Notes

Renal Circulation:
Kidney receives 20% of cardiac output, 950-1100 ml/min
Cortical nephrons have the glomeruli in the outer cortex and are responsible for most of the reabsorption in the kidney. Juxtamedullary nephrons have their glomeruli near the medulla and are responsible for the formation of urine.
Efferent arterioles can drain into the peritubular capillaries or the vasa recta. As blood flows through the glomerulus only 20% of plasma is filters into bowman's capsule. Most continues onto peritubular capillaries. Very small amounts flitered in medulla.
Values for Renal Blood Flow and Glomerular Filtration:
Typical renal blood flow: 1.1ml/min ~ 20% cardiac output
Typical renal plasma flow: 605ml/min
GFR: 125ml/min, most reabsorbed 124ml/min. 1.5L excreted/day
Entire plasma volume is filtered 60 times a day.
There is reduced blood flow from the cortex to the medulla, which is crucial for maintaining the hyperosmolar gradient in medullary interstitium from being washed away.
Crossing the epithelial barrier:
- In cortex (peritubular capillaries): epithelium is fenestrated and underly basement membrane is loose. Transport depends on tubular epithelium.
- In medulla (vasa recta): epithelium is partly fenestrated. Transport depends on tubular epithelium and vascular endothelium.
Capillaries:
Capillaries are a single thin layer of endothelial cells surrounded by a basement membrane. They are the principal site for exchange of gases, nutrients, water, and waste products. Diffusion of small, water-souble solutes across capillary wall depends on the permeability and concentration gradient. Filtration is the second process occurring across the capillaries. Hydrostatic pressure differences between capillary and ISF drive filtration. Differences in protein concentration create oncotic pressure differences that oppose the movement of capillary fluid into interstitial space.
Net filtration pressure:
4 factors that determine net filtration pressure (Starling forces): capillary hydrostatic pressure, interstitial fluid hydrostatic pressure, oncotic pressure due to capillary plasma protein concentration, and oncotic pressure due to interstital fluid protein concentration. If NFP is positive, then net movement is out of capillary, negative is net movement in capillaries.

Imbalances in Starling pressures result in oedema: increased capillary hydrostatic pressure in heart failure, increased capillary hydrostatic pressure and ISF oncotic pressure in injury, and decreased capillary oncotic pressure in decrease of plasma proteins.

Netfiltration pressure in typical renal vascular bed. Along glomerular capillaries, hydrostatic pressure is high relative to non renal vascular beds i.e. 53mmHg. This is due to the presence of a second set of arterioles after glomerular capillaries i.e. efferent arterioles and branching of capillaries increasing cross-sectional area. Pcap decreases only slightly along glomerular capillaries - 50mmHg. In peritiublar capillaries, the hydrostatic pressure is lower than the oncotic pressure so fluid reabsorption occurs.

Fall in mean BP from arteries to Capillaries:
- Glomerular hydrostatic pressure between 45mmHg-60mmHg
- hydrostatic pressure only decreases slightly due to large increase of cross sectional area and resistance to flow by presence of afferent arterioles
- oncotic pressure increase along glomerular capillaries as protein concentration rises
- Net filtration across glomerular capillaries - afferent end (18mmHg) ad efferent end (5mmHg)
- Further reduction in hydrostatic pressure in efferent arterioles where oncotic pressure is high so fluid is reabsorbed