Tuesday, April 7, 2009

Urine Formation

Filtration. The excretory tubule collects a filtrate from the blood. Water and solutes are forced by blood pressure across the selectively permeable membranes of a cluster of capillaries and into the excretory tubule.

Reabsorbtion. The transport epithelium reclaims valuable substances from the filtrate and returns then to the body fluids

Secretion. Other substances such as toxins and excess ions are extracted from the body fluids and added to the contents of the excretory tubule

Excretion. The altered filtrate (urine) leaves the system

In the human kidney, most of the nephrons are cortical nephrons, these are in the renal cortex. The rest are juxtamedullary nephrons, with long loops of Henle that extend into the renal medulla

Capillaries called afferent arterioles are associated with the nephrons, and as they leave the glomerulus, the capillaries converge into an efferent arteriole. This vessel subdivides again to form peritubular capillaries which surround the proximal and ditstal tubules.

There are Five Main Steps in the Transformation of Blood Filtrate to Urine

1.) In the proximal tubule, secretion and reabsorbtion changes the volume and composition of the filtrate. The pH of body fluids is controlled and bicarbonate is absorbed as are NaCl and water.
2.) In the descending loop of Henle, reabsorbtion of water continues
3.) In the ascending loop of Henle, the filtrate loses salt without giving up water and becomes more dilute.
4.) In the distal tubule, K+ and NaCl levels are regulated, as is filtrate pH
5.) The collection duct carries the filtrate though the medulla to the renal pelvis and the filtrate becomes more concentrated by the movement of salt.

Antidiuretic hormone is an important hormone in the regulation of water balance. It is the product in the hypothalamus and stored in and released from the pituitary gland. Two other hormones involved in regulation of water balance are angiotensin and aldersterone.

Chapter 45: Hormones and the Endocrine System

The Body’s Long-Distance Regulators

Animal hormones are chemical signals that are secreted into the circulatory system and communicate regulatory messages within the body.

Hormones reach all parts of the body, but only target cells are equipped to respond

Two systems coordinate communication throughout the body: the endocrine system and the nervous system

The endocrine system secretes hormones that coordinate slower but longer-acting responses including reproduction, development, energy metabolism, growth and behavior.

Types of Secreted Signaling Molecules

Secreted chemical signals include:
• Hormones
• Local regulators
• Neurotransmitters
• Neurohormones
• Pheromones

Endocrine glands are ductless and secrete hormones directly into surrounding fluid

Exocrine glands have ducts and secrete substances onto the body surfaces or into body cavities (for example, tear ducts)

Local regulators are chemical signals that travel over short distances by diffusion

Pheromones are chemical signals that are released from the body and used to communicate with other individuals in the species

Binding of a hormone to its receptor initiates a signals transduction pathway leading to responses in the cytoplasm, enzyme activation, or a change in gene expression

Insulin and Glucagon: Control of Blood Glucose

Insulin and glucagon are antagonistic hormones that help maintain glucose homeostasis

The pancreas has clusters of endocrine cells called islets of Langerhans with aloha wells that produce glucagon and beta cells that produce insulin.

Multiple Choice
1) Loop of Henle is concerned with:
A. excretory system
B. reproductive system
C. nervous system
D. muscular system

2) Glucose is reabsorbed in the kidney mainly by
A. Bowman's capsule
B. Loop of Henle
C. Proximal Convoluted Tubule
D. Distal Convoluted Tubule

3) In the kidney the correct sequence of formation of urine involves the following processes:

A. glomerular filtration, reabsorption, tubular secretion
B. reabsorption, filtration, secretion
C. filtration, secretion, reabsorption
D. reabsorption, secretion, filtration

4) In the distal convoluted tubule of the nephrons

A. Sodium reabsorption requires energy
B. Secretion of potassium does not require energy
C. Water reabsorption requires energy
D. Ammonia is secreted

5) All of the following statements about hormones are correct except:

A. They are carried by the circulatory system.
B. They are used to communicate between different organisms.
C. They elicit specific biological responses from target cells.
D. They are produced by endocrine glands.
E. They are modified amino acids, peptides, or steroid molecules.

ANSWERS: 1) A 2) C 3) A 4) A 5) B

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