Unit Overview:

In this section we will discuss how the kidneys work and how the body reabsorbs and recycles materials and how the body removes waste from its system. You will learn that the kidney is divided into an outer and inner medulla. You will also learn that the kidney contains nephrons. Most of what the kidneys filters, ends up going back into the vascular system.

1. **Structure and Function of the Kidneys**

The kidney is divided into an outer cortex and inner medulla.
- The medulla is composed of renal pyramids, separated by renal columns.
- The renal pyramids empty urine into the calyces that drain into the renal pelvis. From there urine flows into the ureter and is transported to the bladder to be stored.


Each kidney contains more than a million microscopic functional units called nephrons. Nephrons consist of vascular and tubular components.

- Filtration occurs in the glomerulus, which receives blood from an afferent arteriole.
- Glomerular blood is drained by an efferent arteriole, which delivers blood to peritubular capillaries that surround the nephron tubules.

- The glomerular (Bowman's) capsule and the proximal and distal convoluted tubules are located in the cortex.
- The loop of Henle is located in the medulla.
- Filtrate from the distal convoluted tubule is drained into collecting ducts, which plunge through the medulla to empty urine into the calyces.

Glomerular Filtration

A filtrate derived from plasma in the glomerulus must pass though a basement membrane of the glomerular capillaries and through slits in the processes of the podocytes, the cells that compose the inner layer of the glomerular (Bowman's) capsule.

- The glomerular ultrafiltrate, formed under the force of blood pressure, has a low protein concentration.

- The glomerular filtration rate (GFR) is the volume of filtrate produced by both kidneys each minute. It ranges from 115 to 125 ml/min.

The GFR can be regulated by constriction or dilation of the afferent arterioles.

- Sympathetic innervation causes constriction of the afferent arterioles.


- Intrinsic mechanisms help to autoregulate the rate of renal blood flow and the GFR.

2. Reabsorption of Salt and Water-17.3
Your kidneys receive about 180 liters of glomular ultrafiltrate every day and only let go 1-2 liters that was received during the whole day. 99% of what the kidneys receive are filtered back into to vascular system. The remaining 1% in put into the urine. If a lot of water is consumed, the urine is diluted down and the amount increases. Water is returned to the vascular system by means of passive transport. The filtration in the glomular ultrafiltrate is isosmotic because all plasma solutes can go in and out except for proteins. Active transport of sodium is necessary for the reabsorption by osmosis to occur. Epithelial cells form a strong wall of the proximal tubule by forming tight junctions. In the proximal tubule about 65% of the salt and water is reabsorbed back into the vascular system. Since the proximal tubule are freely permeable to water, the salt and water are taken out in certain amounts like around 20% is returned. Fine tuning of reabsorption for what goes in the urine is performed by hormones that are located in the later regions of the nephron.

Water is not able to be actively passed across the tubule wall, but it can be transported passively and reabsorbed if the surrounding fluids need to be hypertonic to each other. The loop of Henle is divided into two regions, the thin and thick segment. The thick segment is located nearest to the tip of the loop. Salt is actively taken out of the thick segment by an ascending limb to the interstitial fluid. With the counter-current multiplication system, it flows in the opposite direction of the Loop of Henle. With the discharge of sodium chloride, it makes the interstitial fluid more concentrated. The descending limb is more passively permeable to water making an increase in the concentration in the surrounding fluid, which in turn makes the interstitial fluid also more concentrated. Water is removed from the blood by way of a vasa recta, which is long, thin vessels located in the loop of Henle. The vasa rectas job is to remove water from the interstitial fluid and renal medulla.

Urea is diffused into the interstitial fluid from the inner collecting duct. From there it to goes to through the ascending limb of the loop of Henle. With urea and NaCl in the interstitial fluid, it makes it very hypertonic so water has to leave the collecting duct by means of passive transport. Because of the antidiuretic hormone (ADH), more water is taken out by passive transport into the hypertonic renal medulla and capillaries. When ADH can't attach to its receptors, water channels are removed by a process called endocytosis. When ADH is increased the more water is reabsorbed. Dehydration is the result of the plasma becoming more concentrated.


3. Renal Plasma Clearance
The volume of plasma from which a particular solute is cleared each minute by the excretion of that solute in the urine. If there is no reabsorption or secretion of that solute by the nephron tubules, the renal plasma clearance is equal to the glomerular filtration rate.
Clearing the blood of excess ions and waste products from the blood is accomplished through their excretion in the urine. Because of renal clearance, the concentrations of substances in the blood leaving the kidneys is lower than their concentrations in the blood entering the kidneys. Secretion is a membrane transport process that affects renal clearance. In terms of it direction of transport, secretion is the opposite of reabsorption. Secreted molecules and ions move out of the peritubular capillaries into the interstitial fluid and then they are transported across the basolateral membrane of the tubular epithelial cells and into the lumen of the nephron tubule.


The rate at which a substance in the plasma is excreted in the urine is equal to the rate at which it enters the filtrate minus the rate at which it is reabsorbed from the filtrate.
Excretion Rate = (filtration rate+ secretion rate) - reabsorption rate

Many foreign molecules, xenobiotics and toxins and drugs, are eliminated in the urine more rapidly than would be possible by just glomerular filtration. They are secreted by membrane carriers that recognize them as foreign to the body. Organic anion transporters are a major group of transport proteins involved in this elimination. These transporters are located in the basolateral membrane of the proximal tubule and function to secrete their transported molecules into the filtrate of the proximal tubule.
Inulin is a compound injected into the blood and filtered by the glomeruli. The amount of inulin excreted per minute is equal to the amount that is filtered per minute.


(a) inulin is present in the blood entering the glomeruli
(b) some of this blood with its dissolved inulin is filtered; all of this filtered inulin enters the urine and most of the filtered water is returned to the vascular system
(c) blood leaving the kidneys in the renal vein contains less inulin than the blood that entered the kidneys in the renal artery; the inulin clearance rate equals the GFR because inulin is filtered but not reabsorbed or secreted


Essential Questions:
-Describe the renal process of filtration, reabsorption, secretion, and excretion. Generally describe each process and then give a specific example of each process occurring in the nephron and/or urinary system organs. JADA
-Why is it so important to maintain a proper electrolyte balance? Give two examples.
Without proper electrolyte balance you will not be able to get the right amount of potassium that is required to help with proper functioning of the cardiac and/or skeletal muscles. Also it is important to keep proper electrolyte balance in order because with out the right amount of balance you could become dehydrated and or have a stroke.

How does this apply to PTA:
This applies to PTA because it is very important for us to know the functions of the kidneys because if the patient had Ascites because their kidneys are not working properly the patient is put on fluid retension. In that case we would not be able to do a vigorous massage because they would need to take in fluids afterward.

1. http://highered.mcgraw-hill.com/sites/0073378119/student_view0/chapter17/textbook_images.html
2. Fox, Stuart I. "Human Physiology." New York: McGraw-Hill, 2011. Print.
3. http://www.google.com/images?hl=en&source=hp&biw=1362&bih=583&q=countercurrent+multiplier+system&gbv=2&aq=1&aqi=g1&aql=&oq=countercurrent+multiplier+s
4. http://www.colorado.edu/intphys/Class/IPHY3430-200/image/19-1.jpg
5. Goodman, Catherine Cavallaro., and Kenda S. Fuller. Pathology: Implications for the Physical Therapist. St. Louis, MO: Saunders/Elsevier, 2009.Print