Key Takeaways

Is this urine? Globule

.The renal system is primarily responsible for regulating blood volume and plasma osmolality, and waste removal via urine is a convenient way that the body performs many functions by utilizing one system. Urine is formed through the following processes:


An afferent arteriole carries blood into the glomerulus, where filterable components of blood, such as water and nitrogenous waste, move into the glomerulus, while nonfilterable components, such as serum albumins and cell debris, exit through an efferent arteriole.These filterable components accumulate in the glomerulus where they form glomerular filtrate.

.The remaining 80% of the blood is carried through the body to facilitate tissue perfusion and gas exchange.


During the next step, cells will reabsorb molecules and ions into their bloodstream.Fluid passes through the elements of nephrons (the proximal/distal convoluted tubules, loop of Henle, the collecting duct) as water and ions are removed, with osmolarity (ion concentration) dependent on fluid movement.As the fluid leaves the ureter in the form of urine, secretion occurs in the collecting duct.


During secretion, some substances-such as hydrogen ions, creatinine, and drugs-will be transferred from the blood into the collecting duct via the peritubular capillary network.A collection of substances that are not reabsorbed during glomerular filtration or tubular reabsorption forms urine as the outcome of all these processes.

Urine is mainly composed of water that has not been reabsorbed by the body, which is how the body lowers blood volume by increasing the amount of water that becomes urine instead of being reabsorbed.The other main component of urine is urea, which is a very soluble molecule composed of ammonia and carbon dioxide. Urea provides a way for nitrogen (which is found in ammonia) to leave the body.There are also many salts and other waste components in urine.Blood cells and sugar are normally not found in urine, but they can indicate glomerulus injury and diabetes mellitus, respectively.

Physiology of the kidney: This illustration illustrates physiology of the kidney by demonstrating where some types of diuretics act, and what they do.

Key Takeaways

Glioblasts Pressure in a fluid system

Glomerular filtration is the first step in forming urine and is the basic physiologic function of the kidneys.In the kidney, glomerular capillaries filter blood, removing fluid, ions, glucose, and waste products.

The body reabsorbed many of these materials as the fluid traveled through the nephron, but the material that wasn't reabsorbed was expelled out in urine.

Glomerulus Structure


Plasma enters the afferent arteriole and flows into the glomerulus, a cluster of intertwined capillaries.

Vistrial layer - located underneath glomerular basement membrane - consists of podocytes that form small slits for fluid to pass through into nephron.

The efferent arterioles then leave the kidney, and become capillaries meant for kidney-oxygen exchange and reabsorption before becoming venous circulation.Also, positively charged podocytes will hinder the filtration of negatively charged particles (such as albumins).

The Mechanisms of Filtration

Renal ultrafiltration is the process of glomerular filtration.

A combination of osmotic and hydrostatic pressure influences how molecules are filtered in the glomerulus. The difference between the two is the effective pressure of the glomerulus, which determines the force by which molecules are filtered.The glomeruluar filtration rate is affected by these factors, as well as others.

Regulation of Glomerular Filtration Rate

GFR regulation requires a mechanism that can detect and correct an inappropriate GFR.

Learning Objectives

List the conditions that affect renal glomerular filtration rate (GFR) and the manner in which it is regulated

Key Takeaways

This is Bowman's capsule Osmosis

Glomerular Filtration Rate

GFR is the measure of how much filtrate is produced by both kidneys per minute by all the renal corpuscles.Because glomerular filtration rate is directly correlated to pressure gradients in glomeruli, changes in pressure will modify GFR.

As GFR is also an indicator of urine production, a higher GFR will result in more urine being produced, and vice versa.

As a result of the Starling equation, GFR is:

No rewrites were found

.Changes in either glomerulus or Bowman's capsule pressure will affect GFR.

Hydrostatic Pressure Changes

Changes in hydrostatic pressure have the potential to affect GFR, as they affect blood flow to the glomerulus.During hydrostatic pressure changes within the glomerulus, the GFR is most sensitive.This is clear in body-wide examples like blood volume.

.Blood volume increases with higher blood pressure, which will be carried through the afferent arteriole into the glomerulus, increasing GFR.Dehydration decreases the GFR of people with low blood volume.

Similarly, changes in pressure within the afferent and efferent arterioles that go into and out of the glomerulus itself will affect GFR.Increased blood flow (and hydrostatic pressure) in the glomerulus will result from afferent arteriole vasodilatation and efferent arteriole vasoconstriction.Vasoconstriction in the afferent arteriole and vasodilation in the efferent arteriole both decrease GFR.

The Bowman's capsule space exerts its own hydrostatic pressure on the glomerulus.GFR decreases with an increased Bowman's capsule hydrostatic pressure, whereas GFR increases with a decreased Bowman's capsule hydrostatic pressure.

Eventually, the accumulation of fluid inside the nephrons is caused by an obstruction of the ureter.A Bowman's capsule obstruction will cause hydrostatic pressure to increase and consequently lower GFR.

Osmotic Pressure Changes

In the case of filtration, osmotic pressure is the force exerted by proteins, which forces water into the filter.An increase in serum albumin in the bloodstream increases osmotic pressure in the glomerulus and decreases GFR, and vice versa.

Due to the fact that albumin cannot enter Bowman's capsule, the osmotic pressure in Bowman's space is generally nonexistent, and is therefore removed when calculating GFR.Due to increased Bowman's capsule osmotic pressure in certain kidney diseases, the basement membrane may be damaged (becoming leaky to proteins), resulting in decreased GFR.

An image of glomerulus filtration shows how the glomerulus (red) filters fluid into the Bowman's capsule (blue), which sends it through the nephron (yellow).GFR refers to the rate at which this filtration occurs.

GFR Feedback

GFR is just one of the ways blood volume and blood pressure may be maintained at an optimal level.Reduced GFR, in particular, is one of the factors that will activate the renin-angiotensin feedback system, a complex process that will increase blood pressure, blood volume, and GFR.A low GFR also stimulates this system, as well as a low blood pressure alone.

Tubular Reabsorption

Tubercular reabsorption occurs when solutes and water leave tubular fluids and are transported to the blood.

Key Takeaways

It is made of 2 molecules: NA+ and K+ Capillaries in the tubules


Filtrate, a fluid filtered from blood, passes through the nephron and much of it is reabsorbed.

Mechanisms of Reabsorption

Passive or active reabsorption occurs in the nephron, and the specific permeability of each part of the nephron varies depending on what is reabsorbed.Mechanisms that result in the reabsorption of substances into peri-tubular capillaries include:

As part of these processes, the substance passes through the luminal barrier and the basolateral membrane, two plasma membranes of kidney epithelial cells, and into the peritubular capillaries on the other side.Various substances can pass through tiny gaps between epithelial cells in the kidney called tight junctions.

Osmolarity Changes

As filtrate passes through the nephron, its osmolarity (ion concentration) changes as ions and water are absorbed.This is the same osmolarity as normal plasma entering the proximal convoluted tubule, which is 300 mOsm/L.

All the glucose from the filtrate is reabsorbed along with an equally high concentration of ions and water (through cotransport) in the proximal convoluted tubules, so that the filtrate remains at 300 mOsm/L when it leaves the tubule.With the water leaving the descending loop of Henle, as well as being impermeable to ions, the filtrate osmolarity is reduced to 1200 mOsm/L.Osmolarity falls to 100–200 mOsm/L in the ascending loop of Henle, a capillary that permits ions to pass through but not water.

Depending on hormone stimulation, a variable amount of ions and water is reabsorbed in the distal convoluted tubule and collecting duct.In this way, the final osmolarity of urine depends on the water permeability of the final collecting tubules and ducts, which is determined by homeostasis.

The mechanisms of reabsorption occurring in the nephron are shown in a diagram.

Key Takeaways

A duct that collects There are capillaries in the peritubular area A luminance

It is the opposite process of reabsorption since it involves the transfer of materials from the capillaries to the renal tubular lumen.This occurs mainly by passive diffusion and active transport.

The majority of substances secreted are waste products.Urine is the substance left in the collecting duct after absorption and secretion.

Mechanisms of Secretion

As with reabsorption, the mechanisms responsible for secretion occur in the opposite direction.

The difference between renal secretion and renal absorption is that renal secretion involves the removal of blood substances, as opposed to retaining them.Tubular fluid contains substances that are secreted into the body for removal:

Hydrogen Ion Secretion

H+ and NH4+ are secreted into the tubular fluid from the blood during pH regulation.Additionally, the movement of these ions helps to conserve sodium bicarbonate (NaHCO3).In urine, the pH usually ranges from 6.0 to 6.5, while the optimum pH ranges from 7.35 to 7.45 for blood.

The respiratory system regulates pH primarily by exchanging carbon dioxide (a carbonic acid component of blood), but tubular secretion also plays an important role.

Following Secretion

The urine produced by these three processes leaves the kidney through the ureter, and is stored in the bladder until it is excreted through the urethra.At this point, it contains approximately one percent of the original filtered volume, primarily water with a small amount of urea, creatinine, and variable concentrations of ions.