Renal System Physiology

The kidneys are excretory and regulatory organs. By excreting water and solutes, the kidneys are responsible for ridding the body of waste products and excess water. The kidneys regulate 1) plasma osmolarity, or the concen- tration of a solution expressed as osmoles of solute per liter of solvent; 2) plasma volume; 3) acid-base balance; 4) electrolyte balance; 5) excretion of metabolic wastes and foreign materials; and 6) the production and secretion of hormones that regulate osmolarity and electrolyte balance. All these activities are extremely im- portant to maintaining homeostasis in the body. 3 The kidneys are located between the posterior abdominal wall and the ab- dominal peritoneum. Although many textbooks depict the kidneys directly across from each other, the right kidney is actually slightly lower than the left. Each human kidney contains approximately 1.2 million nephrons, the functional units of the kidney. Each nephron is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries, called the glomeru- lus, which is enclosed by a fluid-filled capsule called Bowman’s capsule. An af- ferent arteriole supplies blood to the glomerulus. As blood flows through the glomerular capillaries, protein-free plasma filters into the Bowman’s capsule, a process called glomerular filtration. An efferent arteriole then drains the glomerulus of the remaining blood. The filtrate flows from Bowman’s capsule to the start of the renal tubule, called the proximal convoluted tubule, then on to the proximal straight tubule, followed by the loop of Henle, a U-shaped hairpin loop. The filtrate then flows into the distal convoluted tubule before reaching the con- necting tubule and the collecting duct, where urine collects. The distal tubule and collecting duct are composed of two cell types: principal cells and intercalated cells. Principal cells reabsorb Na+ and water and secrete K+. Intercalated cells secrete either H+ or HCO - and are, therefore, very important in the regulation of the acid/base balance. Glomerular Filtration Let’s take a closer look at what happens during glomerular filtration. Blood en- ters the glomerulus from the afferent arteriole. Starling forces (hydrostatic and osmotic pressure gradients) drive protein-free plasma from the blood across the walls of the glomerular capillaries and into the Bowman’s capsule. The glomeru- lar filtration rate is an index of kidney function. In humans, the filtration rate