Kidney structure and function

The kidneys reside against the back muscles in the upper abdominal cavity and are located in the region of the thoracic (T12) and lumbar vertebrae (L3), as shown in Figure 4.1. The right kidney is lower than the left kidney to accommodate the liver. The kidneys receive blood from the aorta via the renal arteries and return blood to the inferior vena cava via the renal veins. Urine is formed in the kidney and passed by a pair of ureters to the uterine bladder, where it is stored until it can be released through the urethra. There are a large number of blood vessels which run through the kidney and supply a vast network of capillaries.

Figure 4.1: Location of the kidneys as viewed from the back. Image adapted from http://philschatz.com/anatomy-book/contents/m46429.html)

Figure 4.2 shows the kidney’s anatomy and structure. The kidney has two distinct regions, the medulla and cortex. The medulla is composed of tubules, collecting ducts and blood vessels which are grouped together to form renal pyramids and is subdivided into the outer and inner medulla, with the inner medulla being furthest from the cortex. The tip of the medullary pyramid forms a renal papilla. The hilum is the medial aspect of each kidney, where the ureter, blood vessels, nerves, and lymphatic vessels enter or leave the kidney.

Figure 4.2: Diagram showing kidney anatomy and structure. (Image adapted from Renal Physiology and Body Fluids, Chapter 22, Kidney Function by George A.

Tanner)

The nephron is the basic structure and functional unit of the kidney, there are about one million nephrons per kidney and they provide an enormous surface area for the exchange of materials. Each nephron is composed of six regions as shown in Figure 4.3. These are the renal capsule, containing the glomerulus and surrounded by the Bowman’s capsule; the proximal convoluted tubule; the descending limb of the loop of Henle; the ascending limb of the

loop of Henle; the distal convoluted tubule; and the collecting duct. There are two types of nephron, cortical nephrons located in the cortex which contain a short loop of Henle, and the juxtomedullary nephrons which have renal corpuscles close to the junction of the cortex and medulla.

Figure 4.3: Components of the nephron and the three processes of ultrafiltration, selective reabsorption and secretion performed in the kidney. (Image modified from

Renal Physiology and Body Fluids, Chapter 22, Kidney Function by George A. Tanner)

The processes that are performed by the nephron are ultrafiltration, selective reabsorption, and secretion, with the transportation of substances being either by active or passive processes. All small molecules (i.e. water, glucose, and urea) are filtered out of the blood plasma in the glomerulus and produce a filtrate in the Bowman’s capsule which passes into the tubule of the nephron. Blood pressure forces fluid from blood in the glomerulus into the

Bowman's capsule. The Bowman's capsule is permeable to water and small solutes and is involved in nonselective filtration. The filtrate contains salts, glucose, amino acids, vitamins, urea, and other small molecules. All substances useful to the body to maintain the water and salt composition of body fluids are removed from the filtrate and reabsorbed into the blood capillaries which surround the kidney tubules. The processes performed by nephrons are described in Figure 4.3, and are (1) In the proximal tubule, NaCl, glucose, amino acids, K+, HCO3¯, and water diffuse into the tissue. This process helps maintain pH (HCO3¯) in the kidney. At the same time, H+ and NH3 are secreted into filtrate; (2) In the descending loop of Henle, water is reabsorbed into the tissue and filtrate becomes more concentrated; (3) In the thick and thin ascending loop of Henle, only NaCl is reabsorbed because the wall of the ascending loop of Henle is impermeable to water; (4) In the distal tubule, NaCl, water, and HCO3¯ are reabsorbed, while K+ is secreted into filtrate. In order to control pH in the tissue, H+ is secreted into the filtrate and HCO3¯ is reabsorbed; (5) In the collecting duct, urea and water is carried though the medulla to the pelvis and NaCl is actively reabsorbed. Finally, urine is excreted into a minor calyx by each renal papilla, which then flows into the renal pelvis and exits the kidney through the ureters.

Figure 4.4 shows the vessels involved in the blood circulation in the kidney. A single renal artery supplies blood to each kidney and branches into interlobar arteries, which pass toward the cortex between the kidney lobes. The interlobar arteries branch to form arcuate arteries at the junction of the cortex and give rise to smaller cortical radial arteries. The cortical radial arteries extend to form afferent arterioles and gives rise to glomerulus, followed by efferent arterioles.

Finally, the efferent arteriole divides into peritubular capillaries, which surround the kidney tubules and vasa recta, which are straight and long capillaries. Some vasa recta reach deep into the inner medulla. The descending and ascending vasa recta are grouped in vascular bundles in the outer medulla. This arrangement provides a medium to exchange substances between blood flowing into and out of the medulla.

The glomerular filtration rate (GFR) is the rate at which plasma is filtered by the kidney glomeruli. GFR is typically used to evaluate kidney function, and can be measured from iohexol or inulin clearance measurements, or estimated from a blood test.

The kidney plays an important role in the human body in maintaining the body fluid composition at a stable internal environment by excreting substances (waste products, foreign compounds, including useful substances)

Figure 4.4: Vessel supply to the kidney and flow of blood in the kidney.

in the urine. It also regulates the concentration of electrolytes in blood plasma, such as, Na+, K+, Ca²+_{, Mg²}+_{, Cl¯, bicarbonate (HCO}

3¯), phosphate, and sulphate. The kidney plays a major role in balancing acid-base in the body by excreting H+ when there is excess acid, or HCO3¯ when there is excess base. The kidney regulates blood volume by controlling Na+ and the amount of water excreted. It excretes nitrogenous compounds such as urea, uric acid, ammonia, and creatinine. It also helps to regulate blood pressure by adjusting Na+ excretion and producing various substances, such as renin, that can affect blood pressure and remove drugs and foreign or toxic compounds. If there is a problem with the kidney, it will manifest in kidney disease in the form of Chronic Kidney Disease (CKD) or Acute Kidney Injury (AKI).