Kidney: master controller of the body’s fluid compartments
Shifting study of anatomy and physiology from bones and muscles to study of the kidney is a challenge, because anatomy of the functional structures in a kidney is only seen with a microscope.
Learning how the kidney uses its microscopic structures to maintain a healthy composition of blood and interstitial fluid requires imagination and the use of cell models.
Fluids maintenance by kidney includes:
- Removal of water-soluble metabolic waste products from blood
- Regulation of the quantity of blood electrolytes, the water-soluble atoms carrying an electrical charge such a sodium (Na+) and potassium (K+)
- Long term maintenance of blood pH (H+)
- Safeguarding of plasma osmolality, and thereby blood and interstitial fluid volume
While accomplishing this, a kidney must not lose any of the valuable water-soluble nutrients delivered by blood to tissues such as amino acids and glucose.
Filtration, resorption and secretion
Removal of water-soluble waste products of metabolism is accomplished by forcing part of the liquid portion of blood into a set of tubules – filtration – and by sending back into the blood from those tubules the molecules that are important to keep like water, amino acids, glucose, Na+ and K+ – resorption. The portion of water and excess molecules not reabsorbed into blood exits the kidney as urine through the ureters.
Large waste molecules that do not filter easily at the glomerulus are moved into the tubules by another process called secretion. The walls of the kidney tubules, in cooperation with proteins in the walls of the blood vessels transfer large waste molecules into the kidney tubules for elimination from the body in urine.
It is extremely important to tissues in the body that the quantity of particular charged particles remains at near constant level in blood and in the protein-free interstitial fluid surrounding cells. For example, both heart muscle contraction and neuron activity require precise interstitial fluid levels of Na+ and K+ and Ca++. The concentration of these ions in interstitial fluid depends to a large extent upon their concentration in blood.
Steady, long-term regulation of blood pH occurs at the kidney. Excess hydrogen ion (H+), that (H+) not buffered by bicarbonate and other blood buffers, is secreted from blood into the tubules where it complexes with positively charged ammonium. Ammonium is synthesized by the cells of the tubules and secreted into their lumen to capture the free (H+) and eliminate it in urine.
Path of blood flow through kidneys
The arteries to the kidney, right and left renal arteries, branch directly from the abdominal aorta. The renal artery divides as it enters the kidney into segmental arteries.
The segmental arteries continue to divide as they move deeper toward the renal cortex which lies just under the kidney’s fibrous capsule. In the renal cortex, the arterial branches form the arterioles of the kidney’s blood filtration unit, the glomerulus.
Although not shown in the above image, nerves of the sympathetic and parasympathetic system also travel through the kidney in close association with the arteries.
Nephrons of the kidney cortex
The functional unit of a kidney is named a nephron. There are approximately one million nephrons in the cortex of each human kidney. Nephrons consist of a capsule containing an elaborate cluster of specialized blood capillaries and a set of tubular structures.
The tubular structures are surrounded by a capillary network that leads to venules. Venules join together to form the veins leaving the kidney. Kidney veins follow the path of the incoming arteries, but in a reverse direction. Venous blood exiting the kidney flows into the inferior vena cava for return to the heart.
The illustration above suggests more open space between the tubules and capillaries than exists in a kidney. Check out the photomicrograph at the beginning of this article and notice how closely the tubules pack around the glomerular capsule allowing only small spaces for the capillaries.
Nephron and Urine Formation – video by Ron Manalastas
Blood filtration at Bowman’s capsule
As shown in the video, the first step in processing blood in the kidney is accomplished by the glomerulus, the cluster of capillaries within the nephron’s capsule called Bowman’s capsule.
Capillaries of Bowman’s capsule possess characteristics different than capillaries elsewhere in the body. Other body capillaries spread out in a net-like pattern with an arteriole at one end feeding blood to them and a soft-walled vessel named a venule at the other end draining blood back toward the heart.
In contrast, the cluster of capillaries in Bowman’s capsule has an arteriole on both ends. This is important because it allows blood pressure within the glomerular capillaries, and therefore the rate of filtration, to be fine-tuned by changing the diameter of either the afferent (incoming blood) or efferent (exiting blood) arteriole.
Capillaries of Bowman’s capsule possess numerous large open holes called fenestrae. The fenestrated capillaries are covered by cells with foot-like projections named podocytes. The foot processes of the podocytes cells form slit-like barriers over the fenestrae that limit the amount of larger molecules such as protein that can be filtered into the capsule.
Arterial control of glomerular filtration
The rate at which blood constituents filter into kidney tubules is determined by blood pressure in the glomerular capillaries. The kidney has multiple mechanisms for regulating glomerular blood pressure. But, the primary mechanism is the response of the afferent arteriole to changes in overall blood pressure in the body, systemic blood pressure.
The wall of arterioles contains smooth muscle cells. The response of these cells to stretch is to shorten their length. High systemic blood pressure stretches muscle of the arteriole, the muscle contracts and the diameter of the arteriole becomes smaller permitting less blood to flow through. The reverse happens when systemic blood pressure is low. As systemic blood pressure drops, the muscle cells of the arteriole relax and blood flow increases.
Hormonal control of glomerular filtration
Refer to the diagram of Bowman’s capsule above, notice the juxtaglomerular cells between the afferent and efferent arteriole and the tubule with some of its cells labeled in green as the macula densa. The tubule pictured in cross section is a portion of the distal convoluted tubule of the nephron. It is the part of the nephron immediately before the urinary collecting duct. In vivo the nephron’s distal tubule circles back close to the Bowman’s capsule.
This arrangement provides an opportunity for the nephron to check on the progress of its work. The macula densa cells of the tubule sense the osmolarity of the tubular fluid. If osmolarity is too high, an indication that too many filtered particles are still in the fluid the macula densa cells release molecules called vasoconstrictors that narrow the diameter of the afferent arteriole.
In the presence of vasoconstrictors the afferent arteriole permits less blood to flow into the glomerulus. This slows the blood filtration rate and allows more time for absorption of molecules from the tubules into the capillaries surrounding the tubules.
In contrast, if osmolarity is too low in the distal tubule, indicating there are too few particles in the fluid due to an inadequate filtration pressure, the macula densa halts the use of vasoconstrictors. The macula densa cells also send signals to the cells of the juxtaglomerular complex to release a molecule named renin. Renin sets in motion the serial conversion of blood molecules to form a hormone named angiotensin II.
Angiotensin II has several effects. At the glomerulus it boosts the filtration rate. It decreases the diameter of the efferent arteriole hindering blood flow out of the glomerulus, thereby increasing pressure in the capillaries. It also improves reabsorption of Na+ and water at the proximal convoluted tubule and stimulates release of another hormone named aldosterone from the adrenal gland, which sits on top of the kidney.
Aldosterone works to augment the action of angiotensin II. Its effect is at the distal tubule and collecting duct of the nephron. It increase the reabsorption of Na+ and water. The net effect is an increase in total blood volume and therefore in blood pressure.
Do you have questions?
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Margaret Thompson Reece PhD, physiologist, former Senior Scientist and Laboratory Director at academic medical centers in California, New York and Massachusetts and CSO at Serometrix LLC is now CEO at Reece Biomedical Consulting LLC.
Dr. Reece is passionate about helping students, online and in person, pursue careers in life sciences. Her books “Physiology: Custom-Designed Chemistry” (2012), “Inside the Closed World of the Brain” (2015) and the workbook (2017) companion to her online course “30-Day Challenge: Craft Your Plan for Learning Physiology” are written for those new to life science. More about her books can be found at amazon.com/author/margaretreece.
Dr. Reece offers a free 30 minute “how-to-get-started” phone conference to students struggling with human anatomy and physiology. Schedule an appointment by email at DrReece@MedicalScienceNavigator.com.