The sinoatrial node is responsible for initiating the contraction of the heart. Depolarization of the sinoatrial node coincides with atrial contraction. The depolarization travels very quickly to the atrioventricular node during this period. The atrioventricular node delays the spread of the impulse, preventing it from triggering ventricular contraction. This time delay allows the atria to fill the ventricles with blood before the impulse causes the ventricles to contract. Without this delay, an inadequate amount of blood would be pumped from the ventricles.

The circulatory system is composed of two primary regions: the systemic circuit and the pulmonary circuit.

The systemic circuit allows blood to travel from the heart to the tissues of the body, delivering nutrients and oxygen, and then returns the deoxygenated blood to the heart. Most arteries, arterioles, capillaries, venules, and veins are part of the systemic circuit. The systemic circuit essentially starts with the aorta and ends with the vena cavae.

The pulmonary circuit receives dexoygenated blood from the body and carries it to the lungs for reoxygentation, before returning it to the heart to enter the systemic circuit through the aorta.

In general circulation, the vena cavae empty into the right atrium. Blood then enters the right ventricle and enters the pulmonary circuit. It travels to the lungs through the pulmonary arteries, becomes reoxygenated in the capillaries of the lungs, then returns to the left atrium via the pulmonary veins. From the left atrium, blood enters the left ventricle and is transferred to the systemic circuit.

Multiplying stroke volume by heart rate gives another measure called cardiac output. Stroke volume will be influenced by variables such as resistance in the arteries and contractility of the heart muscle cells, while heart rate will be influenced by variables such as emotional state and age. Both stroke volume and heart rate increase with exercise.

The left ventricle supplies oxygenated blood from the heart to the aorta so it can be distributed to the body. The left ventricle must overcome the forces in the aorta and arteries (e.g., blood pressure). The higher the blood pressure, the harder the left ventricle must contract to force blood out of the aortic valve. This leads to muscle growth and enlargement of the left ventricle over time.

Cells and large proteins are too large to cross the capillary membranes of the glomerulus, and cannot be filtered into Bowman's capsule. After this initial step of filtration, urine in the nephron is only affected by secretion and reabsorbtion of water, ions, and other small molecules. Finally, each kidney is composed of about one million nephrons.

The kidney-nephron system is one of the few portal systems in the human body. Portal systems link two arteriole or capillary systems together, in this case the afferent and efferent arterioles. The efferent arterioles leave the glomerulus after it has been filtered into Bowman's capsule, and follow along the nephron to pick up any reabsorbed material from the nephrons via the vasa recta capillary bed. Both the glomerulus and vasa recta are groups of capillaries, which qualifies the renal circulation as a portal system.

The proximal convoluted tubule is incredible for its ability to reabsorb glucose at levels of nearly 100%. This function is due to specialized proteins that help transport the glucose out of the filtrate. Damage to the proximal convoluted tubule can lead to glucose in the urine. Another possible cause would be extremely high glucose levels in the filtrate, such that the proximal convoluted tubule is incapable of properly removing all of the solute. This condition is a trademark of diabetes.

The loop of Henle, the distal convoluted tubule, and, under special circumstances, the collection duct are responsible for the reabsorption of other nutrients (such as water and various ions). 

The glomerulus, proximal tubule, distal tubule are all located in the cortex (outer portion) of the kidney, where the osmolarity of the interstitial fluid is relatively low. Both limbs of the loop of Henle and the collecting duct are located in the medulla (central portion) of the kidney, where the osmolarity of the interstitial fluid is much greater.

Filtrate movement through the nephron into different surrounding osmolarities is what allows water and sodium to be retained, if necessary, while other waste products are concentrated in the urine. The evolution of the loop of Henle is specifically designed to create the countercurrent multiplier system that allows for urine concentration and water retention in land animals. Water-based animals generally have shorter loops of Henle that may not intersect with the renal medulla, since water retention is less important.

Together, the glomerulus and Bowman's capsule form the structure known as the renal corpuscle. Blood in the capillaries of the glomerulus is forced against the walls of the vessels, where specialized epithelium and cell junctions allows fluids and small solutes to diffuse across the walls of the glomerulus and into Bowman's capsule. This process is highly dependent on pressure differentials; higher hydrostatic pressure in the glomerulus and greater solute concentration in Bowman's capsule will work to remove water and fluids from the capillary. Blood cells and large proteins are unable to pass through filtration and remain in circulation. Ions, small sugars, amino acids, and nitrogenous wastes pass through filtration, and are either reabsorbed back into the blood or excreted in the filtrate.

The proximal and distal tubules are regions of the nephron that are located in the renal cortex and specialize in ion reabsorption. The renal pelvis is located in the renal medulla and serves as the final collecting point for filtrate from multiple collecting ducts before transferring it to the ureter.

The function of the kidney is to filter wastes out of the blood and concentrate them into a filtrate that can be excreted from the body. Nephrons are the functional unit of the excretory system, meaning that each nephron is capable of concentrating wastes into filtrate. Each nephron is made of a single long tubule, with different regions modified to transport different ions and wastes into or out of the filtrate. The proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule are the principle regions of the nephron.

The glomerulus and Bowman's capsule form the renal corpuscle, the site of blood filtration. While the nephron serves to concentrate filtrate, the renal corpuscle separates the filtrate from the blood. The glomerulus is a system of capillaries, and carries blood rather than filtrate. It is kept separate from the nephron by the barriers in the glomerulus walls and Bowman's capsule.