Renal Module
Kidney Functions
Learning Objectives
By the end of this section, you will be able to:
- Describe the characteristics of a normal urine sample, including normal range of pH, osmolarity, and volume
- Explain the normal physiological role and the five (5) functions of kidneys:
- urine formation
- regulation of fluid and electrolyte balance
- excretion of metabolic waste products (urea and creatinine)
- regulation of acid-base balance
- production of hormones (calcitriol, erythropoietin, renin)
- Describe the utility of a selection of clinical tests associated with renal function
Urine Formation
The physiologic goal is to modify the composition of the plasma and, in doing so, produce the waste product urine. Different parts of the nephron utilize specific processes to produce urine: filtration, reabsorption, and secretion.
- Glomerular Filtration: Water and solutes move from the capillary lumen to Bowman’s space across 3 layers of the glomerular filter.
The outermost part of Bowman’s capsule, the parietal layer, is a simple squamous epithelium. It transitions onto the glomerular capillaries in an intimate embrace to form the visceral layer of the capsule. Here, the cells are not squamous, but uniquely shaped cells (podocytes) extending finger-like arms (pedicels) to cover the glomerular capillaries (Figure 1). These projections interdigitate to form filtration slits, leaving small gaps between the digits to form a sieve. Where the fenestrae (windows) in the glomerular capillaries match the spaces between the podocyte “fingers,” the only thing separating the capillary lumen and the lumen of Bowman’s capsule is their shared basement membrane (Figure 2). These three features comprise what is known as the filtration membrane. Overall, filtration is regulated by fenestrations in capillary endothelial cells, podocytes with filtration slits, membrane charge, and the basement membrane between capillary cells. The result is the creation of a filtrate that does not contain cells or large proteins, and has a slight predominance of positively charged substances.Figure 2. Fenestrated Capillary. Fenestrations allow many substances to diffuse from the blood based primarily on size. - Reabsorption back into the blood: lots in the proximal convoluted tubules (PCT) and some in Loop of Henle; fine-tuning after that [distal convoluted tubules (DCT) and collecting duct]
The filtrate enters the PCT where absorption and secretion of several substances occur. The descending and ascending limbs of the loop of Henle consist of thick and thin segments. Absorption and secretion continue in the DCT but to a lesser extent than in the PCT. - Secretion from blood into tubule lumen: in the PCT, DCT & collecting duct. What remains in the lumen from the collecting duct on is urine which will be excreted.
Various portions of the nephron differ in their capacity to reabsorb water and specific solutes. While much of the reabsorption and secretion occur passively based on con