Module 8 - The Renal System Flashcards
Describe the different parts and functions of the nephron
The nephron is the basic structural and functional unit of the kidney. The function is to control the concentration of water and soluble materials by filtering the blood, reabsorbing needed materials, and excreting waste products as urine. The nephron eliminates wastes from the body, regulates blood volume, pH and pressure, and controls the levels of electrolytes.
It is made of the renal corpuscle and tubules. The renal corpuscle is made of the glomerulus and the glomerular capsule/Bowman’s capsule. The glomerulus is a capillary bed that allows the fluid of the blood solutes and small molecules to flow out of the blood vessels or the capillary. Once it is out of the capillary and into the surrounding corpuscle, it’s called filtrate.
The glomerular/Bowman’s capsule is a cup shaped chamber to hold filtrate; as it is collected, it will then be able to flow through the rest of the tubular nephron.
The renal tubules modify filtrate to create urine. Filtrate first moves through the proximal convoluted tubule (PCT) which has the largest role in reabsorbing nutrients. Then through the loop of Henle , which establishes an osmotic gradient that will allow for water reabsorption and concentration of the urine. Then to the Distal convoluted tubule (DCT) which functions to futher modify the tubule fluid through excretion or reabsorption. The cells of the DCT contain aldosterone receptors to help with regulating blood pressure by creating osmotic gradients.
Each DCT links to the collecting ducts that capture the tubular filtrate from the nephrons, then passes the fluid to the medulla (middle of kidney), and then eventually concentrates the urine and it flows out of the kidney into the ureter.
Explain glomerular filtration, tubular reabsorption, and tubular secretion.
In glomerular filtration, the glomerulus (capillary bed) has fenestrated capillaries with filtering holes that allow plasma to be pushed out via hydrostatic pressure and that plasma contains water and solutes. The fenestrations are small enough that no blood cells or proteins should leave. The normal glomerular filtration rate (GFR) is 120 mL/min.
Tubular reabsorption is the movement of substances that have just been secreted from the blood into the filtrate, it is taken back into the bloodstream - generally electrolytes, glucose, bicarbonate, and amino acids.
Tubular secretion is the movement of substances from the bloodstream in the vasculature that runs around the nephron to the filtrate in the tubules. This is often water, H+ to control pH.
Chemically, how does reabsorption occur?
There are transporters on the lumen side of the proximal tubular cell. As fluid moves by, they can take back particles that are in it. A transporter brings sodium in from the tubular lumen, into the cell, and into the capillary blood. As the transporter brings the sodium, it will also be bringing back glucose and amino acids. On the opposite (vasolateral) side of the bloodstream, we have other transporters, as well. We have a sodium potassium ATPase pump. As sodium comes ito the cell, it is quickly shuttled out into the blood, and so we are setting up a concentration gradient here. As we constantly shuttle sodium out, then the lumenal side transporter is going to be pulling sodium into the concentration gradient, and it will pick up glucose and amino acids as well, bringing them back into the bloodstream
List the three ways the kidney acts as an endocrine organ
Through the renin-angiotensin-aldosterone system
Through the regulation of red blood cell production through the formation of erythropoietin (EPO)
Through calcium metabolism by the activation of vitamin D
Describe the action of ADH
ADH acts on the collecting tubules to increase water absorption. ADH inhibits urine output by increase the number of water channels in the cell membrane of the collecting ducts.
Antidiuretic hormone (ADH)- made by the hypothalamus and it is released by the posterior pituitary gland. It’s triggered by an increase in the blood’s osmolality. It’s also triggered by the RAAS system (renin-angiotensin II - aldosterone - system) which is activated by hypotension. The angiotensin II that is produced, can cause a release of ADH. And so after ADH is released, it travels to the collecting ducts and it places pores in the cells surface, and that allows water to move from the collecting duct back into the bloodstream. As it does so it would be increasing the blood’s volume again to help raise our bloodpressure. At the same time it decreases the volume of the filtrate in urine, and by doing so it would be concentrating the urine and making it a smaller volume
Describe the action of aldosterone
Aldosterone acts to place several types of ion channels inside the cells of the collecting ducts. One type of ion channel is a sodium-hydrogen ion channel. Aldosterone increases Na+ reabsorption through the excretion of H+ ions. Na+ ions are pumped out of the filtrate while hydrogen ions are pumped inside and then excreted. Because water follows salt, Na+ reabsorption will cause water reabsorption. Aldosterone will also increase K+ secretion through Na+/K+ pumps. Na+ is pumped out of the filtrate to be returned to the blood while potassium (K+) is excreted in urine. The main action of aldosterone is to increase the blood volume and, therefore, blood pressure when needed.
Describe how RAAS helps to maintain the GFR
Hypotension or hypovolemia (decrease in blood volume) is going to decrease the renal profusion of blood and so when we have the decrease of blood pressure, we have different receptors by the kidney that can detect it. So first, we have the blood vessels that lead into the glomerulus - afferent blood vessels. They have stretch receptors, and they can pick up on that decrease in stretch and that signals to them there is a decrease in blood pressure.
We also have chemoreceptors in the cells close to the glomerulus. These are going to be the macula densa cells. They can pick up on when there is a decrease in NaCl delivery. They are screening for the presence of those ions. And so if there’s less coming through, they detect there’s less blood/blood pressure, these signals signal to the kidney to release RENIN - the hormone that the kidney can control and release.
Renin is a prohormone, it has to be processed. The process happens in different places, so the renin substrate is going to be cleaved into angiotensin I in the bloodstream. It travels and comes to the lungs where we have ACE (angiotensin converting enzymes) that will cleave it into its final active form, angiotensin II
Angiotensin II is going to act in a few different ways:
It can trigger thirst, vasoconstriction
Releases other hormones
Angiotensin II is going to trigger aldosterone secretion which is a hormone that is going to work in the distal convoluted tubule (DCT) and take back sodium. As we take back sodiu, water is going to follow it by those osmotic principles, so we will reabsorb more water. As we do so, we’ll have an increase in our blood volume and hopefully raise the blood pressure.
Agiontensin II also causes our antidiuretic hormone to be released which will cause water to be reabsorbed as well. And Angiotensin II via vasoconstriction is also going to work to raise the blood pressure.
As these mechanisms take place, there will be a decrease of renin released because we’ll be coming back into our homeostatic balance, but as the blood volume and pressure increase, we will be able to maintain that GFR, which is so important for the kidneys and their function.
Compare and contrast the renal cortex and renal medulla. Discuss the structures found in each.
The outer cortex houses the glomeruli and convoluted tubules (proximal and distal) of the nephron as well as blood vessels. The inner medulla is comprised of the Loop of Henle of the nephron and cone-shaped masses also known as the renal pyramids.
Explain the difference between cortical nephrons and juxtamedullary nephrons.
Cortical nephrons make up 85% of all nephrons. They originate superficially in the cortex and have shorter loops of Henle that extend only a short distance into the medulla. Juxtamedullary nephrons make up the remaining 15% of all nephrons. They originate deeper in the cortex, and their loops of Henle are thinner and extend into the medulla entirely.
Explain the differences in the two systems providing the blood supply to the nephron. How does their structure determine their role?
The glomerulus is located between afferent and efferent arterioles, which are high resistance vessels resulting in an extremely high pressure system which can easily force fluid and solutes out of the blood into the glomerular capillary along its entire length. This allows blood to flow into the glomerulus to be filtered.
The peritubular capillaries are low pressure vessels better suited for reabsorption as opposed to filtration. These capillaries are surrounded by tubules in their entirety allowing rapid movement of solutes and water. Efferent arterioles located deep in the renal cortex turn into long, thin-walled vessels known as the vasa recta, and they run parallel to the loops of Henle in the medullary region and assist in the exchange of solutes and water flowing in and out of the kidney.
Name two specialized structures of the glomerular capillaries that contribute to the filtration of blood.
Fenestrations and podocytes
What is the norm value for the GFR? Discuss why maintenance of this value is important.
A normal glomerular filtration rate (GFR) is 120-125 ml/min or 180 L/day. Maintenance of a relatively constant GFR is important for adequate reabsorption of water and other needed nutrients from the filtrate.
List the 3 regulatory mechanisms of the GFR.
(1) renal autoregulation, (2) nervous system control, and (3) hormonal control.
What are common tests for kidney function?
Urinalysis - test strip can determine pH, presence of protein or glucose (damage to the kidneys or hyperglycemia), white blood cells or nitrites (infection in urinary tract), bilirubin (liver problem), and blood (infection or kidney damage).
Estimated glomerular filtration rate (eGFR) - estimated by timed samples of blood and urine screened for the waste product creatinine, which is a waste product from muscle metabolism and it gets filtered out of the blood and is not reabsorbed into the body, so it should be secreted at a steady rate.
Blood Urea Nitrogen (BUN) - indicates health of kidneys and liver. When proteins are broken down into amino acids, ammonia is produced and then converted into less toxic urea by the liver. If there’s an increased amount of urea - the liver is producing more or the kidneys are not filtering it out as they should. If there’s a decrease, the liver is not functioning properly and producing the urea.
Describe the three layers of the glomerular capillary membrane
The glomerular capillary membrane contains 3 layers: (1) the capillary endothelial layer, (2) the basement membrane, and (3) the single-celled capsular epithelial layer. The endothelial cells contain small pores called fenestrations which allow for the filtration of blood. The epithelial layer surrounding the glomerulus is continuous with the epithelium that lines Bowman capsule. Podocytes (foot processes) are long extensions of the epithelium that embed themselves into the basement membrane. The podocytes form small pores referred to as slit pores which allow the glomerular filtrate to pass. See Figure 8.3B. The basement membrane is situated between the epithelial and endothelial cell layers as seen in Figure 8.3C. Spaces within the structural framework of the basement membrane determine the size-dependent permeability of the glomerulus. The size of these spaces, under normal circumstances, prevent red blood cells and plasma proteins from passing through the glomerular membrane into the filtrate. A compromise to the basement membrane would lead to the leakage of red blood cells and proteins into the filtrate which occurs with glomerular disease.
