Chap 19 Excretory products and their elimination

Definition:

Removal of nitrogenous non gaseous wastes like ammonia, urea, uric acid etc along with excess of water salts and pigments out of body is called excretion.

The main of excretion is to maintain homeostasis.

Need:

Various types of wastes are produced by the living systems during the metabolic processes. Some of the major wastes are:

1. Nitrogenous wastes such as ammonia, urea, uric acid etc due to protein digestion
2. CO₂ from respiratory processes
3. Water from respiratory processes
4. Ions (Na⁺, K⁺ Cl^- phosphates, sulphates etc) from excess intake or metabolism
5. Pigments due to metabolism of haemoglobin from dead RBCs

It is important to remove these wastes completely or partially.

The removal of nitrogenous wastes is stressed more since these are toxic for the living systems.

Types of Excretion:

Nitrogenous wastes can be excreted as ammonia, urea or uric acid. On this basis, there can be three methods of excretion.

1. Ammonotelism:
• Nitrogenous waste excreted as ammonia
• Such animals known as ammonotelic.
• Eg. Aquatic animals such as bony fishes, aquatic amphibians, aquatic insects. Primitive organisms such as protozoans, coelenterates.
• Ammonia is formed as a digestive product of proteins in the liver.
• It is highly toxic due to its high pH.
• It is highly soluble in water so large amount of water is needed for its elimination leading to loss of 300-500ml water per gram ammonia.
• Thus there is a correlation between ammonotelism and aquatic life.
• Ammonia is excreted by diffusion across body surfaces or through gill surfaces as ammonium ions.
• No important role of kidneys.

2. Ureotelism:
• Main nitrogenous waste is urea.
• Such animals known as ureotelic
• Eg. Land animals that can afford to lose large amounts of water such as terrestrial amphibians (frogs) OR animals that can maintain higher concentrations of urea. Also marine fishes, mammals (including humans).
• Urea is used in the latter category for maintaining osmolarity.
• In humans, 80-90% of total nitrogenous waste is ammonia.
• Ammonia formed during metabolism is converted into urea in liver by ammonia cycle.
• It is released in to blood, undergoes filtration and is excreted through kidneys.
• Some urea is retained in the kidney matrix to maintain blood osmolarity.
• Urea has lesser toxicity then ammonia and requires for excretion only 50ml water per gram urea.
• Thus ureotelism enables conservation of water and hence is associated with terrestrial mode of life.

Significance of ureotelism over ammonotelism:

1. Urea is 100,000 times less toxic than ammonia
2. it can be retained for longer duration without harm
3. needs less water for elimination

3. Uricotelism:
• Elimination of nitrogenous wastes as uric acid.
• Such animals are known as uricotelic.
• Eg. Land reptiles (lizards and snakes), birds, land snails and insects.
• Uric acid is formed from ammonia and purines in liver. In insects it is formed in malpigian tubules.
• Uric acid eliminated as a pellet or paste with minimum water loss

Significance of uricotelism

Uric acid is least soluble form of nitrogenous waste. Only 10ml of water is required for expulsion of 1 gram of uric acid. It is also the least toxic hence can be retained in body for longer duration without causing harm. So it is highly advantageous method of excretion for animals that have limited access to water. It is highly suitable adaptation for terrestrial mode of life.

S No.	Animal Group	Main form of nitrogenous waste	Excretory system
1.	Protozoa, Porifera & Coelentrata	Ammonia	Diffusion through cell membrane
2.	Platyhelminthes	Ammonia, fatty acids	Body surface, protonephridia (or flame cells)
3.	Nematoda	Ammonia	Body surface & Renett cells
4.	Annelida	Ammonia, urea in land forms	Nephridia
5.	Arachnida	Guanine	Coxal glanda, malpighian tubules
6.	Crustacea	Ammonia	Antennary or green glands
7.	Insecta	Uric acid in land, ammonia in aquatic forms	Malpighian tubules
8.	Mollusca	Uric acid in land, ammonia in aquatic forms	Kidneys (metanephric system)
9.	Echinodermata	Ammonia	Papulae, podia
10.	Vertebrata	Ammonia, urea, uric acid	Kidneys

HUMAN EXCRETORY SYSTEM

            Comprises of:

• A pair of kidneys
• A pair of ureters
• Urinary bladder
• Urethra

Figure 19.1

1. Kidneys: Primary or major respiratory organs.

• Reddish brown
• Bean shaped
• Large size: length 10-12cm, width 5-7 cm, thickness 2-3 cm. weight 120-170 gm.
• Present in upper part of abdominal cavity, close to dorsal inner wall of abdominal cavity.
• Located between levels of last thoracic and second lumber vertebra, one on each side of vertebral column.

Figure 19.2

The parts of kidney include

• Hilum or the notch at the centre of the inner concave surface of kidney. Ureter, blood vessels and nerves enter kidney through hilum.
• Renal Pelvis: broad funnel shaped space present inner to hilum. It has projections known as Calyces (singular calyx).
• Capsule or the outer tough layer of the kidney.

There internal organization of the kidney comprises into two parts:

a)      Outer Cortex

b)      Inner Medulla

Medulla is divided in to two conical masses or medullary pyramids that project in to calyces.

In between the medullary pyramids, portions of cortex form renal columns known as Columns of Bertini.

Nephrons: Functional units of kidney.

The kidney is made of almost 1 million complex tubular structures called nephrons.

Function: To form urine

2. Ureters: A pair of long narrow tubular structures, each arising from hilum of kidney.

Function: By peristalsis conduct urine from kidneys to urinary bladder.

3. Urinary Bladder: Large thin walled pear shaped sac present in the pelvis region of abdomen.

·        The upper broader part of the bladder is known as body of bladder and it stores urine.

·        The lower narrow par known as neck of bladder is the site of origin of urethra. It is guarded by two sphincters.

4. Urethra: muscular and tubular structure extending from neck of bladder to outside. It is responsible for the discharge of urine.

Structure of Nephron

Structural and functional units of kidneys

Figure 19.3

The nephron can be divided in to two parts:

1.      Glomerulus

2.      Renal Tubules

The glomerulus is a cluster of capillaries formed by the afferent arteriole, which is a smaller branch of renal artery. Blood from glomerulus is carried away by efferent arteriole.

The renal tubule comprises of:

·         Bowman’s capsule: a double walled cup like structure, enclosing the glomerulus. Glomerulus along with Bowman’s capsule is known as Malpigian Body or Renal Corpuscle.

·         Proximal Convoluted Tubule (PCT): The Bowman’s capsule proceeds in to the PCT.

·         Henle’s Loop: The PCT next forms the Henle’s loop which has a descending limb and an ascending limb.

·         Distal Convoluted Tubule (DCT): The ascending limb of Henle’s loop forms a highly coiled tubular region called DCT.

The Malpighian body, along with PCT and DCT are in the cortex of kidney, whereas the Henle’s loop is in the medulla.

On the basis of length of Henle’s loop, the nephrons can be of two types:

1. Cortical nephrons: In about 85% of nephrons, the Henle’s loop is short and extends very little in the medulla.
2. Juxta-Medullary Nephron: In about 15% of the nephrons the loop of Henle is longer and runs deep in to the medulla.

Renal Blood Supply

Each kidney receives blood supple from the renal artery branching from abdominal aorta. Within the kidney, the renal artery divides in to many afferent arterioles. One afferent arteriole enters a Bowman’s capsule and divides in to a tuft of capillaries known as glomerulus.

From glomerulus the blood is drained by efferent arterioles. The efferent arterioles break up in to peritubular network of capillaries. The peritubular capillaries join and form the renal vein.

Abdominal aorta→Renal Artery→Afferent artery→glomerulus→efferent artery→peritubular network→renal vein

The efferent arteriole also forms U shaped blood capillaries close to loop of Henle, known as Vasa recta. Vasa rectae help in counter current mechanism of urine concentration.

URINE FORMATION (UROPOEISIS)

All the body cells produce nitrogenous wastes that are carried by blood to the kidney. Inside the kidney they are converted to urine by three processes:

1. Glomerular Filtration or Ultrafiltration
2. Reabsorption
3. Secretion

1. Glomerular Filtration or Ultrafiltration:

• Filtration of blood carried out by glomerulus.
• It is completely passive and non selective process.
• 1100 – 1200 ml of blood filtered by kidneys per minute. This is 1/5 of the total volume of blood pumped by each ventricle in a minute.
• The blood pressure in capillaries of glomerulus is twice that in other capillaries since the efferent arteriole is much narrower than the afferent arteriole.
• At a pressure of 60mm of Hg the blood undergoes ultrafiltration through three layers:

ü  Glomerular capillary endotheliu m with numerous pores known as fenestrae

ü  The epithelium of the Bowman’s capsule which is formed of two layers of cells: the outer or the parietal layer and the inner or visceral layer. The visceral layer is formed of special cells known as podocytes or foot cells because of their feet like structures. The feet like structures are known as pedicels. They have millions of minute pores known as slit pores that allow the ultrafiltrate to pass through.

ü  The basement membrane between these two layers.

·         The above three membranes together form the filtering or dialyzing membrane that acts as an ultrafilter and is responsible for ultrafiltration.

·         The difference between the pressure at which the blood enters the glomerulus and the pressure that resist it, is known as glomerular filtration pressure (GFP).

·         The GFP causes filtration of:

ü  Water (about 70 litres a day)

ü  Small soluble molecules such as glucose, amino acids, vitamin C, Na+

ü  Harmful substances such as urea, uric acid, creatinine, ammonium salts, pigments, K+ etc.

·         Blood corpuscles, proteins, fats etc. are not filtered out.

·         The filtrate obtained after glomerular filtration is known as glomerular or capsular filtrate, nephric filtrate, ultrafiltrate, or primary urine. It is isotonic to blood plasma.

Ultrafiltrate = Blood – cells – proteins

·         The amount of filtrate formed by the kidneys per minute is called glomerular filtration rate (GFR). GFR in a healthy individual is approximately 125ml/min or 180 litres/day.

Autoregulation of Ultrafiltration: The kidneys have built-in mechanism of regulation of GFR. The GFR is regulated by 3 modes:

1. GFP: GFR is directly proportional to  GFP
2. Juxtaglomerular Apparatus (JGA): it is a special sensitive region formed by cellular modifications in distal convoluted tubule and efferent arteriole at the site of their contact.

The cells of JGA secrete an enzyme Renin that regulates blood pressure, renal blood flow, and rate of Ultrafiltration. A fall in GFR activates the JGA cells to release more rennin causing increased blood flow and raising GFR.

3. Nervous Control: Sympathetic nerve fibres of ANS stimulate vasoconstriction of renal arteries which decreases renal blood flow and GFR.

Selective Reabsorption: The primary urine or ultrafiltrate produced after Ultrafiltration is subjected to reabsorption. The volume of ultrafiltrate produced in the body is 180 l/day, while the amount of urine produced is 1.5 litres. Thus approximately 99% of the ultrafiltrate is reabsorbed by selective reabsorption.

The ultrafiltrate passes through renal tubules by ciliary action. As it passes, the following substances are reabsorbed:

• 99% of water
• whole of glucose
• amino acids
• most of Na+ and Cl-
• some urea and uric acid

Two mechanisms are involved in selective reabsorption:

1. Back diffusion or passive reabsorption: It is a passive process and occurs along the concentration gradient. It is not energy dependent and is therefore a slow process.
• Water and nitrogenous wastes are reabsorbed by this mechanism.
• Urea is reabsorbed mainly in PCT.
• Quantity of water reabsorbed depends on the body needs and environmental temperature.

2. Active reabsorption: It is an energy requiring mechanism that occurs against the concentration gradient. Rapid process.

Glucose, amino acids, ions etc are reabsorbed by this mechanism.

Tubular Secretion:

Highly selective process.

·         Glandular cells of nephrons present in PCT and DCT, extract certain molecules from the peritubular capillaries and return them to ultrafiltrate.

·         This process involves active transport.

·         The substances secreted include uric acid, creatinine, K+, H+, ammonia etc.

·         Tubular secretion helps in maintenance of ionic and acid balance of body fluids.

FUNCTIONS OF THE TUBULES

Proximal Convoluted Tubules:

• PCT is lined by simple cuboidal brush border epithelium that increases the surface area for absorption.
• Almost all the essential nutrients and 70-80% of water and electrolytes are reabsorbed.
• Tubular secretion is also performed here. PCT secretes ions, and ammonia in to primary urine. This helps in maintaining ionic and pH balance of the filtrate.
• This helps in maintaining ionic and pH balance of the filtrate.

Henle’s Loop:

• Minimal reabsorption
• It plays significant role in maintaining high osmolarity of medullary interstitial fluid.
• Comprises of two regions:

ü  Descending Limb: Permeable to water but almost impermeable to electrolytes. Concentrates filtrate.

ü  Ascending Limb: impermeable to water but allows both active and passive transport of electrolytes.

Thus as concentrated fluid passes upward it gets diluted due to passage of electrolytes in to the medullary fluid.

Distal convoluted tubule:

·         Involves conditional reabsorption of Na⁺ and water.

·         Reabsorption of HCO₃^- ions.

·         Selective secretion of H and K ions; and ammonia to maintain pH and sodium potassium balance in blood.

Collecting Duct:

·         Extends from cortex of kidney to medulla.

·         Reabsorption of large amounts of water to produce concentrated urine

·         Allows passage of small amounts of urea in to medullary interstium to maintain osmolarity.

·         Maintains pH and ionic balance by selective secretion of K and H ions.

Mechanism of concentration of filtrate:

A counter current mechanism is used for production of concentrated urine.

2 parts involved:

·         Henle’s loop

·         Vasa recta

Flow of urine in ascending and descending limbs of Henle’s loop is in opposite direction. This forms a counter current.

Flow of blood in two limbs of vasa recta is also in opposite direction; forming a counter current.

The region between cortex and inner medulla medullary interstitium develops an osmolarity gradient from 300 in cortex to 1200 in inner medulla. Two factors are responsible for this gradient:

·         Counter current mechanism

·         Proximity of Henle’s loop and vasa recta

The two compounds responsible for the osmotic gradient are:

·         Urea and

·         NaCl

These two mechanisms help in maintenance of concentration gradient of medullary interstitium. This interstitial gradient is helpful in easy passage of water from collecting tubule. Thus primary urine is concentrated to almost 4 times to form urine.

Regulation of Kidney Function:

Feedback mechanism involving:

·         Hypothalamus (Brain)

·         JGA

·         Heart

Hypothalamus: Our body contains osmoreceptors that are activated by changes in blood volume, body fluid volume and ionic concentration.

During situation of fluid loss these receptors are activated.

They stimulate Hypothalamus

The hypothalamus causes pituitary gland to release ADH (antidiuretic hormone) or vasopressin from its neurohypophysis.

 ADH effects kidney function by two methods:

1.      It causes water reabsorption from the latter part of renal tubule.

This leads to rise in body fluid volume. And the osmoreceptors are switched off; ADH release is thus suppressed.

This is how feedback mechanism is completed.

2.      It causes constriction of blood vessels; leading to rise in blood pressure.

Higher blood pressure leads to rise in glomerular blood flow and thus increase in GFR

JGA: Complex regulatory role: Renin Angiostenin mechanism

·         Fall in glomerular blood flow

·         Fall in glomerular blood pressure

·         Fall in GFR

·         JGA cell activated to release Renin

·         Renin converts angiotensinogen in blood to angiotensin I and then to angiotensin II.

·         Angiotensin II is a powerful vasoconstrictor. It increases blood flow, blood pressure and thus GFR.

·         Angiotensin II also causes release of aldosterone from adrenal cortex

·         Aldosterone causes reabsorption of water and Na⁺ ions from DCT

Renin Angiostenin mechanism

HEART: rise in blood flow to atria of heart leads to release of ANF (atrial Natriuretic Factor). This causes vasodilation, thus reducing blood pressure and GFR.

This mechanism acts as a check on Renin Angiostenin mechanism.

Micturition

Process of release of urine

Control by neural mechanisms. Neural mechanisms causing micturition are called micturition reflex.

Process

·         Urine formed in nephrons carried to urinary bladder by ureter

·         Stored in urinary bladder till a voluntary signal given by CNS – Micturition Reflex (figure below)

·         The signal is initiated by stretching of urinary bladder as it gets filled with urine

·         The effector are smooth muscles of bladder

·         The smooth muscles contract along with simultaneous relaxation of urinary sphincter

·         This causes release of urine

Characteristics of human urine:

·         1 to 1.5 litres /day (with 25-30 gms urea)

·         Light yellow

·         Watery

·         pH slightly acidic (6.0)

·         Various conditions effect urine characteristics and so urine is used for clinical diagnosis of many metabolic disorders. E.g. in diabetes mellitus urine shows Glucose (Glycosuria) and ketone bodies (ketonuria)

Accessory organs of excretion

1.      Lung: CO₂ (upto 18 litres/ day) and water.

2.      Liver: Acts as an excretory organ. Toxins, drugs and alcohol are broken down in the liver for excretion. Bile pigmnets; bilirubin, bilverdin also pass out with digestive wastes and urine.

3.      Skin: sweat and sebaceous glands in skin help in excretion

Sweat contains watery fluid with NaCl, urea, lactic acid etc which are eliminated from the body.

Sebaceous glands excrete sterols, hydrocarbons and waxes along with sebum.

   

Disorders of excretory system