Excretory System

• Function of the urinary system

  • To rid the body of wastes, especially nitrogenous wastes such as urea
  • To regulate the balance of fluid, salt and pH
• How the urinary system achieves its function

  Filtering the blood as it passes through the kidneys
• 1. Glomerular Capsule

  -        Collects filtrate from glomerulus

  -        One cell thick wall

  Function:

  •       Performs the first step in the filtration of blood to form urine.

  •       Holds the glomerulus which supplies blood for the kidney to filter.

  Structure:

  ·       Lining the capsule are specialised cells called podocytes

  ·       These cells have finger-like extensions that wrap around the capillaries of the glomerulus

  ·       The spaces between the “fingers” are filtration slits

  ·       Play an active role in preventing plasma proteins from entering the filtrate
• Renal Tubule- long and convoluted structure from the glomerulus and divided into 3 parts
• Renal Capsule

  A thin membrane that covers the outer surface of each kidney and supports and protects the kidney tissue from injury
• 1.     Proximal convoluted tubule

  -        due to its proximity to the glomerulus;

  -        it stays in the renal cortex.

  -        Maximum reabsorption

  -        Lined with simple cuboidal epithelium

  o   Function: To reabsorb essential substances such as glucose, proteins, amino acids.

  o   Structure:  Microvilli line the proximal tubule and create a brush border, which greatly increases the surface area for reabsorption
• Components of the Urinary System

  • Kidneys x2
  • Urinary bladder x1
  • Ureters x2
  • Urethra x1
•  

  1.     Proximal convoluted tubule (loop of Henle)

  -        A loop (with descending and ascending limbs) that goes through the renal medulla.

  -        Descending limb= permeable to water, impermeable to electrolyte

  -        Ascending limb= permeable to electrolytes but impermeable to water.

  -        Since the electrolytes get reabsorbed at the ascending loop of Henle, the filtrate gets diluted as it moves towards the ascending limb. But reabsorption is limited in this segment.
• Function of the renal capsule

  Support and protect the kidney tissue from injury
•  

  1.     Distal convoluted tubule (DCT)

  -        also restricted to the renal cortex.
• Nephron

  A structure within the renal pyramid
• Urine Formation Process

  1. Glomerular Filtration

  2. Reabsorption

  3. Tubular Secretion
• Blood Supply to Kidneys

  • Oxygenated blood in
  • Abdominal aorta feeds blood to the left and right renal artery
  • Deoxygenated blood out
  • The left and right renal vein feed blood to the inferior vena cava
  • Receives ~ quarter of the blood from the left side of the heart
  • Approx. 1.2 L of blood pass through the two kidneys every minute
• 1.     Reabsorption

  –       Materials are reabsorbed into the peritubular capillaries

  –       Peritubular capillaries wrap around the  convoluted tubules and loop of henle

  –       This process is carried out by the cells that line the renal tube

  –       Materials include water, glucose and amino acids.

  –       For reabsorption a large SA is required, achieved by the large length of the loop of henle and large numbers of nephrons in the kidneys
• Blood Supply to the Nephron

  1. Renal artery supplies blood to the afferent arteriole to the glomerulus

  2. Blood is filtered

  3. Efferent arteriole leaves the glomerulus

  4. Afferent arteriole has a wider diameter than the efferent arteriole

  5. This increases resistance to the flow of blood and produces a higher pressure in the glomerulus
• 1.     Tubular Secretion

  -        Adds substances from the blood to the filtrate

  -        Can be active/passive transport

  -        Functions: Maintains blood pH (Body needs to be in pH range from 7.4-7.5)

  -        Maintains urine pH (pH 6)
• Glomerular Filtration

  • The process where blood is filtered to create a fluid called filtrate which occurs in the renal corpuscle, enhanced by blood pressure
• Urine’s journey past the kidneys

  •       The kidney tubules join into a collecting duct that opens into the renal pelvis.

  •       The renal pelvis

  –       Funnels urine into the ureter

  •       Ureter (x2) refer to as ureters

  –       A tube that carries urine from the kidney to the bladder

  •       Bladder

  –       Function is storage of urine

  •       Urethra

  –       A tube that carries urine from the bladder to the outside world

  –       Male urethra is longer than the female urethra
• • Structural Adaptations for Kidney Function:

  1. Glomerular capsule for collecting fluid.
  2. Afferent arteriole wider than efferent arteriole, increasing blood pressure for filtration.
  3. Nephron's convolutions and loop for a large surface area.
  4. Millions of nephrons in each kidney to enhance reabsorption and secretion.
•  

  Excretion- The removal of the wastes of metabolism from the body-        Essential to remove wastes before they reach harmful concentrations
• Organs of excretion:

  •       Liver

  –        manufactures urea and processes many substances so they can be excreted

  •       Sweat glands

  –       Sweat = water and some waste products of metabolism (salts, urea and lactic acid)

  •       Kidneys

  –       Responsible for maintaining the constant concentration of materials in body fluids and the excretion of nitrogenous wastes

  •       Alimentary canal (bile pigments)

  –       Faeces is not excreted but eliminated as it is not a product of metabolism

  •       Lungs

  –       Carbon dioxide & water
• Glomerulus


  • One cell thick wall
  • Has a differently permeable membrane, meaning it is selective in what it allows in and out, it will only allow small material through
• Liver & Protein

  -        Excess protein cannot be stored in the cell therefor it has to be removed from the body

  -        Some protein is broken down in the body but most of this breakdown is incomplete

  -        The liver prepares protein for excretion
• What is the problem with the process of ammonia in the body and how does the body deal with it?

  •       Deamination- The removal of an amino group (NH2) from an amino acid

  –       Occurs in liver with help of enzymes

  –       Remaining product converted into ammonia (NH3) and carbohydrate.

  •       Carbohydrate component used to generate ATP

  •       Ammonia however is very TOXIC and soluble in water so it must be converted to a less toxic form – Urea

  –       Urea then circulates the body and is removed by the kidneys and eliminated in urine
•  

  What are the other functions of the liver?

  -       Detoxifies alcohol and many other drugs

  -       Deactivates many hormones

  -       Breaks down haemoglobin from dead red blood cells to produce bile pigments which are passed out of the body with faeces
• What are the functions of skin?

  •       Physical barrier protecting from external environment

  •       Also has a role in excretion
•  

  How does skin play a role in excretion?

  •       Sweat glands are located in the lower layer of the skin and a duct transports the sweat

  •       Even with no visible perspiration, the sweat glands secrete about 500mls of water a day

  •       Dissolved in the water are salts, lactic acid and urea