Kidney Anatomy and Physiology: Calyces, Pelvis, Nephron
The kidneys are complex organs responsible for filtering blood and maintaining homeostasis. This document details their key anatomical structures and physiological processes.
Kidney Anatomy
Calyces and Renal Pelvis
Calyces: These are smaller, cup-shaped structures where urine collection begins. Minor calyces surround each renal papilla (the tip of a renal pyramid). They are small membranous ducts that open into the major calyces.
Major Calyces: Typically, there are three major calyces: superior, middle, and inferior.
Renal Pelvis: This is a flattened, funnel-shaped structure oriented downward and inward. It connects to the ureter at the ureteropelvic junction.
Kidney Relations
Superiorly: The kidneys are related to the adrenal glands and diaphragm.
Anteriorly:
- Right Kidney: Related to the liver, right colic flexure, and duodenum.
- Left Kidney: Related to the spleen, tail of the pancreas, and descending colon.
Renal Blood Supply
Each kidney receives blood from a renal artery, a direct branch of the aorta. Renal blood flow is approximately 1,200 mL/min. The renal artery divides into segmental arteries, which further divide into interlobar arteries passing through the renal columns. These give rise to arcuate arteries at the corticomedullary junction, which then branch into interlobular arteries. The interlobular arteries penetrate the cortex and give off afferent arterioles.
Renal veins originate in the renal pyramids and drain into the inferior vena cava.
Kidney Physiology: The Nephron
The nephron is the basic functional unit of the kidney. It is responsible for three primary processes:
1. Filtration
Blood flows through the glomerular capillaries under high pressure. This pressure forces plasma and small solutes into the Bowman’s capsule, forming the filtrate. Factors affecting filtration include:
- Hydrostatic pressure
- Large surface area of glomerular capillaries
- High permeability of the filtration membrane
Histological Correlation: The wall of Bowman’s capsule consists of specialized epithelial cells called podocytes. These cells have numerous extensions that cover most of the glomerular capillary walls. The filtration membrane is formed by the fenestrated endothelium of the glomerular capillaries and the podocytes.
2. Reabsorption
Reabsorption is the process by which filtered substances are returned to the blood. This occurs primarily in the proximal convoluted tubule (PCT) and the loop of Henle. Factors affecting reabsorption include the concentration of substances in the blood.
Histological Correlation: The simple cuboidal epithelial cells lining the renal tubules are adapted for reabsorption. They have abundant microvilli to increase the surface area for reabsorption.
3. Secretion
Secretion is the passage of substances from the blood into the tubular fluid, opposite to reabsorption. This occurs primarily in the distal convoluted tubule (DCT) and collecting ducts. Factors affecting secretion include high concentrations of potassium and hydrogen ions.
Histological Correlation: Cells in these regions have abundant endoplasmic reticulum and Golgi apparatus for active transport and secretion.
Meiosis
Meiosis is a type of cell division that reduces the chromosome number by half, creating four haploid cells from one diploid cell. This process is essential for sexual reproduction. It occurs in two main stages: Meiosis I and Meiosis II. Both stages include prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis. There is no DNA replication (S phase) between these two stages. Errors in meiosis can lead to chromosomal abnormalities. Meiosis ensures that the chromosome number remains constant across generations.