Chapter Notes

Excretory Products and their Elimination

Our bodies, like those of all animals, produce waste products through metabolic activities or by taking in more substances than we need. These wastes include ammonia, urea, uric acid, carbon dioxide, water, and ions like $\text{Na}^{+}$, $\text{K}^{+}$, $\text{Cl}^{-}$, phosphate, and sulphate. The process of removing these substances, either completely or partially, is called excretion. This is vital for maintaining a stable internal environment.

Nitrogenous Wastes

The major nitrogenous (nitrogen-containing) wastes excreted by animals are ammonia, urea, and uric acid. The type of waste an animal produces is often related to its habitat and the availability of water.

• Ammonia: This is the most toxic nitrogenous waste and requires a large amount of water to be eliminated safely.
• Urea: This is less toxic than ammonia and requires less water for removal.
• Uric Acid: This is the least toxic and can be excreted as a paste or pellet with very little water loss, making it ideal for animals in dry environments.

Based on their primary nitrogenous waste, animals are classified into three groups:

1. Ammonotelism (Ammonia Excretion): Animals that excrete ammonia are called ammonotelic. This group includes many bony fishes, aquatic amphibians, and aquatic insects. Since ammonia is highly soluble in water, it is easily excreted by diffusion across body surfaces or through gills (as ammonium ions). Kidneys play a minor role in its removal.
2. Ureotelism (Urea Excretion): Animals that excrete urea are called ureotelic. This adaptation helps conserve water and is found in mammals, many terrestrial amphibians, and marine fishes. In these animals, the liver converts toxic ammonia into less toxic urea. The urea is then released into the blood, filtered by the kidneys, and excreted in urine.
3. Uricotelism (Uric Acid Excretion): Animals that excrete uric acid are called uricotelic. This is the most water-efficient method of excretion. Reptiles, birds, land snails, and insects excrete uric acid as a paste or pellet, minimizing water loss.

Excretory Structures in the Animal Kingdom

Different animals have evolved various structures for excretion and osmoregulation (the regulation of ionic and fluid volume).

• Protonephridia (Flame Cells): These are simple tubular structures found in invertebrates like Platyhelminthes (e.g., Planaria), rotifers, some annelids, and the cephalochordate Amphioxus. They are primarily involved in osmoregulation.
• Nephridia: These are the tubular excretory organs in earthworms and other annelids. They help remove nitrogenous wastes and maintain fluid and ionic balance.
• Malpighian Tubules: Found in most insects, including cockroaches, these structures are involved in removing nitrogenous wastes and osmoregulation.
• Antennal Glands (Green Glands): These perform the excretory function in crustaceans like prawns.
• Kidneys: Vertebrates have complex tubular organs called kidneys for excretion.

Human Excretory System

In humans, the excretory system is composed of:

• A pair of kidneys
• A pair of ureters
• A urinary bladder
• A urethra

Kidneys

Kidneys are the primary excretory organs in humans. They are reddish-brown, bean-shaped organs located in the abdominal cavity, close to the dorsal inner wall, between the last thoracic and third lumbar vertebrae.

• Size and Weight: An adult kidney measures $10-12 \text{ cm}$ in length, $5-7 \text{ cm}$ in width, and $2-3 \text{ cm}$ in thickness, with an average weight of $120-170 \text{ g}$.
• Hilum: The inner, concave surface of the kidney has a notch called the hilum, through which the ureter, blood vessels, and nerves enter and exit.
• Internal Structure: Inside the tough outer capsule, the kidney is divided into two main zones:
  • Cortex: The outer zone.
  • Medulla: The inner zone. The medulla is divided into several conical masses called medullary pyramids. The cortex extends between these pyramids as renal columns, also known as the Columns of Bertini. The pyramids project into funnel-shaped spaces called calyces (singular: calyx), which open into a broader space called the renal pelvis.

The Nephron: Functional Unit of the Kidney

Each kidney contains nearly one million complex, tubular structures called nephrons, which are the functional units responsible for forming urine. Each nephron has two main parts: the glomerulus and the renal tubule.

1. Glomerulus: This is a tuft of capillaries formed by the afferent arteriole (a branch of the renal artery). Blood is carried away from the glomerulus by the efferent arteriole.
2. Renal Tubule: The tubule begins with a double-walled, cup-like structure called Bowman's capsule, which encloses the glomerulus.
   • The glomerulus and Bowman's capsule together are called the Malpighian body or renal corpuscle.
   • The tubule continues as a highly coiled network called the proximal convoluted tubule (PCT).
   • Next is a hairpin-shaped loop called Henle's loop, which has a descending and an ascending limb.
   • The ascending limb continues as another coiled region called the distal convoluted tubule (DCT).
   • The DCTs of many nephrons open into a straight tube called the collecting duct. Many collecting ducts converge and open into the renal pelvis through the medullary pyramids.

Types of Nephrons

Based on the length of the Henle's loop, there are two types of nephrons:

• Cortical Nephrons: These make up the majority of nephrons. Their Henle's loop is very short and extends only a little way into the medulla.
• Juxtamedullary Nephrons: These have a very long Henle's loop that runs deep into the medulla. They play a crucial role in concentrating urine.

Blood Supply to the Nephron

The efferent arteriole (exiting the glomerulus) forms a fine capillary network around the renal tubule called the peritubular capillaries. A tiny vessel from this network runs parallel to Henle's loop, forming a U-shaped structure called the vasa recta. The vasa recta is either absent or highly reduced in cortical nephrons.

Urine Formation

The formation of urine involves three main processes that occur in different parts of the nephron:

1. Glomerular Filtration
2. Reabsorption
3. Secretion

Glomerular Filtration

This is the first step in urine formation. It is the process of filtering blood in the glomerulus.

• Filtration Rate: The kidneys filter about $1100-1200 \text{ ml}$ of blood per minute.
• Filtration Membrane: Blood is filtered through three layers:
  1. The endothelium of the glomerular blood vessels.
  2. The epithelium of Bowman's capsule.
  3. A basement membrane between these two layers.
• Podocytes: The epithelial cells of Bowman's capsule, called podocytes, are arranged with minute spaces between them called filtration slits or slit pores.
• Ultrafiltration: This filtration is so fine that almost all constituents of the blood plasma, except for proteins, pass into the Bowman's capsule. This is why it's called ultrafiltration.
• Glomerular Filtration Rate (GFR): This is the amount of filtrate formed by the kidneys per minute. In a healthy person, the GFR is approximately $125 \text{ ml/minute}$, which amounts to a staggering 180 litres per day!

Regulation of GFR

The kidneys have a built-in mechanism to regulate GFR, primarily carried out by the Juxtaglomerular Apparatus (JGA). The JGA is a sensitive region formed by cellular modifications where the distal convoluted tubule and the afferent arteriole come into contact. If GFR falls, the JGA cells release renin, which helps restore the GFR to normal.

Reabsorption

Although 180 litres of filtrate are formed daily, we only excrete about 1.5 litres of urine. This means that nearly 99% of the filtrate must be reabsorbed back into the blood by the renal tubules. This process is called reabsorption.

• Active Reabsorption: Substances like glucose, amino acids, and $\text{Na}^{+}$ are reabsorbed actively (requiring energy).
• Passive Reabsorption: Nitrogenous wastes and water are reabsorbed passively (without energy).

Secretion

During urine formation, the tubular cells secrete substances like hydrogen ions ($\text{H}^{+}$), potassium ions ($\text{K}^{+}$), and ammonia into the filtrate. Tubular secretion is an important step as it helps maintain the ionic and acid-base balance of body fluids.

Function of the Tubules

Each part of the renal tubule has a specific role in urine formation.

Proximal Convoluted Tubule (PCT)

• Lined by simple cuboidal brush border epithelium, which increases the surface area for reabsorption.
• Reabsorbs nearly all essential nutrients, and 70-80% of electrolytes and water.
• Helps maintain pH and ionic balance by selectively secreting $\text{H}^{+}$ and ammonia into the filtrate and absorbing bicarbonate ($\text{HCO}_3^{-}$) from it.

Henle's Loop

This region is crucial for creating the high osmolarity (concentration of solutes) of the medullary interstitial fluid, which is necessary for concentrating urine.

• Descending Limb: Permeable to water but almost impermeable to electrolytes. As filtrate moves down, water leaves, and the filtrate becomes more concentrated.
• Ascending Limb: Impermeable to water but allows the transport of electrolytes (actively or passively). As the concentrated filtrate moves up, electrolytes leave, and the filtrate becomes diluted.

Distal Convoluted Tubule (DCT)

• Conditional reabsorption of $\text{Na}^{+}$ and water occurs here, meaning it is regulated by hormones.
• It is also capable of reabsorbing $\text{HCO}_3^{-}$ and selectively secreting $\text{H}^{+}$, $\text{K}^{+}$, and $\text{NH}_3$ to maintain the pH and sodium-potassium balance in the blood.

Collecting Duct

• This long duct extends from the cortex to the inner medulla.
• Large amounts of water can be reabsorbed from this region to produce concentrated urine.
• It allows small amounts of urea to pass into the medullary interstitium to help maintain osmolarity.
• It also plays a role in maintaining pH and ionic balance by secreting $\text{H}^{+}$ and $\text{K}^{+}$ ions.

Mechanism of Concentration of the Filtrate

Mammals can produce urine that is much more concentrated than their body fluids. The Henle's loop and vasa recta are key players in this process through a mechanism called the counter current mechanism.

• Counter Current Flow: This refers to the flow of fluid in opposite directions in two parallel limbs.
  • Filtrate flows down the descending limb and up the ascending limb of Henle's loop.
  • Blood flows down the descending limb and up the ascending limb of the vasa recta.
• Creating the Gradient: The proximity of Henle's loop and the vasa recta, along with their counter current flows, helps create and maintain an increasing osmolarity in the inner medullary interstitium. The concentration increases from about $300 \text{ mOsmolL}^{-1}$ in the cortex to about $1200 \text{ mOsmolL}^{-1}$ in the inner medulla.
• Role of NaCl and Urea: This gradient is mainly caused by $\text{NaCl}$ and urea.
  • $\text{NaCl}$ is transported out of the ascending limb of Henle's loop into the interstitium.
  • Small amounts of urea move from the collecting duct into the interstitium.
• Concentrating the Urine: This high concentration gradient in the interstitium allows water to be drawn out from the collecting tubule by osmosis, thereby concentrating the final urine. Human kidneys can produce urine nearly four times more concentrated than the initial filtrate.

Regulation of Kidney Function

The function of the kidneys is carefully monitored and regulated by hormonal feedback mechanisms involving the hypothalamus, JGA, and the heart.

Hypothalamus and ADH

• Osmoreceptors in the body detect changes in blood volume, body fluid volume, and ionic concentration.
• If there is an excessive loss of fluid, these receptors stimulate the hypothalamus to release antidiuretic hormone (ADH), also known as vasopressin, from the neurohypophysis (posterior pituitary).
• Action of ADH: ADH increases the permeability of the DCT and collecting duct to water, facilitating more water reabsorption and preventing diuresis (excessive urine production).
• ADH can also cause vasoconstriction (narrowing of blood vessels), which increases blood pressure and, in turn, can increase GFR.

JGA and the Renin-Angiotensin Mechanism

This is a complex regulatory pathway activated when there is a fall in glomerular blood flow or blood pressure.

1. Renin Release: The JG cells are activated and release an enzyme called renin.
2. Angiotensin Formation: Renin converts a plasma protein called angiotensinogen into angiotensin I. Angiotensin I is then converted to angiotensin II.
3. Effects of Angiotensin II:
   • It is a powerful vasoconstrictor, which increases glomerular blood pressure and GFR.
   • It stimulates the adrenal cortex to release the hormone Aldosterone.
4. Effect of Aldosterone: Aldosterone causes the reabsorption of $\text{Na}^{+}$ and water from the distal parts of the tubule. This also leads to an increase in blood pressure and GFR.

Heart and Atrial Natriuretic Factor (ANF)

• When blood flow to the atria of the heart increases, the atrial walls release Atrial Natriuretic Factor (ANF).
• Action of ANF: ANF causes vasodilation (dilation of blood vessels), which decreases blood pressure.
• The ANF mechanism acts as a check on the renin-angiotensin mechanism, effectively opposing its effects.

Micturition

Micturition is the process of releasing urine from the urinary bladder. It is controlled by a neural mechanism called the micturition reflex.

1. Urine formed by the nephrons is stored in the urinary bladder.
2. As the bladder fills, its walls stretch, activating stretch receptors.
3. These receptors send signals to the central nervous system (CNS).
4. The CNS sends motor messages that cause the smooth muscles of the bladder to contract and the urethral sphincter to relax, allowing urine to be released.

An adult human typically excretes 1 to 1.5 litres of urine per day. Healthy urine is a light yellow, watery fluid that is slightly acidic ($\text{pH } 6.0$) and has a characteristic odour. On average, $25-30 \text{ gm}$ of urea is excreted per day.

Role of other Organs in Excretion

Besides the kidneys, other organs also help eliminate excretory wastes.

• Lungs: Remove large amounts of carbon dioxide (about $200 \text{ mL/minute}$) and significant quantities of water vapor.
• Liver: The liver secretes bile, which contains substances like bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins, and drugs. Most of these are eliminated along with digestive wastes.
• Skin:
  • Sweat Glands: Produce sweat, a watery fluid containing $\text{NaCl}$, small amounts of urea, and lactic acid. While the primary function of sweat is cooling, it also helps in excretion.
  • Sebaceous Glands: Secrete an oily substance called sebum, which eliminates sterols, hydrocarbons, and waxes and provides a protective covering for the skin.
• Saliva: Small amounts of nitrogenous wastes can also be eliminated through saliva.

Disorders of the Excretory System

• Uremia: This is a condition where urea accumulates in the blood due to kidney malfunction. It is highly harmful and can lead to kidney failure.
• Hemodialysis: For patients with kidney failure, urea can be removed using an artificial kidney. In this process, blood is taken from an artery, mixed with an anticoagulant like heparin, and pumped through a dialyzing unit. The unit contains a cellophane tube surrounded by a dialyzing fluid with the same composition as plasma, except for nitrogenous wastes. These wastes diffuse from the blood into the fluid, cleaning the blood, which is then returned to the body through a vein.
• Kidney Transplantation: This is the ultimate treatment for acute kidney failure. A functioning kidney from a donor (preferably a close relative to minimize rejection) is transplanted into the patient.
• Renal Calculi: Commonly known as kidney stones, these are insoluble masses of crystallized salts (like oxalates) formed within the kidney.
• Glomerulonephritis: This is the inflammation of the glomeruli of the kidney.