Nature of Excretory Products in Relation to Habitats

 

The nature of excretory products in different organisms is closely related to their habitats. Different habitats impose varying conditions and challenges that require unique adaptations to maintain homeostasis and eliminate metabolic waste products.

In aquatic organisms, such as fish and amphibians, ammonia is the primary excretory product. This is because ammonia is highly soluble in water and can be easily eliminated through diffusion across the gills or skin. However, the production of ammonia requires a large amount of water to be present for its dilution. Therefore, aquatic animals must continuously excrete ammonia and require large amounts of water to live in.

Terrestrial animals, on the other hand, face the challenge of conserving water. This is why they excrete urea or uric acid as their primary excretory product. Urea and uric acid require less water to excrete and can be stored in the body for longer periods of time. Urea is the primary excretory product in mammals, while uric acid is the primary excretory product in birds, reptiles, and insects.

Insects, due to their small size and limited ability to retain water, excrete uric acid, which is a highly concentrated, dry form of waste. Desert-dwelling animals, such as camels, excrete very little water and use specialized kidneys to concentrate their urine, allowing them to retain as much water as possible.

The nature of excretory products in organisms is closely related to their habitats and the challenges they face in maintaining homeostasis. The ability to adapt to varying conditions is essential for survival in diverse environments.

Ammonia is very toxic and dissolves quickly in body fluids. Thus, it must be kept in low concentration in the body. To maintain its low concentration below that of the body requires a large volume of water also to eliminate it in urine as it is produced. This is possible in a hypotonic environment. Therefore, ammonia is kept as the excretory product of the animals inhabiting hypotonic (e.g. freshwater) environments. About 500 ml water is needed to excrete 1g of ammonia nitrogen.

In a restricted supply of water, ammonia cannot be kept as an excretory product; the other alternative is to change it into a less toxic substance such as urea. Urea requires only 50 ml of water for its 1g of nitrogen removal. Here excretory nitrogen is metabolically converted into urea by urea cycle (Fig.15.6) in the animals inhabiting an environment with restricted supply of water e.g. terrestrial mammals.

Animals inhabiting an environment with acute shortage of water supply require an excretory product which can be excreted with a minimum amount of water. Only 1 ml water is required to eliminate 1g of nitrogen in the form of uric acid. Therefore the reptiles and birds that inhabit arid environments excrete uric acid as excretory product. Animals excreting ammonia, urea and uric acid are called as ammonotelic, ureotelic and uricotelic respectively. Ureotely and uricotely are evolutionary adaptations of nitrogenous waste in their habitats. Animals have adapted not only the chemical nature of excretory products but also the various adaptations have been obtained to provide diversity in excretory structures. The main representative models are described below:

EXCRETION IN REPRESENTATIVE ANIMALS

Excretion in Hydra

Hydra does not have specialized excretory structures. In it waste products simply diffuse into the isosmotic surroundings.

Hydra is a genus of freshwater polyps that belong to the phylum Cnidaria. As a simple aquatic organism, Hydra has a relatively simple excretory system that helps maintain homeostasis and eliminate metabolic waste products.

The main excretory organ in Hydra is called the "foot cell." This cell is located at the base of the animal's body and is responsible for removing excess water, ions, and nitrogenous waste from the body.

Hydra primarily excretes ammonia as its waste product. Ammonia is a highly toxic compound that must be eliminated from the body to maintain homeostasis. The foot cell of Hydra helps eliminate excess ammonia through the process of diffusion.

In addition to excreting waste products, the foot cell of Hydra also helps regulate the water balance in the body. This is important for maintaining proper osmotic pressure and preventing dehydration.

Overall, the excretory system of Hydra is relatively simple but effective. The foot cell plays a vital role in maintaining homeostasis and eliminating waste products from the body. 

Excretion in Planaria

Planaria, the animals of the group of flatworms, have a simple tubular excretory system called protonephridium. A protonephridium is a network of closed tubules without internal openings. Tubular system is spread throughout the body and branches are capped by a cellular setup termed as flame cell. Each flame cell has a tuft of cilia, whose beating propels interstitial fluid into the tubular system (The beating of cilia looks like a flickering flame; therefore these cells are termed flame cells). The tubular system is drained into excretory ducts, which open to the exterior through several nephridiopores (Fig.15.7). 

Freshwater flatworms excrete very dilute urine. The parasitic flatworms, which are isotonic to the host environment mainly, function in disposing of nitrogenous wastes.

Excretion in Earthworm

Earthworm is an ideal example of another type of tubular excretory system called metanephridium. Each segment of earthworm has a pair of metanephridia. This system has an internal ciliated opening of the nephrostome immersed in coelomic fluid and enveloped by a network of capillaries. Nephrostomes collect coelomic fluid. As fluid moves along the tubule, epithelium reabsorbs salt from the lumen and sends it to blood vessels surrounding the nephridium. The leftover appears as urine containing nitrogenous waste (Fig. 15.8).

Excretion in Cockroach

Terrestrial arthropods particularly in the insects, the excretory structures are adapted to collect excretory products from hemolymph in sinuses through suspended tubular structures called Malpighian tubules. These Malpighian tubules remove nitrogenous waste from the hemolymph. These are the only excretory structures in the animal kingdom that are associated with the digestive tract. The epithelial lining of the tubules transports solutes including salts and nitrogenous waste from hemolymph into tubules lumen. Fluid then passes to the hind gut into the rectum. Rectum reabsorbs most of the salts and water, thus nitrogenous wastes are excreted as solid excreta, in the form of uric acid crystals along the feces. This kind of adaptation in excretion is the success of these animals on land with acute shortage of water (Fig. 15.9).

EXCRETION IN VERTEBRATES

The ancestors of vertebrates, the invertebrate chordates, have segmented arranged excretory structures throughout the body like the metanephridia in earthworm. This character is well represented in the primitive vertebrate hagfishes which have kidneys with segmental arranged tubules. However, the contrasting developments proceeded in evolution in other vertebrates with the appearance of kidneys. Kidneys contain numerous tubules, not arranged segmental, closely associated with dense networks of capillaries. The basic functional structure in the kidneys is nephron.

Excretion in Human

Norman Mechanisms: Considering the chemical basis of life and its sustainability on metabolic pathways, the generation of wastes is primarily done at metabolic level and these are called metabolic wastes. These include urea, produced from the metabolism of amino acids; creatinine, produced from muscle creatine; uric acid, from nucleic acids; bilirubin, end products of hemoglobin breakdown and metabolites of various hormones.

Metabolic wastes also include the toxins produced within the body and ingested into the body such as pesticides, drugs and food additives. The presence of wastes in the body causes serious hazards, thus are eliminated by the excretory system.

 Excretory Organs

Liver and kidneys are the primary structure for eliminating waste products. Liver is the central station of metabolism and consequently the body's central metabolic clearing house. Due to this characteristic, liver functions are pivotal to homeostasis and involve interaction with most of body's organs systems. Liver supports the excretory role of the kidney by detoxifying many chemical poisons and produce ammonia, urea and uric acids from the nitrogen of amino acids. Removal of salts with water by the 'sweat glands and of sebum by sebaceous glands seems to be excretory in nature. The removal of water and salts from sweat glands is for the purpose of thermoregulation and of sebum on the skin is for protection against microorganism. Therefore in context of definition of excretion, skin may not be considered as an excretory organ. Among the various nitrogenous wastes described earlier, urea is the principal excretory product and liver form it from the waste nitrogen. The metabolic pathways involved in the Liver are not only involved in the synthesis of nitrogenous wastes to assist the kidney in their disposal, but also has numerous crucial functions of homeostasis importance. These functions belong to synthesis, storage, conversion, recycling and detoxification categories (Table 15.1).

Urinary System

Urea is a detoxified form of ammonia in the urea cycle, which can be retained in the body in greater amounts than ammonia and can be eliminated with 1/10 quantity of water as compared to ammonia.

A pair of kidneys consists of millions of functional units, nephrons. The nephrons have extensive blood supply via the renal arteries, which leave each kidney via the renal vein. The function of kidney and blood in clearing wastes is very evident from the fact that weight of kidneys accounts for less than 1% of the total body weight while receiving 20% of blood supplied with each cardiac beat. Following filtration of blood and further processing through tubular system urine is collected in a central cavity of the kidney, pelvis. Urine leaves the kidney through a duct ureter. The ureters of both the kidneys drain into the urinary bladder through ureteral orifice. Urine leaves the body during urination, from the bladder through a tube called the urethra, which empties near the vagina in females or through the penis in males. Sphincter muscles near the junction of the urethra and the bladder control the urine in the bladder (Fig. 15.10).

Nephron

The functional units, nephrons, in human kidneys are arranged along two distinct regions, an outer cortex and an inner medulla. The nephrons arranged along the cortex are called cortical, however, those arranged along the border of cortex and medulla with the tubular system looping deep in inner medulla are juxtamedullary nephrons These Juxtamedullary nephrons are specifically instrumental in the production of concentrated urine (Fig. 15.11).

In each nephron inner end forms a cup-shaped swelling, called Bowman's capsule and it is around a ball of capillaries called glomerulus. Glomerulus circulates blood through the capsule as it arrives through afferent arteriole and leaves the capsule by efferent arteriole. The blood vessel subdivides again into another network of capillaries, the peritubular capillaries.

Concentration of Excretory Products

In restricted supply of water, the conservation of water is the principal function of the body. This is done by concentration of the filtrate by counter current and hormonal mechanisms. In the sufficient or excess supply of water, re-absorption of water from the filtrate is reduced, specifically due to inhibition of release of antidiuretic hormone in the presence of hypoosmotic body fluids. The reduction in re-absorption causes large volumes of diluted urine. Mammalian kidneys, including humans, are adapted to conserve water by over 99.5% re-absorption of glomerular filtrate.

The interstitial fluid of the kidney is gradually concentrated from cortical to medullary part, thus inner medulla is highly concentrated with the presence of urea and through a mechanism of countercurrent multiplier. This mechanism causes gradual osmotic outflow of water from the filtrate back to the kidney as it passes downward in the descending loop of Henle. Furthermore, the ascending loop of Henle does not allow outflow of water from its filtrate, instead actively transporting Na ions into kidney interstitium to sustain its high concentration.

Hormones

The active uptake of sodium in the ascending limb or thick loop of Henle is promoted by the action of aldosterone, the hormone secreted from adrenal cortex. The other site in the nephron, where reabsorption of water takes place is collecting tubules. ADH released from the posterior pituitary lobe acts to actively transport water from filtrate in collecting tubules back to the kidney. Gradually increasing osmotic concentration from cortex to inner medulla is a main factor for the production of hypertonic (concentrated) urine in mammals including humans.

Kidney Problems and Cures

Unusual situations may arise in the function of the kidney by factors originating within the kidney or outside. These cause serious kidney diseases.

Kidney Stones

Stony materials are found in the kidney and these cause urinary obstruction and are generally complicated by infections. These stones have a specified chemical nature. These are formed in metabolic disease, hypercalcemia i.e. high level of circulating calcium in blood because of other diseases. Hyperoxaluria i.e. higher blood level of oxalates is another contributing factor in the formation of calcium oxalate stones. Oxalates are present in green vegetables and tomatoes therefore may be the source of hyperoxaluria. The incidence of calcium oxalate type stones is 70% of all the kidney stones. The incidence of other types of stones of calcium phosphate and of uric acid is 15% and 10% respectively. These salts are precipitated out during urine formation and accumulate later to form stone (Fig.15.13).

 Lithotripsy

The kidney stones have been removed by kidney surgery. Presently lithotripsy is used for non-surgical removal of kidney stones. It is the technique used to break up stones that form in the kidney, ureter or gallbladder. There are several ways to do it, although the most common is extracorporeal shock wave lithotripsy. High concentrations of X-ray or ultrasound are directed from a machine outside the body to the stone inside. The shock waves break the stone in tiny pieces or into sand, which are passed out of the body in urine. 

Renal Failure

Various factors of pathological and chemical nature may progressively destroy the nephron, particularly its glomerular part. This results in an increase in the plasma level of urea and other nitrogenous wastes. The rise in urea causes complications of increase in blood pressure and anemia etc.

Dialysis

In chronic renal failure, the function of the kidney is completely lost and is unable to remove nitrogenous waste. To remove nitrogenous waste, particularly the urea, the blood of the patient is treated through dialysis. It cleans the blood either by passing it through an artificial kidney or by filtering it within the abdomen. The wastes and excess water are removed during the treatment as is done by the healthy kidneys.

There are two types of dialysis: hemodialysis and peritoneal dialysis. Hemodialysis means 'cleaning the blood'. In this procedure blood is circulated through a machine which contains a dialyzer also called an artificial kidney. Dialyzer has two spaces separated by a thin membrane. Blood passes from one side of the membrane and dialysis fluid on the other. The wastes and excess water pass from the blood through the membrane into the dialysis fluid.

Peritoneal dialysis works on the same principle except that the abdomen has a peritoneal cavity, lined by a thin epithelium called peritoneum. Peritoneal cavity is filled with dialysis fluid that enters the body through a catheter. Excess water and wastes pass through the peritoneum into the dialysis fluid. This process is repeated several times a day. Dialyzer is a kidney machine that works on the same principle as in a kidney for removal of nitrogenous wastes and excess water from the blood. It is used after kidney failure and dialysis is done again and again until a matching donor's kidney is transplanted.

Kidney Transplant

Dialysis may be used as a temporary measure. In high degree renal failure also called uremia or end-stage renal disease, the dialysis cannot be done hence the surgical transplantation of a matching donor kidney is the only option left for as the permanent treatment.

A kidney transplant is a surgical procedure where a healthy kidney from a donor is transplanted into a person with kidney failure. The donor can be a living person, such as a family member or friend, or a deceased person whose organs have been donated.

Kidney transplant is typically reserved for people with end-stage renal disease (ESRD), a condition where the kidneys are no longer able to filter waste products and excess fluids from the blood. It is often the preferred treatment option for ESRD because it can provide a better quality of life and longer lifespan compared to other treatment options, such as dialysis.

Before a kidney transplant, a thorough medical evaluation is conducted to ensure that the person is a suitable candidate for the procedure. This includes assessing the person's overall health, identifying any underlying medical conditions that may affect the success of the transplant, and determining whether the person is compatible with the donor kidney.

The transplant surgery itself typically takes several hours and involves placing the donor kidney into the recipient's abdomen and connecting the blood vessels and ureter to the new kidney. After the surgery, the person will need to take immunosuppressive medications to prevent rejection of the new kidney by the immune system.

Kidney transplant is a major surgical procedure and involves some risks, including infection, bleeding, and rejection of the new kidney. However, with proper medical care and follow-up, many people who receive a kidney transplant are able to lead normal, healthy lives with a functioning kidney.

Kidney transplant is an effective treatment option for people with end-stage renal disease and can provide a better quality of life and longer lifespan compared to other treatment options. However, it is important to carefully consider the risks and benefits of the procedure before making a decision.