Urine is formed in the kidney’s nephrons by a combination of following three processes:
- Tubular Secretion
Note: some books mention excretion and water conservation as processes as well. We feel that the excretion is not a process of urine formation, instead it is waste product discarding function. Similarly we feel that the water conservation is achieved by the reabsorption and hence is not a process in itself.
Usually urine formation is measured as volume of urine produced each minute. There are times when we measure the urine output during longer periods of time, for example 24 hours.
Recall that the output measured in set time intervals is called a rate. For example volume of a substance X excreted in the urine each minute will be called the rate of excretion of the substance X per minute.
As we study the urine formation we will determine the excretion rate of various substances by the following formula:
Excretion rate of substance X = Filtration rate of X + Secretion rate of X – Reabsorption rate of X
Nephron is the functional unit of a kidney. Each kidney contains about one million nephrons (1,000,000 or 10^6). Each individual nephron is able to execute all processes to form urine. A nephron consists of Bowman’s capsule, proximal convoluted tubule, loop of Henle, and the distal convoluted tubule. Distal convoluted tubule opens in the collecting duct.
Urine from the collecting ducts passes through the urinary tract and is excreted by the process of micturition. Surface cells of nephrons are epithelial because the nephrons are continuous with the urinary pathway that opens outside the body.
The loop of Henle enters in the medulla of the kidney. Remaining parts of the nephron are located in the renal cortex.
Types of Nephrons
- Cortical Nephrons (70%-80%)
- Juxtamedullary Nephrons (20%-30%)
70%-80% of the nephrons are cortical. Their primary function is urine formation. Their loop of Henle penetrates for a small distance in the renal medulla. Their participation in the concentration process is minimum. See the blood supply section below to note the difference of the blood supply between the two types of the nephrons.
20%-30% of the nephrons are located very close to the medulla. This is why these are called Juxtamedullary (near the medulla). These nephrons have long loop of Henle that traverses deep in the medulla. Capillary network around these nephrons is designed to support the process of urine concentration.
Physiologic Blood Supply of the Kidney
About 22% of the cardiac output in healthy individuals goes to both of their kidneys. Hence, kidneys receive about 1100ml/minute blood supply for a person with 5L/m cardiac output. Blood flow to the kidneys can be calculated by the following formula:
Blood supply to both kidneys = CO x 0.22
= 5L/m x 0.22
Glomerulus is a tuft of capillaries present in the Bowman’s capsule of a nephron. Blood arrives in the glomerulus via the afferent arterioles. Efferent arterioles take the blood out of the glomerulus and into the peritubular capillaries.
Glomerulus is covered by the visceral epithelial cells of the Bowman’s capsule. Glomerular capillary endothelial cells, basement membrane, and the visceral epithelial cells o the Bowman’s capsule form the interface between the blood and the urine compartments.
Hydrostatic pressure in the glomerular capillaries is higher (60 mmHg) than the other capillaries in the body (30 mmHg).
Cortical Nephron vs. Juxtamedullary Blood Supply
Efferent arterioles from the glomerulus of the cortical nephron form a rich capillary meshwork around the corresponding nephron. This meshwork finally coalesce to form the venous end of the blood supply. These peritubular capillaries help with the urine formation, but have limited role for the urine concentration.
Efferent arterioles from the glomeruli of the Juxtamedullary nephrons on the other hand form the capillary network in the early part, however, the efferent arteriole extend deep in the medulla next to the corresponding nephron’s loop of Henle. This long extension is called vasa recta. Vasa recta helps juxtamedullary nephron with the urine concentration.
Stages of the Urine Formation
Water and other substances are filtered from the glomerular capillaries to enter the bowman space. Initially substances are filtered regardless of the body’s need of them. Think of it as kidney decide to throw everything out in the ruine. As this fluid moves through the nephron reabsorptive processes reuptake the substances that we need and bring them back into the blood stream. Anything that is not picked up is excreted.
As the fluid moves through the nephrons various active and passive processes pick up the substances that are needed in the body. Some substances are completely picked up, while only a partial amount is picked up for some substances.
It is important to note that the proximal convoluted tubules and medullary thick ascending limb of the loop of Henle are the major sites of active reabsorption. These areas require a lot of ATP to function. For this reason hypoxic/ischemic conditions damage cells in these two sites before damaging cells in the other parts of the nephron.
Some of the substances especially poisons and drugs are actively transported from the peritubular capillaries into the lumen of the nephron. This process is called tubular section.
- The renal processes are not uniformly applied to all substances.
- These processes can be altered according to the need of a substance. For example if we need to eliminate a substance then then filtration and secretion can increase, and reabsorption can decrease.
Schematic for further discussion
We will use following schematic diagram to represent this setup.
Some Examples of the Renal Processes
- Glucose: filtered and completely reabsorbed
- Inulin: filtered, neither reabsorbed nor secreted.
- PAH: 20%-30% filtered, rest is completely secreted, not reabsorbed.