A. Every day the kidneys filter nearly 200 liters of fluid from the bloodstream, allowing toxins, metabolic wastes, and excess ions to leave the body in urine while returning needed substances to the blood; of this, only 1-2 L of urine is actually formed daily
B.Location and External Anatomy
1.Kidneys - bean-shaped organs that lie retroperitoneal in the superior lumbar region.
2.The medial surface is concave and has a renal hilus that leads into a renal sinus, where the blood vessels, nerves, and lymphatics lie.
3.The adrenal glands lie on top of the kidneys; a fatty encasement cushions the kidneys and holds them in place, preventing ptosis – dropping of the kidneys to a lower position
C.3 distinct regions of the kidney: the cortex, the medulla, and the renal pelvis.
D.Nephrons -functional units of the kidneys that carry out processes that form urine
1.Over 1 million of these tiny blood-processing units, which function to form urine
2. The renal corpuscle which is composed of the glomerulus (tuft of capillaries) surrounded the glomerular capsule (Bowman’s capsule).
3.Renal tubule begins at the glomerular capsule as the 1)proximal convoluted tubule, continues through a hairpin loop, the 2)loop of Henle (exixts in the medulla), and turns into a 3)distal convoluted tubule before emptying into a collecting duct.
4.Collecting ducts - collect filtrate from many nephrons, and extend through the renal pyramid to the renal papilla, where they empty into a minor calyx; not part of the renal tubules
5.Peritubular capillaries arise from efferent arterioles draining the glomerulus, and absorb solutes and water from the tubules.
6.The filtration membrane lies between the blood and the interior of the glomerular capsule, and allows free passage of water and solutes – fenestrated, or porous
II.Kidney Physiology: Mechanisms of Urine Formation
A.Step 1: Glomerular Filtration
1.Glomerular filtration is a passive, nonselective process in which hydrostatic pressure forces fluids through the glomerular membrane.
a. Filtrate and is the raw material that the renal tubules process to form urine; essentially plasma minus blood cells and large plasma proteins
2.Filtration membrane has a large surface area and is thousands of times more permeable to water and solutes
3.Maintenance of relatively constant glomerular filtration rate is important b/c reabsorption of water & solutes depends on how quickly filtrate flows thru tubules.
4.Glomerular filtration rate – volume of filtrate formed each minute by the combined activity of all 2 million glomeruli of the kidneys. Factors that govern the GFR are:
a.Total surface area available for filtration
b.Filtration memrane permeability
c.Net filtration pressure
5. Anuria – abnormally low urinary output (less than 50ml/day)
B.Step 2: Tubular Reabsorption – 99% of filtrate is returned to blood stream
1. Total plasma volume filters into the renal tubules about every 22 min. Reclamation of most of this fluid occurs here and is returned to the blood.
2.Tubular reabsorption begins as soon as the filtrate enters the PCT, and involves reabsorption of virtually all organic nutrients, such as glucose and amino acids; reabsorption of water and ions continuously regulated and adjusted by hormones
3.Substances that are not reabsorbed or incompletely reabsorbed remain in the filtrate due to a lack of carrier molecules, lipid insolubility, or large size (urea, creatinine, and uric acid).
4. Transport Maximum – when transporters for a particular substance are saturated, the excess is secreted in urine, i.e. glucose in...