Figure 1. Kidney from Labster’s theory page
Renal physiology is the science of the kidney. It covers all the functions of the kidney, including filtering waste products from the bloodstream and recycling nutrients and ions. In addition, the kidneys play an important role in regulating blood pressure, electrolyte balance, and red blood cell production.
The kidney structure is complex and multilayered, and its functional unit is called the nephron. This is achieved when oxygenated blood enters the kidneys through the renal arteries and then flows through the nephrons, where waste products and excess water are filtered from the bloodstream and sent to the ureters and eventually the bladder. Thus, in humans, the kidneys process up to 180 liters per day, with most of the filtrate being reabsorbed into the bloodstream, resulting in an average typical urine volume of only 3- 5 liters per day.
The kidney has a characteristic structure whose primary filter is a nephron located in the medulla and cortex of the inner tissue. In the outer layer we have:
Renal fascia: dense connective tissue that surrounds the kidneys and adrenal glands
Perirenal fat capsule: a layer of adipose tissue that protects the kidneys from damage and trauma
Kidney capsule: a tough fibrous layer that encloses the functional tissues of the kidney
The internal and functional network consists of:
Outer cortex: includes most of the blood vessels and collecting ducts, as well as some structures of the nephron, such as renal blood cells and parts of the proximal and distal convoluted tubules. This is where most of the blood is filtered and larger molecules such as sugar and amino acids are returned to the bloodstream.
Medulla: the innermost tissue of the kidney, divided into recognizable structures called pyramids. Within this pyramid, poorer in blood vessels, are other structures of the renal nephrons, especially the loop of Henle. This is where most of the ions and water are reabsorbed by the body.
Finally, in the concave area of the kidney, there is the hilum where the arteries, veins, and ureters join into the kidney tissue.
Find out why this can be a frustrating topic for teachers and students, five suggestions for changing it, and ideas for why a virtual lab can make things easier.
There are three distinct explanations for why kidney physiology can be hard for even the most energetic student.
We teach students about the renal system and its primary function of removing bodily waste from the blood. However, they cannot see this process even if the kidney is dissected because the process takes place at the cellular level of our body. Everything seems like nonsense to the students.
Figure 2: A snippet from the renal physiology simulation by Labster showing the kidney after a dissection. It is available for School and University/College classes.
Renal nephrons are the main structures that filter the bloodstream, recycling important molecules and ions before metabolites are excreted into the urinary tract. Renal nephrons are found in the medulla ( in the renal pyramids) and the cortical tissue of the kidney. It is estimated that one human kidney contains up to one million nephrons.
Structurally, the nephron is a long tubule, one end of which ends in a spherical shape called Bowman's capsule. The other end connects to the renal pelvis, which leads to the ureter. Going by the filtering, and recycling process, the structure is as follows:
Renal corpuscle: This structure includes Bowman's capsule and a complex network of blood vessels called the glomerulus. Blood flows from the arterioles to the glomerulus, where pressure forces fluid out of the vessels through the three-layer filtration barrier. Most of the fluid enters Bowman's capsule and convoluted tubules, whereas red blood cells and large molecules such as proteins are retained in the bloodstream and exit the blood cells via the arterioles.
Proximal convoluted tubule: Just behind the Renal corpuscle ( hence the name proximal), this part of the nephron is responsible for most of the reabsorption of molecules and ions back into the bloodstream. Glucose, amino acids, and ions such, as sodium, phosphate, potassium, magnesium, and calcium, are mainly absorbed here. The proximal convoluted tubule also secretes urea and ammonium, which are excreted in the urine, and creatine.
Proximal straight tubule: This section mainly absorbs residual phosphate.
Loop of Henle: This structure includes a descending thin loop, a bend, and an ascending thin loop. It passively absorbs water but not ions, thereby concentrating the urine. During warmer climates, the relevance of this part is heightened.
Thick ascending limb: In this section, Na+, K+, and Cl- are actively absorbed through the epithelial wall.
Distal convoluted tubule: structure furthest from renal blood cells ( hence called distal), reabsorbs sodium and chloride and its water permeability depends on the action of antidiuretic hormone (ADH).
Collecting duct: This terminal structure reabsorbs water from the filtrate when necessary to maintain body fluid homeostasis.
Overall, the nephron is a highly efficient recycling station, retaining more than 90% of essential nutrients and ions, and water to, maintain homeostasis and help regulate body fluids.
The first stage of the filtration process in the kidney occurs in a structure known as the glomerulus, in the part of the nephron located in the cortex. Blood enters the tiny glomerular capillaries within Bowman's capsule from the afferent arteriole. Because the capillaries are squeezed between the two narrowed arterioles, strong hydrostatic pressure "pushes" water, ions, and other small molecules across the filter membrane into the nephron duct and further into the proximal tubule. It is a passive process in which the membrane acts as a special net to facilitate the filtration of metabolites in the bloodstream.
The amount of filtrate that crosses the glomerular membrane in one minute for both kidneys is called the glomerular filtration rate (GFR). To demonstrate the efficiency of glomerular filtration, the kidneys filter up to 180 liters per day in the average person, although 99% of water is reabsorbed, limiting the typical daily urine volume to 1-2 liters.
GFR depends primarily on the difference between external pressure ( pressure exerted by the beating heart and contracting arterioles) and internal pressure (hydrostatic pressure exerted by the tubules in Bowman's capsule). This difference is called the net filtration pressure. To measure the filtration rate, we can also use the creatinine loss in the bloodstream. Creatinine is a by-product of metabolic processes in muscles and is almost completely filtered into the urine by the kidneys. By measuring plasma and urine creatinine concentrations and urine volume, we can estimate the glomerular filtration rate. However, this is presumably because some creatinine is actively secreted into the nephrons and not as a result of passive glomerular filtration which contributes to the final concentration of creatinine in the excreted urine.
Figure 3: Glomerular filtration rate estimation
Now that you have the right foundation, let's review five ways to make kidney physiology class engaging, easy, and fun for you and your students.
Very little was known about renal physiology until the mid- 19th century. The turning point occurred in 1842 when the famous German physiologist and physician Karl Ludwig (1816-1895) proposed a theory of a two-step process (filtration and resorption) that lead to the excretion of urine. This document focuses on Ludwig and the time in which he lived. It also tells the story of the study of anatomy in the 17th century and the chemical approach to urine and kidney physiology in the 18th century that preceded Carl Ludwig's research. The theory of filtration and reabsorption contradicts another theory (the theory of secretion) which considers the kidneys to be glands, like the salivary glands. The origins of this theory go back to Marcello Malpighi (1628-1694) in the Renaissance but continued by William Bowman (1816-1892) and especially Rudolf Heidenhain (1834-1897) in the 19th century. Research in the 1920s and 1930s marked the end of the struggle between the two theories. It proved Ludwig right and posthumously gave him the recognition he deserved.
Here we will use the term antidiuretic hormone to summarize the two main hormones that can reduce urine production in the human kidney. The first is vasopressin, also called ADH (antidiuretic hormone), and the second is aldosterone. They have obvious medicinal importance because of their important role in regulating body water.
ADH is a peptide hormone produced in the hypothalamus that acts directly on the kidneys. At higher than normal blood flow osmolarities ( higher than normal concentrations of ions and metabolites), ADH is released from neurons and, upon reaching the renal nephrons, acts on the distal tubule to cause a higher rate of water reabsorption. The extra water retained in the blood dilutes the metabolites and lowers the osmolarity to normal levels, thereby maintaining homeostasis. The obvious side effect is a decrease in urinary volume excretion, hence the antidiuretic effect.
Aldosterone is also a hormone, this time it is produced in the adrenal glands above the kidneys and acts on the distal tubule in the nephron. Below normal blood pressure or an increase in plasma potassium concentration triggers the production and secretion of aldosterone, among other things. Aldosterone, in turn, stimulates the transmembrane Na+/K+ pump of cells lining the distal renal tubule, causing reabsorption of Na+ (and hence water) and excretion of K+. Reabsorption of Na+ and water causes an increase in blood volume and thus an increase in blood pressure and a decrease in the plasma potassium concentration to normal values. Aldosterone also regulates several other processes in the gut, saliva, and sweat glands related to K+ secretion.
Hypertension, also known as high or raised blood pressure, describes abnormally high blood pressure and is a physiological condition in which the walls of the arteries are constantly under pressure. The cause of high blood pressure remains unclear, but many factors have been found to worsen the condition or increase the likelihood of its occurrence. These factors are:
lack of physical activity
ethnicity and family history
smoking and drinking habits
The medical standard for diagnosing hypertension is a blood pressure of 130/ 90 mmHg (systolic/diastolic) or higher outside of exercise compared to a normal value of 120/ 80 mmHg. When blood pressure reaches the level of 180/120, the condition becomes a hypertensive crisis and requires immediate treatment. Symptoms of high blood pressure are not always obvious and raised pressure can cause damage over a long time. Possible complications are:
Higher risk of stroke
Higher chance of heart attack
There are various kinds of treatment depending on the severity of the disease and the combination with medications already being taken for other ailments. However, the most common high blood pressure medications can be broken down into 5 categories.
Diuretics are often the first-line treatment for high blood pressure. By facilitating the release of salts ( such as sodium ions) from the body, it induces the secretion of larger amounts of urine, thereby reducing blood volume and blood pressure.
Angiotensin-converting enzyme (ACE) inhibitors. These inhibitors block the enzyme that converts angiotensin I to angiotensin II. Angiotensin II is a hormone that increases blood pressure by narrowing arterioles (vasoconstriction) and decreasing urine output.
Angiotensin receptor blocker: This compound blocks the angiotensin II receptor and blocks the effects of this hormone.
Calcium channel blockers: Calcium is usually higher in the blood than in the cells, and the entry of calcium into cells through these channels triggers a cellular response. Blocking these types of calcium channels in the heart lowers calcium levels in heart cells, reduces the force of heart contractions, and lowers blood pressure. Blocking other types of calcium channels lowers calcium levels in arteriolar smooth muscle, causing arterioles to widen (vasodilation) and thereby lowering blood pressure.
Beta-blockers: They block beta receptors for epinephrine in the heart, which lowers heart rate and blood pressure.
A good mnemonic for the structural components of the nephron in the progressive sequence RiPe Let DiCe where lowercase letters are omitted and P appears twice
R - Renal corpuscle,
P - Proximal convoluted tubule,
P - Proximal straight tubule,
L - Loop of Henle,
T - Thick ascending limb
D - Distal convoluted tubule
C - Collecting duct
A good resource for memorizing high blood pressure medications is the ABCD, where "A" represents two categories of medications.
A - Angiotensin-converting enzyme
A - Angiotensin receptor blocker
B - Beta-blockers
C - Calcium channel blockers
D - Diuretics
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