Formation of urine takes place in kidneys and it is transferred to the bladder before it is excreted from the body. Urine is formed for various reasons such as excretion of waste product of protein metabolism, water and electrolyte levels are controlled and also Ph level is maintained.
Urine formation takes place in three steps:-
• Filtration:- takes place through the semi permeable walls of Glomerulus and Glomerular capsule. Water and other small molecules pass through it easily but blood cells, plasma proteins and molecules which are larger in size so they do not take part in filtration process and remain in blood capillaries, some of the small molecules and water are later reabsorbed. Filtration takes place due to the difference in the blood pressure in the glomerulus and the pressure of filtrate in the glomerular capsule.
Since the efferent arteriole is narrower that afferent arteriole, a pressure develops in the glomerulus known as capillary hydrostatic pressure( 55 mmHg);this pressure is opposed by two pressures, first is the osmotic pressure of blood(30 mmHg) and second is the filtrate hydrostatic pressure(15 mmHg) in glomerular capsule, so the net filtrate pressure is
The volume of filtrate formed by each kidney is known as glomerular filtration rate which is about 125ml/min in a healthy adult. Nearly about 8 liters for filtrate is formed in a day but only 1-1.5 liter is excreted in form of urine because of re absorption and secretion process.
• Selective re absorption:- mostly the reabsorption from the filtrate takes place in the proximal convoluted tubules(PCT) because it has increased surface area for reabsorption . many substances are reabsorbed here including water electrolytes and nutrients such as glucose. some substances are reabsorbed passively while glucose is reabsorbed through active transport mechanism, from here only 60-70% of filtrate passes ahead and much of water, Na and Cl are reabsorbed in the medullary loop, so at last only 15-20% of the filtrate reaches the DCT. Electrolytes are mainly absorbed in PCT and DCT because of which the filtrate that reaches the ducts is very diluted and the main function of the duct is to reabsorb as much as water as the body needs.
Some ions like Na and Cl are reabsorbed by both active and passive ways while some constituent of the filtrate like glucose are completely reabsorbed unless its level is high in the body while the nitrogenous waste products like urea and uric acid are reabsorbed very limitedly.
The maximum capacity of the kidney for reabsorption is known as transport maximum or renal threshold. For example If glucose level in the body rises above its transport maximum than glucose is found in the urine. The transport maximum for some substances is regulated by the body needs while for some constituents it is regulated by hormones
1. parathyroid hormone – regulates the reabsorption of Ca and phosphate
2. antidiuretic hormone ADH – it increases water reabsorption in DCT and in collecting ducts.
3. aldosterone – increases reabsorption of sodium and water and excretion of potassium.
4. atrial natriuretic peptide ANP – it decreases the reabsorption of sodium and water from PCT and collecting ducts.
• Tubular secretion:- filtration takes place when the blood flows through the glomerulus, substance which are not required and foreign particles like drugs example penicillin and asprin may not be completely filtered in that small duration , that’s why such substances are cleared by secretion from the tubular capillaries within the convoluted tubules. Tubular secretion of hydrogen ions is important for maintaining normal blood Ph level.
Multiple choice questions(MCQs)
1. What are the 3 steps of urine formation?
A. formation, selective reabsorption and tubular secretion
B. filtration, selective absorption and tubular secretion
C. filtration, selective reabsorption and tubular secretion
D. formation, selective absorption and tubular secretion
2. Why do filtration takes place?
A. difference between the BP in glomerulus and the pressure of filtrate
B. similarity between the BP in glomerulus and the pressure of filtrate
C. difference between the pressure in glomerulus and the BP of filtrate
D. similarity between the pressure in glomerulus and the BP of filtrate
3. Where do capillary hydrostatic pressure builds up?
A. glomerulus B. PCT
C. DCT d. collecting ducts
4. Match the following-
a. parathyroid hormone 1. Increases excretion of potassium
b. atrial natriuretic peptide 2. Decreases reabsorption of Na and water
c. aldosterone 3. Reabsorption of Ca
d. antidiuretic hormone 4. Increases water reabsorption
5. Which of the following products is reabsorbed very limitedly?
A. urea B. uric acid
C. creatinine D. all of the above
6. Why there is difference in the volume of filtrate formed and the concentration of urine excreted?
A. due to selective reabsorption and tubular secretion of the constituents
B. due to low transport maximum of some constituents
c. due to higher net filtration pressure
D. the difference is not constant so no particular reason for it
7. Which of the following statement is NOT true?
A. ADH is secreted by posterior pituitary
B. filtrate hydrostatic pressure is found in glomerular capsule
C. the kidney’s maximum capacity is known as glomerular filtration rate
D. ANP is secreted by atria of the heart
8. What factor maintains the normal blood pH?
A. tubular secretion of hydrogen ions
B. tubular secretion of Ca ions
C. tubular secretion of potassium ions
D. tubular secretion of Na ions
9. Which of the following hormones decreases reabsorption?
A. ADH B. parathyroid hormone
C. aldosterone D. none of the above
10. Out of the given options which one is not constituent of glomerular filtration?
A. amino acids B. glucose
C. some drugs (large molecule) D. some drugs (small molecules)
1. filtration, selective reabsorption and tubular secretion
2. difference in the BP in glomerulus and the pressure of filtrate.
4. a – 3 b – 2 c – 1 d – 4
5. all of the above
6. due to selective reabsorption and tubular secretion of constituents
7. the kidney’s max capacity is known as glomerular filtration rate
8. tubular secretion of hydrogen ions
9. none of the above
10. some drugs( large molecules
REFRENCES:- 1. Ross and Wilson-Anatomy and physiology in health and illness; 12th edition; page no.-:
2. Gerard J. Tortora -Principles of anatomy and physiology; edition twelfth ; page no.-: 1030-1034.