Why is the vasculature central to the function of the nephron?
This is because the vasculature is what delivers the fluid that will be transformed into urine.
Unfiltered blood enters the glomerulus through what arteriole? Which arteriole does the filtered blood exit the glomerulus?
Unfiltered blood enters the glomerulus through the AFFERENT ARTERIOLE and filtered blood exits the glomerulus via an EFFERENT ARTERIOLE.
The walls of the glomerulus capillaries act as a filter that retains red blood cells and large molecules but permits _______________.
The walls of the glomerulus permits liquid and small molecules to escape into the Bowman's capsule.
Filtration is possible due to specialized cells called ______.
Podocytes. These podocytes have extensions called FOOT PROCESSES, that act like a filter. The space between the foot process allows water and small molecules to exit, but excludes red blood cells and macromolecules.
What is the filtrate (fluid) that passes from the glomerulus to the tubule called?
It is called the primary urine.
How much of the primary urine is recovered?
About 99% of the volume of the primary urine is recovered and only about 1% contributes to the volume of the FINAL URINE.
Human's make 7.5 liters of primary urine per hour, but generate only 75 ml of final urine. How is this possible?
This is possible because the primary urine is remodeled as it passes through different parts of the tubule. Recall, tubules have properties of epithelial cells, so they are designed to absorb water and solutes.
The ability to transport solutes from primary urine is limited by _________.
Transport capacity.
What is the RENAL THRESHOLD?
This is the limit for solute recovery. Active transporters will become saturated at high substrate levels and this constraint sets a limit for solute recovery called the RENAL THRESHOLD, beyond which solutes are lost to the urine.
Give me an example of when renal threshold can occur.
One example is glucose reabsorption. Glucose transporters become saturated at high blood glucose concentration and reabsorption plateaus. The glucose that is not reabsorbed is lost via urine.
Humans with Type 1 diabetes have high glucose concentrations in the blood, known as ________.
Glucosuria.
Why does the morphology differ across different regions of the tubule?
This is because specific regions of tubules are specialized for different functions.
How do molecules in the tubule get back into the bloodstream?
Molecules are transported from the lumen of the tubule to the extracellular fluid then to surrounding epithelial cells where they re-enter the bloodstream.
What is the PROXIMAL TUBULE specialized for?
The proximal tubule is specialized for transport and is where most water and solute reabsorption occurs. Thus this region is specialized for reabsorption.
How would the specialization for reabsorption in the proximal tubule affect the structure of the epithelial cells in this region?
The epithelial cells of the proximal tubule are tall cuboidal epithelial cells, with abundant mitochondria and microvilli. These features are common in other cells that carry out energy-dependent solute transport processes.
The next region of the tubule after the proximal tubule is the DESCENDING LIMB OF THE LOOP OF HENLE, which is specialized for water absorption and expresses a high number of proteins called ________________________.
Aquaporins, which are channels that allow for the passage of water through the plasma membrane.
The descending limb of the loop of henle is specialized for water reabsorption, how does this affect the structure of the epithelial cells within the descending limb?
As you go further down the descending limb, the cuboidal epithelial cells are with many microvilli are replaced with flatter squamous epithelial cells, that have few microvilli. These types of cells are called principal cells.
Where else can principal cells be found?
They can be found in the collecting duct, another place where there is a lot of water reabsorption.
The ascending limb of the loop of henle, which is specialized for solute absorption expresses a high number of ion transport proteins. What would you expect the structure of the epithelial cells to look like here?
I would expect the ascending limb to be composed of cuboidal epithelial cells that have many microvilli, which would help allow for the reabsorption of solutes.
What happens in the DISTAL TUBULE?
This is an important site for hormone regulated transport of water and solutes.
What types of hormones can be found in the distal tubule?
Hormones like mineralocorticoids (e.g. aldosterone) and vasopressin (ADH) act to alter levels of transport proteins in this region to alter uptake of water and solutes.
When does vasopressin produced?
During dehydration, vasopressin causes translocation of aquaporin proteins and thus increases water uptake and fluid retention in the distal tubule.
Does vasopressin create new aquaporins?
No, vasopressin acts via a G-protein coupled receptor pathway to translocate aquaporin proteins to the membrane. This does not require synthesis of new proteins, but rather the translocation of existing proteins.
What is aldosterone?
It is a steroid hormone involved in altering sodium and potassium balance. Aldosterone is secreted by the adrenal gland.
How does aldosterone work?
Aldosterone stimulates the expression of ion transport genes like sodium and potassium channels. These newly produced proteins are transported and inserted into the tubule membrane.
Why does aldosterone hours in order to take effect compared to vasopressin, which only takes a few minutes to take effect?
There is a slower response because aldosterone activation involves creation of new protein, rather than translocation of existing proteins.
What are are two hormones that act via the kidney to regulate blood pressure?
1. Angiotensin
2. Atrial Naturetic Peptide (ANP).
What is angiotensin?
Angiotensin is a hormone that can can act via the kidney to regulate blood pressure. It can also act on other tissues including the heart, brain and blood vessels.
Angiotensin increases the absorption of sodium in the tubule and can also act as a vasocons
How does ANP work?
Atrial Naturetic Peptide (ANP) is a hormone specific to animals. Cardiac atrial (heart) cells produce ANP, which is released in response to the excessive stretch that accompanies blood volume increase.
ANP travels to the kidney where it increases blood fl
Several regions of the tubule can transport ___________ and ____________ which in turn assists in pH regulation.
1. Protons
2. Bicarbonate
What type of excretory system do flatworms have?
They have PROTONEPHRIDIA, which are analogous to vertebrate tubules. The number of protonephridia can vary widely in different species, from as few as two to several thousand.
How do protonephridia work?
Protonephridia rely on cilia or flagella to draw fluid into the lumen for filtering and absorption.
Molluscs and annelids have METANEPHRIDIA, which are similar to vertebrate tubules. How many metanephridia do mollsucs and annelids have?
Most molluscs possess a single metanephridium while annelids possess one metanephridium per segment.
What is the main difference between proto and meta nephridia?
The main difference is that metanephridia have an opening called the NEPHROSTOME which collects fluid rather than relying on cilia or flagella.
What do insects use to fulfill to get rid of their excretory waste?
They use MALPIGHIAN TUBULES, to fulfill the function of the vertebrate kidneys. These tubes are connected to the hindgut where the filtrate can be further modified.
How does the fluid travel through Malpighian tubules in the fly?
1. The fluid enters blind of tubule via paracellular or transcellular transport.
2. Ion transport proteins move ions against their concentration gradients using ATP, but provide a diffusive gradient for water to follow.
What types of cells mediate transport in the Malpighian tubules?
The transport is mediated by two cell types:
1. PRINCIPAL CELLS: Have abundant mitochondria, microvilli and several different ions.
2. STELLATE CELLS: Transport only chloride and water and therefore have fewer mitochondria and lack microvilli.
What is a diuretic?
It is an agent that promotes urine formation.
What is a hormone?
It is a type of chemical messenger that is carried in the blood and thus can act across long distances. Classically defined as a substance released from an endocrine gland and active at very low concentrations.
Absorption in the Malpighian tubules is controlled by three classes of DIURETIC HORMONES (chemical messengers that promote urine formation). What are they?
1. Corticotropin releasing factor related diuretic hormones: Activate cation transport.
2. Insect Myokinins: Act on the stellate cells to increase chloride transport.
3. Cardioacceleratory hormones: First identified as hormones that increase heart rate, t
The structure and function of fish kidneys differ between freshwater and marine species. How exactly do they differ?
The glomerulus and tubule are much larger in freshwater species. Diffusion of water into the cells of freshwater fish create a need to process more fluid and prevent solute loss. The kidneys of freshwater fish produce large amounts of dilute urine compare
How are amphibian kidneys unique?
Amphibian kidneys change during metamorphosis. Larval frogs possess a pair of PRONEPHROS, a simple tubule, which is replaced by a more mammalian like kidney in adult frogs.
Amphibians use the urinary bladder to store and recover water, rather than to simp
The reabsorption of water in the urinary bladder is under the control of the hormone _______________.
Vasopressin. They use their version of vasopressin in order to reabsorb water.
Variations in kidney structure in reptiles, birds and mammals reflect different solutions to avoid dehydration. How do reptiles and birds accomplish the task of waste excretion?
They excrete nitrogen waste as uric acid. They are uricoteles.
How do mammals conserve water?
Mammals conserve water by lengthening the LOOP of HENLE to allow more surface area for absorption.
Do mammals that live in aquatic environments like beavers have nephrons with long or short loops of henle?
Aquatic mammals such as beavers have nephrons with very short Loop's of Henle. They produce very dilute urine. This makes sense because they live in a much more dilute environment.
Give me an example of an animal with a long loop of henle.
Mammals that live in water-limited environments, like the Kangaroo rat have nephrons with a long Loop of Henle and they produce very hyperosmotic (i.e. concentrated) urine.