Urinary System: Dr Mustafa

1. Urine Formation Results from Glomerular Filtration, Tubular Reabsorption, and Tubular Secretion The rates at which different substances are excreted in the urine represent the sum of three renal processes Glomerular filtration Reabsorption of substances from the renal tubules into the blood Secretion of substances from the blood into the renal tubules. Expressed mathematically, Urinary excretion rate = Filtration rate – Reabsorption rate + Secretion rate

2. Filtration Formation of urine begins with filtration, in which fluids and small solute are forced under pressure to flow from the glomerulus to the capsular space Reabsorption As the filtrate passes through the tubules, specific substances are reabsorbed back into the blood of the peritubular capillaries Secretion Some solute are removed from the blood of the peritubular capillaries and secreted by the tubular cells into the filtrate

3. Urine Formation Results from Glomerular Filtration, Tubular Reabsorption, and Tubular Secretion Most substances in the plasma, except for proteins, are freely filtered, so that their concentration in the glomerular filtrate in Bowman's capsule is almost the same as in the plasma. As filtered fluid leaves Bowman's capsule and passes through the tubules, it is modified by reabsorption of water and specific solutes back into the blood or by secretion of other substances from the peritubular capillaries into the tubules.

4. Filtration, Reabsorption, and Secretion of Different Substances End products of metabolism such as urea, creatinine, and uric acid are poorly reabsorbed and are therefore excreted in large amounts in the urine. Electrolytes, such as sodium ions, chloride ions, and bicarbonate ions, are highly reabsorbed, so that only small amounts appear in the urine.

5. In certain kidney diseases, some of the lower-molecular-weight proteins, especially albumin, are filtered and appear in the urine, a condition known as proteinuria or albuminuria. Filtration, Reabsorption, and Secretion of Different Substances

6. Filtration, Reabsorption, and Secretion of Different Substances Each of the processes-glomerular filtration, tubular reabsorption, and tubular secretion-is regulated according to the needs of the body. When there is excess sodium in the body, the rate at which sodium is filtered increases and a smaller fraction of the filtered sodium is reabsorbed, resulting in increased urinary excretion of sodium.

7. Capillary Beds of the Nephron Blood pressure in the glomerulus is high because: Afferent arterioles have larger diameters than efferent arterioles Fluids and solutes are forced out of the blood throughout the entire length of the glomerulus

8. Net Filtration Pressure (NFP) The pressure responsible for filtrate formation NFP equals the glomerular hydrostatic pressure (HPg) minus the colloid osmotic pressure of glomerular blood (OPg) combined with the capsular hydrostatic pressure (HPc) NFP = HPg – (OPg + HPc)

9. Determination of GFR Net Filtration pressure: (1) hydrostatic pressure inside the glomerular capillaries (glomerular hydrostatic pressure which promotes filtration (2) the hydrostatic pressure in Bowman's capsule outside the capillaries, which opposes filtration (3) the colloid osmotic pressure of the glomerular capillary plasma proteins, which opposes filtration The GFR can therefore be expressed as

10. High hydro-static pressure in the glomerular capillaries (about 60 mm Hg) causes rapid fluid filtration Lower hydrostatic pressure in the peritubular capillaries (about 13 mm Hg) permits rapid fluid reabsorption By adjusting the resistance of the afferent and efferent arterioles, the kidneys can regulate the hydrostatic pressure in both the glomerular and the peritubular capillaries, thereby changing the rate of glomerular filtration, tubular reabsorption, or both in response to body homeostatic demands Approximately 1100 ml/min of systemic cardiac output flows through the kidneys

11. Glomerular Filtration Rate (GFR) The total amount of filtrate formed per minute by the kidneys The filtration coefficient (Kf) is calculated to be 12.5 ml/min/mm Hg of filtration pressure, this value depends on the permeability and the surface area of the filtration barrier GFR = Kf x net filtration pressure

12. Factors determine the GFR Total surface area available for filtration Filtration membrane permeability Net filtration pressure

13. Regulation of Glomerular Filtration If the GFR is too high: Needed substances cannot be reabsorbed quickly enough and are lost in the urine If the GFR is too low: Everything is reabsorbed, including wastes that are normally disposed out

14. Regulation of Glomerular Filtration Three mechanisms control the GFR Renal autoregulation Sympathetic control Hormonal control

15. Renal Autoregulation The ability of nephrons to adjust their own blood flow and GFR without external control Autoregulation entails two types of control Myogenic Mechanism – responds to changes in pressure in the renal blood vessels Tubuloglomerular Feedback Flow-dependent , senses changes in the juxtaglomerular apparatus

16. Myogenic Mechanism When arterial blood pressure rises, it stretches the afferent arteriole and the arteriole contracts and thus prevents blood flow into the glomerulus from changing very much Glomerular blood flow and filtration remain stable

17. Tubuloglomerular Feedback The juxtaglomerular apparatus monitors the fluid entering the distal tubule and adjust the GFR High GFR Reduced GFR Rapid flow of filtrate in renal tubules (NaCl) Constriction of afferent arteriole Sensed by macula densa Paracrine secretion

18. The Renin-Angiotensin Mechanism In response to low flow of filtrate and therefore low NaCl as a result of low GFR caused by low blood pressure, the macula densa cells signal a JG cells to release renin into the blood stream Renin triggers production of angiotensin II In the kidney increase Angiotensin II causes constriction of the efferent arteriole Glomerular hydrostatic pressure to increase and increase GFR Angiotensin II stimulate release of aldosterone