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.
The nephron is the urine-making unit of the kidney. There are about one million nephrons in each kidney. The numbers do not increase after birth nor are they replaced if damaged. Any kidney growth that takes place is the result of an enlargement of the nephrons and not from an increase in numbers.
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 calcitriol (D3) by the kidneys. Calcitriol is the most active hormone form of Vitamin D, increasing the absorption of calcium and phosphorus in the small intestine.
Kidney hormones and their effects
- 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.
The kidneys are not primarily endocrine organs, but are involved in the production of hormones and affected by them.
Major hormones controlling kidney function include:
- ADH (antidiuretic hormone), secreted by the posterior pituitary (neurohypophysis) gland, stimulates the reabsorption of water, and allows the kidneys to concentrate urine.
- Aldosterone is a hormone secreted by the adrenal cortex which regulates electrolyte and water balance. It stimulates the reabsorption of sodium and water and the excretion of potassium. It also increases blood volume thereby increasing blood pressure.
- ANF (atrial natriuretic factor) is also known as ANP (atrial natriuretic peptide). It is secreted by the atria of the heart which then decreases the reabsorption of sodium. ANF also causes greater excretion of sodium and water by the kidney. It has the opposite effect from that of aldosterone and ADH.
- Angiotensin II, converted from angiotensin I, is a substance formed in the blood when renin is released by the kidney. It is a powerful vasocompressor and stimulator of aldosterone secretion.
- Calcitriol is a product converted in the kidneys after the release of D3 by the liver. It is important to calcium ion homeostasis. Its synthesis is dependent on the availability of O3.
- PTH (parathyroid hormone) is secreted by the parathyroid gland. It is involved with two electrolytes: the reabsorption of calcium and excretion of phosphate.
- REF (renal erythropoietic factor) or EPO (erythropoietin) is secreted by the kidney in the adult and by the liver in the fetus. It acts on the stem cells of the bone marrow to stimulate red blood cell production, and, along with renin, is involved in the regulation of blood pressure and volume.
- Renin is synthesized, stored, and secreted by the kidney. It is released in response to a decline in blood volume, blood pressure, or both, thereby, stabalizing blood pressure. Once in the bloodstream, it functions as an enzyme leading to the formation of angiotensin II, which has several functions, including the stimulation of aldosterone production by the adrenal cortex. It is the hormone that initiates a series of events called the renin-angiotensin-aldosterone system.