Sep 27, 2014

Excretion in Vertebrates

Among the vertebrates the main organ that performs the function of excretion is the kidney. Kidney can function with modification in fresh water, in the sea and oh land. The ancestors of vertebrates, the invertebrate chordate have segmental-shaped arranged excretory structures throughout the body like the metanephridia in earthworm. The primitive vertebrate hag fishes have kidney with segmental-form arranged tubules. Kidneys contain numerous tubules not arranged segmental.

Excretion in Man

Excretory organs in man
Metabolic wastes: Metabolic wastes are the substances produced within the body by various chemical processes. The wastes of the digestive process are removed i.e. eliminated. The only true excretory products that are removed from the body during elimination are bile pigment and salts of certain minerals. Bile pigments are called bilirubin produced by breakdown of old worn out red blood cells and metabolites of various hormones. Nitrogen containing molecules or nitrogenous wastes includes urea, produced from deamination of amino acids. Uric acid is formed from the breakdown of nucleic acids. Creatinine is derived from a nitrogen containing molecule called creatine in the muscle cells.

Toxin substances such as pesticides, drugs, food additives ingested into the body, and carbon dioxide and water vapor produced during respiration are also metabolic wastes. The waste present in the body cause serious hazards, thus are eliminated by excretory system.

Excretory Organs: Kidneys, lungs, skin and liver are the structures for the elimination of metabolic waste products.

Urea: Urea is the primary nitrogenous waste product of humans and other living mammals. The advantages of using urea as a nitrogenous waste product are:
(i) Non-toxic: It can therefore be carried round the body in the blood from the liver until it is removed by the kidneys.

(ii) Very soluble: It does not require a great deal of water to get rid of it and it is easily transported.

(iii) A small molecule: It is easily filtered in the kidneys. Ammonia is converted into urea in the liver by a cyclic reaction known as ornithine cycle.

In ornithine cycle, two molecules of ammonia and one molecule of carbon dioxide are used. One ammonia molecule combines with CO2 and already available precursor from previous cycle ornithine to form citrulline, subsequently ammonia combines to form arginine. One molecule of water is made (two made, one used). The arginine is split by arginase to form one molecule of urea. Ornithine is regenerated ready for the next cycle.

Urea is transported in the blood plasma from the liver to the kidneys. The metabolic pathways involved in the production of urea are called urea cycle.

Liver as a Homeostatic Organ

The major homeostatic roles of liver are:
(a) Carbohydrate metabolism: Inside the liver cells the glucose is either built up into glycogen for storage or broken down into CO2 and water with the release of energy. Considerable excess of glucose is converted into lipids.

(b) Fat metabolism:

 (1) Liver converts excess of carbohydrates to fats.
(2) Liver removes excess of cholesterol from the blood and breaking it down or when necessary synthesizing it. If glucose is in short supply, the liver cells breakdown fats into fatty acids and glycerol for respiration.

(c) Detoxification: Detoxification means the removal of toxins or poisons. Detoxification is part of homeostasis and helps to maintain the composition of blood in a steady state. The liver cells detoxify drugs, poisons, chemicals, additives, pesticides, alcohol, nicotine etc. by absorbing them and then changing them chemically. The major toxic substance is ammonia which is converted into urea.

(d) Deamination and urea formation: Excess of amino acids, brought to the liver through the hepatic portal vein, is deaminated by the liver cells. The amino group is converted to ammonia by the specific enzymes in the liver cells. Ammonia enters ornithine cycle in which it reacts with CO2 to form urea. The urea is then shed from the liver cells into the blood stream, and taken to the kidney which eliminates it from the body through urine.

(e) Storage of vitamins: The main vitamins stored in the liver are fat soluble vitamins A, D, E, and K. The liver also stores water soluble vitamin B and C especially those of the B group such as nicotinic acid, vitamin B12 and folic acid.

(f) Breakdown of red blood cells: Red blood cells have a life span of 120 days. They are then broken-down by phagocytic macrophage (special WBCs) cells in the liver, spleen and bone marrow. Inside the macrophages hemoglobin is broken down into heme and globin. Globin is broken down into amino acids. The iron is removed from heme. The remaining part of the molecule is converted to bilirubin, which is yellow and a component of bile. The accumulation of bilirubin in the blood causes jaundice. The iron may be then reused by the cells in the bone marrow to make more hemoglobin, or iron may be stored in the liver. The yellow pigment found in urine called urochrome, is also derived from the break down of heme, but this pigment is deposited in the blood and subsequently excreted by the kidneys.

Urinary System in Man

Excretory system in man
There is a pair of kidneys just above the hipbone, at the back of the abdominal cavity, one on each side of the spine against the abdominal wall. The left kidney lies slightly above the right. The kidneys are red brown in color and bean shaped. The kidneys receive blood from the aorta via renal arteries and the renal veins return blood to the inferior vena cava. Urine formed in the kidneys passes by a pair of ureter to the bladder where it is stored until it is released via the urethra. Two sphincter muscles surround the urethra where it leaves the bladder, one of which is under voluntary controls. The process of control release of urine is known as urination.

Internal Structure of Kidney

A sagittal section of the kidneys shows two distinct regions, an outer darker the cortex and an inner lighter region the medulla. The cortex is covered by tough transparent membrane the capsule. The cortex and medulla of each kidney has nephrons. The medulla is composed of tubular part of the nephron and blood vessels, which together form the renal pyramids. All the pyramids project into the pelvis which leads into the ureter.
Gross anatomy of kidney

Nephron: The nephron is the structural and functional unit of kidney. There are about one million nephrons in each kidney. There are two type of nephrons (a) cortical nephron (b) juxtamedullary nephron.

Cortical nephron: The nephrons arranged along the cortex are known as cortical nephron, have relatively short Loop Of Henle. Juxtamedullary nephron (juxta means, close): These nephrons have their renal (glomerular) capsule close to the junction of the cortex and medulla.

They have long loop of Henle which extend deep into the medulla. The two types of nephrons have different uses. Under normal conditions of water availability the cortical nephrons deal with the control of blood volume, whereas, when water is in short supply, increased water retention occurs, through the juxtamedullary nephrons.

Structure of Nephron

The nephron consists of a long tubule. It is coiled. It is closed at one end and open at the other. At the close end of the tubule in the cortex, the wall of the nephron is expanded and folded into a double walled cup-shaped chamber, the Bowman’s capsule, within the infolded portion of the Bowman’s capsule is a network of capillaries the glomerulus (pl: glomeruli). The tubule consists of:
structure of nephron

(a) Proximal tubule
(b) Loop of Henle
(c) Distal tubule
(d) Collecting duct.

Proximal Tubule: It begins from the Bowman’s capsule. It is highly coiled. This part of the nephron is called proximal tubule. It is present in the cortex.

Loop of Henle: The tubule then plunge down into the medulla, forms a sharp loop, then again straighten up and return to the region of Bowman’s capsule. It is thin walled region of the nephron.

Distal tubule: At the end of the loop, the nephron becomes highly coiled. This part of the nephron is the distal tubule.

Collecting duct: The nephron finally straightens out and merges into a collecting duct. Collecting ducts join with even larger ducts, which eventually empty into the renal pelvis.

Peritubular Capillaries: Blood travels from the renal artery, into afferent arterioles, then into the glomerular capillaries. These capillaries do not merge into veins. Rather, they merge into efferent arterioles (so-called because it conducts blood away from the glomerulus) which divide into a second set of blood capillaries, known as Peritubular capillaries. In nephrons that are positioned almost entirely in the cortex, peritubular capillaries thread profusely around proximal tubule, the loop of Henle, and the distal tubule. From there, the peritubular capillaries merge into renal veins that lead away from the kidney. In juxtamedullary nephrons additional capillaries extend down to form a loop of vessels called vasa recta.

Functioning Of Kidney

 The function of the kidney is to form urine. It involves three processes in the nephrons: glomerulus filtration, tubular re-absorption and tubular secretion.

Glomerular filtration: In filtration all compounds of the blood except blood cells and most proteins can move into the glomerulus. From there the filtrate passes into the nephron tubule. Active transportation is not involved. Hydrostatic pressure in the glomerulus created by the heart contractions is greater in the capillaries than the fluid tissues pressure in the tubule. The pressure drives out a certain amount of fluid and solutes from the blood. Thus the relatively high pressure difference across the glomerulus is the reason of filter of fluid into the tubule of the nephron. The glomerular filtrate contains sugar, amino acids, salts, nitrogenous wastes specially urea and other dissolved substances.

Tubular re-absorption: During re-absorption, about 90% of the water and most of the solutes that enter the nephron are returned to the bloodstream. As the filtrate flows along the nephron, selective re-absorption rapidly alters its composition. For instance, sodium ions are actively transported across the proximal tubule walls and into interstitial fluid. Chloride ions being negatively charged are attracted by the positively charged sodium ions and passively follow them in the same direction. On the average 75% of the Na and Cl ions present in the filtrate leaves the proximal tubule. The outward solute movement sets up an osmotic gradient and water passively move out. The solutes and water flow into capillaries surrounding the tubule. Thus, although re-absorption into capillaries is bulk flow process, water is reabsorbed only because solutes are reabsorbed first. In loop of Henle H2O and Na is reabsorbed.

Tubular secretion: Some substances appear in the urine in greater amounts than were filtered into the nephron. They are actively transported from the peritubular capillaries into the nephron tubule, an event known as tubular secretion. These substances include K and H ions, drugs and some toxic substances. These include many metabolic wastes, such as uric acid, creatinine and assorted products of hemoglobin breakdown.

IMAGE: Filtration, re-absorption and secretion in a nephron

Hormonal Control of Kidney Function

Antidiuretic hormone (ADH): (Diueresis, increased excretion of urine). It is also called vasopressin. When ADH is released from the posterior pituitary, it is picked up by the blood stream and transported to the kidneys. ADH acts on cells of the distal tubules and collecting ducts making them more permeable to water. With increased ADH secretion, more water is reabsorbed. As a result, urine volume decreases, and its salt concentration increases and fluid volume leaving the body is more diluted.

Aldosterone: It is a hormone produced and secreted by the adrenal glands and has major influence over sodium re-absorption. The kidneys themselves help to control aldosterone secretion.

Counter Current Mechanism

It is in the interest of terrestrial animals to conserve water. In mammals, this function depends on loop of Henle. Mammals are the only vertebrates which can produce markedly hypertonic urine. The loop of Henle concentrates sodium chloride in the medulla of the kidney. This then causes osmotic flow of water from the collecting ducts there by concentrating the urine and making it hypertonic to blood. In achieving this loop of Henle follows the principle of countercurrent multiplier.

Counter current mechanism
As the renal fluid flows along the ascending limb, salt is actively removed from it and deposited in the surrounding tissue fluid. From there the salt diffuses into, and equilibrates with, the fluid in the descending limb. This active transfer of salt takes place at all levels of the loop of Henle. At any given level the effect of this is to raise the concentration in the descending limb above that in the ascending limb. The effect at any one level is slight, but the overall result is multiplied by the length of the loop. As the renal fluid flows down the descending limb towards the apex of the loop it becomes more and more diluted. As the renal fluid passes down the collecting duct water passes out of it by osmosis. This raises its solute concentration. Meanwhile, the water is taken away in the blood stream. The characteristics U- shape of loop of Henle provides a counter current system which establishes and maintains this high concentration.

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