Author: drbeeneducation

Glomerular Apparatus

mesengial cells

General Considerations

All nephrons have one glomerular apparatus each. Each glomerular apparatus is a critical contributor to the renal function. Here are the functions performed by the glomerular apparatus. Notice that the tubular secretion and reabsorption is not performed by the glomerular apparatus.

  1. Filtration of the substances. This is the first step in the urine formation. (Filtration Barrier.)
  2. Prevention of filtration for the substances that we don’t want to loose. For example blood cells and proteins. (Filtration Barrier.)
  3. Blood pressure measurement and maintenance. (Juxtaglomerular cells.)
  4. Blood osmolality measurement and maintenance. (Macule Densa cells.)
  5. Renin release to help maintain body fluid volume and blood pressure. (Macula Densa cells.)
  6. Blood flow regulation to the glomerulus to control the rate of filtration. (Mesangial cells.)

Structures forming Glomerular Apparatus

This apparatus is formed by following structures:

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Urine is formed in the kidney’s nephrons by a combination of following three processes:

  • Filtration
  • Tubular Secretion
  • Reabsorption

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:

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Kidneys perform following categories of functions:

  • Homeostasis
  • Urine Formation
  • Hormone Secretion


Kidney play a critical role to maintain healthy serum, interstitial, and intracellular  environments. Here are the some of the homeostatic functions that kidneys perform:

  • Acid-base balances (Davenport diagram is a must for medical students to understand.)
  • Serum electrolyte concentrations in varying external and internal situations. (Knowing kidney’s role to maintain Na+, K+, Ca++, H+, PO4, NH3, etc. is critical.)
  • Total body fluid and its distribution in various compartments is influenced by the kidneys.
  • Body fluid osmolarity

Urine Formation

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Restrictive lung diseases are usually chronic, diffused, lung interstitial diseases, usually effecting the most peripheral and delicate interstitium of the alveolar walls.

What is pulmonary Interstitium?

Pulmonary interstitium is composed of basement membranes of the alveolar epithelial cells, vascular endothelial cells, and the tissue between them, mostly made of elastic fibers, collagen, fibroblasts that make more elastic fibers, smooth muscle cells, mast cells, and sometimes mononuclear cells.

In the peripheral parts of the lung where the alveoli are numerous the interstitium is thin and delicate. Sometimes composed of only the two basement membranes fused together.

Respiratory membrane

Figure: Components of the interstitium. Note the alveolar macrophage is not a part of this interstitium. Also, in peripheral areas the interstitium is minimal causing the two basement membranes to fuse.

Hallmark of the restrictive lung diseases

Reduced compliance of the lungs. That is, the lungs are so stiff that they cannot expand easily. Patient has to exert more energy to pull air in the lungs. This is dyspnea.

Damage to the alveolar epithelium and pulmonary vessels hinders gas exchange leading to hypoxia.

Causes of the restrictive lung diseases

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