test this hypothesis, they compared the water loss rates of unclipped and clipped camels.

RESULTS

Removing the fur of a camel increased

the rate of water loss through

sweating by up to 50%.

CONCLUSION

The fur of camels plays a critical role in

conserving water in the hot desert

environment where they live.

Fig. 2.6 The role of fur in camel in

water conservation. Mimi Sophia 15

Marine birds Transport epithelia are specialized cells that

regulate solute movement. They are essential components of osmotic regulation and metabolic waste disposal. They are arranged into complex tubular networks.

An example of transport epithelia is found in the salt glands of marine birds that remove excess sodium chloride from the blood.

Salt glands is an organ to excrete excess salt.

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(a) An albatross’s salt glands empty via a duct into the nostrils, and the salty solution either drips off the tip of the beak or is exhaled in a fine mist.

(c) The secretory cells actively transport salt from the blood into the tubules. Blood flows counter to the flow of salt secretion. By maintaining a concentration gradient of salt in the tubule (aqua), this countercurrent system enhances salt transfer from the blood to the lumen of the tubule.

(b) One of several thousand secretory tubules in a salt-excreting gland. Each tubule is lined by a transport epithelium surrounded by capillaries, and drains into a central duct.

Fig. 2.7 Salt-excreting glands in birds

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Types of nitrogenous waste

An animal’s nitrogenous wastes reflect its phylogeny and habitat.

The type and quantity of an animal’s waste products may have a large impact on its water balance.

Among the most important wastes are the nitrogenous breakdown products of proteins and nucleic acids.

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NITROGENOUS WASTE PROTEIN

NUCLEIC ACID

Breakdown product from

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Forms of Nitrogenous Wastes

Different animals

excrete different

forms of nitrogenous

wastes:

Ammonia

Urea

Uric acid

Fig. 2.8 Nitrogenous wastes

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i) Ammonia

Animals that excrete nitrogenous wastes as ammonia need access to lots of water. This is because ammonia is very soluble but can be tolerated only at very low concentrations.

Therefore, ammonia excretion is most common in aquatic species.

Ammonia is release across the whole body surface or through the gills. Many invertebrates release ammonia across the whole body surface.

In fishes, most of the ammonia is lost as ammonium ions (NH4+) at the gill epithelium. Freshwater fishes are able to exchange NH4+ for Na+ from the environment, which helps maintain Na+ concentrations in body fluids.

Ammonia excretion is much less suitable for land animals. Since ammonia is so toxic, it can only be transported and excreted in large volumes of very dilute solutions. Most terrestrial animals and many marine organisms (which tend to lose water to their environment by osmosis) do not have access to sufficient water.

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ii) Urea

The liver of mammals and most adult amphibians converts ammonia to less toxic urea, such as in adult amphibians, sharks, and some marine bony fishes and turtles.

Urea is synthesized in the liver by combining ammonia with carbon dioxide and is excreted by the kidneys. The main advantage of urea is its low toxicity, about 100,000 times less than that of ammonia. Urea can be transported and stored safely at high concentrations.

Urea is carried to the kidneys, concentrated and excreted with a minimal loss of water. The main disadvantage of urea is that animals must expend energy to produce it from ammonia.

In weighing the relative advantages of urea versus ammonia as the form of nitrogenous waste, it makes sense that many amphibians excrete mainly ammonia when they are aquatic tadpoles. They switch largely to urea when they are land-dwelling adults.

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iii) Uric Acid

Insects, land snails, and many reptiles, including birds excrete uric acid as their major nitrogenous waste.

Uric acid is largely insoluble in water and can be secreted as a paste with little water loss. While saving even more water than urea, it is even more energetically expensive to produce.

Uric acid and urea represent different adaptations for excreting nitrogenous wastes with minimal water loss.

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The Influence of Evolution and

Environment on Nitrogenous Wastes

The kinds of nitrogenous wastes excreted depend on an animal’s evolutionary history and habitat. The amount of nitrogenous waste produced is coupled to the animal’s energy budget. Mode of reproduction appears to have been important in choosing among these alternatives.

Soluble wastes can diffuse out of a shell-less amphibian egg (ammonia) or be carried away by the mother’s blood in a mammalian embryo (urea). However, the shelled eggs of birds and reptiles are not permeable to liquids, which mean that soluble nitrogenous wastes trapped within the egg could accumulate to dangerous levels. Even urea is toxic at very high concentrations.

Uric acid precipitates out of solution and can be stored within the egg as a harmless solid left behind when the animal hatches.

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Table 1: Nitrogenous wastes

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Diverse excretory systems are variations on a tubular theme

The function of excretory systems is to regulate solute movement between internal fluids and the external environment.

Excretory Processes

Even though excretory systems are varied, most of them produce urine by refining a filtrate derived from body fluids.

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1. Filtration. The excretory tubule collects a

filtrate from the blood. Water and solutes

are forced by blood pressure across the

selectively permeable membranes of a

cluster of capillaries and into the excretory

tubule.

2. Reabsorption. The transport epithelium

reclaims valuable substances from the filtrate

and returns them to the body fluids.

3. Secretion. Other substances, such as

toxins and excess ions, are extracted from

body fluids and added to the contents of the

excretory tubule.

4. Excretion. The filtrate leaves the system

and the body.

Fig. 2.9 Overview of key functions of excretory systems Mimi Sophia 26

Key functions of most excretory systems:

1. Filtration - pressure-filtering of body fluids producing a filtrate

2. Reabsorption - reclaiming valuable solutes from the filtrate

3. Secretion – extraction and addition of toxins and other solutes from the body fluids to the content of filtrate in the excretory tubule.

4. Excretion - the filtrate leaves the system

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Survey of Excretory Systems

The systems that perform basic excretory functions vary widely among animal groups.

They are generally built on a complex network of tubules, for example, protonephridia, metanephridia, malpighian tubules, and vertebrate kidneys

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Protonephridia: Flame-Bulb Systems

Flatworms have an excretory system called protonephridia.

A protonephridium is a network of dead-end tubules lacking internal openings. The tubules branch throughout the body. A cellular unit called a flame bulb caps the smallest branches.

These flame bulb with a tuft (bunch) of cilia draws water and solutes from the interstitial fluid, through the flame bulb, and into the tubule system.

These tubules excrete a dilute fluid and function in osmoregulation.

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The urine in the tubules exits through openings called nephridiopores. Excreted urine is very dilute in freshwater flatworms. In fact, the tubules reabsorb most solutes before the urine exits the body.

In these freshwater flatworms, the major function of the flame-bulb system is osmoregulation, while most metabolic wastes diffuse across the body surface or are excreted into the gastrovascular cavity.

However, in some parasitic flatworms, protonephridia do dispose of nitrogenous wastes. Protonephridia are also found in rotifers, some annelids, larval molluscs, and lancelets.

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Fig. 2.10 Protonephridia; the flame-bulb system of a planarian

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Metanephridia

Metanephridia is another tubular excretory system that

consists of internal openings that collect body fluids from

the coelom through a ciliated funnel, the nephrostome, and

release the fluid to the outside through the nephridiopore.

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Fig. 2.11 Metanephridia of an earthworm 32

Each segment of an annelid worm has a pair of open-ended metanephridia. An earthworm’s metanephridia have both excretory and osmoregulatory functions.

As urine moves along the tubule, the transport epithelium bordering the lumen reabsorbs most solutes and returns them to the blood in the capillaries.

Nitrogenous wastes remain in the tubule and are excreted outside. Since earthworms experience a net uptake of water by osmosis through the skin from damp soil, their metanephridia balance water influx (entry) by producing dilute urine.

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Malpighian Tubules

In insects and other terrestrial arthropods.

Malpighian tubules remove nitrogenous wastes from hemolymph and function in osmoregulation. Insects produce a relatively dry waste matter, which is an important adaptation to terrestrial life.

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Malphigian tubules open into the digestive system and dead-end at tips that are immersed in the hemolymph (circulatory fluid).

The transport epithelium lining the tubules secretes certain solutes, including nitrogenous wastes, from the hemolymph into the lumen of the tubule.

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Water follows the solutes into the tubule by osmosis, and the fluid then passes back to the rectum, where most of the solutes are pumped back into the hemolymph.

Water again follows the solutes, and the nitrogenous wastes, primarily insoluble uric acid, are eliminated as almost dry matter along with the feces.

This system is highly effective in conserving water and is one of several key adaptations contributing to the great success of insects on land.

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Fig. 2.12 Malphigian tubules of insects

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Vertebrate Kidneys

Kidneys as the excretory organs of vertebrates, function in both excretion and osmoregulation.

Nephrons and associated blood vessels are the functional units of the mammalian kidney.

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The mammalian excretory system centers on paired kidneys, which are also the principal site of water balance and salt regulation.

Mammals have a pair of bean-shaped kidneys. Each kidney is supplied with blood by a renal artery and drained by a renal vein.

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In humans, the kidneys account for less than 1% of body weight, but they receive about 20% of resting cardiac output.

Urine exits each kidney through a duct called the ureter, and both ureters drain through a common urinary bladder.

During urination, urine is expelled from the urinary bladder through a tube called the urethra, which empties to the outside near the vagina in females or through the penis in males.

Sphincter muscles near the junction of the urethra and the bladder control urination.

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Structure and Function of the

Nephron and Associated Structures

Each nephron consists of a single long tubule and a ball of capillaries, called the glomerulus.

The blind end of the tubule forms a cup-shaped swelling, called Bowman’s capsule, that surrounds the glomerulus.

Each human kidney contains about a million nephrons, with a total tubule length of 80 km.

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• The mammalian kidney has two distinct regions, an outer

renal cortex and an inner renal medulla.

• Both regions are packed with microscopic excretory

tubules, nephrons, and their associated blood vessels.

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Nephron: functional unit of kidney

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Fig2.13 The mammalian excretory system

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Filtration of the blood

Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of Bowman’s capsule.

The porous capillaries, along with specialized capsule cells called podocytes, are permeable to water and small solutes but not to blood cells or large molecules such as plasma proteins.

The filtrate in Bowman’s capsule contains salt, glucose, amino acids, vitamins, nitrogenous wastes such as urea, and other small molecules.

Filtration of small molecules is nonselective and the filtrate in Bowman’s capsule is a mixture that reflects the concentration of various solutes in the blood plasma.

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Pathway of the Filtrate

From Bowman’s capsule, the filtrate passes through three regions of the nephron: the proximal tubule; the loop of Henle, a hairpin turn with a descending limb and an ascending limb; and the distal tubule.

The distal tubule empties into a collecting duct, which receives processed filtrate from many nephrons. The many collecting ducts empty into the renal pelvis, which is drained by the ureter.

The nephron and the collecting duct are lined by a transport epithelium that processes the filtrate to form the urine.

Their most important task is to reabsorb solutes and water. The nephrons and collecting ducts reabsorb nearly all of the sugar, vitamins, and other organic nutrients from the initial filtrate and about 99% of the water. This reduces 180 L of initial filtrate to about 1.5 L of urine to be voided.

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Blood Vessels Associated with the Nephrons

Each nephron is supplied with blood by

an afferent arteriole, a branch of the renal artery that subdivides into the capillaries of the glomerulus.

The capillaries converge as they leave the glomerulus forming an efferent arteriole.

The vessels subdivide again forming the peritubular capillaries, which surround the proximal and distal tubules.

Additional capillaries extend downward to form the vasa recta, a loop of capillaries that serves the loop of Henle. The tubules and capillaries are immersed in interstitial fluid, through which substances diffuse.

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Although the excretory tubules and their surrounding capillaries are closely associated, they do not exchange materials directly.

The tubules and capillaries are immersed in interstitial fluid, through which various materials diffuse between the plasma in the capillaries and the filtrate within the nephron tubule.

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From Blood Filtrate to Urine:

A Closer Look

Filtrate from Bowman’s capsule becomes urine as it flows through the

mammalian nephron and collecting ducts.

Secretion and reabsorption in the proximal tubule significantly alter the volume and composition of filtrate. For example, the cells of the transport epithelium help maintain a constant pH in body fluids by controlled secretions of hydrogen ions or ammonia. The cells also synthesize and secrete ammonia, which neutralizes the acid.

The proximal tubules reabsorb about 90% of the important buffer bicarbonate (HCO3−).

Drugs and other poisons that have been processed in the liver pass from the peritubular capillaries into the interstitial fluid and then across the epithelium to the nephron’s lumen. Valuable nutrients, including glucose, amino acids, and K+, are actively or passively absorbed from filtrate.

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One of the most important functions of the proximal tubule is reabsorption of most of the NaCl and water from the initial filtrate volume.

Salt in the filtrate diffuses into the cells of the transport epithelium. The epithelial cells actively transport Na+ into the interstitial fluid. This transfer of positive charge is balanced by the passive transport of Cl− out of the tubule. As salt moves from the filtrate to the interstitial fluid, water follows by osmosis.

Reabsorption of water continues as the filtrate moves into the descending limb of the loop of Henle. This transport epithelium is freely permeable to water but not very permeable to salt and other small solutes.

For water to move out of the tubule by osmosis, the interstitial fluid bathing the tubule must be hyperosmotic to the filtrate.

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Since the osmolarity of the interstitial fluid becomes increasingly greater from the outer cortex to the inner medulla, the filtrate moving within the descending loop of Henle continues to lose water.

In contrast to the descending limb, the transport epithelium of the ascending limb of the loop of Henle is permeable to salt, not water.

As filtrate ascends the thin segment of the ascending limb of the loop of Henle, NaCl diffuses out of the permeable tubule into the interstitial fluid, increasing the osmolarity of the medulla.

The active transport of salt from the filtrate into the interstitial fluid continues in the thick segment of the ascending limb.

By losing salt without giving up water, the filtrate becomes more and more dilute as it moves up to the cortex in the ascending limb of the loop.

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The distal tubule plays a key role in regulating the K+ and NaCl concentrations in body fluids by varying the amount of K+ that is secreted into the filtrate and the amount of NaCl reabsorbed from the filtrate.

Like the proximal tubule, the distal tubule also contributes to pH regulation by controlled secretion of H+ and the reabsorption of bicarbonate (HCO3−).

The collecting duct carries the filtrate through the medulla to the renal pelvis and reabsorbs NaCl.

By actively reabsorbing NaCl, the transport epithelium of the collecting duct plays a large role in determining how much salt is actually excreted in the urine. Though the degree of its permeability is under hormonal control, the epithelium is permeable to water but not to salt or (in the renal cortex) to urea.

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As the collecting duct go across the gradient of osmolarity in the kidney, the filtrate becomes increasingly concentrated as it loses more and more water by osmosis to the hyperosmotic interstitial fluid.

In the inner medulla, the duct becomes permeable to urea. Because of the high urea concentration in the filtrate at this point, some urea diffuses out of the duct and into the interstitial fluid. Along with NaCl, this urea contributes to the high osmolarity of the interstitial fluid in the medulla.

This high osmolarity enables the mammalian kidney to conserve water by excreting urine that is hyperosmotic to general body fluids.

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Fig. 2.14 The nephron and collecting duct: regional functions of the transport epithelium.

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The mammalian kidney’s ability to conserve

water is a key terrestrial adaptation

The mammalian kidney can produce urine much more concentrated than body fluids, thus conserving water.

In the human kidney, about 80% of the nephrons, the cortical nephrons, have reduced loops of Henle and are almost entirely confined to the renal cortex.The other 20%, the juxtamedullary nephrons, have well-developed loops that extend deeply into the renal medulla.

Only mammals and birds have juxtamedullary nephrons; the nephrons of other vertebrates lack loops of Henle. The juxtamedullary nephrons enable mammals to produce urine that is hyperosmotic (hypertonic) to body fluids and thus conserving water.

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Solute Gradients and

Water Conservation In a mammalian kidney, the joint action and precise

arrangement of the loops of Henle and the collecting ducts are largely responsible for the osmotic gradient that concentrates the urine

Fig. 2.15 How the human kidney concentrates urine: the two-solute model Mimi Sophia 54

Two solutes, NaCl and urea, contribute to the osmolarity of the interstitial fluid which causes the reabsorption of water in the kidney and concentrates the urine.

Before leaving the kidney, the urine may attain osmolarity of the interstitial fluid in the inner medula, which can be as high as 1200 mosm/L.

Although urine is isoosmotic to the inner medula interstitial fluid, it is hyperosmotic to blood and interstitial fluid elsewhere in the body.

The high osmolarity allows the solutes remaining in the urine to be excreted from the body with minimal water loss.

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The countercurrent multiplier system involving the loop of Henle maintains a high salt concentration in the interior of the kidney, which enables the kidney to form concentrated urine.

The collecting duct, permeable to water but not salt conducts the filtrate through the kidney’s osmolarity gradient, and more water exits the filtrate by osmosis.

Urea diffuses out of the collecting duct as it traverse (go across) the inner medulla. Urea and NaCl, form the osmotic gradient that enables the kidney to produce urine that is hyperosmotic to the blood.

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Regulation of Kidney Function

Fig 2.16 Hormonal Control of Kidney Mimi Sophia

1) Antidiuretic hormone (ADH) 2) Renin-Angiotensin-Aldosterone System

(RAAS)

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Regulation of blood osmolarity is maintained by hormonal control of the kidney by negative feedback circuits.

One hormone important in regulating water balance is antidiuretic hormone (ADH).

[Diuretic: substance tend to increase flow of urine]

ADH…

* produced in the hypothalamus of the brain

* stored in and released from the pituitary gland, which

lies just below the hypothalamus

* osmoreceptor cells in the hypothalamus monitor the

osmolarity of the blood.

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When blood osmolarity rises above a set point, more ADH is released into the bloodstream and reaches the kidney.

ADH induces the epithelium of the distal tubules and collecting ducts to become more permeable to water.

This will increase water reabsorption. This reduces urine volume and helps prevent further increase of blood osmolarity above the set point.

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HIGH:

Osmolarity

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When blood osmolarity rises above a set point, more ADH is released into the bloodstream and reaches the kidney.

ADH induces the epithelium of the distal tubules and collecting ducts to become more permeable to water.

This will increase water reabsorption. This reduces urine volume and helps prevent further increase of blood osmolarity above the set point.

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ADH Level Water

reabsorption

Osmolarity

(High Na+)

HIGH:

Osmolarity

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By negative feedback, the drop in

osmolarity of the blood reduces the

activity of osmoreceptor cells in the

hypothalamus, and less ADH is secreted.

Only a gain of additional water in food and

drink can bring osmolarity all the way back

down to the set point. ADH alone only

prevents further movements away from the

set point.

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Renin-angiotensin-aldosterone system

(RAAS)

A second regulatory mechanism

involves a special tissue called the

juxtaglomerular apparatus

(JGA), located near the afferent

arteriole that supplies blood to the

glomerulus.

When blood pressure or blood

volume in the afferent arteriole drops,

the enzyme renin initiates chemical

reactions that convert a plasma

protein angiotensinogen to a peptide

called angiotensin II.

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LOW:

- Blood volume

- Blood pressure

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Acting as a hormone, angiotensin II increases

blood pressure and blood volume in several ways:

It raises blood pressure by constricting arterioles, decreasing blood flow to many capillaries, including those of the kidney.

Stimulates the proximal tubules to reabsorb more NaCl and water. This reduces the amount of salt and water excreted and, consequently, raises blood pressure and volume.

It also stimulates the adrenal glands, located atop the kidneys, to release a hormone called aldosterone. This acts on the distal tubules, which reabsorb Na+ and water, increasing blood volume and pressure.

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In summary, the renin-angiotensin-aldosterone system (RAAS) is part of a complex feedback circuit that functions in homeostasis.

A drop in blood pressure triggers a release of renin from the JGA. In turn, the rise in blood pressure and volume resulting from the various actions of angiotensin II and aldosterone reduce the release of renin.

Normally, ADH and the RAAS are partners in homeostasis. ADH alone would lower blood Na+ concentration by stimulating water reabsorption in the kidney. But the RAAS helps maintain balance by stimulating Na+ reabsorption.

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EVOLUTION OF VERTEBRATE KIDNEY

Diverse adaptations of the vertebrate kidney have evolved in different environments

The form and function of nephrons in various vertebrate classes are related primarily to the requirements for osmoregulation in the animal’s habitat

Exploring environmental adaptations of the vertebrate kidney

Variations in nephron structure and function equip the kidneys of different vertebrates for osmoregulation in their various habitats.

Mammals that excrete the most hyperosmotic urine, such as hopping mice and other desert mammals, have exceptionally long loops of Henle. This maintains steep osmotic gradients, resulting in very concentrated urine.

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In contrast, beavers, which rarely face problems of dehydration, have

nephrons with short loops, resulting in a much lower ability to

concentrate urine.

Birds, like mammals, have kidneys with juxtamedullary nephrons that

specialize in conserving water. However, the nephrons of birds have much

shorter loops of Henle than do mammalian nephrons.

Bird kidneys cannot concentrate urine to the osmolarities achieved by

mammalian kidneys. The main water conservation adaptation of birds is

the use of uric acid as the nitrogen excretion molecule.

The kidneys of other reptiles, having only cortical nephrons, produce

urine that is, at most, isoosmotic to body fluids. However, the epithelium

of the cloaca helps conserve fluid by reabsorbing some of the water

present in urine and feces. Also, like birds, most other terrestrial reptiles

excrete nitrogenous wastes as uric acid.

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Cloaca is the posterior opening that serves as the only such opening for the intestinal and urinary tracts of certain animal species.

Birds, reptiles, amphibian excrete both urine and feces through cloaca. For placental mammals have different opening for evacuation.

In contrast to mammals and birds, a freshwater fish must excrete excess water because the animal is hyperosmotic to its surroundings. Instead of conserving water, the nephrons produce a large volume of very dilute urine.

Freshwater fishes conserve salts by reabsorption of ions from the filtrate in the nephrons.

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Amphibian kidneys function much like those of freshwater fishes. When in fresh water, the skin of the frog accumulates certain salts from the water by active transport, and the kidneys excrete dilute urine.

On land, where dehydration is the most critical problem, frogs conserve body fluid by reabsorbing water across the epithelium of the urinary bladder.

Marine bony fishes, being hypoosmotic (hypotonic) to their surroundings, have the opposite problem of their freshwater relatives. In many species, nephrons have small glomeruli or lack glomeruli altogether. Concentrated urine is produced by secreting ions into excretory tubules.

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The kidneys of marine fishes excrete very

little urine and function mainly to get rid of

divalent ions such as Ca2+, Mg2+, and SO42−,

which the fish takes in by its nonstop

drinking of seawater.

Its gills excrete mainly monovalent ions

such as Na+ and Cl− and the bulk of its

nitrogenous wastes in the form of NH4+.

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