Explain The Processess Involved In The Formation Of Urine

Question 1

Describe the components of urine.
What does the body balance to maintain constituent levels?

Answer 1

The exchange of substances between filtrate in the nephron and the blood in renal capillaries is a reflection of urine.

Waste products of proteins metabolism are excreted
The balance of essential body constituents is maintained by adjusting the amount of other substances excreted:
- water
- electrolytes
- hydrogen ions

Question 2

What are three processes involved in formation of urine?

Answer 2

1. Filtration
2. Selective absorption
3. Secretions

Question 3

What is filtration? Describe the process

Answer 3

Filtration takes place through the semi-permeable wall of the glomerular capillaries and glomerular capsule.

Water and other small molecules readily pass through

The fluid filtered from the blood into the glomerular capsule (filtrate) is very similar in composition to plasma.

Filtration occurs because there is a difference between blood pressure in the glomerulus and the pressure of the filtrate in the glomerular capsule.

The efferent arteriole is narrower than the afferent arteriole so capillary hydrostatic pressure builds up in the glomerulus

This pressure is opposed by the osmotic pressure if the blood provided mainly by plasma proteins AND by the filtrate hydrostatic pressure in the glomerular capsule.

Question 4

What is glomerular filtration rate?

Answer 4

Volume of filtrate formed by both kidneys each minute

Question 5

What is auto regulation?

Answer 5

Renal blood flow, and therefore glomerular filtration is protected by auto regulation.

Glomerular filtration rate depends on glomerular blood pressure which is controlled independently of systemic blood pressure.

Auto regulation can contract or relax the arteriole to protect the glomerular capillaries from filtering too much or too little. This helps maintain constant blood flow through the glomeruli.

Question 6

What is selective reabsorption? Describe the process.
What substances are reabsorption in which places?

Answer 6

Most reabsorption from the filtrate back into blood takes place in the proximal convoluted tubule.
Epithelial lining possesses microvilli to increase surface area for reabsorption

Water, electrolytes, nutrients, glucose and amino acids are reabsorbed in PROXIMAL CONVOLUTED TUBULE

Reabsorption is both passive and active.
Further reabsorption takes place in the medullary loop, these are:
- water
- sodium
- chloride

Most electrolytes are reabsorbed in the DISTAL CONVOLUTED TUBULE.

Question 7

how does active reabsorption happen?
What is transport maximum or renal threshold?

Answer 7

Active transport takes place at carrier sites in the epithelial membrane.
Substances reabsorbed actively are:
Sodium
calcium
potassium
phosphate
chloride
glucose + amino acids (normally completely reabsorbed)
nitrogenous waste products (limited reabsorption)

Transport maximum is the max capacity the kidneys can reabsorb.

Question 8

What hormones influence selective reabsorption?

Answer 8

1. Parathyroid hormone
   - secreted by the parathyroid gland and, together with calcitonin from the thyroid gland
   -regulates reabsorption of calcium and phosphate to main normal blood levels
2. Anti diuretic hormone - secreted by the posterior pituitary. It increases the permeability of the distal convoluted tubules and collecting ducts to water, increasing water. Osmoreceptors in hypothalamus monitor water content in blood and ADH secretion is adjusted accordingly
3. Aldosterone
   - secreted by adrenal cortex.
   -Increases the reabsorption of sodium and water, and the excretion of potassium
4. Atrial natriuretic peptide (ANP)
   - secreted by atria of the heart in response to stretching in atrial wall when blood volume increases.
   - Decreases reabsorption of sodium and water from the proximal convoluted tubules and collecting ducts. Increases urinary volume and decreases blood volume.

Question 9

What is secretion? Describe the process

Answer 9

Filtration occurs as blood flow through the glomerulus

• Some unwanted substances may not be entirely filtered out of blood because of the short term spent passing through the glomerulus.
  OR
  Because they are too large to pass through the filtration pores.

-Such substances are cleared from the blood into filtrate by secretion from the peritubular capillaries in the convoluted tubule

• Tubular secretion of hydrogen ions is important in maintaining normal blood pH

Question 10

Summarise in simple terms what happens in filtration, reabsorption and secretion.

Answer 10

Filtration: blood —> nephron
Reabsorption: Filtrate - > blood
Secretion: Blood —> filtrate