Kidneys
The indentation of the bean-shaped organ is called the hilus. This is the point where the blood vessels, ureter, and nerves enter or exit the kidney.
Regions of the kidneys
- Renal cortex – outer region
- Renal medulla – the darker triangular structures located deep within the kidney forming the renal pyramids. Each pyramid is separated by renal columns. The lower ends point toward the renal pelvis.
- Renal pelvis – a basin that collects the urine and helps form the upper end of the ureter. The cup-like edges closest to the pyramids are called calyces (singular calyx), which collect the urine formed in the kidney.
Each nephron is composed of a renal tubule and blood vessels. Renal tubules consist of the Bowman's capsule, a C-shaped structure partially surrounded by a cluster of capillaries called a glomerulus. This goes on to form a highly coiled tubule called the proximal convoluted tubule, which dips toward the renal pelvis to form a hairpin-like structure called the loop of Henle. This contains a descending limb and an ascending limb. The ascending limb becomes the distal convoluted tubule. These tubules from several nephrons merge to form a collecting duct, which runs through the renal medulla to the calyx of the renal pelvis.
Blood flow to the kidney is into, through, and out of various structures: renal artery to smaller and smaller arteries, to afferent arteriole, to glomerulus, to efferent arteriole, to peritubular capillary, to the renal venules, to the larger veins, to the renal vein, and to the inferior vena cava.
Functions of the kidneys
- Excrete such nitrogenous waste as urea, ammonia, and creatinine.
- Regulate blood volume by determining the amount of water excreted.
- Help regulate the electrolyte content of the blood.
- Help regulate acid-base balance (blood pH) by controlling the excretion of hydrogen ions.
- Help regulate red blood cell production through the secretion of a hormone called erythropoietin. This hormone is secreted whenever the blood oxygen level decreases (hypoxia). Erythropoietin stimulates the red bone marrow to increase the rate of RBC production. With more RBCs in circulation, the oxygen-carrying capacity of the blood increases alleviating the hypoxic state.
- Help regulate blood pressure through the secretion of renin. When blood pressure decreases, the juxtaglomerular cells in the walls of the afferent arterioles secrete an enzyme called renin, which initiates the renin-angiotensin mechanism to increase blood pressure. The end product is angiotensin II which causes vasoconstriction and increases the secretion of aldosterone, both of which help raise blood pressure. In this way, the kidneys ensure that the heart has enough blood to pump in order to maintain cardiac output and blood pressure.)
- Sequence of Renin-Angiotensin Mechanism:
- Decreased blood pressure stimulates the kidneys to secrete renin.
- Renin splits the plasma protein angiotensinogen (synthesized by the liver) to angiotensin I.
- Angiotensin I is converted to angiotensin II by an enzyme found primarily in lung tissue.
- Angiotensin II causes vasoconstriction and stimulates the adrenal cortex to secrete aldosterone.
- Sequence of Renin-Angiotensin Mechanism:
- Activation of Vitamin D. This vitamin exists in several structural forms which are converted to calciferol (D2) by the kidneys. Calciferol is the most active form of Vitamin D, increasing the absorption of calcium and phosphorus in the small intestine.
- PTH (parathyroid hormone) – parathyroid glands – promotes reabsorption of Ca+ ions from filtrate to the blood and excretion of phosphate ions into the filtrate
- ADH (antidiuretic hormone) – posterior pituitary – promotes reabsorption of water from the filtrate to the blood
- Aldosterone - adrenal cortex – promotes reabsorption of Na+ ions from the filtrate to the blood and excretion of K+into the filtrate. Water is reabsorbed following the reabsorption of sodium.
- ANH (atrial natriuretic hormone) – atria of the heart – decreases reabsorption of Na+ ions, which remain in the filtrate. More sodium and water are eliminated in the urine.
