Human Physiology Lab – Zool 2200L
Lab 10 Excretory Physiology
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The kidneys play a role in maintaining homeostasis through excretion. The kidneys are the major route of excretion for wastes and toxins. The kidneys also play a major role in regulating the volume and composition of extracellular fluid by regulating the volume and composition of urine. The nephron is the functional unit of the kidney. The nephron has a vascular component and a tubular component. The processes of filtration, reabsorption and secretion occurring in the nephron determine the volume and composition of urine.
Define the following in terms of moving from vascular to tubular components in the nephron.
|The Movement of fluid across capillary walls from plasma the interstitium |
|Transport of a substance into blood |
|Movement of substance from the internal environment to the external environment by transport across an epithelium; movement of substances from| |inside a cell to outside the cell by movement across the plasma membrane. |
The kidney can make hyperosmotic or hyposmotic urine depending on the body’s need. If the osmolarity of the extracellular fluid increases the kidneys will produce a low volume highly concentrated urine by reabsorbing most of the water filtered. If the osmolarity of the extracellular fluid instead decreases the kidneys will produce a large volume dilute urine by not reabsorbing most of the filtered water. The kidneys can make hyperosmotic urine because of a vertical osmotic gradient that develops in the renal medulla due to the action of the juxtamedullary nephrons. The loop of Henle in the juxtamedullary nephrons extends deep into the renal medulla. The descending limb of the loop of Henle is permeable to water while the ascending limb is not. The ascending limb of the loop of Henle actively reabsorbs sodium, potassium and chloride while the descending limb does not. The flow through the descending limb runs countercurrent to the ascending limb. The active reabsorption of sodium, potassium and chloride in the ascending limb makes the extracellular fluid hypertonic which draws water out of the descending limb which concentrates the fluid in the loop of Henle. This then increases the amount of sodium, potassium and chloride reabsorbed from the ascending loop which in turns further concentrates the extracellular fluid drawing yet more water out of the descending limb. This counter current multiplication is what sets up the vertical osmotic gradient in the medulla of the kidney with isosmotic conditions at the cortex medulla interface (300mOsm) to hypertonic conditions at the interior of the medulla (1400mOsm).
Label the following on the juxtamedulary nephron shown below: -Bowman’s capsule -distal tubule -proximal tubule
- loop of Henle -collecting duct
1. Bowman’s Capsule
2. Proximal Tubule
3. Distal Tubule
4. Collecting duct
5. Loop of Henle
Notice that the collecting duct runs through the vertical osmotic gradient in the medulla. If the collecting duct is permeable to water, water gets reabsorbed and a small amount of hypertonic urine is produced. If the collecting duct is not permeable to water, water does not get reabsorbed and a large amount of hypotonic urine is produced. Antidiuretic hormone (ADH) controls the permeability of the collecting duct to water. When ADH is present aquaporins are placed on the collecting tubule and water is reabsorbed. Without ADH aquaporins are not present on the collecting tubule so water is not reabsorbed.
Activity 1: Effect of water intake on the specific gravity and production of urine.
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