Question

1. Explain how solutes and water are reabsorbed in the kidney tubules. Be sure to include the reabsorption of Na+, glucose, water, Cl-, Ca++, K+ etc. Include in your discussion how the various solutes are reabsorbed (i.e. sodium, glucose, amino acids) and the concept of renal threshold and saturation. Discuss the process of filtration. In your discussion, describe the filtration membrane in the renal corpuscle and the pressures involved in filtration. Discuss how blood flow may be regulated to the glomerulus. Describe what GFR is and then explain the principle of "clearance" and how it is used to estimate GFR.

Please be descriptive! thank you in advance!

Answer 1

Filtration is the mass movement of water and solutes from plasma to the renal tubule that occurs in the renal corpuscle. About 20% of the plasma volume passing through the glomerulus at any given time is filtered. This means that about 180 liters of fluid are filtered by the kidneys every day. Glomerular filtration is the first step in making urine. It is the process that your kidneys use to filter excess fluid and waste products out of the blood into the urine collecting tubules of the kidney.

The physical characteristics of the glomerular capillary wall determine what is filtered and how much is filtered into the glomerular capsule, the capillary walls are made up of three layers,

a) Endothelium : They have large pores, so solutes, plasma proteins and fluid can pass through, but not blood cells..

b) Basement membrane : This membrane is s fused to the endothelial layer. Its job is to prevent plasma proteins from being filtered out of the bloodstream..

c) Epithelium : this layer consists of specialized cells called podocytes. These cells are attached to the basement membrane by foot processes (pedicels). They wrap around the capillaries, but leave slits between them, known as filtration slits.this is a final filtration barrier before the fluid enters the glomerular space.

As blood passes through the glomerulus, 10 to 20 percent of the plasma filters out of the fenestrations, through the basement membrane and between these sieve-like fingers to be captured by the glomerular capsule and funneled to the PCT.

The force of hydrostatic pressure in the glomerulus is the driving force that pushes filtrate out of the capillaries and into the slits in the nephron.Osmotic pressure, the pulling force exerted by the proteins (albumins) works against the greater force of hydrostatic pressure, and the difference between the two determines the effective pressure of the glomerulus that determines the force by which molecules are filtered.

The rate at which kidneys filter blood is called the glomerular filtration rate.Glomerular filtration rate (GFR) is the measure that describes the total amount of filtrate formed by all the renal corpuscles in both kidneys per minute. The glomerular filtration rate is directly proportional to the pressure gradient in the glomerulus, so changes in pressure will change GFR. Pressure changes within the afferent and efferent arterioles that go into and out of the glomerulus itself will also impact GFR

99 percent of the water and most of the solutes filtered by the nephron must be reabsorbed. Water and substances that are reabsorbed are returned to the circulation by the peritubular and vasa recta capillaries. About 67 percent of the water, Na⁺, and K⁺ entering the nephron is reabsorbed in the PCT and returned to the circulation. Almost 100 percent of glucose, amino acids, and other organic substances such as vitamins are normally recovered here. Some glucose may appear in the urine if circulating glucose levels are high enough especially in a condition like diabetese. The renal threshold is the concentration of a substance dissolved in the blood above which the kidneys begin to remove it into the urine. When the renal threshold of a substance is exceeded, reabsorption of the substance by the proximal convoluted tubule is incomplete; consequently, part of the substance remains in the urine. Renal thresholds vary by substance, the most common reason for the glucose renal threshold ever being exceeded is diabetes, which is called glycosuria.      Fifty percent of Cl^– and variable quantities of Ca⁺⁺, Mg⁺⁺, and HPO42− are also recovered in the PCT.Other substances, such as urea, K⁺, ammonia (NH₃₎, creatinine, and some drugs are secreted into the filtrate as waste products. Descending loop of henle reabsorbs some amount of water, but they impermeable to ions, while the asscending loop of henle reabsorbs ions like sodium, potassium but they are impermeable to water.

Kidneys have mechanism to preserve GFR, if GFR is low, metabolic wastes will not be filtered from the blood into the renal tubules, on the other hand if GFR is high, the absorptive capacity of salt and water by the renal tubules becomes overwhelmed, autoregulation which manages these changes are known as Myogenic mechanism. The other mechanism involved in the regulation of GFR is known as Tuboglomerular feedback mechanism.

Renal clearance uses the rate at which a compound is cleared from the body, i.e., is excreted in the urine, to determine aspects of renal function. The practical aspect of the clearance principle is that by applying it to select compounds, one can estimate glomerular filtration rate and renal plasma flow. If a substance in plasma is filtered but is neither reabsorbed nor secreted, than its excretion rate must be equal to its filteration rate, this fact is used to measure the volume of blood plasma filtered per minute by the kidneys called glomerular filteration rate.