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Anatomy located in the cortex physiology it is the

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---(Anatomy) located in the cortex; (Physiology) it is the next part of the nephron due to receive the now cell free and essentially protein free blood – this filtration contains waste products which are toxic to the body above a certain concentration, and substances that are valuable to the body – 1 st function of the proximal tubule is to return the bulk of each valuable substance back to the blood circulation – thus 75% of the water, sodium, and chloride; 100 % of the glucose (up to the renal threshold when there is an overabundance of glucose in the body – or when a concentration of a substance within the blood which, when reached, causes the substance to appear in the urine) – almost all of the amino acids, vitamins, and proteins, and varying amounts of urea, uric acid, and ions, such as magnesium, calcium, potassium, and bicarbonate, are reabsorbed – almost all (98 – 100%) of uric acid, a waste product, is actively reabsorbed, only to be secreted at the distal end of the proximal tubule – when the substances move from the tubular lumen to the peritubular capillary plasma the process is called tubular reabsorption – w/ the exception of water and chloride ions the process is active, that is, the tubular epithelial cells use energy to bind and transport the substances across the plasma membrane to the blood – the transport processes that are involved normally have sufficient reserve for efficient reabsorption but they are saturable – when the concentration of the filtered substance exceeds the capacity of the transport system, the substance is then excreted in urine – the plasma concentration above which the substance appears in urine is known as the renal threshold and its determination is useful in assessing both tubular function and non-renal disease states – a renal threshold does not exist for water b/c it is always transported passively thru diffusion down a concentration gradient – chloride ions in this instance diffuse in the wake of sodium – a 2 nd function of the proximal tubule is to secrete products of kidney tubular cell metabolism such as hydrogen ions and drugs such as penicillin – the term tubular secretion is used in 2 ways: (1) tubular secretion describes the movement of substances from peritubular capillary plasma to the tubular lumen, and (2) tubular secretion also describes when tubule cells secrete products of their own cellular metabolism into the filtrate in the tubular lumen – transport across the membrane of the cell is again either active or passive LOOP OF HENLE --- (Anatomy) composed of the thin descending limb which spans the medulla and the ascending limb which is located in both the medulla and the cortex composed of a region that is thin and then thick (Physiology) (Countercurrent Multiplier System ) the osmolality in the medulla in this portion of the nephron increases steadily from the corticomedullary junction inward and facilitates the reabsorption of water, sodium, and chloride – the hyperosmolality that develops in the medulla
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