ADVANCED EXCRETION

             

                               EXCRETION

The Concept of Excretion

              Chemical reactions which occur in the cells of living organisms all the time to carry out the life processes. The sum of these reactions is called metabolism. Metabolism produces useful products as well as toxic (poisonous) by-products.

            These toxic substances have to be removed as they are harmful if allowed to accumulate. The removal of metabolic waste products from the body of an organism is known as excretion.

             The major excretory products are carbon dioxide, excess water, and nitrogenous compounds like ammonia, urea, uric acid, etc. Carbon dioxide and water are produced in the process of tissue respiration. Nitrogenous compounds are formed from the breakdown of proteins and amino acids. Water and salts in excess of the body’s needs are also excreted.

             Other excretory products include chemicals from medicines, toxic substances, and circulating hormones that have already served their purpose. We will learn how metabolic wastes get eliminated.

               In concise, excretion is the process by which waste products of metabolism and other non-useful materials are eliminated from an organism.

Examples of Excretory Products Eliminated by Organisms

                 Give examples of excretory products eliminated by organisms

Living organisms excrete various excretory products of diverse chemical nature. The following are examples of excretory products excreted by living organisms:

Carbon dioxide

        The is a by-product of respiration of both plants and animals. It is excreted through the pores of the stomata in plants (some of the carbon dioxide produced by respiration is used in photosynthesis). In man, carbon dioxide is eliminated from the body by lungs.

Water

      The concentration of water in cells must be kept within narrow limits. Too little or too much water can have a negative effect on the osmotic condition in and around the cell. Therefore, it has to be regulated.

Plant cells are protected from bursting by their cell walls. Animals do not have cell walls, and will burst if they have too much water. Excess water is lost from the surface of gaseous exchange in both plants and animals. In mammals, water is also lost through sweat and through osmoregulation controlled by the kidneys.

          Urea:

   This is a compound produced in mammals from the breakdown of excess amino acids. Amino acids cannot be stored because their accumulation is toxic. They are therefore converted into a less toxic substance. This process occurs in the liver and is called de-amination.Ammonia is converted to urea by the liver.

  Urea is transported by blood to the kidneys where they are excreted. The kidneys are also used to remove uric acid, water, excess salts, excess hormones and bile pigments.

     Calcium oxalate:

  This is a waste material produced by plants and is stored as an insoluble crystalline structure in the cells. Calcium oxalate is stored in aging leaves, stems and roots, flowers or fruits.

     Oxygen:

   Through the process of photosynthesis, oxygen is produced as a by-product. Some of the oxygen is used for respiration, and the remainder is excreted through the stomata of the leaves.In plants, some waste substances are stored in parts of the plant that are dead. Examples of this are the tannin in the bark of trees such as mangroves and the dyes in the heartwood of trees such as logwood.

     The purpose of the storage of waste material ranges from protection to a decreased risk of being consumed.

        Kidney

    Kidneys are the bean shaped organ located behind the abdominal cavity known as retroperitoneal. Kidney has the concave notch called the hilus which is supplied with blood, lymphatic vessels and nerves enter and exit and ureter carry urine from the kidney to the gall bladder.

                                     

                                                     

     LAYERS OF KIDNEY

There are three layers of tissues which surround the kidney.

     1) Renal capsule

             T his is innermost layer which is transparent fibrous membrane (redish in colour)

   2) adipose capsule

              This is the middle layer occupied with a mass of tissues surrounding the renal capsule .

 3) Renal fasci

       This is the outer most thin layer of connective tissue that anchors the kidneys to the surrounding structure and abdominal wall.

                          RAGIONS OF KIDNEY

Kidney has three main ragions which are;

                          1) Medulla

                          2) cortex

                          3) pelvis

1) MEDULLA

     Medulla is the middle part between the cortex and pelvis which is consist of loop of henle and collecting ducting ducts. The medulla in the kidney arranged in such a way that the base of pyramid face the cortex and appex called renal papillae face the calyx

   This medulla or renal pyramid look stripped because of presence of tubules and blood vessels.

       2) CORTEX

            Cortex is the outer most ragion of kidney which is redish in colour due to high supply of blood capillaries.  The portion of the kidney cortex extend between the reanal pyramid or medulla is called  renal column

   The renal sinus of kidney has a large cavity called renal pelvis,. and the edge of pelvis contain cup like extensios Calle minor and major calyces.

   Each minor calyces receive urine from collecting duct of one phramids and deliver urine to major calyces

   From the major calyces the urine drains into renal pelvis and out through ureter into urinary bladder.

        NEPHRON

The nephron is the structural and functional unit of the kidney. There are about two million nephrons in each kidney. Nephrons begin in the cortex; the tubules dip down to the medulla, then return to the cortex before draining into the collecting duct. The collecting ducts then descend towards the renal pelvis and empty urine into the ureter.

 Nephron has three basic function;

                    1) filtration

                    2) secretion

                    3) reabsorption.

The components of a single nephron include:

    -  renal corpuscle

    -  proximal convoluted tubule

     - loop of Henle

      - distal convoluted tubule

       

   

Different sections of nephrons are located in different parts of the kidney:

The cortex contains the renal corpuscle, proximal, and distal convoluted tubules.

The medulla and medullary rays contain the loops of Henle and collecting ducts.

     Throughout the length of the nephron, capillaries called peritubular capillaries lie adjacent to all segments of the tubule. They originate from the efferent arteriole and are important for solute transport throughout the tubule.

             Renal Corpuscle

The renal corpuscle is responsible for the filtration of the plasma. It contains two structures: the glormerulus and Bowman's capsule. The glomerulus is a cluster of capillary loops enclosed by Bowman's capsule, which is part of the renal tubule.

Bowman's capsule has two layers:

The visceral layer

      Is in contact with the glormerulus, and is composed of specialized epithelial cells known as podocytes.

The parietal layer

      Is the outer layer, and is composed of simple squamous epithelial cells. This layer is continuous with the epithelium of the proximal convoluted tubule.

     The space between the two layers is named Bowman's space, and this space contains the ultrafiltrate of plasma.

     The plasma has to pass through a filtration barrier of three layers to enter Bowman's space: the capillary endothelium, the podocyte layer, and their fused basement membrane. Bowman's space is continuous with the proximal convoluted tubule.

Blood enters the renal corpuscle via afferent arterioles and then leaves via efferent arterioles. The part of renal corpuscle where afferent and efferent arterioles are located is known as the vascular pole. On the opposite end of the vascular pole is where the renal tubule begins and is known as the urinary pole.

Mesangial cells can also be found within the glomerulus. These cells secrete a matrix of basement membrane-like material to support the structure of the glomerulus.

     Promixal Convoluted Tubule

The proximal convoluted tubule is the first segment of renal tubule. It begins at the urinary pole of the glomerulus. This is where the majority (65%) of the glomerular filtrate is reabsorbed.

       The convoluted portion of the tubule leads into a straight segment that descends into the medulla within a medullary ray and becomes the loop of Henle.

         Loop of Henle

     The loop of Henle forms a hair-pin structure that dips down into the medulla. It contains four segments: the pars recta (the straight descending limb of proximal tubule), the thin descending limb, the thin ascending limb, and the thick ascending limb. The turn of the loop of Henle usually occurs in the thin segment within the medulla, and the tubule then ascends toward the cortex parallel to the descending limb.

         The end of the loop of Henle becomes the distal convoluted tubule near its original glomerulus. The loops of Henle run in parallel to capillary loops known as the vasa recta. Recall from Physiology that the loop of Henle serves to create high osmotic pressure in the renal medulla via the counter-current multiplier system. Such high osmotic pressure is important for the reabsorption of water in the later segments of the renal tubule.

Distal Convoluted Tubule

The distal convoluted tubule is shorter and less convoluted than the proximal convoluted tubule. Further reabsorption and secretion of ions occur in this segment. The initial segment of the distal convoluted tubule lies right next to the glomerulus and forms the juxtaglomerular apparatus.

Juxtaglomerular Apparatus

           The juxtaglomerular apparatus is a specialized structure formed by the distal convoluted tubule and the glomerular afferent arteriole. It is located near the vascular pole of the glomerulus.

     The main function of the apparatus is the secretion of renin, which regulates systemic blood pressure via the renin-angiotensin-alodosterone system.

The juxtaglomerular apparatus is composed of:

               1) macula densa

                2) Juxtaglomerular cells.

Macula densa

    The macula densa, a collection of specialized epithelial cells of the distal convoluted tubule. These cells are enlarged as compared to surrounding tubular cells. The cells of the macula densa sense sodium chloride concentration in the tubule, which in turn reflects the systemic blood pressure.

     Juxtaglomerular cells.

   The juxtaglomerular cells of the afferent arterioles, which are responsible for secreting renin. These cells are derived from smooth muscles cells of afferent arterioles.

The extraglomerular mesangial cells, which are flat and elongated cells located near the macula densa. Their function is currently unclear.

Collecting Ducts

     The terminal portion of the distal tubule empties through collecting tubules into a straight collecting duct in the medullary ray. The collecting duct system is under the control of antidiuretic hormone (ADH). When ADH is present, the collecting duct becomes permeable to water.

       The high osmotic pressure in the medulla (generated by the counter-current multiplier system/loop of Henle) then draws out water from the renal tubule, back to vasa recta.

      Renal Pelvis and Ureter

        Numerous collecting ducts merge into the renal pelvis, which then becomes the ureter. The ureter is a muscular tube, composed of an inner longitudinal layer and an outer circular layer. The lumen of the ureter is covered by transitional epithelium (also called urothelium). Recall from the Laboratory on Epithelia that the transitional epithelium is unique to the conducting passages of the urinary system. Its ability to stretch allows the dilation of the conducting passages when necessary. The ureter connects the kidney and the urinary bladder.

Urinary Bladder

The ureter empties the urine into the bladder. The transitional epithelium continues over the surface of this organ. The thickened muscular layers become interwoven and cannot be clearly identified at this point.

Urethra

The urethra carries the urine away from the bladder to the outside of the body. In the male, it is joined by the genital system. The epithelium changes from transitional to stratified or pseudostratified columnar in the urethra, and to stratified squamous in the distal end of the urethra.

       URINE FORMATION

Urine formation in human passes through three stages which are;

                  1) filtration

                  2)  Reabsorption

                  3) Secretion

1. Filtration

At the glomerulus there is very high pressure, thus this type of filtration is called pressure filtration.

         

The substances removed create a plasma-like filtrate in the Bowman’s capsule

Things that are filtered into the Bowman’s capsule from the blood:

               -Water

               - NaCl

               - Glucose

                - H+

                - Urea/Uric acid

Things that are not filtered into the Bowman’s capsule from the blood:

               - Plasma proteins (too big)

               - Blood cells (too big)

                - Some water, salts, glucose, amino acids, and H+ stay

      2. Reabsorption

Occurs at the proximal convoluted tubule and the Loop of Henle.

     In the proximal convoluted tubule:

Selective reabsorption: Nephron actively transports glucose, amino acids, and Na+ ions back into the blood (useful molecules – takes ATP).

Negative ions (i.e. Cl-) follow the positive ion (Na+) passively

More ions/molecules moving back into the blood concentrates the blood making an osmotic gradient (Difference in concentration between two solutions )

This causes water to reenter the blood vis osmosis.

This causes the filtrate to become concentrated as it moves through the proximal convoluted tubule.

     In the Loop of Henle:

       In the descending loop: not permeable to ions, permeable to water.

Water leaves nephron, urine becomes more concentrated

In the ascending loop: permeable to ions, not permeable to water.

Na+ leaves the nephron, fluid around descending loop becomes concentrated

This allows for more water reabsorption (back into the blood) anytime the nephron passes back into that region (even the collecting duct!)

3. Secretion

Occurs in the distal convoluted tubule (+ little in collecting duct).

Movement of waste still in blood into nephron

Active Transport: Urea, Uric acid, excess K+, vitamin C, drugs, H+.

Some water enters the urine again

The urine is now collected in the collecting ducts and carried to the bladder through the ureter for excretion.

  Regulation of  Glomerular filtration rate (GRF)

The regulation of GRF is achived by modulating blood flow in the afferent arteriole by;

                        1) Intrisic control

                        2) Hormone Regulation.

     INTRISIC CONTROL

An increase in blood pressure, increases the flow of blood the glomerulus.

The myogenic walls of the efferent arteriole responds to stetch thus reducing the diameter of the arteriole.

     EXTRISIC CONTROL

The afferent arterioles are innervated by sympathetic nervous system .

A drop in renal blood pressure causes;

       1) activation of sympathetic renal nerves

       2) low sodium ion(Na+) concentration delivery to distal convulated tubules is sensed by macula densa cells which will stimulate the Juxtaglomerular cell of afferent arterioles.

This induces the release of a protein enzymes renin from Juxtaglomerular cell located in the afferent arterioles.

      RENIN-ANGIOTENSIN SYSTEM

Renin produced from Juxtaglomerular cells cleaves angiotensinogen. Cleavage of angiotensinogen release 10-residue peptide (angiotensin I ). A blood passes through lungs

angiotensin I cleavage by angiotensin-converting enzymes to 8-residue peptide angiotensin II .

Angiotensin II cause the following action;

        - Construction of arterioles through out the body which raises the mean blood

            pressure, there by increasing the glomerulus filtration rate.

     - constriction of efferent arteriole raises glomerulur pressure and filtration rate

    - stimulate release of aldosterone from adrenal cortex and vasopressin (ADH)

      from the posterior pituitary gland.

       ALDOSTERONE

   Aldosterone acts on epithelial cells of distal convulated tubule prodding them to open Na+ channels.

  Sodium ions Na+ reabsorption occurs as it passively enters the epithelial cells. In the absence of aldosterone means that no sodium ions is reabsorbed across the distal convulated tubule cells. As sodium ions reabsorbed the high osmotic pressure of interstial fluid causes reabsorption of water.

        ATRIAL NATRIURETIC PEPTIDE (ANP)

In contrast to aldosterone which act to conserve Na+,, the atrial natriuretic peptide is the Hormone that released by the atrial cardiac cells when blood volume or blood osmotic pressure is elevated inhibt Na+ reabsorption by closing Na+ channels.

      There fore the atrial natriuretic peptide has the following actions;

         1) Reduce water reabsorption and blood volume

         2) Increase urinary production and Na+ excretion

        3) Inhibit renin release ;

                           a) Circulating angiotensin II

                           b) Circulating aldosterone.

VASOPRESSIN ( ANTDIURETIC HORMONE)

      The blood ADH level is function of

               - increasing the blood level of ADH

               - reabsorption of water from DCT.

     ADH increase the permiability of epithelial cells and water is osmotically drwan from the lumen at a high rate, hence hyperosmosis urine is produced.

The hypothalamus neuroscretory cells that produce ADH receive inhbitory input from arterial and atrial baroreceptors that respond to change in blood pressure.

EXCRETION OF OF NITROGENOUS WASTE

    When amino acid are catanolized, the amino group is released or transferred to another molecules for removal.

 In most animals there is a link between osmoregulatory functions and function involved in the elimination of excess nitrogen.

- The waste amino groups are excreted in one of the three forms;

                1) ammonia

                2) Urea

               3) uric acid

           

Most teleost produce little or no urea, but instead excrete their nitrogenous waste primary as ammonia through the gills. This approach is feasible in aquatic environment, but not in most land animals, since ammonia is highly toxic and soluable.

   The toxcity of ammonia is due to evelation of pH which causes changes in the tertiary structure of protein.

  Large quantities of water is required to dissolve and carry off ammonia ~300-500ml for 1g of nitrogen.

                      Urea

  Although urea is quite soluable in water, it is far less toxic than ammonia ~ 50ml of water is required to excrete 1g of nitrogen. Urea contain 2 nitrogen atom of molecules

                    Uric acid

Uric acid is the choice of most of birds and reptiles, it has advantage of carring away 4 nitrogen atom atoms per molecules. Because it is less soluable in water it can be excreted as apasty precipitate with only 10ml of water required to excrete 1g of nitrogen.

     

                    Ammonotelic  ( ammonia excreting)

Most fresh water teleost are ammonotellic, the amino group of various amino acid is transferred with the aid of an enzymes transminase to glutamate which is converted to glutamine

      Glutamine serve as the amino group carrier in the blood, within the cells of the kidney tubule, glutamine is deaminated- where one amino group is removed to yield glutamate and ammonium ion (NH4+)

      The NH+ dissociate within the cell to NH3 and H+. NH3 crosses the cell membrane into the urinary by diffusion. H+ is removed from the cell by counter-transport against Na+ into the lumen of the kidney tubule.

       In the kidney tubule NH3 can take a portion H+ to form ammonium ion NH4+ which cannot diffuse back into the tissues, so it leaves via the urine. In teleost the NH3 is also excreted by the epithelium of the gills in exchange for Na+ which is actively excreted via the gills.

             Uricotelic

Uricotelic animals excrete ammonia chiefly as uric acid.

-Nitrogen atoms arise from the breakdown of amino acids as incorporated into uric acid

-since these animals do not have uricase enzymes to break down the uric acid, the catalysis of Nitrogen molecules is terminated.

     Uric acid precipitates because of it's low solubility and does not contribute to the tonicity of urine or feces and thus excretion require little water. Means that adapted to low availability of water.

  UREOTELIC ( UREA EXCRETION).

UREOTELIC animals excrete nitrogenous wastes through the uricolytic pathway in case of some marine teleost and ornithine cycle by mammals. Some marine teleost have the enzymes uricase in which the uric acid is converted to allantoin and finally into urea.

      In mammal the synthesis of urea take place in Liver. 2 amino group and 1 molecules of CO2 are added to ornithine to form arginine. The urea molecules is removed with the aid of the enzymes arginase, the formation of urea enable elimination of two wastes products CO2 and NH3