Kidneys

Kidneys

Bean-shaped excretory organs in vertebrates

Kidney function

-Part of the urinary system

-Kidneys filter waste (especially urea) from the blood and excrete them, along with water, as urine

Nephrology

Medical field that studies kidneys and diseases affecting the kidney

Kidney-related

Renal

Kidney location

-In the posterior part of the abdomen

-One on each side of the spine

-Kidneys are retroperitoneal: they lie behind the peritoneum (the lining of the abdominal cavity)

Right kidney location

Sits just below the liver

Left kidney location

Below the diaphragm and adjacent to the spleen

Adrenal gland

Located above each kidney

Kidney protection

-Upper parts of kidneys are partially protected by the 11th and 12th ribs

-Each kidney is surrounded by 2 layers of fat (perirenal fat and pararenal fat) to cushion them

Kidney structure

Each kidney is about 11cm long, 5 cm thick, and weigh around 150g

Hilium

-Opening that each kidney has

-Admits the renal artery, the renal veins, nerves, and the ureter

Renal cortex

-Outermost portion of the kidney

-Sits directly beneath the kidney’s loose connective tissue capsule

Renal medulla

• Deep to the cortex

• Divided into 10-20 renal pyramids in humans

• Each pyramid together with the associated overlying cortex forms a renal lobe

• The tip of each pyramid (called a papilla) empties into the calyx, and the calyces empty into the renal pelvis

  • Pelvis transmits urine to the urinary bladder via the ureter

Kidney blood supply

-Each kidney receives its blood supply from a renal artery, two of which branch from the abdominal aorta

Renal artery

-Upon entering the hilum of each kidney, the renal artery divides into smaller arteries

Afferent arterioles

-Branch off of the arteries

-Supply the glomerular capillaries, which drain into efferent arterioles

Efferent arterioles

-Divide into peritubular capillaries that provide an extensive blood supply to the cortex

Blood from capillaries

Blood from these capillaries collects in renal venules and leaves the kidney via the renal vein

Nephron

-Basic functional unit of the kidney

-There are more than a million in each normal adult human kidney

Nephron function

Nephrons regulate water and soluble matter (especially electrolytes) in the body by first filtering the blood, then reabsorbing some necessary fluid and molecules while secreting other, unneeded molecules

How is reabsorption and secretion accomplished?

-Accomplished with both cotransport and countertransport mechanisms established in the nephron and associated collecting ducts

Glomerulus

-Main filter of the nephron and is located within the Bowman’s capsule

-Looks like a twisted mass of tiny tubes through which the blood passes

-Is semipermeable: allows water and soluble wastes to pass through and be excreted out of the Bowman’s capsule as urine

-Filtered blood passes out of the glomerulus into the efferent arteriole

Bowman’s Capsule

-Contains the primary filtering device of the nephron, the glomerulus

-Blood is transported into the Bowman’s capsule from the afferent arteriole (branching off of the interlobular artery)

-Within the capsule, the blood is filtered through the glomerulus and then passes out via the efferent arteriole

-The filtered water and aqueous wastes are passed out of the Bowman’s capsule into the proximal convoluted tubule

Reabsorption and secretion + Excretion

• Reabsorption and secretion

-Most of the water and dissolved solutes that enter the glomerulus filtrate must be returned to the blood

-Reabsorption of glucose and amino acids is driven by active transport carriers

-Secretion of waste products involves transport across capillary membranes and kidney tubules

• Excretion

Proximal convoluted tubule

-Globular filtrate passes first into the proximal convolute tubule

-Energy dependent mechanisms (active transport) reabsorb all of the following: glucose, amino acids, potassium, sodium (about 80% reabsorbed), bicarbonate

Loop of Henle

-U-shaped tube that consists of a descending limb and an ascending limb

-It begins in the cortex, receives filtrate from the proximal convoluted tubule, extends into the medulla, and then returns to the cortex to empty in the distal convoluted tubule

-Its primary role is to concentrate the salt in the interstitium, the tissue surrounding the loop

Descending limb

-Permeable to water, impermeable to salt

-As the filtrate descends deeper into the hypertonic interstitium, water flows freely out of the descending limb by osmosis until the tonicity of the filtrate and interstitium equilibrate

-Longer descending limbs allow more time for water to flow out of the filtrate, so longer limbs make the fluids more hypertonic than shorter limbs

Ascending limb

-Impermeable to water

-Ascending limb actively pumps sodium out of the filtrate, generating the hypertonic interstitium that drives countercurrent exchange

-In passing through the ascending limb, the filtrate grows hypotonic since it has lost much of its sodium content

-Hypotonic filtrate is passed to the distal convoluted tubule in the renal cortex

Distal convoluted tubule

-Distal convoluted tubule is similar to the proximal convoluted tubule in structure and function

-Cells lining the tubule have multiple mitochondria, enabling active transport to take place by the energy supplied by ATP

-Much of ion transport taking place in distal convoluted tubule is regulated by the endocrine system

-In the presence of parathyroid hormone, the tubule reabsorbs more calcium and excretes more phosphate

-When aldosterone is present, more sodium is reabsorbed and more potassium is excreted

-Tubule secretes ammonia and hydrogen to regulate pH

-After travelling the distal tubule, 3% of the water remains, and the remaining salt content is negligible

Collecting duct

-Each distal convoluted tubule drains into the collecting duct, beginning with the connecting tubule

-Normally impermeable to water, but becomes permeable in the presence of ADH

-ADH levels regulate urine concentration: dehydration increases ADH and produces concentrated urine, adequate hydration lowers ADH and causes dilute urine

-Also permeable to urea, which helps maintain the medulla’s high ion concentration

ADH (Antidiuretic hormone)

-Polypeptide hormone secreted by the posterior pituitary gland

-Regulate the amount of water excreted by the kidneys

-Causes the kidneys to reabsorb water directly from the distal tubules, thus concentrating the salts and waste products, which will eventually become urine

Regulation of ADH

-Dehydration or body stress will raise ADH secretion and water will be retained

-Stimulate reabsorption of water by the kidneys

Aldosterone

-Steroid hormone to regulate sodium and potassium balance in the blood

-Increases sodium reabsorption from the distal tubule and the collecting duct

-Produced by the adrenal gland