Excretory Products and their Elimination

Animals accumulate ammonia, urea, uric acid, carbon dioxide, water and ions like Na⁺, K⁺, Cl^–, phosphate, sulphate, etc., either by metabolic activities or by other means like excess ingestion. These substances have to be removed totally or partially. 

Ammonia, urea and uric acid are the major forms of nitrogenous wastes excreted by the animals. Ammonia is the most toxic form and requires a large amount of water for its elimination, whereas uric acid, being the least toxic, can be removed with a minimum loss of water. 

1.0Introduction

• The process of excreting ammonia is Ammonotelism. 
• Many bony fishes, aquatic amphibians and aquatic insects are ammonotelic in nature. Ammonia, as it is readily soluble, is generally excreted by diffusion across body surfaces or through gill surfaces (in fish) as ammonium ions. Kidneys do not play any significant role in its removal. 
• Terrestrial adaptation necessitates the production of lesser toxic nitrogenous wastes like urea and uric acid for conservation of water. Mammals, many terrestrial amphibians and marine fishes mainly excrete urea are called ureotelic animals. 
• Ammonia produced by metabolism is converted into urea in the liver of these animals and released into the blood which is filtered and excreted out by the kidneys. 
• Some amount of urea may be retained in the kidney matrix of some of these animals to maintain a desired osmolarity.
• The process of excreting ammonia is Ammonotelism. Many bony fishes, aquatic amphibians and aquatic insects are ammonotelic in nature. Ammonia, as it is readily soluble, is generally excreted by diffusion across body surfaces or through gill surfaces (in fish) as ammonium ions. Kidneys do not play any significant role in its removal. 
• Terrestrial adaptation necessitates the production of lesser toxic nitrogenous wastes like urea and uric acid for conservation of water. Mammals, many terrestrial amphibians and marine fishes mainly excrete urea are called ureotelic animals. 
• Ammonia produced by metabolism is converted into urea in the liver of these animals and released into the blood which is filtered and excreted out by the kidneys. Some amount of urea may be retained in the kidney matrix of some of these animals to maintain a desired osmolarity.

Excretion: Removal of mainly nitrogenous substances from the body which are end products of metabolic activity. 

Homeostasis : Maintenance of steady state (Walter Cannon). Homeostatic mechanisms are important for normal life as they maintain condition within a range in which the animal’s metabolic processes can occur.

Ornithine Cycle- Ammonia is converted into urea in the liver. It's also called the urea cycle.

$\underset{(Ammonia)}{2N{H}_3}+C{o}_2\to \underset{(Urea)}{N{H}_{2}CON{H}_2}+H_{2}O$

2.0Excretory Organs in Animals

3.0Human Excretory System

The human excretory system consists of : 

1. A pair of kidneys.

2. A pair of ureters.

3. A urinary bladder

4. A urethra

Kidneys and Its Location

• Mammalian kidneys are bean shaped; reddish brown coloured with a tough fibrous connective tissue covering called renal capsule.  
• Kidneys are located laterally on either side of vertebral column levels between the last thoracic (T12) and third lumbar (L3) vertebrae close to the dorsal inner wall of the abdominal cavity.  

Post Renal Urinary Tract

(i) Ureter : Urine passes from the renal pelvis into the ureter. Both the ureters open through separate oblique openings into the urinary bladder. The obliquity of the openings prevents the backflow of urine. 

(ii) Urinary bladder : Externally, the bladder is lined by detrusor muscle. It is involuntary in nature while internally the bladder is lined by transitional epithelium or urothelium. This epithelium has great capacity to expand so that a large volume of urine can be stored. The opening of the urinary bladder is controlled by sphincters made of circular muscles. In humans two sphincters are present. 

(iii) Urethra : Urinary bladder opens into a membranous duct called Urethra. The urethra leads to the end of the penis in males and into the vulva in females.

Function of Kidneys

• Excretion of metabolic waste and foreign chemicals.
• Regulation of body fluid osmolarity and electrolyte concentration.
• Regulation of acid base balance.
• Secretion of hormones like Erythropoietin and Renin.
• Regulation of arterial pressure.

Nephrons

• Nephron is the structural and functional unit of the kidney. 
• It is an epithelial tube which is about 3 cm long and 20-60 m in diameter.

Structure Of Nephron (Urniferous Tubules)

Each nephron has two parts –

(1) Glomerulus – Is a tuft of capillaries formed by the afferent arteriole (a fine branch of

renal artery). Blood from the glomerulus is carried away by an efferent

arteriole.

(2) Renal tubule – Begins with a double walled cup-like structure called Bowman’s capsule, which encloses the glomerulus.

(A) Bowman's Capsule

• At the proximal or closed end the nephron is expanded and curved inwardly to form a double walled cup shaped Bowman's capsule.
• Within the Bowman's capsule a network or tuft of capillaries is present, called Glomerulus.
• Malpighian Body : Glomerulus and its surrounding
• Bowman's capsule together forms the Malpighian body or Renal corpuscle. It is responsible for the first step of urine formation (Ultrafiltration).
• The outer wall of Bowman's capsule is composed of flattened squamous cells.
• The inner, invaginated wall that lines the concavity of Bowman's capsule is composed of a special type of cells called Podocytes. Which are arranged in an intricate manner so as to leave some minute spaces called filtration slits or slit pores.

Proximal Convoluted Tubule (PCT)

• The tubule continues further to form a highly coiled network – proximal convoluted tubule (PCT). 
• The epithelial cells of this region are specialised for transport of salts and other substances from the lumen to the interstitial fluid. It is lined by a simple cuboidal brush border epithelium. 
• The membranes of these cells facing the tubule lumen has numerous microvilli (finger like projections or Brush Borders) which increase the surface area for absorption. 

Loop of Henle

• A hairpin shaped Henle’s loop is the next part of the tubule which has a descending and an ascending limb.

Distal Convoluted Tubule(DCT)

• The ascending limb of Henle's loop continues as another highly coiled tubular region called distal convoluted tubule. This is lined by cuboidal epithelial cells.
• The DCT of many nephrons opens into a straight tube called a collecting duct. 
• Collecting ducts (present in medullary pyramids) are long tubules which traverse through the medulla in the pyramids. 
• In the papilla of the medullary pyramid, several adjacent collecting ducts converge and open into a common short and thick duct of Bellini (present in papilla of medulla). 
• All ducts of Bellini then open at the tip of the papillae into the pelvis. 
• Renal cortex – The Malpighian corpuscle, PCT & DCT of the nephrons are located here. 
• Renal medulla – Loop of Henle, collecting duct and ducts of Bellini are found in this region.

• The efferent arteriole emerging from the glomerulus forms a fine capillary network around the renal tubule called peritubular capillaries. 
• A minute vessel of this network runs parallel to the Henle's loop forming a "U" shaped Vasa recta. 

Types of Nephrons

• In Cortical Nephrons:  The efferent arterioles break up into dense Peritubular network of capillaries around their tubules.
• In Juxtamedullary Nephrons:  This vasa recta runs deep into the medulla or its pyramids surround the loop of Henle.  
• Both peritubular capillaries of cortical nephrons and vasa recta of juxtamedullary nephrons lead into venules which join and rejoin to form small and large veins, all of which ultimately join to form renal vein.

4.0Blood Flow in Kidney

5.0Mechanism Of Concentration Of The Filtrate (Counter Current Mechanism)

• The proximity between the Henle's loop and Vasa recta, as well as the counter current in them help in maintaining an increasing osmolarity towards the inner medullary interstitium. 
• Interstitium gradient of caused by NaCl & urea :-

6.0Regulation of Kidney Function

It consist of:- 

• Juxtaglomerular cells + Macula densa + Lacis / Polkissen / Mesangial cells.  
• JGA is a special sensitive region formed by cellular modifications in the DCT and the afferent arteriole at the location of their contact. 
• (i) Juxtaglomerular cells : The smooth muscle cells of the wall of both arterioles where it comes in contact with DCT are swollen and contain dark granules of inactive renin. These are called juxtaglomerular cells. 
• (ii) Macula densa : The cells of DCT epithelium in contact with the arteriolar wall are denser than other epithelial cells. These are collectively called macula densa. 

7.0Micturition

8.0Role of Other organs In Excretion

9.0Disorders Of The Excretory System

(ii) Renal failure

(iii) Renal calculi (Urolithiasis)

(iv) Glomerulonephritis

(v) Oligouria

(vi) Anuria 

(vii) Polyuria

(vii) Glycosuria 

(ix) Haematuria 

(x) Diuresis

(xi) Ketonuria

10.0Hemodialysis

• In uremia patients, urea can be removed by a process called hemodialysis. Blood drained from a convenient artery is pumped into a dialysing unit after adding an anticoagulant like heparin. 
• The unit contains a coiled cellophane tube surrounded by a fluid (dialysing fluid) having the same composition as that of plasma except the nitrogenous wastes. 
• The porous cellophane membrane of the tube allows the passage of molecules based on concentration gradient. As nitrogenous wastes are absent in the dialysing fluid, these substances freely move out, thereby clearing the blood. 
• The cleared blood is pumped back to the body through a vein after adding anti-heparin to it. This method is a boon for thousands of uremic patients all over the world.