Organism level systems Study Guide

The body maintains a stable internal environment, including blood glucose levels, through a process called homeostasis. Two key hormones, insulin and glucagon, produced by the pancreas, play opposing roles in regulating blood sugar. When blood glucose levels rise, typically after a meal, the pancreas releases insulin. Insulin acts on liver and muscle cells, causing them to take up glucose from the blood and convert it into glycogen for storage. This lowers blood glucose back to normal. Conversely, when blood glucose levels fall, such as during fasting or strenuous exercise, the pancreas releases glucagon. Glucagon instructs the liver to convert stored glycogen back into glucose and release it into the bloodstream. It also stimulates the liver to produce glucose from other non-carbohydrate sources (gluconeogenesis). This increases blood glucose levels back to the normal range. The coordinated action of insulin and glucagon forms a negative feedback loop, ensuring that blood glucose levels remain within a narrow, healthy range, preventing both hyperglycemia (high blood sugar) and hypoglycemia (low blood sugar).

The human kidneys are vital organs responsible for filtering blood, removing waste products, and maintaining the body's fluid and electrolyte balance. Each kidney is a bean-shaped organ, roughly the size of a fist, located on either side of the spine, just below the rib cage. The outer protective layer of the kidney is called the renal capsule. Internally, the kidney is divided into two main regions: the renal cortex and the renal medulla. The renal cortex is the outer, lighter-coloured region, while the renal medulla is the inner, darker region. The medulla is further composed of conical structures known as renal pyramids. Urine formed in these regions drains into the renal pelvis, a funnel-shaped structure, and then exits the kidney via the ureter to the bladder.

The functional unit of the kidney is the nephron, sometimes referred to as the kidney tubule. There are millions of nephrons in each kidney. Each nephron begins in the cortex with a structure called the Bowman's capsule (or renal capsule). This cup-shaped structure surrounds a network of capillaries called the glomerulus, where initial blood filtration occurs. From the Bowman's capsule, the filtered fluid (filtrate) enters the renal tubule, which is a long, convoluted tube. This highly coiled tubule has several distinct sections: the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule. The proximal convoluted tubule is where most reabsorption of useful substances like glucose, amino acids, and some salts occurs. The loop of Henle extends into the medulla and is crucial for creating a concentration gradient that helps in water reabsorption. The distal convoluted tubule plays a role in fine-tuning the reabsorption of water and salts. Finally, the distal convoluted tubules from several nephrons drain into a collecting duct, which carries the urine to the renal pelvis.