Secretion - A cell pumps stuff out (Neurotransmitters, Hormones, Ions, etc.) In the kidney, refers to secretion into the tubule (out of the blood)
Reabsorption - cell pumps stuff from the tubule (filtrate/urine) into the blood
Excretion - the organism gets rid of stuff from the body (into urine, gut/stool, sweat)
Gross Section of the Kidney. Notice the very dark red swath between the cortex and medulla. This is where the glomeruli are.
Diagrammatic of organ-level anatomy (upper left), and the part of the nephron labeled at the lower-left.
Glomerulus - Filters the serum into the nephron (filtrate is chemically identical to serum)
PCT - Reabsorbs the “good stuff” (Na+, glucose, aa’s, HCO3-). Convolution ⇒ ↑ Surface Area for reabsorption.
DCT - Fine-tunes the concentration of Na+, K+, and H+/HCO3-(pH) under the influence of aldosterone, a steroid hormone responsible for upregulating the transport proteins that exchange Na for K, and Na for H.
Important: The Loop and the Collecting duct work hand-in-hand. Via a countercurrent exchange mechanism, the loop sets up a hyperconcentrated medullary interstitium which the CCD needs to drive water through water-holes (aquaporins) when they are present.
Loop of Henle - sets up a high concentration of stuff (osmolarity) in the medullary interstitium
CCD (cortical collecting duct) - takes what is coming out of the DCT (dilute urine). CCD epithelium is normally impermeable to water. ADH, which binds to V2 receptors (vasopressin-type-2) on the blood side (basolateral surface) of the CCD epithelial cells, activates a 2nd messenger pathway that causes vesicles with pre-formed aquaporins embedded in their membranes to exocytose and fuse with the apical (urine side) of the cell, thus making the epithelium permeable to water. On the other side of the CCD is the hyperosmolar medullary interstitium. With the aquaporins present, water flows down its concentration gradient from the dilute urine coming from the DCT into the medullary interstitium, where it is picked up by the blood in the peritubular capillaries. This effect only happens when the body is making ADH (dehydration). If you are super-hydrated, ADH isn’t produced, and that dilute urine is allowed to flow unchanged into the renal pelvis, where it eventually leaves the body, taking the extra water with it.
For more detail on the endocrine control of the kidney via ADH and Aldosterone, see the Endocrine Page.
The kidney produces two hormones of its own.
Erythropoietin (EPO): In response to low Oxygen delivery to the kidney, EPO is produced to stimulate the Bone Marrow to make more red blood cells, a process called erythropoiesis (Gr.: red + making).
↓ O2 Delivery ⇒ ↑ EPO ⇒ ↑ RBC production
Vitamin D3: The kidney produces an enzyme called 1,25 Hydroxylase that converts the Vitamin D2 from the Liver into Vitamin D3 (the active form). The subscripts refer to the number of Hydroxyl groups, as you can see from the structures below:
Vitamin D2 (Calcidiol)
Vitamin D3 (Calcitriol)
For full details on Calcium Regulation, see the Endocrine Page.
Last update: 10-10-14