Excretory Products and Their Elimination

Types of Excretion

• Ammoneotelic : most of aquatic organisms
• Ureotelic: mammals and terrestrial organisms
• Uricotelic : birds, reptiles, snails

Types of excretory organs

• Invertebrates: Tubular structures
• Crustaceans (Prawn) : Antennal glands
• Platyhelminthes (Liver fluke, Flatworms): Flame cells
• Insects: Malphighion tubules
• Cephalochordates (Amphioxus) : Protonephridia
• Earthworms: Nephridia
• Vertebrates: Kidney

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Human Excretory System

• The kidneys regulate the amount of water, salts and other substances in the blood and 120-170 g.
• The kidneys are fist-sized, bean shaped structures (10-12 cm long, 5-7 cm in width and 2-3 cm thick)
• Remove nitrogenous wastes (urine) and excess salts from the blood.
• The ureters are tubes that carry urine from the pelvis of the kidneys to the urinary bladder.

• The urinary bladder temporarily stores urine until it is released from the body.
• The urethra is the tube that carries urine from the urinary bladder to the outside of the body.
• The outer end of the urethra is controlled by a circular muscle called a sphincter.

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Excretory System

The Kidney

• Each kidney is composed of three sections:
• the outer (renal) cortex, the (renal) medulla (middle part) and the hollow inner (renal) pelvis.
• The cortex is where the blood is filtered.
• The medulla contains the collecting ducts which carry filtrate (filtered substances) to the pelvis.
• The pelvis is a hollow cavity where urine accumulates and drains into the ureter.

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Function of the Kidney

• The principal function of the kidney is to filter blood in order to remove cellular waste products from the body.
• At any given time, 20 % of blood is in the kidneys. Humans can function with one kidney.
• If one ceases to work, the other increases in size to handle the workload.

• The kidney has other functions but it is usually associated with the excretion of cellular waste such as :
• 1) urea (a nitrogenous waste produced in the liver from the breakdown of protein. It is the main component of urine) ;
• 2) uric acid (usually produced from breakdown of DNA or RNA) and
• 3) creatinine (waste product of muscle action).

• All of these compounds have nitrogen as a major component.
• The kidneys are more than excretory organs.
• They are one of the major homeostatic organs of the body.
• They control water pH, secrete erythropoietin (a hormone that stimulates red blood cell production) and activate vitamin D production in the skin.
• That is why a doctor can tell so much from a urine sample.

The Parts of the Kidney

How blood is Filtered

• The filtering units of the kidneys are the nephrons.
• There are approximately one million nephrons in each kidney.
• The nephrons are located within the cortex and medulla of each kidney.
• The tubes of the nephron are surrounded by cells and a network of blood vessels spreads throughout the tissue. Therefore, material that leaves the nephron enters the surrounding cells and returns to the bloodstream by a network of vessels.

Parts of the Nephron

• Each nephron consists of the following parts:
• 1) glomerulus ;
• 2) Bowman’s capsule ;
• 3) proximal tubule ;
• 4) loop of Henle ;
• 5) distal tubule ;
• 6) collecting duct.

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Outline

• Uriniferous tubule (anatomical unit for forming urine)
• Nephron
• Renal corpuscle (in cortex)
• Glomerulus (tuft of capillaries)
• Glomerular (Bowman’s) capsule
• Tubular section
• Proximal convoluted tubule
• Loop of Henle
• Distal convoluted tubule
• Collecting duct

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Kidney action (cont)

• Only materials needed by the body are returned to bloodstream — for example, 99 % of water, all glucose and amino acids and many salts are reabsorbed.
• Osmosis , diffusion, and active transport draw water , glucose , amino acids and ions from filtrate into surrounding cells.
• Small villi like projections help in the active transport of glucose out of the filtrate and speed up the reabsorption process.
• From here, these components return to the bloodstream.

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Kidney action (cont)

• The filtrate reaches the end of the proximal tubule and the fluid is isotonic with the surrounding cells.
• Glucose and amino acids have been removed from the filtrate.
• The filtrate then moves to the loop of Henle whose primary function is to remove water from the filtrate by osmosis.

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Kidney action (cont)

• There is in increase in sodium concentration in the cells of the loop of Henle as we move from the area of the cortex to the inner pelvis of the kidney.
• This causes water to be drawn from the filtrate.
• These high levels of sodium are a result of active transport and results in increased concentration of filtrate (due to water loss).

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Understand at least this much:

Filtration

a. Fluid is squeezed out of the glomerular capillary bed

Resorption

b. Most nutrients, water ad essential ions are returned to the blood of the peritubular capillaries

Secretion

c. Moves additional undesirable molecules into tubule from blood of peritubular capillaries

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The nephron

In the cortex

In the medulla

© 2008 Paul Billiet ODWS

The nephron

© 2008 Paul Billiet ODWS

Filtration in the glomerulus

• Blood enters the glomerulus from a branch of the renal artery
• This blood is under high pressure
• The capillary walls are one cell thick
• They are pierced with openings (fenestrations)
• The plasma filters though the membrane under pressure
• Proteins do not pass

© 2008 Paul Billiet ODWS

Filtration in the glomerulus

The filtration membrane is held in place by specialised podocytes

© 2008 Paul Billiet ODWS

The Glomerulus

• The glomerulus is a mass of thin-walled capillaries.
• The Bowman’s capsule is a double-walled, cup-shaped structure.
• The proximal tubule leads from the Bowman’s capsule to the Loop of Henle.
• The loop of Henle is a long loop which extends into the medulla.
• The distal tubule connects the loop of Henle to the collecting duct.

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FUNCTION OF THE TUBULES

• Proximal Convoluted Tubule (PCT): PCT is lined by simple cuboidal
• brush border epithelium which increases the surface area for reabsorption.
• Nearly all of the essential nutrients, and 70-80 per cent of electrolytes and water are reabsorbed by this segment. PCT also helps to maintain the pH and ionic balance of the body fluids by selective secretion of
• hydrogen ions, ammonia and potassium ions into the filtrate and by absorption of HCO3 from it.

The nephron functions

Variable permeability to water

Impermeable to water

Freely permeable to water

© 2008 Paul Billiet ODWS

The nephron osmoregulation

Na⁺ Na⁺ Na⁺

Active reabsorption

H₂O H₂O

Passive osmosis

80% of water reabsorbed

Ultrafiltration under pressure

© 2008 Paul Billiet ODWS

The nephron osmoregulation

Collecting duct

Loop of Henlé

© 2008 Paul Billiet ODWS

Henle’s Loop: Reabsorption in this segment is minimum.

• This region plays a significant role in the maintenance of high osmolarity of medullary interstitial fluid.
• The descending limb of loop of Henle is permeable to water but almost impermeable to electrolytes.
• The ascending limb is impermeable to water but allows transport of electrolytes actively or
• passively. Therefore, as the concentrated filtrate pass upward, it gets diluted due to the passage of electrolytes to the medullary fluid.

Distal Convoluted Tubule (DCT)

• Conditional re-absorption of Na+
• and water takes place in this segment. DCT is also capable of re-absorption
• of HCO3 – and selective secretion of hydrogen and potassium ions and
• NH3 to maintain the pH and sodium-potassium balance in blood.

Counter current Mechanism

Counter current mechanism

• Vasa recta and loop of Henle play very important role in concentrating the urine.
• Two loops of henle’s descending and ascending loop formed the counter current.
• This counter current help in maintaining an increase in osmolarity towards inner medulary interstitium i.e 300 mOsmol/L in the cortex and 1200 mOsmol/L

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• This is mainly maintained by NaCl and Urea
• NaCl is transported by ascending loop of Henle which exchanged with descending loop of vasa recta
• Similarly small amount of urea also enteres into ascending loop of Henle which is transported back to the interstitium by the collecting tubule.
• This special arrangement made by loop of Henle and vasa recta called CCM

Micturation

• The process by which urine is discharged out from the body that called micturation.
• When ever bladder get filled with urine then signal goes to CNS this signal started by the stretching of urinary bladder.
• An adult person discharge about 1-1.5 l urine per day having urea 25-30g that is yellowish colored and a having pH 6.0

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The nephron osmoregulation

• The blood concentration is monitored by osmoreceptors in the hypothalamus,
• The amount of filtrate formed by the kidney per minute is called GFR i.e 125 ml/minute, i.e., 180 litres per day.
• If the concentration rises the hypothalamus releases Antidiuretic hormone (ADH)
• ADH makes the collecting duct walls more permeable
• More water is reabsorbed from the filtrate as the ducts pass through the salty tissues of the medulla

Kidney reabsorption

Role of Hormone IN EXCRITION

• ADH: Also called vasopresin secreted by hypothalamus facilitate water reabsorption
• Renin-Angiotensin Mechanism: Juxta glomerular cells Secreted renin that convert angiostensiogen to angiotensin I then to angiostensin II that acts as vasocontrictor increase the blood pressure in Glomerla and finally GFR

• Angiostensin II also secretes Aldosterone cause reabsorption of Na and water in DCT
• Atrial Natriuretic factor (ANF) cause vasodilation and decrease the blood pressure. This keep an check on renin-angiostensin mechanism.

ROLE OF OTHER ORGANS IN EXCRITION

• Lungs remove CO₂ (18 L/ day) and water too
• Liver secretes bilirubin, biliverdin, cholesterol, hormone , drugs and vitamin that pass out through digestive waste
• Sweat and subaseous glands eliminates NaCl, Urea, lactic acid etc
• Subaseous glands remove sterol, hydrocarbon and waxes through sebum

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Disorders of the Excretory System

• Uremia : Accumulation of urea in blood due to malfunctioning of kidney
• Renal failure: Non functioning of kidney
• Kidney stone: Also called renal calculi accumulation of stone or insoluble mass of crystallized salts e.g. oxalates that finally formed kidney stone
• Glomerulonephritis : Inflammation of glomeruli of kidney

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Kidney Stones

• Are also a common kidney disorder. They form when chemicals in the urine precipitate out and form crystals.
• The most common crystals are from calcium oxalate, while others could be from uric acid and cystine.
• Kidney stones are more common in men than women and can reoccur at any time.
• Factors such as recurrent urinary bladder infections, insufficient water intake and consumption, low levels of physical activity, and too much Vitamin C and D intake can lead to kidney stones.
• One of the best ways to decrease the occurrence of stones is to drink lots of water and to change your dietary habits.

Kidney Stones (cont)

• Symptoms include severe back or abdomen pain, blood in the urine, nausea and vomiting.
• Diagnosis involves a complete medical examination, including X-rays.
• Treatment may vary from letting the stones pass through the urinary tract to ultrasound shock (or lithotripsy) to disintegrate the stones to a small size that can be passed through the urinary tract.
• Real large stones require surgery for removal.

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