Tubular Reabsorption This graphic depicts the formation of

Tubular Reabsorption • This graphic depicts the formation of a dilute urine, mostly through obligatory reabsorption of water. • Compare this process to the one depicted on the next slide where urine is concentrated by the action of ADH on the DCT and collecting ducts of juxtamedullary nephrons.

Tubular Reabsorption • Urine can be up to 4 times more concentrated than blood plasma.

Vasa recta Loop of Henle Juxtamedullary nephron and its blood supply together Glomerular (Bowman’s) capsule H 2 O Glomerulus Afferent arteriole Na+CI– Blood flow Distal convoluted tubule Presense of Na +-K+-2 CI– symporters Interstitial fluid in renal cortex 200 H 2 O Efferent arteriole 300 Collecting duct 300 100 H 2 O 320 Na+CI– 400 Interstitial fluid in renal medulla 380 200 H 2 O 400 800 600 Na+CI– 700 780 600 H 2 O Urea H 2 O 980 1000 Na+CI– 400 500 600 ascending limb cause buildup of Na+ and Cl– H 2 O collecting duct reabsorb more water when ADH is present H 2 O 1 Symporters in thick Osmotic gradient 3 Principal cells in H 2 O 580 600 320 300 H 2 O Proximal convoluted tubule Flow of tubular fluid 800 H 2 O 800 900 4 Urea recycling 1000 Na+CI– H 2 O causes buildup of urea in the renal medulla 1000 1100 H 2 O 1200 2 Countercurrent flow through loop of Henle establishes an osmotic gradient 1200 Loop of Henle 1200 Papillary duct 1200 Concentrated urine (a) Reabsorption of Na+CI– and water in a long-loop juxtamedullary nephron 1200 (b) Recycling of salts and urea in the vasa recta

Tubular Reabsorption (Interactions Animation) • Water Homeostasis You must be connected to the internet to run this animation

Tubular Reabsorption • If higher than normal amounts of a substance are present in the filtrate, then the renal threshold for reabsorption of that substance may be surpassed. – When that happens, the substance cannot be reabsorbed fast enough, and it will be excreted in the urine. – For example, the renal [reabsorption] threshold of glucose is 180 -200 mg/dl. When this level is exceeded (as in diabetes mellitus), the glucose is said to “spill” into the urine (meaning a substance which is not normally present in urine begins to appear).

Tubular Secretion • Tubular secretion is the movement of substances from the capillaries which surround the nephron into the filtrate. – It occurs at a site other than the filtration membrane (in the proximal convoluted tubule, distal convoluted tubule and collecting ducts) by active transport. • The process of tubular secretion controls p. H. – Hydrogen and ammonium ions are secreted to decrease the acidity in the body, and bicarbonate is conserved. • Secreted substances include H+, K+, NH 4+, and some drugs; the amount often depends on body needs.

Tubular Secretion • Maintaining the body’s proper p. H requires cooperation mainly between the lungs and the kidneys. – The lungs eliminate CO 2. • Provides a rapid response (minutes) – The kidneys eliminate H+ and NH 4+ ions and conserve bicarbonate. • This is a slower response (hours-days). – The alimentary canal (digestive), and integumentary system (skin) provide minor contributions.

Hormones and Homeostasis • Five hormones affect the extent of Na+, Cl–, Ca 2+, and water reabsorption as well as K+ secretion by the renal tubules. These hormones, all of which are key to maintaining homeostasis of not only renal blood flow and B. P. , but systemic blood flow and B. P. , are: – angiotensin II – antidiuretic hormone (ADH) – aldosterone – atrial natriuretic peptide (ANP) – parathyroid hormone (PTH)

Hormones and Homeostasis § We have already mentioned the effect of ANP on GFR. § ADH is released by the posterior pituitary in response to low blood flow in this part of the brain. l ADH affects facultative water reabsorption by increasing the water permeability of principal cells in the last part of the distal convoluted tubule and throughout the collecting duct. • In the absence of ADH, the apical membranes of principal cells are almost impermeable to water.

Hormones and Homeostasis • Secretion of the hormones angiotensin II and aldosterone are tied to one another: When blood volume and blood pressure decrease or the sympathetic NS is stimulated, the walls of the afferent arterioles are stretched less, and the cells of the JGA secrete renin.

Hormones and Homeostasis • Renin clips off a 10 -amino-acid peptide called angiotensin I from angiotensinogen, which is synthesized by hepatocytes. By clipping off two more amino acids, angiotensin converting enzyme (ACE) converts angiotensin I to angiotensin II, which is the active form of the hormone. Angiotensin II has 3 main effects: 1. Vasoconstriction decreases GFR. 2. It increases blood volume by increasing reabsorption of water and electrolytes in the PCT. 3. It stimulates the adrenal cortex to release aldosterone.

Hormones and Homeostasis • Aldosterone stimulates the principal cells in the collecting ducts to reabsorb more Na+ and Cl– and secrete more K+. The osmotic consequence of reabsorbing more Na+ and Cl– is that more water is reabsorbed, which increases blood volume and blood pressure.

Summary of Renal Function • The events of filtration, absorption, and secretion are summarized in this graphic.

Urine • Increases in GFR usually result in an increase in urine production. However, as we have seen, the mechanisms which control electrolyte and water reabsorption in the various parts of the nephron and collecting ducts are subject to many complex controls. –Normal urine output (UOP) is 1 -2 L/d.

Urine • A urinalysis analyzes the physical, chemical and microscopic properties of urine. – Water accounts for 95% of total urine volume. – The solutes normally present in urine are filtered and secreted substances that are not reabsorbed. – If disease alters metabolism or kidney function, traces of substances normally not present or normal constituents in abnormal amounts may appear (bacteria, albumin protein, glucose, white blood cells, red blood cells to name a few).

Urine • In addition to a urinalysis, two blood tests are commonly done clinically to assess the adequacy of renal function. – Blood urea nitrogen (BUN) measures nitrogen wastes in blood from catabolism and deamination of amino acids. – Creatinine levels appear in the blood as a result of catabolism of creatine phosphate in skeletal muscle. • The serum creatinine test measures the amount of creatinine in the blood, which increases in states of renal dysfunction.

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